Identifying old tube amp - possibly console?

nadams

I got this old tube amp from a HS electronics teacher. I'm trying to figure out what it came from and if it'd be suitable for some small bookshelf speakers.







So, I'm not sure if it was in a console stereo, or just in a large case at one time. The main rectifier is a 5Y3.... not sure about the rest, other than they're made in U.S.A. and are Zenith brand.

mhardy6647

Looks like a Zenith to me. (if so, I've a very similar one a-sittin' in the basement!)
Single-ended EL84 (6BQ5) stereo amplifier; quite nice actually and the "inspiration" for Steve Deckert's (Decware's) little "Zen" SE-84 stereo amplifier. http://www.decware.com/newsite/SE84C.htm

Appears to be missing one of its output tubes :-(

Single-ended EL84 in pentode mode is good for about 4 watts. High efficiency speakers required but it is capable of delivering quite good sound. Will need to have the coupling capacitors at minimum changed; power supply electrolytic(s) - the silver metal multisection "can" capacitor may well need to be replaced as well (though they can often be salvaged by "reforming"). Do you know if the amplifier's been used recently (say in the past 12 months)?

inspiredsports

And maybe early 50's by the looks of the caps. Can you get any date codes off of the tubes?

RuSsMaN

Cool amp.

Rev. Hayes

That looks very similar to one I pulled out of a (takes a deep breath) "RCA Victor Duel Amplifier Stereo Orthophonic High Fidelity" record player that I use just for the cabinet.

It uses 35C5's and12AU6's though.
(though i have only the faintest notion of what that may signify)

The camera batteries are dead but when I get a charge I'll post it so you can compare for yourself.

nadams

I found the other output tube in the box I had it stored in. It appears to be shot, as theres a large deformity on the side.

The amp has not been powered on in the last 12 months... been sitting in a box in my dining room. I don't have a variac to bring voltage up slowly, I have no tube tester, and I'm not brave enough to just plug it in and turn it on. I've got a friend locally that is an EE, so I think he's going to look it over for me.

mhardy6647

Some cross-references on this (or a very similar) amplifier:
http://www.audiokarma.org/forums/showthread.php?t=290949
http://www.audiokarma.org/forums/showthread.php?t=264627
http://www.audiokarma.org/forums/showthread.php?t=190991

nadams wrote: »

I found the other output tube in the box I had it stored in. It appears to be shot, as theres a large deformity on the side.

The amp has not been powered on in the last 12 months... been sitting in a box in my dining room. I don't have a variac to bring voltage up slowly, I have no tube tester, and I'm not brave enough to just plug it in and turn it on. I've got a friend locally that is an EE, so I think he's going to look it over for me.

"large deformity" as in a hole? :-) Please post a photo (if you would) and let the hive mind evaluate it.

Your plan sounds good (although "modern" EEs probably know about as much about that amp as you do - no offense EEs!)... you could also consider building yourself a "dim bulb tester" aka a "poor man's Variac" (some poor women might also use them...). Google the terms or take a look at www.audiokarma.org (or perhaps even here).

mhardy6647

Rev. Hayes wrote: »

That looks very similar to one I pulled out of a (takes a deep breath) "RCA Victor Duel Amplifier Stereo Orthophonic High Fidelity" record player that I use just for the cabinet.

It uses 35C5's and12AU6's though.
(though i have only the faintest notion of what that may signify)

The camera batteries are dead but when I get a charge I'll post it so you can compare for yourself.

The 35C5s and 12AU6s signify (most likely) a so-called "AC-DC", series string, power transformerless amplifier - better known in the trade as a "killer". No isolation from the AC line. Pretty sure the OP's amp is a Zenith (but not that RCA, Maggotbox, and a host of others didn't make very similar little single-ended console stereo hifi amps).

inspiredsports wrote: »

And maybe early 50's by the looks of the caps. Can you get any date codes off of the tubes?

FWIW, I think the date is more like late-1950s, even very early 1960s, FWIW.

Oh... as to good speakers to use... there are suitable bookshelf speakers, both commercial (e.g., the small Tektons) or DIY (e.g., with Fostex fullrange drivers, as used by Tekton)....



(kitteh not absolutely required for top-notch performance)

....but generally a wee little fleapower amplifier like that is better suited to big ol' hugemongus high sensitivity loudspeakers like this :-)




You might be surprised what a couple-three watts can do with the right loudspeakers.

nadams

Found this picture of a '59-'60 console with what appears to be my amp in it-



But I'm really only basing that off the one control knob I still have.

In looking closer at the deformity/burn mark, it would appear that some glue got blobbed on there at some point and burnt to the tube. The tube itself appears undamaged.

In terms of writing on the tubes, the silkscreening is almost gone. On one of the small tubes, I can read "62-13" and "274". One of the other small tubes is a 12AU7A, and has "322c" and "621?" on it, where the "?" is a worn-off number. The 5Y3 has the following on its base - "62-09" and "188-20"

Is the "62-xx" the date code? Like, 1962, 9th week or 13th week?

The amp chassis also has a sticker "5G29", which is what I was searching for when I found the above picture.

mhardy6647

That should be a similar if not identical model console to the one that held your amp.
That chassis appears to be in Sams (schematic) set 604 folder 14.

https://www.samswebsite.com/en/photofact/search?s=Search&t=model&o=0&k=5G29&submit=Search

I may well have that one downstairs...
Tom Bavis might well have the schematic at his fine website as well.
http://www.audiophool.cjb.net/Techno.html

nadams

So, my buddy looked over the amp and got it running - however, we have an issue with it red-plating the power tubes. The amp is cathode-biased, and we didn't have an appropriate resistor to bring the voltage down a bit. I did get to hear it, though, and it sounds pretty sweet, even driving the RTiA1's, which are not really efficient enough for it.

mhardy6647

redplating could be due to bad coupling capacitors, too... or it could be really tired output tubes.
I'd definitely also recommend changing the bypass electrolytic in the bias circuit.

Silvertone77

I have this Zenith Hi Fi phonograph console. Looks just like the photo above. Chassis is also 5G29. I want give it a tune up. My idea is to replace the electrolytics and replace the needle and/or cartridge. Also the power tubes. I have worked on guitar amps some. There are 2 electrolytics in the chassis. They are marked 40-25DC. Can I assume these are 40uf? The cap can seems like it might be a pain to change. The one for the amp I can find a replacement for at AES. It's a 80/40/20 at 450V/350V. Or I may use single caps.
The can in the tuner chassis (9H20C21) seems like it will be harder to find the parts for. It's a 150/150/60 at 150VDC. Don't see any can like that and even seems hard to find single caps to replace. Does anyone know what values will acceptable to replace if I can't find the same? Also looking for the schematic for 5G29. I'll deal with the needle once I figure out the caps. Thanks. Any help appreciated.

mhardy6647

Match the capacitances (within at least about 20%) but don't worry about the rated working voltage - let me rephrase that! :-) A higher working voltage is fine. If you need a 150 uF @ 150 WVDC, it is perfectly fine to replace it with a 150 uF @ 200V or 250V or 400V or 500V...

Silvertone77

Thanks. I know about the voltage thing, I'm not clear on how the capacitance value affects things though. At AES, Sprague only goes to 100uF. F&T goes from 100 to 220UF. Should I use a 220 for a 150? For the 60 uF, they offer 80uf (450V, overkill I know) as the closest value. Will I be OK with 80uf and 220 uF? And the small cap must be a 40uF right? (40-25DC). Muchas Gracias.

mhardy6647

It actually depends on where in the circuit each section is. The one you have to be most careful with is the first filter cap after a vacuum tube high voltage rectifier. Too much capacitance can cause problems; tube manuals tabulate the maximum capacitance for the rectifiers. You can download tube reference manuals at http://www.tubebooks.org/tube_data.htm if you don't have any.

Did you check other distributors such as DigiKey or Mouser?

Silvertone77

Thanks. I have looked around. I think I'll use single caps for the 80/40/20 can. Found a 60uf 150v. Having a hard time still with the 150uf. Most people only sell a 100uf and 220uf. I can't find a close value or 2 caps to run in parallel for a reasonable price. Maybe I won't bother changing the can on the tuner. Mostly want to use the record player anyways. For the OP, let me know if you have any questions about the console. It's sitting in the other room.

mhardy6647

if worse comes to worst, remember that there's an Ohm's Law analog for capacitances, too! :-)

for (e.g., two) capacitors in parallel: Ct = C1 + C2
for (e.g., two capacitors in series: Ct = 1/(1/C1 + 1/C2)

e.g., a 10 uF plus a 50 uF cap wired in parallel = 60 uF total capacitance
a 10 uF plus a 50 uF cap wired in series = 8.33 uF total capacitance

So if you're patient enough (and have enough space below decks) you can construct pretty much any capacitance you want.

The other thing to bear in mind is that 40 to 50 years ago, electrolytics were very imprecisely manufactured components - e.g., the rated tolerance range on a 50 uF electrolytic might well have been +/- 100% (i.e., a value of 25 to 100 uF for a given 50 uF capacitor would meet specifications). The designers of the day knew that, and most of them would've built enough "slop" into their circuit designs to tolerate it. Today's are much more precisely spec'd. And, for most circuits (except oscillators and other "tuned"/resonant circuits, as in radios &c.) "close enough" is, in fact, close enough.

Silvertone77

Thanks, I ordered individual caps to replace all the cap cans. Found 150uf/150v at mouser although they cost $8 each. Ouch. Will report when everything is back together. Now to fit em in there and figure out replacing the needle.