Fender Bassman 10 modification

[Randy]

A while ago, I got a hold of a Fender Bassman 10. It's a decent guitar amp, but I already have a Bassman head and it sounds better than the B10. I made a 18 watt amp before and love it, but it isn't loud enough to play with the band I'm in. I figured it would be fun to gut the B-10's preamp section and wire up the Marshall 1974x preamp circuit, then do some tweaks to get the power section to run on either 6L6's or EL34's.

Oddly enough, it worked! It sounds and feels a lot like the 18 watt, but it is painfully loud. I have only played with it a few hours, but all indications are that it's going to hold. I made up a schematic and a layout of what I had done if anyone wants to try this modification. Last I looked, Bassman 10's are pretty common and fairly cheap.

Word of caution: if you haven't worked with high-voltage tube amps before, don't try this until you've had proper instruction. You can kill yourself or burn down your house doing this stuff.

[nclayton]

Just checking out your schematic, and I notice your master volume is a little unorthodox. It looks like if you plug into channel one (the 1 knob tone control channel) turning down the master volume will cut a ton of treble and a good bit of bass and leave behind just a little low level midrange. Is this intentional?

Seems like you'd want to put channel 2's 220K mixing resistor AFTER the master volume rather than before, and tie the two 220K resistors directly together followed by a single blocking cap (rather than the two .01 and .1 caps) going into the phase inverter in order to minimize the tonal effect of the master volume's position on the non-master channel. Still, if it works for you as is, obviously there's no particular reason you'd HAVE to do it any other way.

Also, I think you got the negative feedback shunt resistor mislabelled on the schematic? Probably it should be 100 rather than 100K, I'm guessing? If the amp's super loud, I'm sure you didn't mix them up on the build, but if you put this up to let other people copy, you might want to update that to save some potential confusion.

Awesome!


Ned

[Randy]

Just checking out your schematic, and I notice your master volume is a little unorthodox. It looks like if you plug into channel one (the 1 knob tone control channel) turning down the master volume will cut a ton of treble and a good bit of bass and leave behind just a little low level midrange. Is this intentional?

Seems like you'd want to put channel 2's 220K mixing resistor AFTER the master volume rather than before, and tie the two 220K resistors directly together followed by a single blocking cap (rather than the two .01 and .1 caps) going into the phase inverter in order to minimize the tonal effect of the master volume's position on the non-master channel. Still, if it works for you as is, obviously there's no particular reason you'd HAVE to do it any other way.

Also, I think you got the negative feedback shunt resistor mislabelled on the schematic? Probably it should be 100 rather than 100K, I'm guessing? If the amp's super loud, I'm sure you didn't mix them up on the build, but if you put this up to let other people copy, you might want to update that to save some potential confusion.

Awesome!


Ned

Thanks Ned!

I haven't noticed the loss of bass and treble, but yeah, I'm going to try your suggestion. That's just a couple of minutes on the soldering iron and possibly a big improvement.

The 100k thing is definitely a typo.

I'll put up a fixed schematic and layout soon.

[nclayton]

After I looked at the schematic and posted my reply up there -- only AFTER that -- I also looked at your layout, and noticed on that you had the 220K resistor AFTER the master volume, so I assume you actually did it that way. If you'd done it with the 220K BEFORE the MV I'm sure you'd have noticed a big change in channel one as you turned it down.

With the 2 separate caps feeding into the phase inverter, I still think the MV position will affect the tone of the first channel, but the way it is, as you turn it down it will probably just drop channel 1's overall volume a bit, but leaving behind a little low-mid bump, but not a very big one.

If you run the 2 220k resistors directly together and only use 1 cap going into the phase inverter, that will definitely minimize the MV's affect on channel 1...it still could slightly affect overall level, but not tone so much. You might have chosen the 2 different values of blocking cap there to have one channel end up with a little less bass, but in reality I think if you just use a single .01uf cap for both channels, it probably won't affect the sound at all. You'd think that cap is feeding a 1M impedance since that's the value of the grid bias resistor, but in reality the negative feedback coming back from the speaker and developed across the 22K cathod resistor makes the effective impedance much higher than 1M. So even with a .01 cap there, the corner frequency is going to be probably less than 5hz. That's why I don't think you'd notice a difference in sound between the .1 and .01uf.

[Randy]

After I looked at the schematic and posted my reply up there -- only AFTER that -- I also looked at your layout, and noticed on that you had the 220K resistor AFTER the master volume, so I assume you actually did it that way. If you'd done it with the 220K BEFORE the MV I'm sure you'd have noticed a big change in channel one as you turned it down.

With the 2 separate caps feeding into the phase inverter, I still think the MV position will affect the tone of the first channel, but the way it is, as you turn it down it will probably just drop channel 1's overall volume a bit, but leaving behind a little low-mid bump, but not a very big one.

If you run the 2 220k resistors directly together and only use 1 cap going into the phase inverter, that will definitely minimize the MV's affect on channel 1...it still could slightly affect overall level, but not tone so much. You might have chosen the 2 different values of blocking cap there to have one channel end up with a little less bass, but in reality I think if you just use a single .01uf cap for both channels, it probably won't affect the sound at all. You'd think that cap is feeding a 1M impedance since that's the value of the grid bias resistor, but in reality the negative feedback coming back from the speaker and developed across the 22K cathod resistor makes the effective impedance much higher than 1M. So even with a .01 cap there, the corner frequency is going to be probably less than 5hz. That's why I don't think you'd notice a difference in sound between the .1 and .01uf.

Yep, the layout was the correct one on that. I did the schematic afterwards and got a little turned around. I should have seen that resistance going to ground and said "hmmm".

Thanks for the explanation on the NFB's effect on impedance. I overlooked that when going through this. There's so much to take into account when working on this stuff.