Western Electric 121A, power supply time

[Butterylicious]

http://www.butterylicious.com/WE121A/index.htm
There's the link to the picture gallery.

I finally feel like I have the cajones to tackle this restoration. I've had the #2 unit (the one with the JAN5693) fired up B4. I haven't even tested the valves in the other one. Both units have been slightly modified where in R5 is now a pot and the wiper is connected to the grid of V2 (a little attenuator). I think each one has had at least 1 canister capacitor replaced. Obviously none of the tubes are original.

So the questions:

1. Is a 6SJ7 pretty much the same as a 6J7 except for the pin configs?
The manual calls for 6J7's as substitutes for the WE breeds, but these have 6SJ7's. From what this novice can tell, it appears the biggest difference is the grid is on a pin on the SJ where as it's on top of the J.

2. To make a pentode into a triode, do you tie the screen and the supressor to the plate?

3. Points 4, 5, & 6 are to allow plate current measurements. This baffels me. A, they're not even connected to the plates. B, I thought to measure current you either had to get in series within the circuit or use an inductive meter (an amprobe in my day to day world).

4. Anybody have a clue as to just how old these thangs are? There is an ink stamp on one of them with the number 39. They can't be that old?

5. I kinda suspect these thangs won't sound all that great. Do you think these are of any real value?

Thanks again for all the advice you guys give out. This site has been a god send for me. I've learned a ton from just learking about. I love fixing old equipment & I'm starting to feel like I'm understanding what I'm doing vs. the trial-n-error repair method.

 

[NewYorkDave]

Manual available at this link:

PDF

1940s era seems about right.

More comments later if time permits.

 

[Butterylicious]

Thanks for the link. That's a different manual from the one I have.

 

[NewYorkDave]

Points 4, 5, & 6 are to allow plate current measurements.

The voltage drop across the cathode resistor is proportional to the plate current (actually the cathode current, which is the combined plate and screen current, but the screen current is fairly negligible).

Is a 6SJ7 pretty much the same as a 6J7 except for the pin configs?

They're similar.

I kinda suspect these thangs won't sound all that great.

Why do you say that?

If you're looking for a platform for mods, I suggest you look elsewhere. The old WE "speech input" equipment has value and there's a couple of Japanese guys on evilBay who'd probably pay top dollar for your amps.

But before you sell 'em, you should scan and post your manual (if it's different from the one I linked) for... you know, the greater good :wink:

 

[CJ]

6J7, 6SJ7 or even 12SJ7 go for less tha 5 bucks at tubesandmore.com.

That old iron could sound good. You might have to wake it up if because it's probably M6 or barn roof material.

 

[Butterylicious]

NYD,

I intend to scan and post ALL my WE, Ampex, Langvin, & JW Davis manuals. My 121A manual has a chasis layout/wiring diagram. It also specs out 30 to 15KHz vs the 50 to 15k in yours. it's only a matter of time B4 I get a round tuit.

CJ,

What do you mean "wake it up" & "barn roof material"?

Please remember I'm a 1983 tech school drop out turned master electrician & electronic repair enthusiast. A total novice but with a love for the stuff. I'm learning more here than in 2 years of school.

 

[rafafredd]

Call me ridiculous, but I would pull that iron out and sell on ebay for THOUSANDS, then buy a LUNDAHL or SOWTER substitutes for a little hundred and be happy with this...

I mean, I´ve seen this 618B go on ebay for $1600 once...

 

[CJ]

"wake it up" means saturate the hell out of it with some 60 cycles off a variac. Be careful.
How do you know when it is saturating? It will get warm. It might even talk back to you.
Give it about ten minutes.
Measure the inductance before and after if you have the facilities.

"Barn roof material" - some of that WeCo iron that foreigners pay a month's salary for was mass produced garbage that sat on telephone poles. But some of that "garbage" sounds good. Cheap lams were sometimes used. Cold rolled steel is pretty cheap, hence, the phrase "barn roof material", borrowed from Doc Hoyer, who is a transformer winder.

 

[Butterylicious]

About this "wake it up" business......

This sounds like a good way for me to destroy some transformers. Roughly how many volts we talking about here? I don't know ifin I can do this w/o adult supervision.


I brought both preamps up for only a minute or so w/o smoke. Decide I needed to get the project into some sort of chasis to (a) make it safer to work on & (b) kinda steer the project towards "what's it gonna look like" when it's all said and done. It's a bit on the rough side but once I get it working right, I'll disassemble it and pretty it up real good like. Here's the pictures(click me). I plan on using those 2 VU meters in the first picture.

 

[Butterylicious]

[quote author="rafafredd"]Call me ridiculous, but I would pull that iron out and sell on ebay for THOUSANDS, then buy a LUNDAHL or SOWTER substitutes for a little hundred and be happy with this...

I mean, I´ve seen this 618B go on ebay for $1600 once...[/quote]

Man you weren't kidding. I had no idea this stuff was of such value. Here's an evilBay completed auction for a 132A which is basically the same as the 121A less 1 tube stage. It has no power supply, no tubes, and no input transformer yet it sold for $2250.00! My collection just got a new center piece.

 

[Butterylicious]

It's alive! Both pre amps are working and they seem a lot more closely matched that I would have expected. I loaded the outputs with 470Ω resistors. A 1KHz 0.4V p-p input gets me 4.0V p-p out. Frequency response is kinda all over the place. At about 15K, it stops and if you roll the frequency higher, the audible tone actually goes down in pitch B4 it turns to garbage. Heading back down the other way it roller coasters +/- 12dB all the way down to about 150Hz, at which point it rolls off hard.

All in all, this is going much better than expected.

 

[rafafredd]

[quote author="Butterylicious"]+/- 12dB all the way down to about 150Hz, at which point it rolls off hard.

All in all, this is going much better than expected.[/quote]

Come on, sell this thing for $5000 on Ebay. List like this "tested, including very rare input transformer"... You can do much better with real iron. Buy some sowters from Prodigy Pro. Or try finding some old UTC LS series for a frequence responce of +-1dB at 7Hz-50kHz. The hell with those old WE junk. Let japan have it.

Then put all the money in DIY. :green:

 

[NewYorkDave]

Why'd you load with 470 ohms? Try it again with 600.

Anyway, I agree with Rafa. Put those bad boys up on evilBay and prepare yourself for a large cash payment FedExed to you from Japan :grin:

 

[Butterylicious]

Here's a quick graph of the frequency response. I powered it down, poped the first stage loose, and probed the secondary of the input transformer. It appears all my low end roll off is happening within the input transformers.

The hi end thang that's happening right at 13KHz is rather odd. What I physically hear is the pitch start to go down, even though the scope shows the frequency still increasing. Right at 15K, the sound starts going back up before it turns to garbage, followed by silence (yes, I'm sorta deaf). The whole time the output looks good on the scope.

I don't know why I loaded the outputs at 470Ω. How's that for an answer? I may end up putting this on evilBay, but not B4 I'm finished playing around with it. I gotta get it all prettied up with a matching fleet of tubes first. Chances are I'll sit on it until I need the dough.

-Richard

 

[PRR]

That construction and the schematic sure scream late-1930s. WE could have clung to that all through the 1940s, but not far into the 1950s.

Your new-drawn schematic has the wrong value at R4. (I didn't check them all; I just spotted 500K as being a bit odd.)

It is possible a 6SJ7 is a single-ended (no grid cap) 6J7. Certainly it will make very little difference inside the feedback loop. Note that they trust the feedback so much, they don't care if you use 6F6 or 6V6 outputs, even though the 6V6 is much stronger flavor than 6F6.

That's a 3-watt output stage. Good old WE, rating about 12dB below maximum output.

> probed the secondary of the input transformer. It appears all my low end roll off is happening within the input transformers.

"Probed" with what? The transformer is wound for an open-grid. The input impedance of a 'scope is much lower, lots of capacitance to roll-off the highs. And if your source is high impedance (higher than 150Ω), that will lose the lows.

Alternatively: someone has been poking an ohmmeter at the transformer and magnetized the core. Bring up 20Hz to about 3V (tube out, power off) and sloooowly bring it all the way to zero volts.

But the transformer alone can NOT produce that bass-bump. That has to be dying capacitors. On something that old, even WE quality, I would replace all electrolytics. (Actually, considering the Collector's Value, I might not touch it except to ship it to a rich buyer.)

> 0.4V p-p input gets me 4.0V p-p out.

Not at all right. It should have a gain over 1,000, unless you did the low-gain strap. Another reason to suspect sick caps.

Ah! you have a gain-pot! And you are probably over-driving the input! The input level MUST NOT exceed 70 milliVolts at 50Hz, 100mV at 400Hz. If you are putting high levels in, it is going to distort and whine.

> a lot more closely matched that I would have expected.

It's the Telephone Company! At the time, by far the biggest audio system in the world. When you have tens of thousands of amplifiers in 24/7 service, they BETTER be matched or you will go crazy.

> To make a pentode into a triode, do you tie the screen and the supressor to the plate?

Often just the Screen, especially when the Suppressor is hard-tied to the Cathode. In this case, you have a choice; AFAIK it does not matter at these powers.

> I kinda suspect these thangs won't sound all that great.

They should sound very-fine.

> Do you think these are of any real value?

I'd give you $100 each as-is; a bit more if I had a profitable studio with enough space. Classic high-quality stuff. I gather that the "WE" logo is now worth more than the equipment's honest value; stuff happens.

> allow plate current measurements. ...they're not even connected to the plates. ...to measure current you either had to get in series within the circuit or use an inductive meter...

The manual Dave linked does not say "plate current", it says "checking the tubes". IAC: the current is either about-right or you replace the tube, without worrying if it is plate, screen, or cathode current that is wrong.

You are "in series". They put a 111Ω or 5Ω resistor in-series with the tube. You measure the voltage across that resistor. They use different values, so the low-current V1 and V2 and the high current V3 all wind up about the same voltage (~0.140V) at their test-points. All the test-points in a rack go to a many-pole switch and a 0.2VDC meter with a green-zone around 0.140V. You can check all tubes in all amps quickly, even while critical audio is flowing through the system. Couple times a day, you sweep the switch and be sure all tube are green-zone. Every night or every week, you note all the exact readings on a check-list for logging. (In smaller systems without union technicians, of course, you ignore the check-points.)

Ain't much wrong with barn-roof iron, if well used.

 

[Butterylicious]

>That construction and the schematic sure scream late-1930s. WE could have clung to that all through the 1940s, but not far into the 1950s.,

Their serial numbers are 451 & 465. There's an obsured ink stamp, "something something 39" on one of the chasis. Several pieces of iron have 4-44 stamped on them.

>Your new-drawn schematic has the wrong value at R4.

Thank you. I'm still learning how to use that software. You have a keen eye.

>"Probed" with what?

I have a handy dandy octal extender with test points on it. I slapped it in on V1 and put my scope on the grid. I did notice the audio output dropped when I did this.

>Another reason to suspect sick caps.

As many guitar amps as I've serviced over the years, I should know this.


Thanks for your feedback. Next step = cap job.

 

[PRR]

> the audio output dropped when I did this.

Clue that the measurement is affecting the circuit; performance may not be "normal".

> As many guitar amps as I've serviced

Fender made amps for guitarists: cheap/poor, or doing well enough to buy new amps every few years. Guitar amps don't work 24/7. And a guitarist is in-touch with his amp. Fender did good, but not TOO-good. If he built them to last a lifetime, nobody could afford them; building to last a few years between repairs was the way to go.

WE/AT&T was building for their OWN use, huge quantity, much of it 24/7. If they didn't put the reliability in at the factory, they'd end up spending much more in field repairs. Much WE stuff was made to last nearly forever. The electrolytic caps are a little odd: 1939 electros did not last long. Even as late as 1970, we find Langevin boasting about no electrolytic caps in thier amps. I assume WE/AT&T had a maintenance schedule to replace electrolytics every few years. And that these are special-purpose amps: studio, not telephone system.

"Speech amp" in WE lingo does not mean speech-only. Primarily it means "audio" instead of "control" (over half a telephone system is control-systems, not audio). There is also an AT&T decision to focus entirely on speech-related systems, leaving movie-sound and recording to others (in part because the government and other shops did not want AT&T's great monopoly-money in competition for non-monopoly sales). But you note that the specs say +/-1dB over 50-15KC. This is very-very good speech. It is better than most "music" speakers or microphones. It aint a bad music amp, though you do have to be careful not to put unlikely (for 1940) levels into the input transformer.

Shifted from this thread:

> The voltage odometer that came with all this stuff is at 17,817 hours so it's safe to say they've been busy.

That's no time at all. Only 45 minutes per day for 65 years. Or 8/5 for 8 years.

Late-1930s tubes should run many thousand hours, by the 1950s 10,000 hours was expected and tubes often ran much-much longer.

> Keyboard not found, press F1 to continue.

You aint supposed to do this, but: most IBM-ish machines, you can plug-in the keyboard at that point, wait for it to self-initialize, and press F1.

 

[Butterylicious]

Now you've got me wondering if that odometer has rolled over. The fella I bought this stuff from told me it was a radio jingle studio. I can't picture Topeka having too many recording studios back then.

I've been surfin' for caps. Gosh dang multi-section can caps are hard to come by. I'm current high bidder on a box of 100 misc, but I'm sure I'll get sniped, not to mention they're likely to be unusable values. Sometimes you get lucky with the mystery boxes. I'll probably just use some regular ol axial lead devices for the time being.

I know you can usually hot swap a keyboard. That's just my favorite error message. I figure whoever wrote has a pretty good sense of humor.

 

[Butterylicious]

Here are the scans from my instruction bulletin 959P. I also have an orginal instruction bulletin No. 959, but the P version is pretty much identical and in better condition.

http://www.butterylicious.com/Schematics/WE121A_Manual/index.htm

I truly intend to get a lot of schematics posted in the next 2 weeks. The vast majority are Ampex, Langevin, WE, & JW Davis.

Today's 1st Question
Plate Current Metering
If I could please turn your attention to page 3 from the link above, specifically the paragraph on metering plate current. I would like to know if these meters on evilBay can be used. Or perhaps this meter also on evilBay is more suitable.

Today's 2nd Question
Attenuation
Turning your attention to the schematic on page 5, the previous owner replaced R5 (0.5M) with a pot of the same value with the wiper connected to the screen of the 2nd stage. In it's current state (I still haven't had a chance to reCap em yet), as the wiper approaches zero, all hell breaks loose(worse than motor-boating). I have not measured the resistive value at which this occurs but I was thinking of placing a fixed resistor whose value is slightly greater than the measured value at which the problem happens in series with the pot on the grounded side so that the adjustment cannot reach that critical point. I would replace the pot with one whose valuse in addition to the fixed resistor equals 0.5M. Is this a feasable solution? Also, should I consider padding the inputs? As luck would have it, I have some sweet 600 ohm Daven pots.

I'm hoping that my evilBay capacitor crap-shoot purchase shows up soon so I can procede with the cap job. I've heard that sometimes packages get held up at the Canadian border for like an eternity. Hide and watch, I'll wait & wait and then none of them will be usable. :roll:

 

[NewYorkDave]

I'm really looking forward to seeing the other documentation you're going to post! :thumb:

I took the liberty of downconverting your 121A manual images into B&W and putting them into a PDF for those who prefer a more "portable" format. There's a little quality loss but it's all still legible:

272k PDF

the wiper connected to the screen of the 2nd stage

Please tell me that's a typo and you meant grid, 'cause it would be bad news if he really connected it to the screen.