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Old 18th September 2011, 01:17 AM   #1
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Default a Fender bassman amp question

Hello everyone! I'm new here, I just stumbled in to ask a question that is probably pretty simple.

i have a 1965 Fender bassman AB165 model, and after reading entirely too much on the internet about these amps. I decided on doing a Conversion to the AA864 model power section. seems simple enough and there appears to be a lot of folks out there that like the results.

My question is, while I've repaired a lot of these older fenders over the years. and I can fell my way around an amp and make it work well.
I'm not much into the math of these mods and I'm not well versed in the terminology of what resistor or cap does what just by its location in the circuit.

So, I'm a little bit puzzled by something I see in the AB165 scehmatic verses the AA864 schematic.

I understand that most Fenders use a pair of 220K resistors to "sum" the two channels into the Phase inverter input cap...

but on the AB165. there is this extra gain stage with what apears to be a 470K local feedback resistor that is behind the 220K "summing" resistors from the channels.

I photoed shopped the two schematics together at the point where i logically would think it should work.. but I'd like a second opinion, am i missing something? I also removed the pre-amps 100K power supply resistors Bypass caps (500PF and .001) it was something that was stated would be of value to the tone. other than that the two schemes appear to line up almost a little easy.

that extra gain stages coupleing takes me out of my comfort zone.. but there seems to be a general concensus that its a cool thing to have and is one of the main reasons for doing this conversion..

Thanks in advance for any help!

Click the image to open in full size.

Last edited by krayon; 18th September 2011 at 01:37 AM.
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Old 18th September 2011, 03:50 PM   #2
jjman is offline jjman  United States
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Your schematic looks good to me. If you don't like the feedback approach you could reduce that stage's input instead. The approach used on the AA164's bass channel does that with the dropper string before that stage. I'm not hip to the tonal differences between the two but that's probably the only Fender scheme I've seen with such local feedback. I would imaging that some have used an additional volume control in place of the 220k-220k dropping string on the grid of the AA164. This would allow more clipping potential to the 3rd stage. But It's possible it already clips their adequately. I know I've heard some very overdriven samples of stock BF Bassman setups.
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Old 18th September 2011, 05:06 PM   #3
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Thanks!

I'm sorry but terms like "dropper string" have no meaning for me.

I learned everything I know from a set of "basic electronics" Books by Van Valkenburgh Nooger and neville inc. never saw the term "dropper string" in those books.

supposedly, this is called the "Allen mod" refering to Allen Amps in KY. but he doesn't publish it and the details are a bit sketchy on the web..

Plus my amp seems to have been partially done this way before i got it. but they didn't finish the PI part of and it was blowing fuses after getting to full op- temp. and sounded terrible.

so i was a bit torn between trying to "restore" it to AB165 specs to make it work properly by the original schematic, or doing my first ever "mod" to rescue it.
I finished the work on it and all I need to do now is to fire it up.. I'm just a little nervous about it is all. and thought maybe a second pair of eyes will notice something that I'm not aware of. I'll let you know whow it works out shortly.
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Old 18th September 2011, 08:03 PM   #4
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First off, there is a couple of 2200k resistors from the 6L6 plates feeding back to the PI plates, you want to get rid of these.

Now the input stages between the versions of amps are not too far apart, they can be summed together with the 200k resistors just fine. What you might also want to do is get rid of the 470k resistor from the plate to the grid of the third stage. It provides negative feedback which most agree does not sound good.

You may want to google the following

BASSMANHYPERON-1.gif

You will find a cool schematic and info on the build also some more info on the Allen mod. I would go more into it but I am a little busy right now. Might get back to it later though.
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Old 18th September 2011, 09:19 PM   #5
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Quote:
Originally Posted by Printer2 View Post
First off, there is a couple of 2200k resistors from the 6L6 plates feeding back to the PI plates, you want to get rid of these.

Now the input stages between the versions of amps are not too far apart, they can be summed together with the 200k resistors just fine. What you might also want to do is get rid of the 470k resistor from the plate to the grid of the third stage. It provides negative feedback which most agree does not sound good.

You may want to google the following

BASSMANHYPERON-1.gif

You will find a cool schematic and info on the build also some more info on the Allen mod. I would go more into it but I am a little busy right now. Might get back to it later though.

Thanks printer2 but I'm kinda past that point.. I've already done the full conversion to the AA864 power and PI sections. my only question now is.

based on the schematic shown above. is my coupleing between the
"extra buffer stage" and the PI input ok?
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Old 18th September 2011, 09:40 PM   #6
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here is a photoshopped version of the layout with my changes. I try hard to keep things simple and easy.

Click the image to open in full size.
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Old 19th September 2011, 01:21 AM   #7
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Please note the "X" and "Y" points at the PI input. and go back along the dotted line to the 470K resistor and the two 220Ks for that "extra gain stage"

is this arrangment OK for coupleing to the PI input?
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Old 24th October 2011, 01:59 PM   #8
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Update and quick question.

I fired up the bassman and it worked perfectly. SUPER LOUD, clean and quiet. TONS of clean headroom. this actually might be a decent amp for BASS now.

BUT, The voltages are a little higher than I'd like, the schematic calls for 425 + at pin 3 and I'm getting 452. most other voltages are within 5 volts on the high side so thats not bother me too much,

So, while I can live with that.. I'm also seeing -61 volts on the Bias supply cap which is only rated at 50VDC.. but I wondered about putting a 100/100 cap on there, (only because thats what I have instock)

will that help or hurt? seems like it could make the voltage go even higher.

maybe I should replace the 470 AC side bias supply resistor with a 500 or larger?

or should I just order a 50MFD/100VDC cap for it.
or is it really that big a deal. I mean 11 volts over spec seems like it could become a "long term" trouble.. but I'd like some opinions.

again I'm no mathmatician, I can follow schematics and I've built and repaired lots of amps over the last 22 years from basically following the Fender schematics. so I always have "questions" when I run into something "outside" the schematic.

thanks in advance!
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