Fender Bassman SP-3100 output transistors

[jimmyjazz]

A buddy's Fender Bassman SP-3100 has blown output transistors. This is a late 60's SS amp.

They are marked:

RCA 7806

Anyone know where I can get replacements or something similar? The amp is only for a monitor so anything close will probably be fine.

V+ = 40V, 100w TO-3 package

I like Mouser so extra points for something from them.

Thanks!

[J M Fahey]

Hi Jimmy, "RCA 7806" means they were made by RCA on the 6th week of 1978 (so maybe they are not originals), but we still need a little more info.
Is anything else written on them?
As of the power supply: is it a single +40V one or you have two: +40 and -40?
It's very different situation, the first one hints to a 25/30W amp; the second one to 80/100W.
If possible post a couple pictures.
Also tell us the other transistors on the board, specially the drivers.
*Maybe* you are lucky and somebody has the schematic.
Given the date , it might use a driver transformer and some 2N3055's.
I remember hearing one of them, it was *loud* but had no bass.

[Sock Puppet]

*Maybe* you are lucky and somebody has the schematic.

Fender will send you copy if you email them:

Hard Copy Archives

[jimmyjazz]

I did get a schematic from Fender but no part numbers listed. Let me snap some pics and post them along with the other transistors.
Thanks!

[jimmyjazz]

I got the following from my buddy:

The driver transistors are the smaller version of the TO-3 case (not sure what that case number is), with the number RCA 037077 on them.

The power supply is +/- 40 volts.

I'll try to post the schematic.

[J M Fahey]

Heeyy !!! That's quite a good design !!
Very good for the era, very usable even today.
It had Fet input, another Fet in the preamp output, transformer-driven power amp.
That sounds good .
There was also the active notch filter which raises a few questions: if I read the values well, it was set on around 100Hz.
*Maybe* that was the resonant frequency of the speakers when mounted in that box and they had a "farting" problem.
I remember hearing one of them around 1970 and my impression was of a loud and very articulate amp, but with "no bass" (now I understand why); this guy's "other amp" was an Ampeg B15, much rounder sounding but not even half the volume.
I'm sure the original output transistors are the "heavy" version of that era's "king of the hill", the omnipresent 2N3055.
They probably used the very robust (impossible to find today) 2N3055H , factory selected for them, or RCA 40411 or 2N6254.
Today you should use the much beefier MJ15024 or similar.
That "small TO3" case is called TO66, quite obsolete today.
Maybe they are still good; but if they died, with some care you can install TIP31C there, by bending Base and Emitter legs 90 degrees so they fit through the original holes, clipping away the unused middle leg and making the collector connexion through the mounting screw, which must touch the metallic flange.
You'll need a rectangular mica of course, plus giving it some thought as to how to mount it in the neatest way.
But the good news is that you will be able to revive and use it.
Post whatever happens.
PS: the only "trap" you may find is that sometimes PCB designers used the metallic TO66 case as a "bridge" between 2 tracks; just in case, after mounting, solder a very short piece of wire joining both pads where screws originally made contact, unless one of them clearly goes nowhere.

[Enzo]

And as much as I hate NTE, they may have a TO66 sub available for those parts.

I would not hesitate to use MJ15024 or MJ15003 for the outputs.

[jrfrond]

Looks like they borrowed the design of the power amp from the Fender Rhodes Suitcase model. I'm referring to the driver xfmr design. The Rhodes design used germanium, but I've upgraded many to silicon for better stability.

[jimmyjazz]

Thanks everyone for the replies. I have a few things to try. I'll post what I find out.

[ombolikiki]

I have a similar amp, a Bassman 3100 and I just love it.

However not so long ago it started to work funny, (I have bought a new bass, which has a fairly bigger signal, than the previous, not that great however to blow the amp) and from week to week it becomes worse. Here's what it does:
If I play on the bass (volume max, passive pickup) but more notably when I turn my Woolly Mammoth clone on (same volume as the dry signal) it starts to moan under my bass sound. Like 5 notes (or an octave and 5 notes) below the original sound. What do you think, what can be the problem?
And it does this more and more often...

The transistors are the same (2n3055) and I think they have to do something with it, but I don't know.

Also the previous owner used it as an organ amp and made a few modifications on it. I think the preamp stage is still in its original condition, however I have to look at it closer. It worked well for a long time, even with bass.

What would change if I would replace the 2n3055s with MJ15024 or MJ15003?

Sorry for the loads of questions!

[52 Bill]

Welcome to the place.

You might want to start your own thread, so we are not discussing 2 different amps with 2 different problems.

In any case, the symptom that you describe sounds more like a problem with your power supply filter caps and not the output transistors.

[ombolikiki]

Sorry, I will start a new thread then.