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If you compare the AA964 and AA270 schematics it looks like Fender added some turns to the 125P26A. On paper the silverface version seems to have about 10% more B+ in spite of the lossier rectifier. Anybody know if this is some kind of typo or if there really were two different versions of the 125P26A?
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Yes, Higher Voltage
You are right. The schematics show the following for the 125P26A
AA964 630VCT
AB568 690VCT
AA270 690VCT -
maybe not
As it happens I got a '71 Vibrolux Reverb to blackface yesterday. Its just one amp, but it would seem the voltages on the AA270 schematic are bogus...
The '71 PT is labelled 022723. HV-wise it is within a few percent of the 125P26A and the only real difference is that there is no center tap on the heater winding. I don't actually think you'd want the voltages on the AA270 schematic anyway, at least not with the Vibrolux OT.Comment
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I've seen as much as 475vdc on the plates of a SF Vibrolux (and as little as 445v). If you've got the higher voltage type you can always zener it down.Comment
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475vdc?! Wow. I've worked on and rebuilt a fair number of these but I don't think I've seen one that high when it was used with a proper 5U4GB rectifier and the power tubes idling at around 15-17watts.... or something like 34-38ma.Originally posted by MWJB View Post
Remember, they did not come with a GZ34/5AR4 rectifier, but I've seen them come in that way.
Normally they will be in the <445vdc range and some as low as 420vdc with a used but still decent 5U4GB.
BruceComment
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Aha! Good point, if memory serves that particular amp did have a GZ34.Comment
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thats weird
...because the '71 Vibrolux Reverb I worked on a couple days ago had an original 022723 PT with a Shumacher '71 date on it. Stamped below the date it said "CSA TESTED." All the voltages in the amp were slightly under the blackface spec with the 5U4 and slightly over with the GZ34.
So as of '71 they were still making at least some amps with the blackface spec transformer.Comment
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A little weird, I tend to be a bit suspicious of amps that run under Fender's published voltages, but at the end of the day there are several factors that contribute to your final B+ such as...
Brand of power tubes
How those tubes are biased (as Bruce previously mentioned)
The bias circuit itself (e.g. pre-reverb Vibroluxes took the bias supply from the B+ secondary)
Wall AC voltage at the time you actually take the voltages
How your filter caps are wired
...so there's always a reasonable margin, as long as everything else checks out.Comment
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