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 Hoffman's hot switch but 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


BASSMAN_70 Schematic.pdf

[pdf64]

These UL amps aren't a great platform for mods; the combination of stiff power supply, UL and current production tubes doesn't take kindly to being overdriven, as the screen grids get heavily overloaded and the tube quickly fails.
If it must be, then 1st step should be to install a choke for a regular screen grid supply and re-arrange to a regular tetrode output stage.
Or find a supply of Sylvania STR387, the tube that the amps were designed around.
But best to use them as intended, as a clean bass amp.
Pete.

[loudthud]

The Bassman 70 is not a bad platform for a mods but the UL output and the PI should be changed to the more normal arrangement like the AB864 with a choke. Filtering in the power supply is a bit heavy so change those 220uF @ 285V to 100uF @ 350V. There is an extra triode in V2 to add gain to the normal channel.

[Racing]

Easiest remedy with a 70....for the "mixer" stage there is a loop by the form of a 220k resistor that runs from the anode to pin 2 i believe.
Remove that resistor and you´ve all of a sudden got one gainy amp.

[DPM309]

Thanks for the tip. Turns out the owner did not want to spend the money at this time but I will keep this for future refenence.

[Racing]

My pleasure.



My own 70 i modded the living behemoth out of. Skipped the mixerstage setup entirely. Normal channel is now "clean" and setup via a 220k/3.3k/22uF first stage and a 150k/1.5k/22uF second and from there via a 220k mixer resistor heading for the master pot.

Bass channel became the lead channel (guitar amp only) that is built around all in all 4 gainstages due coarse. Major difference was through altering the PI tail though. Went from 22k to 5k and then into a 20kA pot with a 22k resistor in para for inbound signal. Then a 100nF cap vs ground from the wiper.
Works like a friggin charm and through that presence setup i can dial in basicaly anything from mild to wild.

Low,dirty n easy tho...is to just remove said 220k resistor for the mixerstage. Flipside is that BOTH channels turn "dirty".