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After doing one of these yesterday I was wondering what you guys think of this topology with the 220k resistors coming off the plates of the power tubes and OT Primary. The fixed bias balance and the feedback is a little different. I'm reverting it back to AA165 Blackface standards and I never got to hear this amp as it had a blown power tranny in it. Do you like this circuit and how does it sound compared to the AA165 or other Blackface circuits ?
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KB -
I prefer the AA864 circuit myself. I converted my AB165 bassmans to that. The AB165 is probably a little cleaner sounding if you like that sort of thing. -
Nice site Regis and lots of helpful info there that's hard to find on the net. Nice job ! After studying and working on this amp for ahile I have one major question. What the hell was Fender thinking when they did this one ?
I don't know what it sounds like but it can't be good even with a Bass and what's up with the plate feedback resistors. I think this is the only amp that uses them that I could find. Both on the power tubes and the 3rd stage of the preamp tubes. It really is much different than most of the blackfaces and silver for that matter. The bias supply on this amp didn't work so I reverted it back to blackface. It sounds really good but was more work than I was in for.
KBComment
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The AB165 was a "crossover" circuit.You will see it in the late Blackfaces and early SF's.It still has a lot of the BF circuitry,so when doing a "BF mod" its a lot less work than Blackfacing a later SF,like the AA568 or later.I actually did some work for a friend on a SF version of the 165 and it didnt have the 220k's,I cant say for sure but the amp appeared to be stock,I dont think they were ever installed.Fender did a lot of "on the fly" changes during that period when they switched from BF to SF.They didnt waste anything,so you see alot of BF things in early SF's,and amps with BF cosmetics and SF circuitry.Depended on what they had left over on any given day.Comment
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KB thanks for the kudos!!
Something to remember is that back in the sixties Fender and pretty much all the other amp manufacturers were striving to build amps without distortion. Especially in a bass amp. That is why all that extra shit was added to the amp lines, to reduce that nasty distortion crap.
It took Fender and all the other builders many years to realize we liked that kind of tone, and even longer to build amps that were usable and enjoyable to most guitarists.
It's bizarre that Leo built amps that had great tone that guitarists liked, then moved away from it to an amp line that everybody hated.Comment
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"It's bizarre that Leo built amps that had great tone that guitarists liked, then moved away from it to an amp line that everybody hated."
Leo didnt "move away from it..,",it was CBS when they took over.Comment
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If you look back at what Leo was doing,even though he pioneered mass production to some degree what he was turning out was closer to "botique" at that time,even his prices were higher than the early Marshalls,his biggest competitor at the time.CBS really put the corporation end of things into motion and was more concerned with the profit margin than Leo ever was.Comment
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Kerry... I think the AA864 is better too but I've also found that with any minor mods to the AB165, the amps seem to be susceptible to hum. ?Originally posted by Amp Kat View Post
Earlier this January, I did a AA864 mod and complete refurb to an AB165 using inexpensive M150s and generic power supply caps, etc. Virtually no mods.
!! That bass channel can be modded into a screamer.... call me sometime for that, but I digress...
With a pair of NOS 5881 power tubes, idling at about 17 watts each, it sounded like a "whole number" step up in tone, girth and overall usefulness.Comment
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It's pretty hum free Bruce but it has some ocean noise which I'm thinking is a lead dress issue but have to do some testing for sure. It's got the 5881's and has a very nice tone of course all the filter caps and bypass are all new. Any clues on the ocean noise ?KBComment
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Lead dress is a common issue in these amps,not as bad in the AB165,as the later versions,but still rears its ugly head when modding them.Thats why the SF's had the snubber caps,to make up for careless lead dress issues.Could be what those 220k's on the power tubes were about.I've only seen them in the schem for that amp,and that amp only,the one I worked on didnt have them.Originally posted by Amp Kat View PostComment
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Hi. I know this is out of the blue, but did you ever find out what was causing the "ocean noise" issue? I am having the same problem... ThanksOriginally posted by Amp Kat View PostComment
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The only time I have had ocean noise is with my Twin Rev and it was a prelude to a power tube dying a horrible death...followed by a couple days on the workbench to make sure I replaced everything it took out with it.Comment
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I actually PREFER the 220 K local feedback resistors, but I am not everyone else. I play my Bassman up quite loud, and I am playing lap-steel type slide on it. The local feedback lets the slide ring and sustain without quickly blasting into uncontrolled feedback and also tightens the low end a bit. As for Ampkat's remark that the bias circuit doesn't work- It works great, but isn't easily adjustable for voltage, just balance. Most players would probably prefer the amp without the resistors. They're easy to remove/install so why not try both ways?Comment
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