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So I'm working on a 1969 bassman and noticed there's one more filter cap than on the schematic, or on any of the bassman schematics that I've seen. I've noticed this in several amps, and I'm wondering why its not on the schematic. I guess they realized after the fact that they needed more filtering? Someone shed some light on this.
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Okay, which schematics are you looking/have you looked at? Where in the circuit is This extra filter cap? Without more details, the best we can say is, don't be surprised if a vintage Fender doesn't match exactly with ANY schematic. Transitions and changes basically ALWAYS outpaced the updating of literature.
Every schem that I have from AA864 thru AA371 has 6 filter caps (though the first 2 count as 1 in circuit).
Justin"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister - -
I've noticed that many have 6, but 5 are only in the schematic. For example; 2 x 70uf (80-100uf) first, then every schematic lists 3 20uf's. This one has an additional 16uf, though many just use a 20uf. Where's the extra 16-20uf on the schematic? I get why its there, I just don't get why no schematic I have lists 4 parallel 20uf's. And in this case, I'm looking at the 64-71 bassman schematics.Comment
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Well, looking at the AA270 (1970) I see the pair of 70s in series. Then 20,20,20, and an 8 off to the far left.
http://bmamps.com/Schematics/fender/...a270_schem.pdf
AA371, AC5568 are the same.
Even the 6G6a has a parallel pair of 20s to start, then 20, 20, 20, and another 20 over to the left.
The Bassman 50 has the series pair of 70s, then 20, 20, 20, and a 16 way over on the left.
http://bmamps.com/Schematics/fender/bassman50.pdf
If you are looking for four caps in parallel, you won't find it, in fact you are not looking at three caps in parallel either. Those are separate nodes separated by resistors.
Amp fans like to look at amps by the year they were made, like cars. But I have no idea what date most amps were made, unless I tear it apart and look for date codes on the parts. Note how Justin referred to specific schematics. I suggest you do the same. If you are looking at a "Bassman 50" diagram, then say so, I haven't a clue what year it was drawn. The circuit is what matters, not the year.
The exception is when Fender makes a Reissue of a specific year type, and the year is part of the name, such as "59 Bassman".
http://bmamps.com/Schematics/fender/...man_manual.pdf
In that 1990 example, there were two stacked pairs, plus three 22uf.Education is what you're left with after you have forgotten what you have learned.Comment
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Can't believe I didn't see that cap on the far left of the schematic. Now I feel dumb. So if I'm converting the AC568 circuit to the AA864, can I omit that cap? There's not one on the AA864. What difference would it make if I took it out?Originally posted by Enzo View PostComment
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There is something you need to learn, it is not about the parts, it is about the circuit.
You want to convert to AA864 from AC568. The difference is not just the number of caps. One fundamental difference is that the AA864 has 30v lower B+. Note that on that drawing there is no resistor on the way to the last cap. When they raised the B+ overall, they felt the need to reduce it to the input stages, so they added the resistor and extra cap. That brought the B+ for the input stages back down some, and also decoupled them from later stages.
That extra filter cap is ther more for decoupling than for any ripple reduction. By the time you get through the choke, your ripple should be pretty much gone.
There are a bunch of detail differences between the 568 and the earlier 864, and if you want the 864, just build it. Don't look at it piece by piece.Education is what you're left with after you have forgotten what you have learned.Comment
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Where do you stand on the Weber zener diode B+ droppers? What if I dropped the B+ by 30v to match the 864? Ive done this to a twin before and had great results. Also, someone might be giving me this amp, and regardless how inexperienced I sound, I've worked on hundreds of BF and SV fenders, and I'm aware that what I'm about to say might have some people upset, but, I kinda wanna give this amp a tube rectifier and try to get it more along the lines of a tremolux on terms of wattage. 35W vs 50W. If I could afford a tremolux I'd buy one. I've had 2 and I love em. Hypothetically, if I were to try that, what would be your advice? If I drop the B+ will the output decrease, or do I need a proper PT and OT, plus a 5V transformer for the tube rectifier? I'm not concerned with the collectibility of this amp. I'd rather use it as a platform for furthering my knowledge on amps, mods, etc. Enzo, what do you think?Originally posted by Enzo View PostComment
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I have no opinion either way. I just wanted to point out the circuits are different. There is no reason you cannot build the circuit to run on either voltage, after all Fender did. They made certain changes, that is all. Perhaps if you want to change the signal circuits, you could leave the power supply circuits intact.Education is what you're left with after you have forgotten what you have learned.Comment
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