I posted in the Hammond thread about converting an AO-29 to a 5E3 but found a loaded chassis that is smaller in size and may be much closer to what I want, and it's dirt cheap. My question is with the 6J5 on the input. I may be a novice, but to me that's an odd tube for a guitar amp and if I need to deal with it I'd welcome some expert advice.
Tube specs and what few articles I've found say it's a low gain tube and I suspect that might be the reason I don't see them in guitar amps. Can I sub the 6J5 for one with higher gain if needed, maybe a 6S7, or should I plan on scrap the circuit for a 12xx? Thanks!
My 1954 Gibson GA77 with a 15" driver uses a 6J5 for the phase splitter , so they where used in guitars amps, yes!
In regards to your other question , yes I would use a higher gain tube if for no other reason then getting a better noise floor from the amp.
You will then likely need to attenuate that stages output signal some once you do that so that you don't even at low volume levels drive the output stage into clipping too early.
Also I would say stay with a Octal type tube since I find they have a fat sound and certainly with there much bigger cathodes and plates structures last a lot longer then a 9 pin type preamp tube.
You should pick up a copy of the RCA tube manual RC-30 as it has a great section in rear of it with info on setting up audio gain stages with common tubes , but gives the detail of the needed component values in each circuit and with 2 to 3 different plate voltages.
This book is a must have if you ask me!
Also that 220 ohm 2 watt output tube cathode resistor should be upped to a 5 watt when you do the build and a at minimum a 25 uf cap needs to placed across it , this cap will be fine even with just a 25 volt rating , but be sure to keep it 3/4" away from that cathode resistor.
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Since the 6J5 is an octal tube, you could rewire the socket for 6SL7. That's a dual triode with more gain than the 12AY7 originally used in the 5E3, but less than the 12AX7 that many people substitute. They can be microphonic, but as Stevem said, they have a nice beefy sound that should work well with the 5E3 circuit.
Tie the diode plates to the triode cathode, or ground, or even ignore them. But don't connect to the triode plate. Doing so puts a forward biased diode directly across the triode plate/cathode. Not good.
Thanks everyone for the info. I'm feeling a little better about the socket now that I know there are options. I was thinking about a 6Sxx of some kind and that was going to be my next question but you're way ahead of me.
Springfield wrote: ↑Sat Sep 28, 2019 7:40 am
Thanks everyone for the info. I'm feeling a little better about the socket now that I know there are options. I was thinking about a 6Sxx of some kind and that was going to be my next question but you're way ahead of me.
6SC7 is another suitable preamp octal tube (with gm and mu resembling a 5751)
Great info. frank.pocnet.net is currently my favorite site of the week.
Received the amp today. It's clean and in great shape physically but doesn't look like it's ever been serviced. I haven't tested anything yet but the wax filter caps are all sticky and gummy so they will be replaced which brings me to my next question - parts.
There are dozens of US stores on the web. Can someone recommend a good parts page? I used to buy parts from Hoffman and a occasionally a few pots from Eby but it looks like Doug is scaling back. Lots of OoS or discontinued items.
I'm going to fire it up later with dim bulb current limiter and verify voltages just to confirm I have something to work with.
