Bassman AA864 Question

[Pryde]

Just curious if anyone might know why this mod was done to this silverface bassman?

schematic:
http://vintagefenderamprepair.com/wp...-schematic.pdf

Layout:
http://vintagefenderamprepair.com/wp...864-layout.pdf

In the picture off of pin 3 bias pot there is a 80uf cap to ground(+) and then a 3.3k (I think) resistor going to the bias board (which has correct value caps, etc on it. The amp bias functions and is correct but not sure what this "mod" is about? Should I hack it out of there? Any reason to keep it?

[dwmorrin]

Appears to just be extra filtering for the bias voltage.
Why did they need to do it? Don't know. What power tubes are installed?
Check the bias voltage, but more importantly the power tubes idle current draw. If its OK, then I wouldn't worry about the mod.

[g1]

That doesn't look like the AA864 bias circuit, too many differences aside from the mod.
Are you sure it's an AA864?

[g1]

Looks like it may have been AA371 changed from bias "balance" to regular type bias.

[Pryde]

Looks like it may have been AA371 changed from bias "balance" to regular type bias.

Good call I believe you are correct. This amp has A LOT of mods done so I could not figure out what it might have been originally. It looks like the aa371 had a 3.3k and a 50uf cap off of lug 2. The bias current is ok as is but it is some pretty shoddy work (black tape, poor soldering, etc) so I considered removing the cap and resistor if OK.

[g1]

It will probably be ok, you will have a little less filtering on the bias circuit. If going to single cap you should replace it with a new one for reliability, having 2 there right now may be masking any defects.
Bypassing the extra resistor may affect the range of bias available. If you can still compensate with the pot, then you are ok.

Edit: also forgot to mention, the original AA371 bias pot had 4 lugs.

[Justin Thomas]

Those 4-lug pots are a pain to find, and it looks like it might still be in place? There's something under the tape in the photo, but maybe it's just a shadow?

The two caps on the SF amps is normal, at least with the 1971 & later ones. Everey one I've seen is from 74 or later, and they all had both. I don't think it was to cover up a bad part or defect, except maybe to try to clean up hum when the lead dress started getting sloppy... Personally, I make em all balance AND adjust, but if it works good, don't fix it. And the black tape in that one is loads better than some I've seen!

Justin

[Pryde]

Those 4-lug pots are a pain to find, and it looks like it might still be in place? There's something under the tape in the photo, but maybe it's just a shadow?

The two caps on the SF amps is normal, at least with the 1971 & later ones. Everey one I've seen is from 74 or later, and they all had both. I don't think it was to cover up a bad part or defect, except maybe to try to clean up hum when the lead dress started getting sloppy... Personally, I make em all balance AND adjust, but if it works good, don't fix it. And the black tape in that one is loads better than some I've seen!

Justin

Indeed it is a 4 lug pot. Not sure if I will alter this one as tube current idle is just fine. It came with a red plating tube but the tube itself was faulty so so far all is good.

[Justin Thomas]

If it sounds awesome without self-immolation, it's perfectly fine. And if you must have a Bassman, I recommend the AA864/AB165 hybrid w. shared 3rd stage. But plain old 4x12ax7 SF Bassmans are great amps, too!

Justin