Planning out modification roadmap for my AB165 Bassman...any suggestions?

[bikehorn]

So the story goes:

 

3 years ago, a month before graduating high school, i found a '67 bassman and '72 traynor guitar mate in a storage room in my high school. luckily, the school let me "dispose" of them. neither were in working condition.

 

The traynor only needed a new power cord and pilot lamp(although it actually needs a recap now) but the bassman was in rougher shape. supposedly it was shocking people at one point so i have never powered it up. I didn't have the money to get it all tuned up in those days so it sat in my parents' basement for all this time while i've been away at university...but now i have a much better understanding of electronics and a slightly bigger budget too. as i've been reading up on the AB165 it seems like most people modify them to AA684 specs. I don't care about exactly matching vintage specifications and i also don't care about "strictly keeping it original". the sounds i am shooting for are amped-up 60's and 70's rock 'n roll with some help from a fuzzbox.

 

So what i've gathered so far is:

 

-recap everything

-replace crappy balance-style bias circuit with a normal variable bias

-replace ground-lift switch with footswitch socket to allow tone control bypass and boost

-remove local feedback loop for power tubes and first input stage

-remove cap in global feedback loop and adjust resistor values

-try 12AX7 phase inverter

-replace selenium rectifier diodes with new silicon devices

-change plate load resistors to slightly higher values

 

I've been wondering about the power supply though. I can find film caps that are the same value as the electrolytics in there presently, and from my other hobby, which is audio i have come to believe that film caps do a better job of filtering high-frequency noise out of the supply than equivalent value electros. I know they cost more, but i would be happy to pay a little more for some Solens if there would be a noticeable difference. Is there? has anyone done this?

 

Also i noticed in the schematic a pair of 40uF 350v caps in series in the power supply....presumably to give a higher voltage rating....do i need this series connection if i simply put a single 22uF 630v cap in there to replace them both? i would think not but i wanted to ask anyway.

 

I am also interested in replacing the old PS rectifier diodes...i don't believe the hype surrounding ultrafast diodes, but i can get them cheap so i plan to use them anyway. but here's one thing i don't understand...why are there three diodes in series with each other on each side of the PS? it does not appear to be a conventional bridge.

 

i'll also have to find the source of the "shocking" too...

 

I have also wondered about going to DC filaments since i have read that they result in lower noise. is this true, or hype? i imagine it wouldn't be hard to throw a bridge rectifier on the 6.3v supply and then bring the voltage down with a resistor or regulator. anyone tried this?

 

thanks

[Mxr20]

get an aa864 board put in

[JamesPeters]

My suggestion is to get it working properly, modify the bias supply so it can be adjustable (it's still fixed bias though, just with an adjustment pot), and then decide on the rest later. Removing the NFB loop and increasing gain in the preamp by increasing plate resistors etc....I don't know about that. Sounds like a halfassed way to go about making it "higher gain". Either have a circuit in mind to modify it to, or don't really start at all, IMO.

 

Don't bother with a dc supply for the heaters either. Retube it, recap it, bias it properly, make sure the output tubes' heaters are actually wired correctly (so that the heater hum actually cancels like it should; lots of Fenders are wired wrong), and that should take care of most of the noise.