70's Vibrolux Reverb AA864 schematic

[Randall]

I re-tubed a Vibrolux Reverb AA864. When I went to bias it I found the bias pot is set up to balance -48v between the outputs, but no way to adjust the overall voltage. As it sits it is dissipating 20 mA per side. My friend is the local Fender authorized shop, so I asked him to call Fender. They told him they don't have the schematic, and the tech didn't know the amp. Odd.

Here's how it is laid out: bias board feeds the input side of the pot, on the other side are three lugs. Center is grounded to shell via 15K R. Right lug is wired to the eyelet where there is usually the two splitter resistors that feed the bias voltage to the grids, except in this case there is only one 100K there feeding the Right tube. The left lug on the pot is soldered to a 100K that flies to the eyelet that is wired to the Left tube.

Does anyone know if this was intended, or if it has been altered? Or better yet, where on earth can I find the schematic? It's one that Schematic Heaven is looking for also, so that doesn't bode well.

[JoeM]

Have your tech convert to a regular bias adjust, rather than the balance arrangement.

Some info here on the typical conversion:

Bias conversions

[Enzo]

MANY many Fender amps were made with the balance rather than absolute adjustment for bias. It is a latter day thing to bias all amps to the brink of tube meltdown. Relatively cool setting were the norm. Leo Fender was not interested in power tube breakup.


AA864 seems to be one of those variants that has no schematic. It is not alone in that. Here is the AA964, which I bet is real similar, other than it has the adjustable bias you want.

http://www.webphix.com/schematic%20h...a964_schem.pdf

[g1]

If you want to see the circuit with balance rather than bias, the AB568 schematic has it.
http://elektrotanya.com/PREVIEWS/634...b568.pdf_1.png

[Enzo]

Or this might be easier to open:

http://www.webphix.com/schematic%20h...b568_schem.pdf

[Randall]

Thanks guys. I found the AA964, and that was what the Fender rep wanted to give me. Wrong bias circuit. I see from the earlier AA270 that yes, it is a balancing pot. Is this a CBS hifi thing? I notice they go back to the normal circuit in the AA964. The complaint with the amp is that it sounds anemic, and idling at 20mA I can understand why.

I think I am going to revert it back to the adjustable bias circuit. 20mA or 9 watts from an idling 6L6 doesn't work for me, balanced or not.