Bassman Homebrew

[BillyBob]

Hello,
I’m a new member. I’ve enjoyed reading the site for years.
I’m an untrained tube amp experimenter with a “homebrew” amp that needs some help.

The "donor" amp was a garbage-picked Fender Bassman AB165. Its been heavily modified. I’ve attached a “gut shot” and schematic with this message.

Main Mod Features:
-Single channel
-Bassman 864 type adjustable bias system
-Cascaded gain stages (with fixed voltage dividers at a couple points)
-Addition of a “Mid” knob
-LarMar PPIMV
-Output tube mod to run 6L6 or EL34 output tubes by just adjusting bias

All of the components other than transformers, chassis, some wire, and turret board are new.

I’ve got great NOS tubes installed. The amp is quiet (noise floor) and sounds good.!!..with tons of gain available.

My problems is:
When I try to lift off the two remaining AB165 220k (NFB.?) resistors coming off the plates of the output tubes, the 2A slow-blow fuse blows with the volume (both volumes) are turned up halfway or more.

I substituted some 470k resistors and got the volume levels higher before the fuse blew again.

The amp sounds better with those 220k resistors removed..

I’d like to find a way to remove them permanently.

Bill

[xtian]

Welcome, Bill!

You certainly should be able to remove those 220K NFB resistors. Maybe high bias is causing trouble? Turn bias all the way cold, remove the NFB resistors, turn the MV up to full, and then bias up to 60 or 70%. Also, monitor the bias voltage at pin 5 while you spin the MV control--voltage should stay steady.

I can't see in the photo where those resistors are. But I can see a lot of very sketchy floating connections, with lots of opportunities for shorts. Living dangerously, are we?

I LOVE that Mid lift control. Fender to Marshall.

All of your preamp cathodes are unbypassed. That's very unusual. You like it that way?

[BillyBob]

Hello,
Thanks for the reply.

I gave that a try; bias all the way down, lifted those 220k plate resistors, MV up to full, and rebias to 60-70%--plate voltage 435, bias current 38 ( NOTE: Ive got EL34 tubes in there at the moment)

I followed voltage at pin 5 while turning the MV. Voltage varies slightly; -38.1 all way down, to -37.1 all way up

I plugged in a Les Paul and tested it out....fuse blew again with MV half way and pre volume slightly less than half way.

I then tried some other EL34 tubes,...same thing blown fuse.
I am using a Bassman spec 4ohm speaker cabinet, but have also tried different cabs and cables with the same result in the past.

I put those resistors back in and it works fine but sounds a bit neutered..
If I can run without them this amp is over the Finish Line..!

Where can I start sniffing around to understand better why the fuse is blowing..?

Regarding the unbypassed preamp cathodes, Im not clever enough to know any better if I should not like it that way or not. Is it not kosher.?

I am planning to install a new turrett board and propperly secure the components.

Regards,

Bill

[ToneMerc]

I don't it will 100% work correctly as drawn unless you till you further modify the PI and PA sections to be more inline with an 864. You need to totally get rid of that 220k positive feedback loop and then move the PI input the the other side of the PI tube.

Basically scratch the AB165 topology all together and config it as an AA165 or 864.

TM

[BillyBob]

Thanks for the reply TM,

Yeah, that gives me something to think about.
Ill prepare to give that a try. Should be relatively easy, and Ill see what results it gives.

I am still getting wrapped around the axle with the "theory" of why the overcurrent is occuring.. But if that repair works, case closed! Time to enjoy good jam sesson.

Regards,

Bill

[xtian]

In addition to setting the bias for EL34, you also have to consider the speaker load. EL34s expect about half the load (3K-ish) than 6L6s (6K-ish). If the Bassman is made to use a 8ohm load when running 6L6, you'd need to use a 4ohm load with EL34s.

I THINK I got that right. someone please check my brain.

[BillyBob]

Hey Xtian,
Thanks for the message.

The Bassman is a 4ohm machine. I left that unchanged when I worked on the amp. I am running it into a factory Bassman 2x12 cab at 4ohm.

Later this afternoon Ill see if it blows the fuse with 6L6 output tubes also..
I havent tried that in a while.

Regards,

Bill

[ToneMerc]

.......

[BillyBob]

Hello,
I fired up the homebrew again with some 6L6 output tubes this afternoon, biased to 60-70%. This time I was able to get the volume up near max (neighbors!). It sounded good, and did not blow the fuse.

Then I tried the EL34 tubes again, biased to 60-70%, and after a short while the fuse blew with volumes between 1/2 and 2/3rd. way up.
Id prefer to run the EL34s. They sound better to me.

NOTE: During run trials I did see something going on with the bias readings between the output tubes V5 and V6 but Im not sure its relevant. Checking back and forth I saw consistently different current readings between those tubes, with the tube closest to the transformer (V6) at consistently higher bias current, regardless of bias pot setting and through all brands of test tubes I tried.

The current difference runs consistently about 5ma with EL34 and 8ma with 6L6 tubes.

Is that normal?

Regards,

Bill

[TUBEDUDE]

The output transformer windings are not identical on each side. Swap output tube positions to get the best balance. Or you could use a bias balance pot scheme.

[JazzGuitarGimp]

Then I tried the EL34 tubes again, biased to 60-70%, and after a short while the fuse blew with volumes between 1/2 and 2/3rd. way up.

I'm no expert here, but I have to wonder if this ties in with what xtian said about optimal speaker load impedance requirements of the EL34 vs the 6L6.

[ToneMerc]

Then I tried the EL34 tubes again, biased to 60-70%, and after a short while the fuse blew with volumes between 1/2 and 2/3rd. way up.

I'm no expert here, but I have to wonder if this ties in with what xtian said about optimal speaker load impedance requirements of the EL34 vs the 6L6.

Using a pair of El34's to push a 4 ohm cabinet with a Bassman 4.2K/4 ohm OT should not be an issue. I have run EL34's in my 6G6B, both 4 and 8 ohm loads without an issue, just bias accordingly.

TM

[BillyBob]

Hello,
Thanks for the message.

It may take me a short while, but my next step will be to "864" the PI and power amp sections and see what results that brings.

Regards,

Bill

[Stevem]

A EL34 needs a good bunch more Heater current then a 6L6 does , in fact a whole 1.200 amps more and when your Crank the amp with the 34s in the PT can't take it!

If you keep trying the 34s out and blowing fuses you will be needing a new PT soon.
Also 34s need s good bunch less drive signal then a 6L6 so your pushing the Hell out of them which is only taxing the amps power supply even more!

[BillyBob]

Hello,
Well I made some progress with this thing. Thanks for the input.
Im up and running now and not blowing fuses.

When I got back inside the chassis with the idea to rework the circuit towards 864 spec in PI and output sections, I noticed some scorched/burn marks.

I have attached some photos to this message.

I saw that the small vertical AB165 standoff terminal board which receives the NFB wires from the power tube plates and joins with the 200K resistors coming off the board had one terminal arcing down to the chassis, during loud playing I assume. I believe that was probably blowing the fuses.

Im guessing it could have been sloppy solder dripped down near the base close engough to cause an arc..? Im not sure, but the pictures are self evident. The top outside of the chassis at that point looks unaffected.

I removed that terminal standoff and associated wire runs to the output tube socket pins. Previously I had just been lifting one end of the resistors off for testing.

I biased up the amp again with EL34 and it sounds great.! I was also able to reach max volume. It also sounds great with 6L6. And its not blowing fuses anymore.

What do you reckon.? (See how it goes till something else blows up.!)

Bill