Amp with switchable tone stack

[Bieworm]

Hi!

I have built an amp recently of which I'm not so happy with the sound. I want to rebuild it using the same tube layout and faceplate.
Since I have both a gain and raw control on the faceplate I'm thinking about giving it an extra TS+vol control circuit, switchable with a relay. It would be basically a switch between AB763 and Tweed/brown tone stack.

Don't worry about the 6G16 tremolo circuit's operation. I have a fixed bias board in the amp in case the cathode biased system would compromise the tremolo..

See anything in this schematic that needs attention?

Thanks in advance!

[sluckey]

Speakers are shorted.

[Bieworm]

Speakers are shorted.

Lol.. you’re right😊

[shaun]

For what it's worth, I'd be re-voicing the tone stack in the schematic you already have to see why it wasn't what you wanted. Assuming the attached schematic is what you used, the .047 coupling cap in the first stage, combined with the typical Fender value caps in the tone stack, could make for a bit of cotton wool in the bottom end, and not much mid-range definition. As I say, only my 2 cents worth, but I'd try a .022 in the first coupling cap, then a 500pF, .047uF, and .022uF in the TS, which should tighten up the tone considerably.

I'd also be tempted to add an extra power filter node to the preamp rather than run it all off node D. Just seems to make an amp sound smoother when I do that, and a bit more responsive.

Dunno if any of that is of interest, but I always want to know how to diagnose the amp before adding extras. In any case, it sounds like a great project.

[Bieworm]

For what it's worth, I'd be re-voicing the tone stack in the schematic you already have to see why it wasn't what you wanted. Assuming the attached schematic is what you used, the .047 coupling cap in the first stage, combined with the typical Fender value caps in the tone stack, could make for a bit of cotton wool in the bottom end, and not much mid-range definition. As I say, only my 2 cents worth, but I'd try a .022 in the first coupling cap, then a 500pF, .047uF, and .022uF in the TS, which should tighten up the tone considerably.

I'd also be tempted to add an extra power filter node to the preamp rather than run it all off node D. Just seems to make an amp sound smoother when I do that, and a bit more responsive.

Dunno if any of that is of interest, but I always want to know how to diagnose the amp before adding extras. In any case, it sounds like a great project.

Thanks! that is some useful advice. I'll look into that while tweaking the TS. OTOH since the "I don't like" build already has some extra controls and a relays switching circuit on board I seems pretty neat to switch between BF and Tweed/Brown and have that extra gain on tap with the single tone control in Tweed/brown mode. I will know when I did the mods... If it's not what I expected I can always turn the amp into something else (5F4 Super "reverb/tremolo" for instance) with this bandmaster iron and make another faceplate to taste.. that's my plan B

FWIW I think the aesthetics came out pretty cool and I want to make it a useful amp for my purposes

[d95err]

Some notes/ideas:

The 0.047 caps after each tone stack look unnecessary. There’s no DC there.

You could skip one of the 10M resistors around the relay, as either will provide a path to ground for both sides if the relay. Or, if you skip the 0.047 caps, you don’t need any of the 10Ms.

The mixing resistors will cause 6dB attenuation. Is that intentional?

If you don’t want the attenuation, you could skip the mixing resistors and use the relay in series, selecting which tonestack output to connect to the grid of the second stage. That would cause no additional attenuation.

There may be some interaction between the two tonestacks. If you have a DPDT relay, you could use the second switch to disconnect the unused tonestack from ground.

[Bieworm]

Some notes/ideas:

The 0.047 caps after each tone stack look unnecessary. There’s no DC there.

You could skip one of the 10M resistors around the relay, as either will provide a path to ground for both sides if the relay. Or, if you skip the 0.047 caps, you don’t need any of the 10Ms.

The mixing resistors will cause 6dB attenuation. Is that intentional?

If you don’t want the attenuation, you could skip the mixing resistors and use the relay in series, selecting which tonestack output to connect to the grid of the second stage. That would cause no additional attenuation.

There may be some interaction between the two tonestacks. If you have a DPDT relay, you could use the second switch to disconnect the unused tonestack from ground.

Thanks!! That's why I threw this out there.. more people, better ideas :)
Is this in line with what you mean?

[d95err]

The output selection switch is OK. However, you don’t need to connect the unused tonestack output to ground.

What I suggested was to use the second relay switch to disconnect the ground connection of the unused tonestack. I.e. at the bottom of Gain pot and the 6k8 resistor.

Connect the middle lug of the switch to ground and the outside lugs to each tonestack.

[Bieworm]

Ok... I have built this circuit and it looks like a nice configuration. However, the single volume/tone circuit is A LOT louder than the AB763 part of the circuit.
I have always heard about the gain loss due to the T(M)B tone stack compared to the single tone control circuit, but now I really know the amount of gain gets lost.. WOW! I'm astonished!
Is it a good approach to attenuate it by bypassing V4b, adding a relays that switches simultaneously? I know the reverb will be attenuated too, but for more overdrive that could be a good thing?

One thing you should know is that I have used a dual ganged A1M pot for the volume/tone circuit.. so the treble increases with the volume. That will likely need to be tweaked when the volume is more in harmony with the AB763 tone circuit part.

OR/AND... thinking out loud. Is there something beneficial I could do with a split load at the plate resistor of V1? I know it's done with certain 6G15 mods to tame the gain. Just a thought...

I have also added a fixed bias mode, for the tremolo wouldn't wiggle the 6L6 tubes in cathode bias.

[sluckey]

Using a split load for V1A plate will solve the unequal gain of the two tone stacks. I suggest replace the 100K plate resistor with a 100K pot. Leave the TMB stack connected directly to the plate as it is now. But connect the .022µF cap to the pot wiper. Adjust the pot until the loudness of both tone circuits is the same. Then carefully measure the resistance between the wiper and each end of the pot. Now replace the pot with two appropriate value resistors. The .022µF cap will connect to the junction of the two resistors.

Or, you could do as I did on my Warbler. Use a good quality locking pot and mount it permanently. See link...

     http://sluckeyamps.com/warbler/warbler_rev2.pdf

[Bieworm]

Using a split load for V1A plate will solve the unequal gain of the two tone stacks. I suggest replace the 100K plate resistor with a 100K pot. Leave the TMB stack connected directly to the plate as it is now. But connect the .022µF cap to the pot wiper. Adjust the pot until the loudness of both tone circuits is the same. Then carefully measure the resistance between the wiper and each end of the pot. Now replace the pot with two appropriate value resistors. The .022µF cap will connect to the junction of the two resistors.

Or, you could do as I did on my Warbler. Use a good quality locking pot and mount it permanently. See link...

     http://sluckeyamps.com/warbler/warbler_rev2.pdf

thanks! like this? I don't have any 100 linear pot above 0.4 watt.. so I will do the 1st suggestion and measure the resistances of both halves of the pot when I have the balance set.

[Bieworm]

Did the split load thing with a 100k pot. The values landed on 82k + 18k, so the vol/tone circuit needs 5x less load…