Help sorting out whats going on with heavily modded bassman

[john l]

I got this thing super cheap a while back for a project amp since its already been pretty hacked up and was wondering what you guys think is going on circuit wise. Theres an added AX7 as well as some sort of MV on the back. The story goes its got some kind of marshall preamp added to the normal channel although Im not sure how acurate this is. Id like to start with rewiriing the bias to a typical adjustable one since its still a balance type but Im a little cofused as to what the electrolyics on the bias pot were put there for and would like to know before I start ripping parts out.. Thanks guys

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[Firestorm]

The electrolytics on the bias pot are one of the few things that weren't modified. Standard Fender for many models of this era (though I can't read the values of anything in the pic). If you want to know what's going on circuit-wise, I think you're going to have to draw out the components as a schematic and decide what to keep. The lead dress is pretty awful, so there's plenty of room for improvement.

[Structo]

How does it sound?

There is supposed to be a 50uF cap on the bias circuit.

You have a typical silverface rats nest of wiring.
When CBS bought Fender they must have hired all new people to wire the amps.

Do you know which amp this is? Like a AB165, AA270 or ?

Is that bias/ balance pot the weird tapped pot?

The model I hear praised the most is the AA864.

You might want to check that out and shift your amp towards that.

You could Dumblize it by converting one of the channels to an Overdrive.

This amp definitely has some potential.

[NickC]

I converted my 1966 to hybrid AB165 / AA864. Best of both worlds. This link shows how:

http://acruhl.freeshell.org/mga/main/AA ... AB165.html

Recommended.

[john l]

How does it sound?

There is supposed to be a 50uF cap on the bias circuit.

You have a typical silverface rats nest of wiring.
When CBS bought Fender they must have hired all new people to wire the amps.

Do you know which amp this is? Like a AB165, AA270 or ?

Is that bias/ balance pot the weird tapped pot?

The model I hear praised the most is the AA864.

You might want to check that out and shift your amp towards that.

You could Dumblize it by converting one of the channels to an Overdrive.

This amp definitely has some potential.

Thats exactly what I was thinking when I bought the thing, that it deffinatlely had potiential. Believe it or not its sounds really good, and more surprizingly its quiet with no oscillation even after I started clipping out all the supression caps CBS put in these. I believe the circuit may be a AA371 now that firestorm chimed in with bias pot electrolyits being stock. Looks like the guy that modded it just went from 50uf to 80uf is all. Can anyone help with rewireing the bias to adjustable? Im temped to just make it look like a typical black face ditching the other electrolytic and the 3.3k from the board. Is my thinking correct/safe?Theres alot of holes in my little knowledge of tube amplification but I can safely work on a hot amp and can solder well. Thanks for any help

[Cliff Schecht]

I just completely gutted a Silverface PA100 head. It was full of factory errors and (maybe IMO) was just a poorly built amp. Seemed to be a far stretch from the relative tidiness of your typical tweed or blackface amp. It also sounded for shit which I eventually narrowed down to a lame circuit .

This is going to be a donor for my buddies bass project. Sellout has convinced me that 200W will be better than 100W for a bass amp. I was going to try to keep the power section stock but as I started to strip the chassis I noticed the power tube sockets were crumbling. At that point I completely removed everything from the chassis to clean whatever is being reused (preamp sockets, jacks, etc..) and am starting over from scratch. I usually don't like to hurt vintage equipment but this thing had already been modified and really was just an 80+ pound door stop as is (boat anchor maybe).


The OP's amp is pretty rats-nesty too. If it isn't usable and/or safely built then I probably wouldn't have any qualms about stripping it out and starting over. It's already been hacked on and all, and not very well from the looks of it. I'm a stickler for safety when working with these high voltages, some people seem to have less regard for their own lives. I haven't been shocked for a few years now and I'm hoping it never happens again.

[NickC]

Don't know if it'll help, but here's a shot of adjustable bias mod (not bias balance) in my '66 (sorry about poor photography, ancient digital camera).


[img1728]http://homepage.mac.com/swamptone/ModsBassmanMiddle.jpg[/img]

[img1728]http://homepage.mac.com/swamptone/ModsBassmanRight.jpg[/img]

[img1728]http://homepage.mac.com/swamptone/ModsBassmanDone.jpg[/img]

[john l]

I just completely gutted a Silverface PA100 head. It was full of factory errors and (maybe IMO) was just a poorly built amp. Seemed to be a far stretch from the relative tidiness of your typical tweed or blackface amp. It also sounded for shit which I eventually narrowed down to a lame circuit .

This is going to be a donor for my buddies bass project. Sellout has convinced me that 200W will be better than 100W for a bass amp. I was going to try to keep the power section stock but as I started to strip the chassis I noticed the power tube sockets were crumbling. At that point I completely removed everything from the chassis to clean whatever is being reused (preamp sockets, jacks, etc..) and am starting over from scratch. I usually don't like to hurt vintage equipment but this thing had already been modified and really was just an 80+ pound door stop as is (boat anchor maybe).


The OP's amp is pretty rats-nesty too. If it isn't usable and/or safely built then I probably wouldn't have any qualms about stripping it out and starting over. It's already been hacked on and all, and not very well from the looks of it. I'm a stickler for safety when working with these high voltages, some people seem to have less regard for their own lives. I haven't been shocked for a few years now and I'm hoping it never happens again.

Is it really that bad. Most of the silverfaces Ive seen look just like this minus my little bonus board. Thanks for your concern but really Im good to go here.

[Structo]

john i,

Does your amp have the strange tapped pot with four lugs?

As for making the bias adjustable, I would copy the AA864 circuit.

For what it's worth, Hoffman has some good Fender schematics in his data files.
The ones that are all capital letters are cleaned up versions most likely from the Pittman book, much easier to read.

http://www.el34world.com/charts/Schematics2.php

[john l]

john i,

Does your amp have the strange tapped pot with four lugs?

As for making the bias adjustable, I would copy the AA864 circuit.

Yes Tom it does. Can I just bypass it and use the 3 standards? I ask because Ive just gotten it rewired and going over things before I fire it up lol. Ill up load pics in sec

[john l]

So here I am. I went nuts and replaced some of the turds while I was there lol. Im about to reflow some of the shady chassis grounds and then throw it on a light bulb current limiter to check for shorts. How do I look? Again Im kinda just soldering by numbers here so if you see anything weird please speak up.

[IMG:799:448]http://img.photobucket.com/albums/v299/ ... 51_628.jpg[/img]

[Structo]

john i,

Does your amp have the strange tapped pot with four lugs?

As for making the bias adjustable, I would copy the AA864 circuit.

Yes Tom it does. Can I just bypass it and use the 3 standards? I ask because Ive just gotten it rewired and going over things before I fire it up lol. Ill up load pics in sec

I don't have any experience with those pots so I can't say.

I would have ohmed it out to make sure the three remaining lugs function as they should before firing it up.
Because that pot is all that stands in the way of your power tubes running away.

[Cliff Schecht]

I just completely gutted a Silverface PA100 head. It was full of factory errors and (maybe IMO) was just a poorly built amp. Seemed to be a far stretch from the relative tidiness of your typical tweed or blackface amp. It also sounded for shit which I eventually narrowed down to a lame circuit .

This is going to be a donor for my buddies bass project. Sellout has convinced me that 200W will be better than 100W for a bass amp. I was going to try to keep the power section stock but as I started to strip the chassis I noticed the power tube sockets were crumbling. At that point I completely removed everything from the chassis to clean whatever is being reused (preamp sockets, jacks, etc..) and am starting over from scratch. I usually don't like to hurt vintage equipment but this thing had already been modified and really was just an 80+ pound door stop as is (boat anchor maybe).


The OP's amp is pretty rats-nesty too. If it isn't usable and/or safely built then I probably wouldn't have any qualms about stripping it out and starting over. It's already been hacked on and all, and not very well from the looks of it. I'm a stickler for safety when working with these high voltages, some people seem to have less regard for their own lives. I haven't been shocked for a few years now and I'm hoping it never happens again.

Is it really that bad. Most of the silverfaces Ive seen look just like this minus my little bonus board. Thanks for your concern but really Im good to go here.

Nah it's not that bad, I just like a tidy amp. Those Silverface amps can be quite annoying to repair if you have to carefully dig into a big bundle of wires. The later 80's blackface amps are just as bad if not worse about lead dress and big wire bundles and such.

[john l]

Pulled the Tubes and fired it up on the limiter the bulb dimmed normally so no shorts. Guess Ill stand in a bucket of water and throw the switch! Ill post back in a few.

[john l]

Well it works its got a nice sweep and no red plating. I supose if anything was wired wrong id find out pretty quick. I did a little low volume playing and it sounds the same so I think Im done with this part. So far Ive
-done the bias conversion
-replaced all the turd caps I could get to with 150s
-Moved the tone stack of the normal channel off the board and pot mounted it (just cuz Gerald Weber says its better layout and this things already got enough hinky stuff going on)
- replaced all the electrolytics on the board with NOS Mallorys that I had lying around and brought the amp up slowly on a variac to reform them. (Its all I had Id prefer to used new makes)
-ripped out the hinky guitar cable and replaced it with more typical stuff
- Reflowed any bad looking chassis grounds

Ive put in an order for replacement filters so I have to wait to do that untill i get here. What else do you guys think I should do as far as lead dress.
Heres where Im at now

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