Ab763 simplified design

[Mikante]

Hello,
Since my first build turned to be good, i fell like i need to add more challenge for my next one.
I decided to use the ab763 deluxe design, i don t need either tremolo and reverb, i like ss rectified amp better, i like to jump channels, i like to have a mid control for the eq.
So i have simplified the schematic and added my mods.
I have doubts and i d like to use your help.
Here the schematic

55E0D850-EB3E-4C14-B303-B30E661F7DE7.jpeg

What do you think?
My questions:
Is the 10k resistor after the bias pot usually replaced with a trim pot? I have seen few design with it.
I am not sure if i should use an on/off switch for the bright cap on the vibrato channel, thought the cap was added in this design to have a better sounding reverb, so maybe it is not necessary.
How can i put the two channels in phase so that i can use jumpers?
The first channel will only have a 5f2a design tone control.
I didn t draw the power section since i m going to use ss rectificatio and the 50v out going to the bias.
Thanks.

[sluckey]

This may give you some ideas. Schematic at bottom of page...

[Mikante]

This may give you some ideas. Schematic at bottom of page...

http://sluckeyamps.com/phoenix/phoenix.htm

Cool thanks, this design does not include a third stage preamp tube,
I really like the idea of being able to use the first channel for cleans and the second for more overdriven tones and then use a jumper so i can sort of blend the sound i want.
I don t understand how the bias work on this sluckey amp design.

[sluckey]

Maybe something like this...

[Mikante]

Maybe something like this...

Hello, thank you, you have modified the part right after the third gain stage where the tremolo was mixed in the original design, that part was a bitconfusing for me.
Did you change anything else?
Ok i have noticed now, both channels are going into the third pre amp tube, this will allow me to use both at the same time. Will they sound exactly the same? bright cap aside.
The two coupling caps marked red are different, one is .047 and the other is .022. Should i leave it this way?

3AC2820C-DAAF-4F9B-8666-84EAE902C7EE.jpeg

[sluckey]

I made a few minor changes to the drawing. These two preamps will not sound the same. They will both have a lot of gain, maybe too much. You should be able to set the gain for each preamp by just changing the size of the 220K mixing resistors. Bigger resistors will lower the gain. May need to go as large as 3.3M.

It's your call about the two different size coupling caps.

Notice that I gave every triode it's own cathode cap/resistor. No more sharing.

[Mikante]

Thank you, i see now.
If i get rid of the cathode cap in the second preamp stage, like the early 5f1, wouldn t that tame the gain a little bit?
I could do that on both channels, they will both go through a further gain stage and as you said, there is going to be plenty of it anyway.
I did that on my 5f2a and it cleans up the tone quite a lot.
As for the other parts, I think I am going to follow the schematic and perhaps do some change by ear.

[sluckey]

If i get rid of the cathode cap in the second preamp stage, like the early 5f1, wouldn t that tame the gain a little bit?

Yes, because with no bypass cap you will have degenerative feedback at all frequencies. That's something to play with using your ears, not only for gain, but also tone. The 5F1 has no bypass cap on the second triode because that's where the NFB loop is connected.

Anytime you cobble two amps together such as you propose, there will always be some fine tuning by ear. I think the schematic is workable. The main issue should be trimming the gain for each preamp.

When you removed that 3.3M from the AB763 channel, you caused the gain to increase significantly. The 5F2A channel should be even hotter than the AB763 channel just because the simple tone circuit is not as lossy as the AB763 tone circuit. For this reason, you may need a larger mixing resistor for the 5F2A channel if you are interested in equalizing the volume of both channels. It's OK if the two mixers are different values.

Have fun.

[Mikante]

Sorry but i thought you removed the 3.3m resistor, if it was me i did it by mistake. I will put it back.

[johnnyreece]

I did something like this a while ago with a Princeton Reverb. I lowered that to 2.2M in mine to get a little more hair out of it.

[sluckey]

Sorry but i thought you removed the 3.3m resistor, if it was me i did it by mistake. I will put it back.

No! Your original pic showed the 10pF cap but you had clipped out the 3.3M resistor. And that left the third gain stage crippled with no grid bias. That's the main reason I decided to fix your drawing.

Now you don't need that 3.3M and I told you how to tame the gain so everything is good to go.

[Mikante]

It wasn t my intention, I understand how to adjust the gain now but I prefer to keep the schematic design as genuine as possible an maybe work from there. I did the opposite with my first amp and I had hard time understand what was going on untill I brought everything back to original. I will put it back in its place, the 3.3m resistor I mean.

[Roe]

I made a few minor changes to the drawing. These two preamps will not sound the same. They will both have a lot of gain, maybe too much. You should be able to set the gain for each preamp by just changing the size of the 220K mixing resistors. Bigger resistors will lower the gain. May need to go as large as 3.3M.

It's your call about the two different size coupling caps.

Notice that I gave every triode it's own cathode cap/resistor. No more sharing.

I'd remove the 470k to ground connected to the channel mixing resistors. and increase the 25k mid pot to 100ka. and adjust the mixing resistors to 270k/180k

[sluckey]

I'd remove the 470k to ground connected to the channel mixing resistors.

That 470K provides the ground reference for the tube grid. Without it the tube cannot establish bias.

[sluckey]

It wasn t my intention, I understand how to adjust the gain now but I prefer to keep the schematic design as genuine as possible an maybe work from there. I did the opposite with my first amp and I had hard time understand what was going on untill I brought everything back to original. I will put it back in its place, the 3.3m resistor I mean.

Well it's your project so build it as you like. I was just trying to help fix some fatal flaws with your original schematic and put the two channels in phase so you could jump channels. FYI, that third gain stage (V4) cannot possibly work in your original schematic even if you put the 3.3M back in place.