BF Amp Filter Design Differences

[mbratch]

I was looking at the BF Pro Reverb versus the Vibrolux Reverb and noticed that, off the rectifier, the ProR has a series-parallel circuit with two 70uF caps and two 220Kohm resistors. The Vibrolux Reverb has two 16uF (or 20uF) caps in parallel and that's it. The Super Reverb is the same as the VR, and the Twin Reverb (with SS rectifier) is the same as the Pro Reverb.

What's the basic concept behind this difference? How does it affect the sound of the amp? Or is it just a dependency on the transformer used?

[Wakarusa]

What's the basic concept behind this difference? How does it affect the sound of the amp? Or is it just a dependency on the transformer used?

The concept is the voltage rating (WVDC) of the electrolytics.

The actual capacitance in all cases is pretty similar -- 35uF for the series 70uF bypassed by 220K resistors, 40uF for the parallel 20uF setup and 32uF for the parallel 16uF setup. Not enough difference to matter.

However, you'll note that nominal B+ in the AA964 VR is 410VDC, under the 450WVDC rating of the caps. In the Super Reverb you're looking at the AA763 circuit which has a B+ of 465VDC but calls for caps with a 575WVDC rating. The AB763 and all later Super Reverbs use the 70uF/350WVDC series setup.

Affect on sound? Virtually nil.

Transformer difference? Maybe, but only in the fact that the VR power transformer is supposed to develop a lower B+ (maybe. read on). What's screwy is that on the bench the VR B+ may or may not be lower than the other 2x6L6 BF Fenders, and the schematics show the same 315VAC secondary for different amps running GZ34 rectifiers. Now to REALLY mess things up if you look at the AA964 and AA270 VR schematics, one claims 315VAC secondary, 410VDC B+ and the other claims 345VAC secondary and 465VDC B+, but both use the same part number (125P26A) for the power transformer. Even more fun is that the AA270 model still calls for parallel 16uF/450WVDC caps on a B+ line marked for 465VDC. Oops.

So logically we have to conclude that either the VR schematics are junk or Fender messed up the design :)

[mbratch]

Thanks Todd. I was mainly curious why Fender followed two distinctly different ways of doing this small piece of the circuit in these particular (and similar) amps.