Fender AB165 Bassman

[ToneMerc]

I also noticed that this amp as in 5 or 6 others I have worked on (AA165) does not have the resistor from pin 6 to pin 7 on that 3rd gain stage on the bass channel, has anyone ever seen this resistor in a amp or is it a change that never made it out the door on these amps?

Stevem, I believe you are getting the AA/AB circuits mixed up again. The AB165 that you referenced in this original thread does have the 470K feedback resistor between pins 6&7. On my BF AB165 it does, it's partially hidden under the dual 25/25 cap.

Whereas, the AA165 does not have the feedback at the 3rd stage between pins 6&7, my BF AA165 matches the layout and schematic.

TM

[Stevem]

Yes sorry, but this ab165 and at least 4 others I have worked on thru the years did not have that 470k resistors, have you seen any that do?

[ToneMerc]

Yes sorry, but this ab165 and at least 4 others I have worked on thru the years did not have that 470k resistors, have you seen any that do?

I have seen at least one other 66' BF AB165 that has it as well. The ones that you are referencing, were they the later silverface AB165's?

TM

[martin manning]

Looking at available schematics:

aa165 no FB on 3rd stage, no FB on power tubes, GNFB to PI tail 820-100, dated January 1964, Fender/CRD
ab165 FB on 3rd stage (470k) and power tubes (220k), GNFB to PI input 47k-0.1u, dated April 1964, Fender/CRD
aa864 no FB on 3rd stage, no FB on power tubes, GNFB to PI tail 820-100, dated August 1964, Fender/CRD
(aa864 differences from aa165 are in bass channel preamp- 10n//plate load on input stage, and stage 2 to stage 3 coupling)
aa270 FB on 3rd stage (470k), no FB on power tubes, GNFB to PI input 47k-0.1u w/ 100pf//47k, 1n to ground from power tube grids, no date, Fender/CBS
aa371 FB on 3rd stage (220k), no FB on power tubes, GNFB to 3rd stage plate 47k-0.1u w/ 100pf//47k, 1n to ground from power tube grids, no date, Fender/CBS

[Stevem]

I found that darn 470k resistor across V3, the sub terrain wires on these Bassmans is something else as compared to other Fenders!
This amp is a mess to put it lightly ( very hard to read resistor and cap values! )as it was left out of its case in the rain for all of last Summer or longer due to bad PT and I guess given up on!
The filters where the worst shape I have ever seen in a amp, so it's no wonder, and the Chassis is rust covered inside and out, but yet all the tube sockets are fine even though I found it with no tubes in it!

[ToneMerc]

I found that darn 470k resistor

Yee of little faith.....LOL. It was partially hidden under the dual 25/25 wasn't it?

TM

[Stevem]

Well I had changed out those big bugger duel 25s with a small modern cap so it was not right under it, lol!

[strelok]

I have a '66 AB165 as well. I picked it up from my guitar teacher several years ago cheap as it was nonoperational and needed some serious work. I rebuilt the entire thing and got it working again, though I didn't care for how it sounded, so I did the usual thing of trying to mod it back to the aa164 circuit, however when I got down to removing the 220k resistors it became very micro-phonic at any volume if I remember correctly so I was forced to leave them in. I haven't done much of anything with it in quite a while, however I remember reading that the purpose of the feedback resistors was to stabilize the output section given that the OT's used were not of the best quality. I don't know if there is any truth to that though, it certainly seemed to be the case with my amp. It suffered from instability problems for one reason or another in the output section without the local feedback.

[pdf64]

That may just have been microphonic tubes; did you try swapping in a known good one to each slot in turn?

[Stevem]

Strelok the issue with your amp is not due to the OT, as Fender used the same part number OT from the first the first BF Bassman in late 64 right into the at least the early SF amps!
As posted your issue is likely to be a preamp tube or a preamp tube socket in need of a stiff cleaning or replacement!
Getting those two resistors out of the amp and having it put out it full 40 watts will put a smile on your face!
You may be 9 bucks ( cost of a new tube socket ) and 35 minutes away from having a nice amp!

[ToneMerc]

I haven't done much of anything with it in quite a while, however I remember reading that the purpose of the feedback resistors was to stabilize the output section given that the OT's used were not of the best quality. I don't know if there is any truth to that though, it certainly seemed to be the case with my amp. It suffered from instability problems for one reason or another in the output section without the local feedback.

I don't believe that there's any truth to that. My 65 AA165 has 1964 iron in it and the 66 AB165 has 1966 iron, with of course with the same model of Shumacher output iron only different codes.

This was a transitional circuit that CBS engineers mucked up because they thought they knew better.

When you converted it to AA165?, did you tranpose the PI and output transformer connections to match the AA165 as well? Are you sure the "microphonics" isn't a howl from the feedback circuit?

TM

[pdf64]

When you converted it to AA165?, did you tranpose the PI and output transformer connections to match the AA165 as well? Are you sure the "microphonics" isn't a howl from the feedback circuit?

Good thinking, I bet that may be it!

[pdf64]

Stevem, did you ever try a 12AT7 in the phase splitter with the 220k local feedback resistors?
Thinking about it, there's no way a 12AX7 could pull its plate down, when it's got a 220k resistor with a power tube plate at the other end of it working in opposite polarity. So its voltage swing would rather limited.

[Stevem]

I did, but I only had a weak at7 on hand and did not have time to pull a hood one out of one of my own Amps, next week at some point I will get back into it!

[strelok]

Strelok the issue with your amp is not due to the OT, as Fender used the same part number OT from the first the first BF Bassman in late 64 right into the at least the early SF amps!
As posted your issue is likely to be a preamp tube or a preamp tube socket in need of a stiff cleaning or replacement!
Getting those two resistors out of the amp and having it put out it full 40 watts will put a smile on your face!
You may be 9 bucks ( cost of a new tube socket ) and 35 minutes away from having a nice amp!

That's good to know, I was thinking it probably wasn't the issue, I just never had a chance to diagnose it thoroughly as right around that time my 12000 series Metro 100 watt kit had arrived and needless to say I was quite distracted I moved recently and most of my gear is sitting in storage still so I'm going to have to go back and get it this summer, not looking forward to that drive haha.