Fender Bassman 135, need layout diagrams/photos

[Matthews Guitars]

Thanks!

Coincidentally, today I spent about eight hours on this amp, rebuilding it and getting pretty close to completion. Along the way I figured out what the last guy who was in it was up to,
more or less. I had to go back and restore the tube matching control circuit, and found that there are mismatches between diagrams, schematics, and photos of what's in the doghouse and where everything connects up. It's like people were making diagrams of amps that already weren't in original condition.

This did require some changes to be made to the phase inverter circuit.

That's the fun part, when neither the photo nor the diagram match the schematic, and you have to rebuild it from the schematic which makes you REALLY learn how the amp is built.

I'm rebuilding it to be totally stock because I intend to sell it. Whomever gets it next will probably want to mod it, and by restoring it to a stock circuit, that will make it easier for the next guy who gets into it as he won't have any surprises to figure out.

[Matthews Guitars]

The bias balance circuit is for the birds. Screw it, I'm putting in a standard variable bias. It's not hard to do. As it is, the bias is way off, power tubes try to redplate, and rather than troubleshoot DUMB bias circuit design, I'm just going to rip that out and do it better.

I'll use a Marshall JMP bias circuit. I know those by heart.

[sluckey]

It's easy to add a bias adjust to that balance pot. Then you have the benefits of both. Just replace that 33K with a 22K in series with a 25K pot.

[Ten Over]

The bias balance circuit is for the birds. Screw it, I'm putting in a standard variable bias. It's not hard to do. As it is, the bias is way off, power tubes try to redplate, and rather than troubleshoot DUMB bias circuit design, I'm just going to rip that out and do it better.

I'll use a Marshall JMP bias circuit. I know those by heart.

The Marshall bias winding is around 100V and the Bassman 135 is 53V, so the Marshall values won't work in the Fender.

Fender Bassman 135 Bias 2.png

[Matthews Guitars]

I know, but I know the circuit and can adjust values to make it work.

Actually today I got the amp running with the stock bias balance circuit.

It's low on power output, and I need to look into that. And its tone shaping is a bit off. I'm using stock values across the board, but it doesn't sound right.

I'll check on signal levels.

I don't think there's anything wrong with the OT. It doesn't feel or sound like an OT problem. But I can crank it and the volume level is comfortable. That's wrong. It should be LOUD.

[Ten Over]

I know, but I know the circuit and can adjust values to make it work.

Scaling the parts down also scales down the bias voltage range. By the time you start with a low voltage, have two filter nodes and use a 10k pot, you can't get an acceptable range using Marshall's topology. However, you can get a decent range by using the 10k pot as shown below.

Fender Bassman 135 Bias 1.png

[pdf64]

The bias winding voltage on that Fender era can be somewhat marginal for current production valves. The STR387s they seem to have been designed around don’t seem to have needed as much bias voltage. Previously I’ve brought that 470 resistor down to 10ohms and changed to a FWB rectifier to squeeze a few more volts from the bias supply, which got things to a happy place.
The next step otherwise would have been a voltage doubler, which is what’s needed on some Sound Citys with the same issue.

[Stevem]

Well how many watts rms is it outputting into the needed 4 ohm load?

Is the amp oscillating way up high when viewed on a scope?
A amp doing such can eat up a lot of available power and you will not even hear it.

I had a customers 1956 Gibson years ago that was doing such to the point that the amp was doing so using up all the wattage that the amp could muster and to the point that you could barely hear the guitar trying to be played thru it!!

I have never found these amps when set up with new fully formed filters and great testing tubes to output anything more then 125 watts, so don’t be looking for 135 !

If your only testing thru a 8 ohm load then you will only see about 84 watts of rms power, but either way the amp should be loud thru 2 decent 12” drivers.

[Matthews Guitars]

I rolled a few tubes through it. It seems to be working well but I have to do a power measurement. There's no trace of oscillation on the scope. The amp seems to be perfectly well behaved other than seeming to be low on power. I ran it for quite a while today and both my hand and my thermal camera didn't think that the output transformer is running warmer than it should.

(I highly recommend a thermal camera for a quick diagnostic tool. Great for seeing if the power tubes are running at different temperatures, etc.)

I'm plugging into one or two efficient 1x12s with EVM-12Ls in them. So there's my 8 ohm or 4 ohm loads.

It's loud enough to gig with, but it's not as loud as my 35 watt Vibrolux plugged into one EV cabinet, and that's not right.

To get a power measurement I'll scope its max clean sine wave voltage at the speaker jacks and do the math.

[pdf64]

A full power test into an EVM12 - why?!
Your hearing’s a consumable, why waste even a dB/second of it on that?
Whatever, frequency should be somewhere in the range 200-400Hz, so that the speaker’s impedance is close to nominal.

[Matthews Guitars]

I do have and use hearing protection, of course.

The last amp I full power tested is my 400 PS...435 watts RMS. I used three cabinets that can all handle 145 watts RMS or more, each.

It got silly loud. Even with ear protection it was not comfortable.