Shared Cathode Resistor

[Littlewyan]

I'm looking to rebuild a Hammond AO44 into a different amp but need to save space as the chassis is quite small! The amp will have two pre amp stages in series, both of which have 2.7K/22uF cathode resistor/cap combos. I'm wondering if anyone has tried doing this but with shared cathode resistors and caps? Given they're the same values, I'm wondering about just using a single 1.5K/47uF or if that will just oscillate and not be very happy?

[Reeltarded]

Nah.. it will work!

[Lauri]

If the cathode cap has low esr and has high enough value it will probably work but also it might work as a multivibrator circuit.

[Reeltarded]

It can do weird things, but it will work at least as good as a cascade on a JTM, which I have never had but one issue out of many simple one-wire mods.

Of course YMMV because I am pretty average.

Plus, that British power. It has more gain...

[sluckey]

It can do weird things, but it will work at least as good as a cascade on a JTM

The JTM amps do not have a shared cathode R||C on cascaded (series) gain stages. Sure, the two input preamp triodes share a cathode R||C, but that's quite different and not applicable to the OP's question.

[B Ingram]

I'm looking to rebuild a Hammond AO44 into a different amp but need to save space as the chassis is quite small! The amp will have two pre amp stages in series, both of which have 2.7K/22uF cathode resistor/cap combos. ...

The original AO44 had room for every stage to have its own cathode resistor. So I'm thinking you, too, will have enough room. See Sluckey's page on modifying the AO44.




I've also built stuff into small spaces, but I usually take a different approach to layout when I go that route:

[Reeltarded]

It can do weird things, but it will work at least as good as a cascade on a JTM

The JTM amps do not have a shared cathode R||C on cascaded (series) gain stages. Sure, the two input preamp triodes share a cathode R||C, but that's quite different and not applicable to the OP's question.

I am saying I have cascaded stages with (JTM 45/50) shared cathode a bunch of times and after one gave me trouble they all worked fine.

[Littlewyan]

Sounds like it's going to be a suck it and see thing. I might have room for both, we'll see. I have seen Sluckey's conversion and studied it! Along with every other conversion I can find!

I'll do a new thread when I start this but my plan for this build is the following:
- Remove the EZ81 rectifier and replace with a 220Ohm sag resistor.
- Replace the ECL86s with EL84s
- Add two new sockets for 12AX7s
- Wire it up as the normal channel of an AC15 but with a switchable boost stage

From what I've read the PT and OT should be able to handle this, just about.

[pdf64]

Cathodes in cascaded circuits can be shared, provided they're fully bypassed.
Every 60s 70s Fender 2 channel reverb channel V4 has such an arrangement.
When the bypass cap ages and its ESR increases, a positive feedback loop is formed, and the circuit may oscillate.
https://el34world.com/charts/Schematics ... _schem.pdf

[B Ingram]

I am saying I have cascaded stages with (JTM 45/50) shared cathode a bunch of times and after one gave me trouble they all worked fine.

Cathodes in cascaded circuits can be shared, provided they're fully bypassed.
Every 60s 70s Fender 2 channel reverb channel V4 has such an arrangement.
When the bypass cap ages and its ESR increases, a positive feedback loop is formed, and the circuit may oscillate.
https://el34world.com/charts/Schematics ... _schem.pdf

"It will work, until it stops working..."

The question is do you want to build something that works today, needs tinkering tomorrow? Or do you want to build something that won't need a touch-up in a few years?

Funny about that point on Fender amps (which is absolutely true): a recent thread featured an oscillating reissue amp because the filter caps were no longer low-enough impedance (and both the bypass caps & filter caps have to be working right for those models).