You don't know how many years I tried to get Fuzzy (Radio Shop Cat) to sit here - -
Just too busy for that nonsense. And a bit too large.
Now, if I can just get her to sit there when people visit.
Dolly is a very looong cat. Wrap around radio warmer - -
No job is too difficult.
Ready to go, 24-7.
Let's go fix something. The more mice - er - tubes, the better.
P.S. To my friends over at ARF: Shortly after posting a link to this blog, my login became "inactive". Maybe someone over there does not like cats. I don't have time to mess with it, so you won't see me posting there anymore.
- - - or any large signal tube.
If you can tolerate shopping for tubes on E-Pay you will often encounter the claim that an audio output tube is "tested" or that a pair is "matched", OR that a pair is "NOS", tested and matched. So what does this mean and why is it important?
Well - - "matched" can and often is used to describe tubes as appearing the same, same manufacture, maybe the same date code. Sometimes "NIB" (new in box) is claimed because the tube is in a box with the manufacture matching the label on the tube. These claims are often made without "testing" at all. Often an inexperienced seller will use what he has seen as KEY WORDS in ads placed by experienced sellers. These sellers often don't know if the tube is good or not.
Some sellers are actually trying to be accurate, which is a good thing considering the consequences of making false claims - the loss of a sale + round trip shipping. The problem lies in both understanding and capability of the person making the claim.
First let's consider new tubes being sold by a real manufacture. I certainly think that claims made by RCA, GE, Mullard and other traditional manufactures of a bygone time were mostly legitimate and accurate. Todays manufactures should also be able to make legitimate claims. Retailers may muddy this picture, but for the purposes of this article, I will exclude claims made by the actual tube manufacture. What's more, the expected test values are often those stated in manufacturer's spec. sheets. Here we will often refer to the specifications (specs) listed in the RCA tube manuals.
Everyone interested in tube-type equipment should have one of these manuals
There is much more about tube testing already published. My point here is to help you determine what claims are accurate and how to verify them. So, what do the terms mean and how can you test tubes against values given by sellers or just for your own application?
N.I.B. - New In Box, never used beyond testing.
Matched Set - A pair of tubes of the same manufacture, similar date of manufacture, which perform nearly equally in all respects throughout their operating range. - for all practical purposes, identical.
N.O.S. - New Old Stock, same as N.I.B. only the box may be missing.
Tested - Covers all extremes from "testing" the filament with an ohm meter to testing with a high-end tube tester.
A matched set of tubes makes for an easier installation especially in an amp that only has one bias pot per channel.
There are many more terms that refer to condition in some way. Many of them don't make much difference to the average user.
A "tube tester" , often depicted with a tube being tested and a finger pushing the button by people selling tubes. Hickok I-177 US Government issued tester from the 1940s
"A tube is best tested in the appliance in which it is meant to operate." Which is true as long as that test isn't simply PLUG & PLAY. A "small signal" tube like a 6BA6 or even a 12AX7 can generally be accepted as OK if the device it is in operates normally. This means that it does work to some extent but the tube may still be weak. It probably does not have a shorted or open filament (heater) or the tube would not light/work at all and the pilot lights are on and the power transformer is not smoking (!! there are many drawbacks to testing in the device for which the tube is intended).
Simply stated - the more testing you can do prior to placing a tube in service the less likely you are to experience a bad/smoky outcome.
Even the simplest tube testers have a filament check function or this can be done with a V.O.M. (volt ohm meter). Yet, even here frustration can mount do to a high impedance short that can't even be seen until the tube is hot. I have swapped out so many 227 tubes or even 12AX7s because of a filament cathode leak which produces a "leaves rustling" sound in the output. So even this simplest of tests is not to be taken lightly.
But there is so much more. The issue above is one found in both large and small signal tubes. What else should I consider while testing a tube? Just what can a tube tester tell me? Here is list of tests and terms associated with testing and using vacuum tubes.
Shorts - Most common and probably most important. Results range from low resistance (seen on your V.O.M.) to high impedance which requires accurate calibration even on your high-end tester. Remember, this test not only covers a direct short as seen on your VOM but high impedance shorts like the noisy heater/cathode short that could measure in the tens or even hundreds of thousands of ohms. Tests should be done with the filament lit at normal operating temperature.
Filament/heater voltage - An AC or DC voltage applied to the tubes filament. Needs to be an ACCURATE setting for ALL tubes listed in the tester's charts. Assuming that the builders took filament voltage sag into consideration is a fools errand. If the tube is designed to operate at 6.3 volts anything outside of the normal range (slightly more or less than 6.3V) Will lead to a poor prediction of actual performance. I once heard "I wish that I had known that prior to throwing out a lot of low-testing 245 tubes."
This is probably the most unreliable function of MOST tube testers. It will be of particular concern when testing power tubes with low voltage filaments like the UX-245 (early 45).
Most testers have a "line voltage adjustment" beyond the rotary switch that indicates which filament voltage tap you are using. So this addresses the problem - right? Yes, up until the time that you push the "test" button. Even a small signal tube with a 2.5V filament is going to draw considerably less B+ current than the UX-245. So when you set the filament at 2.5V, then use the line setting to adjust the meter to the "set" point the difference in B+ current is going to cause the filament voltage to sag since most testers use the same transformer to generate all test voltages. For the test to be accurate a tester would need to realign the set point while the test is being performed. Few testers will allow this, having only one meter, you cant push "test" and look at the filament set-point at the same time. Beyond that, most testers only have limited calibration adjustment for the filament transformer. Most techs try to get the filament voltage right in the 6.3 to 12.6V range, where most tubes operate. This usually leaves the ends of the range - below 5V and above 12.6 - to settle where they will. Some testers have a filament transformer separate from the high voltage transformer to reduce sag in the filament supply caused by a load increase on the B+ while testing.
Hint: While testing a tube in one socket, use another socket to monitor the filament voltage while the test is performed.
Example: While testing a UX-245 in the 4-pin socket (UX), use your VOM to check the filament voltage on the 5-pin socket. A tube with the Hickok "J and R" settings will work but a tube like the 12AX7 , "E and V" settings will not. But you can modify this to work in most cases.
B+ or Plate Voltage - This is the high voltage applied to the plate during a test. Generally the B- voltage (or close to it) will be on the cathode. The screen voltage is often derived from the B+ source. It is DC. It needs to be close to the plate voltage seen in actual operation in a device. It is also subject to sag.
B+ in most tube testers is limited to around 100V. With many small signal tubes like the 12AX7 this is adequate to simulate conditions in a device. For a tube like the 6L6GC this is far below normal operating conditions which can approach 500V.
Screen Grid (G2) - Usually derived from the B+. A positive voltage that is often about 80% of the plate voltage. Screen voltage in a power pentode is applied to the middle of 3 grids. It's value can influence the output of the tube greatly. Also poorly addressed in most tube testers due to the low B+ available.
Control Grid (G1) - The signal voltage is applied to the grid closest to the cathode. This is the signal used to drive the tube. Most tube testers used an AC signal. Early testers like the I-177 injecting 1/2 V which can overdrive some small signal tubes. Bias is the - (negative) DC voltage applied to this grid to control the tube and prevent excessive current flow. With 450V applied to the plate of a 6L6GC, the negative bias voltage is around -37V. A tube tester capable of testing a tube to its max values must also generate this voltage. Most tube testers can't but don't need to because of the low B+ they use.
Suppressor Grid (G3) - Usually at or near the cathode voltage. Close to 0V (B-). Not usually accessible or adjustable in tube testers. It helps to keep stray electrons from hitting and changing the operation of the other electrodes.
Supreme 500 Tube Tester 1937
The old adage says that if your tube tester indicates that a tube is bad it probably is. If your tube tester indicates that a tube is good, it might be.
Other than detecting shorts, what can a tube tester tell us?
An Emissions Tester will will display a value on it's meter that is often a number between 0 and 100. I am resisting calling this a percentage, but some do. Generally the scale is red-fail, green-pass and yellow-?. The yellow area is usually centered around 50 or 60.
Emissions testers are generally OK for small signal tubes, pass/fail. They can fall short when it comes to power output tube testing. I'll give you an example: One of the OP tubes I often use is the 7868. It is closely related to the 6L6 and really close to the 7591 with a different base (novar). My Hickok testers all lack a novar socket. The only tester I had that could test the 7868 was a Dyna-Jet 707. Though it is a mutual conductance tester, there are a few types that it will only test for emissions. The 7868 is one of them.
Mutual Conductance - Measured in mhos or micromhos this is the preferred measurement for tube users. Mhos is the opposite of ohms and ohm spelled backward. The process was made popular on Hickok designed testers though some Hickok designed testers were built by other companies (contract TV7s). Others also designed circuits to achieve a similar end. Some testers, like the TV7 used the Hickok circuit but displayed the readings on a numeric scale rather than mhos. Also, to add confusion, some test charts specify a reading that is what they thought a new tube should read, so you had to apply your own failure calculation - now is that 50% or 60%. Some gave you a minimum value below which a tube was considered "bad". The expected mhos reading can vary from one tester to another even in the same brand. That could be expected due to the difference in signal level and plate voltage applied in the test. - - - Confused? You'll get used to it. I recommend further reading for sure.
Following the shorts test, all of the 7868 tubes that I tested on the 707 would pass at around 85. It did not matter whether the tube was new, used, or very used. This really points to the shortcomings of this type tester. I had one very used tube that read around 80. The fact that all of the others were slightly higher made me uneasy about using this tube - but it still passed - for what that is worth. Keep this tube in mind, I'll get back to it later.
The 707 has a very useful feature not found on many tube testers, a Grid Emissions display function.
All of the drawbacks found in moderately priced classic testers were known to techs and manufactures. Eventually Hickok produced the 539 series of testers which became more versatile as the series progressed. They knew that a tester that more closely matched actual operating conditions was needed.
There were a few lab-grade testers but they were quite limited in availability and had high prices THEN. Now they are rare and WAY beyond most peoples budget. Hickok introduced the Model 539 in the middle/late 1950s. It was still expensive and probably beyond the average amateur tech in complexity of operation. The 539C was the latest model. It is a upgrade to earlier models in many respects.
In modern times several testers were developed to accommodate the need for testing power tubes. The Amplitrex is one that is sometimes mentioned in references to matched and tested tubes. It appears to still be available "new manufactured in small quantities". Price is not listed by the manufacture but my research found prices in the thousands of $. I have never tested one of these but they seem to be able to overcome some of the earlier testers shortcomings. Having not used it, I can't offer a review, though there are reviews on-line. Here is the company's site: https://amplitrex.com/
Features of the 539 were:
B+ supply could provide about 180V to the plate (closer to 200V on the B,C models)
Negative bias could be set by the user and it is displayed in volts on 2 scales up to about -45V (actual).
Signal voltage can be 2.5V or .5V (AC)
Line voltage is displayed and adjustable during a test (red "set" line = 100V). "Test" button can lock.
Tube performance in mhos on 5 scales
Output for plate current meter/shunt - meter optional
Output for grid current meter/shunt - meter optional
Separate high voltage and filament transformers
I mentioned above that no tube could be expected to perform to spec with a luke-warm filament caused by sag. You can see here that the test parameters were adjusted to compensate for the lower B+. Note that the -bias for a 6L6 is only -3V which is in line with lower voltage (180V) on the plate. The expected output in mhos is also lower than a tube with higher voltage on the plate. (The mutual conductance varies with the test parameters). Filament supply sag is addressed by being able to set the line during a test and a filament supply separate from the HV supply. So manufactures were able to adjust the expected results to accommodate some of the limitations.
Variable mho scales as well as variable signal level
I had to replace all of the meters due to really poor packaging. The guy I bought this tester from broke down a box and then wrapped it around the tester with no padding. You don't have to dent the case to ruin the meters - but that is another story.
Reset the line level - missed a little didn't I?
This 6L6GC tests at 4200 micromhos. The minimum listed on the roll chart is 3800
Around 44 mA using a piece of nichrome wire for a shunt
The plate current meter is not incorporated into the 539 but a set of contacts are. If you install a meter (recommended) you will have to change the value of the shunt, which comes with a shorting bar (remove). I used about an inch of nichrome wire as I recall about 29 ohms/ft. The meter is labeled in fractions of a milliamp but with the right shunt I made it read in mA x 10 without the decimal. Fuzzy, in the lower right is the break-out for grid current. I need to get another meter in there. IMHO this mA measurement is at least as important as the measurement of mhos because this is what you need to set up your amp. Tubes should be matched to output in mA at rated power with negative bias the same or very close.
What is a reasonable solution to the shortcomings of tube testers?
Build your own tube tester. The best vintage testers, like the Hickok 539, A,B,C and the more modern testers like the Amplitrex are EXPENSIVE(! ) And difficult to maintain and calibrate. Some don't even have a clearly understandable calibration method (like my 539A). Some have limited testing charts (like my 539A). You can avoid all of this building your own.
Cost to build my tester was a little more than $100. You may already own an old PA amp which is not really suited to use in your hi-fi setup. (Audio quality aside, They are kind of ugly especially when you stack up 2 of them). The meters I used are cheap on Pay-Bay (around $10ea for Chinese made). All of the rest of the parts came out of the junk drawer (inventory). It came with 2 VERY used 7868s. I have a push-pull 7189 amp which I intend to configure for the 6BQ5 family of tubes. My 6BQ5 tester will use the same plug in display.
The adaptors, 7868 to 7591 and 7591 to 6l6G,GB,GC are also available on P-Bay, or build your own.
Bogan CHP-35 Amp in it's new life as a tube tester but still allowing the user to listen to the tube/tubes being tested.
This project will take some tube experience/knowledge and the associated tools and skill. I think that a lot of my readers do have the tools and ability required. If you are new to tube circuits, save it for a while, but be aware of the shortcomings of what is available now and the claims made by tube sellers.
Very used 6L6GCs. Adaptors reduce the need to punch out a lot of tube socket positions, associated wiring and switches.
This pair of 6L6GCs is pretty well matched and will make full rated power, but they aren't new (or in a box). Generally, you will find that a new tube will make rated power at a higher negative bias setting. Well used tubes may not make rated power even at vey low bias settings. Because of the condition of the getter, I would use these tubes in seldom used equipment but not in a frequently used amp.
Note the brown edges on the getter. This indicates a used tube. My theory is that, temperatures of the electrodes and electron bombardment vaporize coatings and possibly metals. These can precipitate on cooler surfaces like the mica insulators unless absorbed by the getter. Note also the difference from a tube that has "gone to air" (loss of vacuum) in which the getter turns white. Red plating can make this much worse than "normal" use. Deposits on the micas of my 7868 tube probably lead to it's runaway condition.
HEY! those old tubes match pretty well
From left to right, top to bottom, the meters display:
B+ Plate Voltage Screen Voltage
Bias (1) Bias (2)
Milliamps(1) Milliamps(2) Both must be /10 to account for the 10ohm cathode resistor, .2332 = .02332 amps = 23.3mA Display is mV which converts to mA.
Finding meters that display down to 3 digits before the decimal is necessary for the output current and there seems to be limited choices at this time. (accounting for the smaller size meters)
You will need to add:
A filtered supply ~ 5 - 12V DC for the meter display. In my case removing the 6EU7 tube from the Bogan saved about as much power as was required. The 6EU7 was the preamp used with a phono. The 6.3V AC filament supply is good for this.
A filtered negative bias supply. Use a full wave or bridge rectifier and maybe a voltage doubler on the existing bias supply. It needs to be adjustable from ~3V to -45 or -50V somewhat independent on each side (tube 1 & 2)
Variable control of the screen supply. There is little current needed. 1 control.
I kept my tube rectifier and added a filament on/off switch. This allows the filaments of the tubes being tested to warm prior to turning the rectifier filament on. Watching the display through warm-up allows for a quick shutdown if trouble exists. The tube rectifier protects the power trans better than a solid state rect. But, more plate voltage would be available using sand (solid state).
Tone controls can be replaced with the new controls required for testing. (no need to drill)
I had installed test ports on the Bogan. They are handy but redundant and were used prior to building the meter box.
No more need for outdated roll charts. You can use the RCA tube manual for settings instead.
34.75W exceeding the 30W plate dissipation rating - single tube 6L6GC, Note that this test is for tube 1 only - 0s displayed for tube 2 current
This is about all you can expect from this tester. Exceeding the max ratings will damage the tube/redplate/BAD. Don't try this at home. You might want to keep your calculator handy.
Inject a clean sine wave at a known level
You can either use a speaker on the Bogan's output or install a load resistor. DO NOT LEAVE IT OPEN. Using a speaker is a real plus. You can hear what happens if you use too little screen voltage. To much screen will make more power but most people would rather save their expensive tubes rather than getting the full 35 watts out. If to little screen voltage is used, power output distortion will occur at a lower level. You won't be able to get adequate idle current.
The Bogan was built to supply 35 watts output from the pair of 7868 tubes. This was really putting the squeeze on them (She won't take much more of this Captain.). The schematic spec. was lower but testing reveled that about 410V on the screen would get you that 35W and that is where it was when found. I would recommend not exceeding the values in the RCA manual. Dropping the screen and raising the negative bias slightly will not make a noticeable change in sound but will save your expensive toobes. With this tester you can listen to the results and make changes accordingly. Don't forget - this is the tester. Anything you decide will have to be applied to the actual amp in which tubes are to be used.
AND - - should I mention that you need to know the values used in the actual device (amp) prior to setting up this tester. It is easier to adapt a 6L6GC amp to this function than a 6L6G amp. Higher B+ will already be available as well as screen and negative bias.
Use a dual trace scope to compare the input signal with the output
Will your average mutual conductance or emissions tester match tubes used in high level audio outputs? No. This includes the ever popular TV7.
Will your average mutual conductance or emissions tester test tubes for quality/performance in high level, real world applications? No.
Will your average mutual conductance or emissions tester test small signal tubes. Yes
Will a cheap tube tester perform any useful test? Yes, if the shorts tester is working properly.
Can you build a reliable tester? Yes
You can also set up your receiver/amp to test its own tubes using a VOM and a handful of parts but this is risky if you ever encounter a tube with a high impedance short (not found by a shorts tester). A runaway tube can take out an output transformer before you get your meter on the right pins. (don't ignore that hum in the output. One side of the PP outputs may be conducting a lot of DC through the primary winding).
I tested the previously mentioned 7868. Along side of another 7868 it was doing 8mA when the other tube was near full power. I began turning down the - bias (becoming more +), still 8mA. Eventually I hit a bias point that the output began to rise. Without adjusting the bias further it continued to rise - quickly. I did not wait to see how far it would go. And it hit the trash can a bit later. Remember, the DJ 707 said it was OK. I did not have an adaptor to sub it into an octal socket on the Hickok 533, but I suspect, passing the shorts test, a failure would not have been indicated, though I do not know what the GM would have been. The 533 also has a relatively low plate voltage.
There are a lot of testers not covered here. There are a lot of tubes not covered here. I have included a few of the modifications to the Bogan amp I used since the one you use will probably be different. If you have any questions, you can post them below.
I think that this will always be a work in progress.
The Dial Belt was slipping. If you don't change the dial position every once in a while even the new silicone dial belts get out-of-round and slip, especially on the Zenith Strat since there is so much moving mass.
Don't tell Dolly that she is looking at the wrong end.
Rather than have this information/update separate from the rest of the story, I have posted it ate the end of the previous entries - "Zenith Got it Wrong" and "Zenith got it wrong Part Deux" (Duh):
Actually acorns - Oak nuts,
I did not think they would last long, even after slathering them with polyurethane.
The birds never thought much of the Crosley Buccaneer. So it quietly disintegrated.. The birds found the Fiver much more interesting. Turns out it was not an eagle nesting in there ;-) but a bunch of Acorn Woodpeckers decided to use it as a store-house.
The Fiver is now bursting at the seams - filled to the top with acorns. When it was filled to the dial they began stuffing acorns through the cracks, which became wider and wider (it is near complete structural failure).
Acorn woodpeckers prefer location to tube count.
I knew that Sue's radio was unusual but it is more special than I had thought:
We now know the the Aetna model is 252P under Walgreens. It is a 4-tube + ballast Tuned Radio Freq receiver from 1936. Most of the radios found in this cabinet are 550s, which is a superheterodyne rather than a TRF. The super is relatively rare. The TRF is much more unusual.
I know that a lot of people have wondered what would happen if a person was to design a receiver - a TRF receiver, that had one high gain tube driving another and so-on. With this chassis it is easy to find out. Like the front end in a Zenith Stratosphere this chassis has a tuned 6D6 driving another, in this case a 6C6. Well, the answer is that LOTS of RF gain can get you - squealing, motorboating and eventually - nothing. Even in the Strat, they had to turn off one of the 6D6s on some bands to keep the noise down to a controllable level. Unlike the Strat, or any super, this radio has no IF to narrow and amplify a specific freq so strong stations can be over powering to weak stations.
With this radio it is best to decide what kind of antenna you are going to use and align the radio to that antenna. A longer antenna will provide the noises mentioned above and a shorter one will make weak stations - weak. While the schematic in Riders says not to bend the plates on the tuning cap, linearity across the band is poor. Maybe someone already bent the plates or maybe they need some adjustment, but the spot (freq) you choose to align to is going to be the sweet spot all else going down hill from there.
No wonder they replaced this chassis with the 550 super.
I thought that I took some chassis pics as we updated the radio with re-stuffed caps we found over the last 7 years. I'll try to find them.
For internet searchers:
What the hell is wrong with my -
OR any early 5516 chassis.
Use this post and the Part 1 below to correct later 5516 chassis parts discrepancy.
It seems that Zenith never posted a corrected schematic or parts list. All of these issues come from the early use of a 2125 ohm field coil speaker. Most of these chassis came with a 1000 ohm speaker.
IMPORTANT NOTE: When servicing one of these chassis it is not mix and match. If your speaker has a 2125 ohm FC use the original schematic . If your chassis has a 1000 ohm speaker use the corrected schematic and parts list.
In all honesty, the original configuration does not work as well as the latter. I suspect that the speaker spec was an error and , rather than eating the cost the wrong part, Zenith used them anyway. From what I have seen radios having the early speaker (2125 ohm) are far less common. I have observed both the 6.5" and the 10" speakers with 2125 ohm FC, but I have not seen an 8" ((49-152) speaker with the 2125 ohm FC.
Original graphics courtesy Nostalgia Air/Ryders
And Now for Something Completely Different
I have been studying these radios for a while along with their battery powered brethren. So I ended up with quite a few "extra" parts.
I have (had) a 5648 chassis out of a 6-S-330 complete with speaker, all restored. I didn't have a cabinet and never really liked the photo finish offered on the 1938 model. So I put the slightly better 6 toob chassis in a 5-S-127 cabinet and the result is 6-SX-127, the radio they never built but should have.
The cabinet for this one came from PayBay. There is a seller offering a lot of parted-out cabinets and chassis parts. This one looked ok from the sale pictures, but when it got here, well, it was a mess. It had been refinished poorly and sanded hard and dropped at least once. So I decided to reveneer the top and front with book-matched walnut burl, which is a lot more figured than the Zenith stock veneers.
Fuzzy, the ATV riding shop-cat and friend for almost 20 years had just passed so I had no inspiration to move on. The cabinet just kept getting smoother and shinier until it was hard to photograph, especially outside like I normally do.
I miss that old cat.
Say it isn't so!
This is a follow-up to the chassis restorations presented below:
In that post we discussed differences in the 5516 chassis - and replated a couple of them.
Obviously, each cabinet is different and this chassis was placed into 6 different models. Production changes are expected and were not particular to Zenith. Philco was notorious for dozens of changes in some 1930's models as were many smaller manufactures.
What is unusual are the undocumented changes to this chassis in that the published schematic represents a version that had a significant flaw and most radios found do not comply.
Pictured above is the 5-S-127 chassis that I replated and restored in the previously mentioned post. This layout is what is typically found.
But , take a look at the schematic. Many restorers have come to this point and then spent days trying to figure out why their radio is significantly different. And it appears that the largest fraction of chassis produced DO NOT conform to this schematic or parts list, BUT their radio seems to work fine.
What is worse is that if you try to follow this schematic the result is a radio that works - poorly.
Most noticeable is the difference in the field coil impedance, most as-found with 1000 ohm field coils. In these radios the power transformer is significantly larger. In the second picture (from the top) you can see both the size difference in the PTs and different speakers.
The "spec" field coil is 2125 ohms (or 2150).
When you dig deeper a person finds that R5 and R10 are way off from spec - not that they drifted, it was built that way! R10 is most often 490k ohms rather than 990k ohms and R5 is 240k ohms rather than 190k ohms.
I have heard it said that these differences were to accommodate the different field coil values and that is probably true since you would need to change these parts to accommodate the larger field coil impedance.
Well, then why doesn't it work? Or, why does the radio with the 1k ohm F.C. work so well with the wrong resistor values?
I should note at this point that there are several other differences, like band switches that are different enough to require different knobs in order to point to "A", "B" and "C" and a lot of minor stuff.
Above, as found - as most are found not conforming to the parts list. This radio would have (does have) a speaker with 1K ohm F.C.
Above, conforms to the schematic, has a 2150 field coil and works poorly.
So what is the deal?
Most people speculate that parts availability explains the need to "redesign" the radio in mid production, but here is the problem:
A radio with the 2125K F.C. and the non-spec resistors (R10 240K, R5 490K) will have about 285V on the 6F6G plate and -37V bias on the grid. This is almost twice the allowable bias according to the RCA tube manual which shows -20V on the grid.
A radio with the 2125 F.C. and the resistors on the schematic (R10 190K, R5 990K) will have 167V on the 6F6G plate and -27V on pin 5, the grid. This is better but the audio is still limited and settings in the upper third of the vol. cont. range are distorted.
A radio with the 2125 F.C. and the resistors on the schematic (R10 190K, R5 990K) and the 1.5K ohm resistor placed in parallel with the F.C. will have 255V on the 6F6G plate, 265V on the screen and -21V on the Grid. The radio will work normally and the volume is good through the range.
A radio built with the " non-spec" resistors (R10 240K, R5 490k)) but with a 1000 ohm F.C. (49-165) will work just fine as-is..
Higher line voltages today will also raise these voltages somewhat.
The speaker F.C., though in the return leg, was designed to avoid the typical candohm used to generate a negative bias voltage and provide some ripple filtering on the B+..
A resistor in parallel with the F.C. does not offer the same inductance so some hum might appear, though this was not the case on mine.
I checked both 5-S-119 and 5-S-126 radios on display in the museum and they were the 1000 ohm F.C. variety.
I checked as many radio pictures as I could find on-line and they were consistent though I don't know their history and a lot of pictured radios had owners/restorers that I referred to earlier (WTH is going on here???) so my first hand observations of original and mostly original radios is the basis of my conclusions. Input is invited.
B+ on the detector was also high. Spec. is 75V and it was as high as 150V in some configurations. B+ was high everywhere do to the decreased load presented by the 2125 ohm F.C.
There would be better ways to "fix" this problem but most would result in several visible changes to the layout. What I chose to do was use the parts as speced in the schematic and add a 1.5k ohm 3W resistor in parallel to the 1K field coil. This increases the current draw on the B+ by 20ma and does not seem to cause a noticeable temperature rise in the stock (smaller) P.T.. I partially offset the total draw on the P.T. by using LED lamps for dial lights. (small light green WW resistor above)
Above is my 5-S-151 console who's chassis is described by this schematic. It calls out another 2125 F.C. speaker, 49-144, a 10 inch model.
This is the speaker that is actually in the console - a 1000 ohm unit, (49-118AB). I have never seen a console with the 2125 ohm field coil speaker, (49-144). I do not know if they exist.
So what percentage of chassis/speaker combinations are compliant with the schematic - I do not know. At this time, I have the one pictured. From my files I found one that was sold about 10 years ago to a local fellow that used it at night to drown the high pitched whine caused by tinnitus. I don't recall much more about that unit though it was a "looker" (cabinets really do vary a lot). If he was to read this post, I would like to do a check-up on that radio too.
In conclusion, for what ever reason, Zenith built quite a few of these chassis/speaker combinations that worked poorly. The change was probably to correct this error. The power transformer change was probably called for by a slightly increased B+ load. The error is well documented. It would make sense that the error was in early production. In mine it appears on the plated chassis rather than the painted chassis though I am not certain if this is always true.
See also: https://www.antiqueradios.com/forums/viewtopic.php?f=1&t=401068
Has anyone ever seen a schematic with the proper parts?
P.S. I want to tell you about the lighter colored 5-S-127 cabinet with the replated chassis. I really like those veneer choices. So many of them are really dark.
And what is it with the 1937 Zenith band switch drawings - in particular the antenna switch section on the multiband radios? It seems to be in an impossible position. Those connections - the one connection alone - never occurs in any of the switched positions - maybe in radios with an X-band, but still that would have been specified. Best I can tell it was drawn with the contact rotating from the wrong direction BUT this is not the only contact on the wafer, those are correct.
AND have you checked a 6A8 on your Hickok tester lately. I found the settings on my roll chart ARE WRONG, particularly for the bias which is 30-something on the chart but only 11 on the trusted Consolidated Test Data For Hickok Model 533A-600A-605A Tube Testers. No wonder I've seen so many sales of "low testing" tubes.
I am still looking for 5516 chassis to further study (any variety) and complete projects. I also need period correct Zenith tubes. Thanks to all who have helped.
But Wait! There's More
Here are some more changes that apply to the later, large transformer/1000 ohm field coil units.
This chassis has a shield, wrapped in a fiber insulator, that extends over the 6K7G IF tube's base.
Here is a close up of the shield.
Also note the bottom of the antenna coil (upper right under the trimmer cap). There is no external "Antenna Choke" part number 2 on the schematic. It is incorporated into the antenna coil and as a result the antenna coil has an extra ground contact which is missing from the unit pictured below.
The schematic above highlights some additional parts changes besides the "antenna choke" #2.
C 15 .1 MFD 22-294 might be .05 MFD part # 22-523
R4 30000 ohms at 1/2 W might be 1W
R11 80000 ohms at 1W might be 100000 ohms
Chassis W/O shield and with external "Antenna Choke" external to the Antenna Coil.
Close up of External "Antenna Choke" part # 2.
This part is wired as drawn on the schematic, just external to the antenna coil can.
IN a previous post I mentioned that one of my favorite Zeniths is the 1937 5-S-127. It was far from top-of-the-line, in fact,, closer to the bottom. I suppose that the real attraction is the cabinet design, though the radio is not bad for only having 5 tubes.
The Crosley 6H2 is probably my favorite Crosley. Like the Zenith, it is the cabinet design with a not-too-bad radio that appeals to me. And, like the zenith, it seemed to have been popular at the time of sale. Far from the WLW (or stratosphere), it has good performance in a 6-tube chassis.
There is some confusion over the number of this model. Instead of giving it a name, like Buddy Boy, Crosley used a model number. 6H2 was assigned to the chassis and this is usually the "Model" on the tag, though there are certainly variations of this chassis. In some cases the model is called the 61 which seems to refer to the radio/cabinet configuration. Confusion is amplified by the question of model-year. I call the radio above a 1934 model, but there were likely to have been versions of this radio sold several years later
I did replace the broken pointer after this picture was taken..
So, the real mystery, for me, is - Why did they change this popular cabinet during its production run?
I have always wondered why Crosley slapped the hardwood panel over the top of the scroll work.. Possibly more to the point, why did they do the scroll work in the first place? It was not a common feature of radio cabinets at the time. It would have cost production time/money (which Crosley was famous for conserving). Was the original appearance so objectionable (to somebody) that it needed to be covered by the panel which would have cost more time and money?
The radio cabinet without the enhancements appears to be produced later than the more common version. Did they finally run out of cabinets with the panel and just decide to skip the whole thing? Or was the change made simply to save production costs?
What do you think?
Tired of being locked-up. Me too.
Just as we were seeing signs that the SARS-CoV-2 thing might be coming to an end, it appears that our county is going to move back into the "extreme" category.
Since we are not a business, anymore, we are only constrained by good sense, but, we have been adhering to the OR Governor's "gathering" guidelines. We don't want to have anything to do with making one of our radio friends sick.
Still, we remain optimistic. We hope that, by June, we will be able to reopen the Radio Shop/Museum to visitors. If we start with small groups, maybe by the middle of summer we can accommodate larger groups and clubs.
At first we will require masks. We will also require a completed Covid vaccination card ( you show me yours and I'll show you mine). Politics are not part of this and we don't care to discuss that aspect anymore - or ever.
We still have a lot of restored radios sitting around looking for a display spot - or a new home. By mid summer we hope to have a "Radio Shop" sale, possibly attended by other local radio collectors who also have accumulated WAY TOO MUCH STUFF over the last year (or years). Dates will depend on so much - If we have another fire season like last year - you might want to bring a fire-hose.
So - - if you would like to set up a visit, send us a message using the contact form. Or if you already have our email address, use that. Tours are still free. If you have an old radio that needs help, bring it along. Any advise (good or bad) or restoration assistance, is also free of charge.
Looking forward to "normal",
Russ & Sue
Russ Webb & Fuzzy
Best Buddy, Radio fixer