Bassman aa165/ab165 circuit restoration

[Groover]

I have a Bassman head which is labeled AA165 on the tube chart. However, the phase inverter looks more like the ab165 diagram.

The section to the left of the PI (as the layout diagram is oriented) containing the plate resistors appears to have been partially converted from ab165 to aa165 circuit. The ab165 has 220k resistors connecting the plates of the PI to the plates of the 6L6s. It appears in the diagram that these run from the eyelet board to terminal strips. The terminal strips are present, but the resistors aren't in that location now. While the resistor network was rebuilt, the coupling caps were not changed. (The aa165 has .1, the ab165 has .022) The amp was also converted properly to use EL34s.

The ab165 has a slightly more complex bias circuit too.

I want to restore the amp to use 6L6s and optimize it for bass use.

My questions are:
1) Which a phase inverter circuit will sound better for bass? (In "Inside Tube Amps" Dan Torres claims the Bassman PI circuit does not sound very good, and he recommends some mods to optimize it, but he is talking about use for guitar.)
2) which post-PI plate resistor/power amp circuit variation would be preferable?

[mac1amps]

Contrary to their name, Bassman's aren't really very good amps for bass use, unless you're purposely going for some bass crunch. Why Leo decided to call these amps Bassman's will always be a mystery to me. I own four different versions of Bassman heads, but I would NEVER think of plugging any of my basses into them. At the same time, they are well known as great sounding guitar amps (again, primarily due to the extremely limited "headroom" they have). Actually, as far as "classic" Fenders go, a MUCH better amp for bass is a [Dual] Showman. With their moderate gain preamp, and a (4) 6L6 output network (as opposed to 2), they provide a lot more headroom before they start to break-up. No, they won't "keep-up" if you're playing with guitarists using Marshall stacks (you'd need something with more power than that......'like an SVT), but Showman's work great for a "medium volume" band........'just my 2-cents.

[Groover]

Thank you mac1amps for your $0.02, but the question you answered was not among the questions I asked.

[loudthud]

IMHO it's not just the PI, but the whole power amp circuit back to the previous stage. I think the AA165 is the way to go. The only thing I don't like is the .003 plate caps. Leave them out.

In the AA165 there is a contour network to voice the Bass channel. It's not for everybody but I find it usefull and there are endless tweek possibilities.

I could go either way on the bias circuit. It's nice to have a balance pot but then you have to select resistors to actually set the bias or add a trim pot.

[Groover]

thanks loudthud

The only .003 plate cap I see on the schematic is the cap bypassing the plate 100k resistor on the 3rd preamp stage, but you said caps plural, so what a I missing?

In comparing the two schematics, I see an RC network between the 2nd and 3rd stages that is not the same on the ab165. I'm guessing this is the contour circuit you are referring to?

[loudthud]

The AA165 schematic I have is a tatered Fender original with the letters A-FE (some kind of version code) under the word Bassman in the title. There can be slight differences in some models. Think of the schematic as a snapshot of a constantly evolving product. Mine shows a cap across the plate resistor of first triode on the Bass channel as well as the third triode. You are right about the network at between the second and third triode. It gives a gentle treble and bass boost.

[Groover]

my aa165 schematic downloaded from the Fender Amp field guide is marked A-FE also. It shows a .01mF across the 1st stage plate resistor.

You think it's better to take them out?

[loudthud]

You are correct. I guess I need new glasses

[trem]

Thank you mac1amps for your $0.02, but the question you answered was not among the questions I asked.

If my opinion means anything, he really did answer your questions. That amp just is not a bass amp, unless you intend to record with it or use it in very small rooms. If you want to mod it for 6550's and buy different transformers, your headroom might not go off so fast. But in the end you still need a bas amp.
I would suggest making the first channel a JTM45 type of circuit, and keep the second channel as it was. You will have a great two channel "guitar" amp.
There are so many excellent bass heads available today. Have you not been able to find a bass head you like? What bass cab(s) do you have?

[Groover]

Please - no more posts opining that this won't make a good bass amp.

I am aware of it's limitations, and you may not know my intentions.

[Modorange]

Anyone who is hell-bent on turning his Bassman into an excellent guitar amp is certainly welcome to chime in here:

http://music-electronics-forum.com/t14743/

My intention is to use my Bassman for what it really excels at, and to remove a bunch of stinky mods..

[loudthud]

Most of the schematics on this page:

http://www.el34world.com/charts/Diagrams.htm

have layout diagrams as part of the pdf. The blackface Bassman is a good sounding guitar amp but the Holly Grail of Bassmans is the tweed 5F6A.

[Modorange]

Well, what I've got to work with is this blackface bassman. Back to the topic, the bassman is a decent bass amp but won't provide the output that you might want if you're playing live. Recording with one, though I would think you could sound great. And that's just with a stock Bassman. BTW Thanks for the link loudthud, the bassman schematics are on that page but none of the bassman schems include the wiring layout. I already found them elsewhere, though.

[Sweetfinger]

Please - no more posts opining that this won't make a good bass amp.

I am aware of it's limitations, and you may not know my intentions.

Those 220K plate to PI resistors are a local feedback circuit that most guitar players would take out, but you might like to try the amp with, and without. Its pretty easy to connect/disconnect and listen. In the circuit they make the amp a little more smooth and "manageable" at higher volumes.

To the people who wonder why the amp was called a Bassman, it is a continuation of the model name. What bass amp WOULD you use in 1964? Your guitar amp was probably 20 watts. Your PA (if you were lucky enough to have one) was probably 100 watts. All of a sudden, a Bassman starts to look like a reasonable choice for amplification. I wish a lot of bands would still play at those volumes.

[Groover]

What are the differences, generally speaking, between a BF Fender pre-amp circuit and a JTM45 type of circuit? Just individual component values, or a whole different architecture?