Bassman rebuild/hotrod for review

[clarkleach]

A friend brought me this ratty, old (1970) Bassman amp to resurrect. He said he wanted some mods to allow a greater variety of tones. Down the rabbit hole I went.
I'm going to pretty much gut it and build new boards.
I have a preliminary schematic of the pre-amp and output stages that I would like to offer up for peer review/roast.

pre-amp-doodle.pdf

output-doodle.pdf

20200423_104030.jpg

[ChopSauce]

That's ratty, but I like it(very much)...

I don't feel like I have the knowledge to comment, neither do I know what your friend expects as a range of sound to come from the rebuilt version, though...

[Mark]

I was wondering about the purpose of R10 (470K) on V2.2, there seems to be a mistake in the circuit. As far as I can see it will put DC on your pots causing them to be scratchy.

IMG_1363.PNG

[clarkleach]

The original AB165 and AA270 circuits had this resistor. It's local negative feedback which tames that stage a bit and offers some compression. Thank you for pointing out this flaw; it made more sense with the volume pot before the second gain stage. Not so much with this configuration. I'm gonna have to rethink this.

[Paul G.]

Lots of versatility there. The creamy/crunchy switch -- I've not had any success putting this on my amp without a terrifying pop when switching. Maybe a make-before-break switch would work (I just thought of that now)?

[clarkleach]

Lots of versatility there. The creamy/crunchy switch -- I've not had any success putting this on my amp without a terrifying pop when switching. Maybe a make-before-break switch would work (I just thought of that now)?

I was wondering if that would be the case. This is exactly why I submitted this for review.
I'll have to see if there's a way around it. Make before break switches are akin to hen's teeth.
Thanks for looking.

[clarkleach]

Lots of versatility there. The creamy/crunchy switch -- I've not had any success putting this on my amp without a terrifying pop when switching. Maybe a make-before-break switch would work (I just thought of that now)?

Seems like a 1M resistor across the switch would allow the capacitor to charge, thus eliminating the pop.
Ya think?

[clarkleach]

I was wondering about the purpose of R10 (470K) on V2.2, there seems to be a mistake in the circuit. As far as I can see it will put DC on your pots causing them to be scratchy.

IMG_1363.PNG

Do you suppose it's as simple as adding a series capacitor on the "cold" side of the 470K resistor?
That would certainly remove any DC from that path.

[Mark]

The original AB165 and AA270 circuits had this resistor. It's local negative feedback which tames that stage a bit and offers some compression. Thank you for pointing out this flaw; it made more sense with the volume pot before the second gain stage. Not so much with this configuration. I'm gonna have to rethink this.

I would put a 0.047uF or 0.1uF cap in series with the 470K resistor to isolate the DC voltage. In the case of the AB165 there are no pots between stages, Fender are also not too concerned about one stage affecting the DC voltage in another stage. Driving down cost is the main factor here.

[Paul G.]

Lots of versatility there. The creamy/crunchy switch -- I've not had any success putting this on my amp without a terrifying pop when switching. Maybe a make-before-break switch would work (I just thought of that now)?

Seems like a 1M resistor across the switch would allow the capacitor to charge, thus eliminating the pop.
Ya think?

I tried that, didn't work. Even with the amp on standby it popped, hard.

[clarkleach]

I would put a 0.047uF or 0.1uF cap in series with the 470K resistor to isolate the DC voltage.

Why so big?

In the case of the AB165 there are no pots between stages

wut? ALL the pots are between the first two preamp stages...

Fender are also not too concerned about one stage affecting the DC voltage in another stage. Driving down cost is the main factor here.

Classic Leo.

[clarkleach]

Seems like a 1M resistor across the switch would allow the capacitor to charge, thus eliminating the pop.
Ya think?

I tried that, didn't work. Even with the amp on standby it popped, hard.

I found this in another thread in el34world:
"The ol' switched cathode bypass cap on a preamp tube or CF stage has been done a bazillion times, nothing novel here. It's been labeled a "fat" switch, "boost" and many others. There's several ways to wire it so it will not loudly "pop" when you switch it in the first time. I say first time because the cap charges then keeps it's charge for a while so then no popping thereafter. But to never hear a pop - put a 1M 1/8 or 1/4 watt resistor across the switch. That is all."

Back to the rabbit hole...

[clarkleach]

Here's another cathode switching configuration...

pre-amp.pdf

[Mark]

I would put a 0.047uF or 0.1uF cap in series with the 470K resistor to isolate the DC voltage.

Why so big?

To ensure you pass all the signal. You can use a much smaller cap but that will affect your mids and highs. Try experimenting with cap values and find a value that appeals to you.

In the case of the AB165 there are no pots between stages

wut? ALL the pots are between the first two preamp stages...

So the error isn't affecting Fender's circuit in the way it will affect your circuit.

Fender are also not too concerned about one stage affecting the DC voltage in another stage. Driving down cost is the main factor here.

Classic Leo.

Absolutely.

I'm not trying to be critical here, but point out problems you will encounter in the amp.

[shane]

The creamy/crunchy switch pops because youre changing the bias of the stage suddenly, not because of the cap is charging or discharging, however you implement the switch its gonna pop.
Dont know why youve put the volume control on ch A before the first stage because as drawn v1.2 has no grid reference to ground so wont bias properly. It would surely be better to move all the circuitry shown prior to the input of v1.1 between the stages.