EH-125 Tone and Volume

[Ted]

So:

I have this EH-125. It has had a tone control added at some point. Wired from the grid of one of the output 6V6s to ground. The effect is interesting and not entirely musical.
The original volume control is just a pot on one the input jacks ("Mic")before the pre-amp 6SJ7. The other two inputs "Instr1" and "Instr2" go straight to the grid of the 6SJ7 with no volume control. Changing the volume control very definitely changes the tone on the mic input.


I'm trying to keep this thing as stock as possible. But make it sound as good as it can be while being usable in the modern world. I know that there's a huge element of subjectivity in all of this, so opinions are welcomed.
The valve lineup is 6SJ7 pre-amp, 6J5 Cathodyne PI and 2 x 6V6 O/P. With the tone and volume controls bypassed, the gain is adequate but not spectacular. It's a 1940s jazzers amp for heavens sake, not a 5150.

I'm thinking of moving the volume control so that it's between the 6SJ7 and the 6J5 and wiring the tone like an AC30 "cut" control - i.e. between the grids of the two O/P valves. I reckon that this is my best bet for not losing too much gain through insertion loss. I'm guessing that this was why the original tone control was wired like it is. (Likewise putting the volume control at the front minimises losses there). But that 6SJ7 has a 1M anode resistor - so even a 1M volume pot after it will instantly lose 6dB of gain.... Looking at a 5C1 champ, which I just happen to have lying around, that uses a 250K plate on the 6SJ7 and a 1M volume control

Of course that 6J5 socket could be rewired for a 6SN7 which would let me use the lossiest, best sounding tone control in the history of the universe. But that would be wrong, wouldn't it?

Any thoughts?

(apologies for mixing anode/plate and valve/tube)

[g1]

Have you compared to the EH185? Might give you some ideas although it uses an inductor for the tone control.

[nickb]

The schematic I have shows the mic input going to one 6SQ7 and the volume control is after that.

eh125.pdf

You could try this simple treble cut /bass cut single tone knob + volume arrangement:

[Ted]

I don't have the 6SQ7 version, as I mentioned. I posted the schematic in the schematic request part of the forum if you're interested.

This tone stack would have about 20dB of insertion loss when flat from a 1M source. (According to the Duncan Amps Tone Stack Calculator).

[nickb]

I don't have the 6SQ7 version, as I mentioned. I posted the schematic in the schematic request part of the forum if you're interested.

This tone stack would have about 20dB of insertion loss when flat from a 1M source. (According to the Duncan Amps Tone Stack Calculator).

I'm interested, but not enough to go hunting for it. It best to post it in your OP. Any passive tone network is going to involves loss - it's unavoidable. You may be able to pick up the gain elsewhere, if needed.

OK found it :



You could replace the 6SJ7 with two 6SL7 gain stages. That will help to compensate, but not fully I suspect, for the insertion loss of the tone control(s). If you did that it might be a good idea remove the volume control from it's current position as it's not good for signal/noise ratio.

[Gingertube]

Suggestions:
Use a bootstrapped volume control: That is, replace the 6J5 grid leak 680K with a 1M Audio pot with the wiper to to the 6J5 grid. You may want to put a 1K grid stop between wiper and 6J5 grid.
For tone control your suggestion of a "proper" cross line post phase inverter should work well.
Cheers,
Ian

[Ted]

Suggestions:
Use a bootstrapped volume control:

i was thinking about that. I've never actually seen it done. But of course googling comes up with plenty of examples....

[Ted]

The bootstrapped volume control works a treat. Great idea. Slight bleed with the control at zero, but perfectly usable. Thanks for that.