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 on a pair of new JJ 6L6's. 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?

[martin manning]

I think you have the same set-up as this AA371 Bassman: http://ampgarage.com/forum/viewtopic.ph ... 374#252374 The diagram shows a pretty easy mod to add a bias level trimmer.

[Randall]

I see from the earlier AA270, this is indeed a bias balancing pot. Perhaps an early CBS hifi influenced change to get rid of the nasty distortion? I see that it was reverted back to the standard bias circuit in the AA 964. Probably everyone except who's idea it was hated it. I know I do.

I think I'll just talk the customer into letting me roll it back to the standard circuit rather than fuss with a trim pot.

[Firestorm]

Obviously not AA864. Raises some questions.

[martin manning]

Balance + Level is a pretty cool set-up, and that would be my choice. Adding the trimmer does not require any new eyelets, and the remainder of the circuit (tapped pot and main board wiring) stays as-is.

[Randall]

Obviously not AA864. Raises some questions.

???? Explain? It all looks factory.

[Firestorm]

Obviously not AA864. Raises some questions.

???? Explain? It all looks factory.

Sorry. I didn't mean it wasn't factory. Just not 1964. I've never seen an 864 schematic, just 964, and that is straight bias, not balance, so it's interesting. What are the pot dates?

[Randall]

It appears that the AA864 schematic does not exist, especially if Fender doesn't have one. I have the AA964 schematic, and yes, it is straight bias, however the AA864 does not. I have found a AA270 that shows the balancing circuit this AA864 has, so IMHO it is as it was intended.

[Structo]

I always thought the AA864 was a Bassman circuit?

[Firestorm]

By the blackface era, the circuit designation no longer tells you what the amp model is (there are several AB763 amps for example). The first bias balance circuit I can see is the AB165 Bassman, but it seems to use a conventional pot. The first tapped pot bias balance seems to be the AA270 models 5 years later. So if this amp has a tapped pot in 1964, it's pretty interesting to us students of Fender peculiarities.

[NickC]

I always thought the AA864 was a Bassman circuit?

It is.

http://thevintagesound.com/ffg/schem/ba ... _schem.gif

[Firestorm]

I just looked at the massive table Greg Gagliano did years ago and it shows AA864, AA964, AB568 and AA270 as documented circuit numbers for the Vibrolux Reverb.

[Structo]

Thanks, I just looked that up.
Great resource.

[Sone Aura]

Vibrolux AA864 circuit. It is not just an AA964 with Jensens or Oxford or Utah Coil or whatever "different" speakers. I just had a 1965 in and it has a higher voltage preamp rail. Fender was NOT overly diligent about gluing in the proper tube charts either so you may or may not have an AA964 with an AA864 circuit or vise versa, an AA864 with an AA964 chart glued into the cabinet. It simply runs the phase inverter 12AT7 and the 7025s and everything else kind of up around Twin Reverb preamp voltages or for that matter look at the Bassman AA864 preamp voltages, and drop a few volts off for the 5AR4 rectifier tube, because the AA864 Vibrolux Reverb chart shows a 5AR4, two 6L6GCs, and the usual two 12AT7s PI and reverb driver and the three 7025s which were usurped some time ago by the 12AX7A tube. For Gits & Shins: The 7025 was supposedly designed to hold the hum down to an average of 1.8 microVolts with a maximum allowable hum of 7 microVolts. The 12AX7A was also designed to have the same 1.8 microVolts of hum, however they were NOT tested to cull out anything that was over 7 microVolts of hum, so there can be more noisy 12AX7As than 7025s. Since no one makes a real 7025 anymore that is kind of a moot point now. To make a 7025 just test the tubes and reject anything with more than 7 uV of 60Hz hum in the 10KHz bandwidth of amplification. I digress, sorry.

[ampdan]

https://i.imgur.com/nc8kE0c.jpg
https://i.imgur.com/lEq5sTe.jpg

My 1965