Old Epi / Gibs Mighty Mite question - 6k8 Kr and 470k plate

[chopstuck]

I know Gibson / Epi was into doing thing differently. Multiple examples everywhere..
I've been on a roll fixing old gear and had a question about the use of these wonky cathode and plate resistor values on an old EA-22rvt.
This amp in like new but sounds like an old 9v transistor radio.
Also the grids on the first stage have 1M in parallel with a .0047 input cap and 1M to ground. Doesn't this really pad down the signal and make the amp work harder to get limited overall gain?
This thing has big iron and a pair of EL-34s so It should be a monster.

[Structo]

Well, with the preamp 12au7's the gain won't be that much as well as the plate resistors and cathode resistors limit the overall gain.

I see some modifications ahead if you want more gain.

[Phil_S]

Try some bypass caps on cathodes of V1, both sides. That will give you some bass where it is lacking. A pair of 32u (the JJ 2x 32 can is a good choice) for the 1st and 2nd filter caps will stiffen the power supply and crisp things up a bit, also improving the bottom end. The simple truth is, I believe this series of amps wasn't meant for R&R. I think they had jazz players in mind. If you want it to rock, it will need some help. The good news is there is plenty of potential as the basic topology is pretty good. The parts values are all "wrong" and some parts are "missing" if you get my drift.

There is some collector's value in these amps if in good shape. Try to stick with things that are easy to reverse or remove.

[chopstuck]

Structo, the writing is pretty small but the tubes are

V1 and V2 are 12AY7s
V3 and V4 are 6EU7s
V5 and V6 are EL34s

It has Sprague looking orange drops - better caps than the ceramic disks found in my other Gibsons of the period.
It has 447v on the plates and 451 on the screens- (Screen resistors are coming).

Most of the preamp tubes are at 167-180 on the plates.
Except the PI and the two reverb driver triodes (in series no less), which are higher.
I tested all the tubes - all good.
It's not just the gain that's missing but power too. Sounds like a weak Princeton.
It has some shoddy repairs I must see to first.

[tictac]

Changing those plate loads to 270K, do a shared 1K ohm w/ 47uF bypass for V1A & V1B and lowering the value of those 1M input resistors to 33k - 68k is the direction I'd be heading in...

The PI may be OK the way it is and with that 470K plateload you could consider direct coupling the 1st stage to the splitter section...

Other than that it looks like a cool amp; the reverb section is interesting, looks like it could be good sounding....

Let us know...

TT

[Stevem]

Those amps as in many Gib/Epi amps are the diamond in the ruff once rebuilt from its Hi Fi like start!

[Structo]

It's missing a tube, that's why it sounds weak!

J/K, I'm sure it has some great potential.

[gui_tarzan]

What's to stop the can from bouncing right onto the board connections underneath it? Yikes!

They put all the OD caps in and left the brown cardboard cap in?

[David Root]

Tom, that's V2 that's missing. I imagine the OP pulled it trying to get more oomph out of the amp.

+1 on the input changes and plate resistors, and bypassing the cathodes on V1.

I just ran the original V1B with 12AY7 in Tubecad. Had to assume 300V B+ to get 170V plate, 0.28mA, gain 40 (32dB).

Then I changed 12AY7 to 12AX7, boom, gain is 79 (38dB), 183V plate, .25mA.

So even with 12AY7 the gain is not real low. I suspect something else is causing the "transistor radio" tone & power.

[talbany]

I don't think the 1M .0047 grid feed is the problem nor the RP/RK values and tubes..I suspect output section.. can you somehow inject a signal into the output section (speaker too)..I would start there and then work back..

Tony

[chopstuck]

Thanks everyone for the input. Sorry I did leave V2 out to get more gain. I 'm running a 12ax7 in V1 to get still more gain, it works a little.
I have two of these old tweed generation, tolex covered Epiphones. The other is a EA-28RVT. They both need some repairs undone before I really dig in.

Dave, thanks for the simulation work. I will try the signal injection idea.

Marc

[angelodp]

why not put another 12ax7 in v2. I am wiring on an old editing console ( moviola ) amp that has 2 12 ax7's 1 6v6 1 5y3 - tons of gain in tho amp.