Bassman 70 Hot Rod

[dpm309]

I have been asked to"hot-rod" a 1980 Fender Bassman 70.  After searching the internet, I came across a reference to a hot rodding mod done by Jt. Mouradian but little else.  This amp already has a master volume but he wants to add more gain to the guitar channel.  I looked at Doug's hot switch and I don't think this would would work on this amp.  There are several other mods I can do such as tweaking the circuit to resemble the AA864 circuit which would improve the tone.  Torres sells a kit for a AB165 to AA864 conversion but this schematic is not the same as a AB165.  Has anyone heard of Mouradian's mods?  I have attached the schematic for reference.  Any suggestions or ideas?

Thanks,

Dan

[jjasilli]

Not clear what tone the customer wants:  vintage Marshall; Metal; better vintage Fender?

[dpm309]

Talked to owner last night and he wants a better vintage Fender sound with additional gain.  I came across a link about adding a post PI master volume.  This would give 2 master volumes.  The article also mentions that the had to mod the original bias “balance” circuit to make it a bias “level” adjustment.  Here is a link to the bias level adjustment: http://bassman70.com/bias_mod.html.  I have attached the schematic of the post PIMP mod.  In order to avoid adding an additional pot, could the pre PIMV be removed and install a dual ganged pot in its place?

Thanks,

Dan

[PRR]

.

[jjasilli]

You can also get more gain by changing plate resistors to 220K; cathode resistors to 2.2K - 3.9K.  If for regular guitar (not bass), cathode bypass caps can be reduced to 1uF.  This will concentrate gain in the guitar frequency spectrum.  If extra gain causes oscillation, some fixes: local NFB; global NFB; put a resistor up to 10K under the cathode bypass cap.

[dpm309]

Thanks jjasilli and PRR.  I will use your suggestions and report back on how this works.  Do you all think the bias mod is worth doing also?

Dan

[jjasilli]

Yes to the bias mod.  You might consider dual adjust pots, so you can both adjust & balance bias.