Epiphone Electar

[LazarusLong]

Hello. I recently acquired a 1939 Epiphone Electar amp. I just replaced the capacitors, which was pretty straight forward. Heres the issue,  when I plug in the guitar I am getting very low volume even with amp cranked all the way up. Low volume and a little crackling. If i remove the cable from the guitar and touch the end I can hear a buzz, and it will get louder when turning the volume up... As for the tubes, I know that the 6L6 is good, the 80 and the 7S7 both light up but they are untested. I can have the 7S7 tested locally I believe, but the two local shops that have testers aren't able to test the old 4 pin tubes.. Any help here is appreciated, I of course was hoping that the amp would spring back to life with the new caps, but luck was not on my side...

[bmccowan]

I would likely do three things. Check voltages; check the signal path with a listening amp; and buy some tubes on the Bay. You'll want spare tubes anyway, and those are not too expensive. I assume you cleaned the pots - that's an old amp!

[LazarusLong]

There is one pot, the volume pot. I did clean it with deoxit.. I don't have any test equipment other than a couple of basic multi-meters. I can't determine if the 7S7 is actually the correct tube for this amp. I found a post on another forum and he states that his amp had an 80, 6V6, and 6C6...

here are a couple photos before i replaced the caps..

[LazarusLong]

I also can't find a schematic for this amp. There are 3 or 4 different schematics floating out there, but I think they are all later versions.. Any help would be appreciated.

[LazarusLong]

I looked up the 6c6 tube. This amp certainly never used one of those.. According to this: https://www.radiomuseum.org/tubes/tube_7s7.html  The 7S7 was introduced in 1941. I found some other info out there that dated this amp to 1939.. So I'm not sure what is correct here. The base that the 7S7 uses was introduced in 1938, so that matches up. Maybe it should have a 7J7?

[sluckey]

Another pic...

[PRR]

> I can't determine if the 7S7 is actually the correct tube for this amp.

Please report voltages.

This will probably show that 7S7 is totally wrong (being a radio RF/IF converter!).

I agree that 6J7 is the go-to pentode here. Except you have the Loctal socket, not 6J7's Octal. In that era builders used either type depending on price. (Loctal seemed like a good idea, but didn't sell as expected, so bargains could be had....)
sharp cutoff pentodes: 7AB7 7AG7 7AJ7 7C7 7G7 7L7 7T7 7V7 7W7
twin triode like 6SL7: 7F7

7C7 (31.Mar.1939) is IMHO the most likely guy for that socket. Compare pinout.
https://frank.pocnet.net/sheets/127/7/7C7.pdf

[LazarusLong]

Excellent info, thanks guys.. Regarding voltages, please walk me through the procedure.

[shooter]

walk me through the procedure.

start with these;

There are 3 or 4 different schematics floating out there, but I think they are all later versions

compare and make notes on the differences with yours, post your "updated" version of the schematic then we can better say "pin 2 should be close to......."

[bmccowan]

Did you find this site: http://wiedler.ch/nyepireg/closeup14.html I see it lists 6c6 and it seems that 7c7 is a Loctal equivalent - another clue pointing to PRR's suggestion. He knows more than I ever will about this stuff. And he's a fellow Mainer to boot!
As far as walking you through checking voltages. First step is learning about safety when working on tube equipment. This includes safely bringing the power up on old equipment and learning how and where you can get zapped. All else follows that.
Cool looking amp and I love one knob amps!

[LazarusLong]

Did you find this site: http://wiedler.ch/nyepireg/closeup14.html I see it lists 6c6 and it seems that 7c7 is a Loctal equivalent - another clue pointing to PRR's suggestion. He knows more than I ever will about this stuff. And he's a fellow Mainer to boot!
As far as walking you through checking voltages. First step is learning about safety when working on tube equipment. This includes safely bringing the power up on old equipment and learning how and where you can get zapped. All else follows that.
Cool looking amp and I love one knob amps!

I have a lot to learn, but I am well aware of the risks to digging into an old amp.. I have a 7C7 on order. I am going to wait for that to arrive before going any farther. If I have time I might make an attempt at creating a diagram of this amp.

[bmccowan]

Good that you are aware of the risks. If you go here http://el34world.com/Forum/index.php?topic=21999.msg234089#msg234089 you will find lots of great info.

[LazarusLong]

The 7C7 tube made no difference...

[bmccowan]

The 7C7 tube made no difference...

I'm not sure what you mean. But this is an old amp, IMO You should go through a methodical process of checking out what is functioning ok and what is not. The links I referred to previously will guide you through that, but there is other guidance out there on the Web too.

[LazarusLong]

I of course hoped the tube would cure it! And of course you are correct. I will start with the basics and work up from there. This troubleshooting guide seems pretty well done. https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Bad_Guitar_Audio

[bmccowan]

Yes - Rob's info is very good. It is amazing how he finds time for all he does.
I have rebuilt a few amps that were late 30s early 40s. I typically find that the transformers are ok; the field coil speaker is shot or replaced with a PM speaker; and everything else is suspect: wires, sockets, capacitors, resistors, and tubes. Fortunately, except for rare tubes, the parts are all cheap.
Also - if you search around schematics for other amps from that era, you may find something really similar. There was a lot of crossover in the early days.

[LazarusLong]

I've found at least 4 different schematics. I've seen at least 6 different variations of tubes and layouts on these also! I think they made small batches and then made small mods to subsequent builds depending on what the shop had laying around for parts..

I do get some sound, its just very very muted. If I remove the jack from the guitar and touch the end I can hear a buzz when I touch it. The buzz will increase in volume with the vol control.. Its hard to find the time to dedicate right now, but hopefully this weekend I can sit down for a couple hours and try to figure this out.

[LazarusLong]

So, I had a chance to get back into this amp. Turns out, the speaker coil was rusted in place. Ideally I would find another Jensen field coil speaker, but that might be next to impossible. Any suggestions?

[shooter]

you can replace with a modern speaker, but you need to know your FC speakers resistance because it's part of the PS rail, so some form of schematic might help.

[bmccowan]

I think Shooter is right that knowing the resistance and/or having a schematic is a help. But I have rarely seen the specs on the field coil on a schematic. In the past I have used a 1K or 1.5k 5 watt resistor or choke in that position and it has worked out.

[LazarusLong]

I never did find a schematic that matched this amp. The speaker is a Jensen g10rs c2092. I don't think i can get a measurement from it at this point..

[LazarusLong]

I never did find a schematic that matched this amp. The speaker is a Jensen g10rs c2092. I don't think i can get a measurement from it at this point..

I found this image online.. It sure looks like the same speaker.

[bmccowan]

Some info that may help:
There is an image here https://www.worthpoint.com/worthopedia/nos-nib-jensen-electrodynamic-speaker-116802583 that suggests it might be 1.25K.
I have a 10" Jensen Field coil that needs a recone - its a F10rs. It came out of a 1940s Valco. I replaced it with a P10R and used a 1K resistor in the power supply. I'm not saying that's what you should do, just that it's what I did and it worked out fine.
And old Valcos often had jensen field coil speakers and their schematics often show the resistance value of the field coil. From what I recall, it usually ranges from 500R to 1.25K. If you search through old Valco schematics (Valco, National, Gretsch, Oahu, Supro) you may find that model # speaker with the resistance spec.

[bmccowan]

Your image showed up as I was typing mine - same image that I saw - pretty good evidence I'd say.

[LazarusLong]

So, a bit over a year later I finally have the amp working. Last spring I repaired and restored the cabinet, and today I decided to get the amp out and figure it out.. Well, the amp was working all along. The problem was that the original Jensen speaker was faulty. The voice coil was rusted! Beyond repair rusted. I tested by simply extending some speaker wire from the OT to a test speaker, and its working fine.. I'm going to put a more modern speaker in it. Jensen P10R seems like a decent option, but I'm open to suggestions.

[bmccowan]

Glad to hear it is working. I like the Weber 10" alnicos, including the inexpensive Signature Series, better than the new Italian Jensens.