Fender Schematic Voltages - Question

[Dr Tony Balls]

I'm looking into a Showman build and have sort of a two part question relating to Fender's published voltages for the AA763 Showman: https://ampwares.com/schematics/showman_aa763.pdf

1) The schematic shows a 345-0-345 PT and then a B+ of 450VDC. That seems off to me, but i'm probably missing something. If that B+ was an unloaded voltage i'd expect to see more (~480). If it was a loaded voltage i'd expect less (~420). Am I missing something here?

2) Question 2 is probably answered by question 1, essentially which voltage on the schematic is correct. I see replacement PTs that match the published secondary voltages (345-0-345) and then I see others that look designed to provide the published B+ (320-0-320)

[sluckey]

It all depends on how stiff the PT is and how loaded the B+ is.

[martin manning]

I'm looking into a Showman build and have sort of a two part question relating to Fender's published voltages for the AA763 Showman: https://ampwares.com/schematics/showman_aa763.pdf

1) The schematic shows a 345-0-345 PT and then a B+ of 450VDC. That seems off to me, but i'm probably missing something. If that B+ was an unloaded voltage i'd expect to see more (~480). If it was a loaded voltage i'd expect less (~420). Am I missing something here?

As a guess, it looks like the DC voltage is with standby on play, and the AC is with standby on standby.

2) Question 2 is probably answered by question 1, essentially which voltage on the schematic is correct. I see replacement PTs that match the published secondary voltages (345-0-345) and then I see others that look designed to provide the published B+ (320-0-320)

Transformer voltage specs may be loaded (at rated current) or unloaded, and the difference is usually somewhere around 5-10%.

Hammond's Showman PT (290FX) data sheet gives both loaded and unloaded voltages: Loaded voltage 320 VAC at 540 mA, which would be ~452 peak, minus the rectifier voltage drop and half the ripple for the average DC. Unloaded voltage is 333 VAC, which would be ~471 peak. An idling amp is a part-loaded case, so it should wind up somewhere between loaded and unloaded.

[jabguit]

I'm looking into a Showman build and have sort of a two part question relating to Fender's published voltages for the AA763 Showman: https://ampwares.com/schematics/showman_aa763.pdf

1) The schematic shows a 345-0-345 PT and then a B+ of 450VDC. That seems off to me, but i'm probably missing something. If that B+ was an unloaded voltage i'd expect to see more (~480). If it was a loaded voltage i'd expect less (~420). Am I missing something here?

2) Question 2 is probably answered by question 1, essentially which voltage on the schematic is correct. I see replacement PTs that match the published secondary voltages (345-0-345) and then I see others that look designed to provide the published B+ (320-0-320)

Font characters are hard to read on many older Fender schematics. I see 480 off the rectification where it may look like 450 to your eyes....

cheers,

[sluckey]

Font characters are hard to read on many older Fender schematics. I see 480 off the rectification where it may look like 450 to your eyes....

I doubt it is 480. That would mean 30V dropped across the OT primary and 35V dropped across the choke. That's not likely. And if you look at an "8" elsewhere on the schematic you will see an obvious difference to that "5". The "fonts on that schematic are actual draftsman pen strokes so naturally there will be a slight difference even between the same characters. But I'm pretty sure that the voltage shown out of the rectifier is 450V, not 480V.

[Stevem]

Until CBS/ Fender had Sylvania produce there STR 6I6 tubes made for them for the silver face 70 watt and 135 watt amps there was never a Fender amp that had unloaded V+ voltages near 480.

Of course with today’s wall voltages like where I work being 122, things are going to creep up !

500 volt rated filters used at the second filter node in these 70 and 135 watt amps may be pushed well above there 500 volt rating in standby!

I have not had one fail yet, but I have considered stacking caps for this screen node also.

[Dr Tony Balls]

Thanks for the input, everyone!

[jabguit]

Font characters are hard to read on many older Fender schematics. I see 480 off the rectification where it may look like 450 to your eyes....

I doubt it is 480. That would mean 30V dropped across the OT primary and 35V dropped across the choke. That's not likely. And if you look at an "8" elsewhere on the schematic you will see an obvious difference to that "5". The "fonts on that schematic are actual draftsman pen strokes so naturally there will be a slight difference even between the same characters. But I'm pretty sure that the voltage shown out of the rectifier is 450V, not 480V.

Yes - the more I look at it the more it looks like 450, but with the SS recto it should be closer to 480.

cheers

[sluckey]

Yes - the more I look at it the more it looks like 450, but with the SS recto it should be closer to 480.

A ss rectifier with a 345-0-345 PT would put out 487V unloaded. Lot's of factors in the amp, mainly the PA, PT, and biasing, will cause that voltage to drop.

As an example, my Phoenix is an AB763 based amp with a PT that puts out 340-0-340 and uses a SS rectifier. Unloaded output should be 480V, but my actual B+ is only 423V.