Attic find - 40's tube amp

Discussion in 'Amp Tech Center' started by AndyPanda, Sep 4, 2019.

  1. AndyPanda

    AndyPanda TDPRI Member

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    I've been repairing an old tube amp that was abandoned and missing some parts. There were no schematics online and only a few mentions of the amp and brand at all. I ended up drawing my own schematic by drawing everything that was still there and making some educated guesses about what was missing. And adjusting a few values after I got it working.

    It's an Ampower Tremolo 80 and seems to have been made in Chicago from what I can guess.

    When I found this amp, it had 6v6 tubes (dead) and 5y3 rectifier (also dead) and the push pull pair was driven by a cathodyne circuit around a 6j5 tube. It took me some time to understand how the push pull was being driven by the single 6j5 tube and it was really tempting to rewire it for something like a 12au7 or even a 6sn7 (two 6j5 in same package) but it does work even though I can't quite get my brain wrapped around how that single tube is able to drive both sides of the push-pull pair ... and it sounds really good so I'm glad I left it stock just to get a feel for what amps were like back then. The preamp is three 6SJ7 tubes with FOUR instrument inputs and TWO microphone inputs. I'm guessing this was the only amp on stage back then and the pedal steel player had it in front of him (there is a lever that pokes out the back where you can change the tonality of the output section - it patches in capacitors between the output tube plates to bleed off the highs) and the singers plugged their mics into the amp too. (just a guess - I wasn't alive in the 40's). Oh, I forgot, it also has tremolo ---- you can only adjust intensity, not the speed, but it sounds good and must be one of the first amps with tremolo.

    Once I got it running, the voltages are high enough that this would have fried any vintage 6v6 tubes but I was using Ruby 6v6GTA-R and they were right close to the limit (and sounded awesome). With 6L6GC tubes and a 5U4GB rectifier the output tubes are just loafing along dissipating 19 watts each --- and I think that's about exactly what the 6L6 tubes back then were able to handle.

    The speaker that was in it can't be the original - though it is really old, but it's a HiFi speaker with a whizzer cone. And that brings me to my question. I have three candidates here and curious if you guys have strong opinions on which speaker would be best. I have a vintage '56 Jensen P12Q, a vintage Fender maroon/gold Jensen C12N and an old EVM12L (the EVM just barely fits - but it does fit).

    The amp is super clean sounding and not enough gain to really overdrive things - I moved the output from the instrument channel to drive the input of the mic channel and you can now overdrive that second stage really well - it still isn't able to drive the power amp as hard as I'd like though. I'll have to think about increasing the gain of the driver but that cathodyne circuit still baffles me. Any thoughts on driving the power section harder?

    AmpowerBasicSchematic.jpg
     
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  2. Milspec

    Milspec Friend of Leo's

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    I would think that an amp of that age would have a field coil speaker so changing to the ceramics would require some changes to the circuitry. I have a '49 Epiphone Century with a field coil and uses those 6sn7 tubes as part of the tube complement along with a unique loctal tube which was normally reserved for portable radio units. Anyway, as long as the circuitry is adapted to a ceramic speaker, I would think anything that was bright, articulate, and could carry strong mids should work. Microphones were very mids-biased back then so the amplifiers would be designed for that kind of sound.

    Mine has the original Rola 12 inch which can still be found on fleabay, but there is at least one amp builder that still uses field coil speakers (newly built) in their amps, so that might be worth locating. I just can't recall who that builder is right now.
     
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  3. Random1643

    Random1643 Tele-Afflicted Silver Supporter

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    I have no technical expertise but that amp is just way cool. Sound samples?
     
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  4. P-Nutz

    P-Nutz Tele-Afflicted Silver Supporter

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    So, please may we have some pics ... ?
     
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  5. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Cool score and great that you were able to bring it back from the dead. Sounds like quite the amp. My vote is for the Jensen P12Q. Can't help re driving the power section harder. I was going to suggest eliminating the negative feedback loop tho I see it doesn't have one.
     
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  6. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Were there such things as instrument speakers and Hi-Fi speakers then?, or were there just speakers?
    Go with what your ears tell you. Oh, pics needed or the continuum pixies will get restless.
     
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  7. radiocaster

    radiocaster Poster Extraordinaire

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  8. Bikersluggo

    Bikersluggo Tele-Holic

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    Cool project. Welcome to TDPRI
     
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  9. Telekarster

    Telekarster Tele-Meister

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    What an awesome find! Ok, about the speaker, I'd definitely put a Jensen in that baby. But, are you talking an all original 56 Jensen speaker? If so, are you sure the mags and cone are going to be able to handle play time? I'd recommend putting in a new Jensen P12Q, or a Weber of some flavor, and leave the vintage speaker alone IMO. bell'd Alnico for sure either way if it were me ;) Looked up some pics and that's a really neat looking little amp! Looks like a lot of fun.
     
    Last edited: Sep 5, 2019
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  10. Wally

    Wally Telefied Ad Free Member

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  11. radiocaster

    radiocaster Poster Extraordinaire

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    Could be.
     
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  12. Wally

    Wally Telefied Ad Free Member

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    And...it just occurred to me. That 4-way switch is controlling the value of the capacitance in a conjunctive filter circuit, isn’t it?
     
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  13. Lord_Ingipz

    Lord_Ingipz Tele-Meister

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    Nice find! Love me one of those!
     
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  14. timfred

    timfred TDPRI Member

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    Have you checked/replaced the power supply resistors and caps? The voltages on the preamp tubes seem really low. Tube spec sheet says more like 180-250V+ on the plates.
     
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  15. printer2

    printer2 Poster Extraordinaire

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    The PI is a Cathodyne circuit, look it up, it was in the tweed Fenders and still in the Princeton. Wally has it on the switch. For some gain put the mic input into the instrument channel.
     
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  16. gusfinley

    gusfinley Tele-Holic

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    When dealing with vintage tube gear, I would suggest an isolation transformer.

    I once had a 40's Philco Tube radio that took out the aux output on my mixer. The chassis was HOT (about 170 VDC! )

    At least this amp has a power transformer, many early circuits do not.
     
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  17. AndyPanda

    AndyPanda TDPRI Member

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    The lever is that 4 position switch in the schematic I drew up. It inserts the capacitor network across the output and darkens the sound.

    The Jensen is a real 1955 P12Q that I pulled from a Hammond Organ Tone Cabinet and it's in like new condition and sounds really good to my ears. Since my first post, I've tried a couple of 50's Rolla speakers (also from Hammond Organ Tone Cabinets) and a Celestion G12L which sounded very similar to the Jensen and I settled on the P12Q.

    It seemed really odd to me that the voltages on the Preamp tubes were so low - but I've looked at several photos of other amps insides and they all have similar value dropping resistors lowering those voltages. I've replaced all the caps of course - especially the power supply filter caps and the voltages to those preamp tubes are being dropped deliberately by the components in the original circuit. It would be easy enough to change those dropping resistor values to bring the voltages up - but the designers had some reason (?!?)

    Here's a recording running into the Instrument channel which then feeds the mic channel - Instrument channel turned up to overdrive the mic channel and mic channel turned very low to keep overall amp volume low - recorded with an SM57



    And here it is with the amp turned up to drive the power tubes a bit and loud enough to have the neighbors across the street complaining:
     
    Last edited: Sep 11, 2019
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  18. Wally

    Wally Telefied Ad Free Member

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    Andy, if you go to the ‘more options’ box, you can insert the link.
    As for the cap switching in the output, I will correct myself. That circuit lacks the resistor which would make it a conjunctive or corrective filter.
     
  19. AndyPanda

    AndyPanda TDPRI Member

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    I was able to put a link to the mp3 file on dropbox - but I would have preferred to have attached the mp3 file itself to the post - but when uploading an mp3 to the post I get "The uploaded file does not have an allowed extension" So are mp3 files not allowed in the forum?
     
  20. AndyPanda

    AndyPanda TDPRI Member

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    Since my original post, I've looked at a few more pictures online and realized that my Output Transformer is a replacement. It's a "Universal Output" with a variety of taps and reading the spec sheet I discovered that it has a max of 40mA per side and 15 watt output. (the 6L6 were running 47mA per side)

    I'd been going back and forth between running 6v6 tubes with 5y3 (running them at higher plate voltage than 6v6 ought to be run) and 6L6 tubes with a 5u4 (running much cooler bias than 6L6 are usually run) .... but now I see that OT can only handle 15 watts. I could always put a 25 watt OT in there - but I like this universal OT and between reading the spec sheet and monitoring waveforms with an oscilloscope I learned that I use different taps for 6L6 vs 6V6 outputs.

    So I increased the bias resistor to 450 ohm and with 6L6 it is just making 15 watts now.
     
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