Silverface Twin Reverb plate voltage

[punkykatt]

Hello, can anyone explain the difference in power tube plate voltages on the AB568 (470v) vs the AA270 (405v) they both have the same part number power transformers and power supply. How can that be?  65 volt difference???

[HotBluePlates]

Trust measurements on an amp in front of you...

But anecdotal evidence suggests later silverface amps did have a lower supply voltage than blackface/first silverface amps.

That tells me that "125P34A" doesn't equal "125P34A"; perhaps the specified winding voltage was different between the different years. Look again at the schematics: earlier windings are 340-0-340v (or 350-0-350v), later windings are 315-0-315v.

It was Fender's internal part number. I bet Schumacher wound whatever Fender said, stamped whatever number Fender said stamp.

[punkykatt]

Gotcha  HPB, Makes a bit more sence now.   On this 68/69 Twin Reverb I have on the bench, the sticker label says AB763, but it has all the silverface bandaids on the power tube sockets. Plates at 405v , sounds great, still is all original, Has slight hum , pops and burbs here and there . Just got the OK for new filter caps and three conductor grounding chord to start with. YEAH!!! fun stuff.

[HotBluePlates]

That amp might follow the AB568 schematic, which says it has the higher voltage PT. You would know better than I since you can check the amp itself.

[EL34]

It's hard to say on some silvers
Fender may have been using up existing supplies

Some had way higher voltages than blacks

You have to get the amp biased correctly before you can get the exact working voltages

Having higher voltages keeps them a bit cleaner which is not a bad thing, IMO on fenders

[punkykatt]

Right now its biased way cold  with only the balance pot adjustment.   I suspect the plate voltages will go up when i replace the filter caps.

[Geezer]

The '74-ish SFMV Twin I'm modding right now has 400v on the plates, for which I'm thankful, as its not going to be a clean machine.

Punky....if you're interested, I have come up with a way to have the best of both worlds....bias-balance AND true bias voltage adjustment. Works really well, and allows the use of mismatched tubes with better bias results. All it takes is adding a small 25k pot or trimmer to the existing SF bias circuit.

G

[punkykatt]

Geezer, Yes I`m interested in your adjustable/balance bias mod.  Do you have a drawing?

[Geezer]

I'll draw one up as soon as I can and post it here

It's basically a 25k trimmer and a 2.7k resistor off the center lug of the bias balance pot.

You balance the bias voltage with the old pot to match on each tube, then you use the new trimmer to actually set the correct bias voltage....works great

G