Ampeg B25b Voltages

[Jack1962]

does anyone here have experience working on the Ampeg B25b ? On the schematic is shows the Voltage to be 500 volts on the center tap of the output transformer , what I actually have here is 574 volts , is this normal with these amps?

[jbrew73]

mine was 525v with 5ar4 rectifier

[Jack1962]

well what concerns me it the maximum voltage rating on the 6L6's is 600 volts and I'm not sure but I think it's pretty close to the max voltage rating on the 7199 as well.

[sluckey]

6L6s??? Shouldn't that be 7027s?

[RicharD]

A "Must Read" concerning Ampegs & 7027's.

Help, Uncle Ned, My amp has 7027's

http://www.triodeel.com/7027.htm

[PRR]

> the maximum voltage rating on the 6L6's is 600 volts

No. 400V in 1937, 360V for most of the run, 450V on 6L6GC.

It is possible that good new 6L6GC will survive this tube-killer, but I think it would be far wiser to run 6550, which were factory upgrades on similar models.

> I think it's pretty close to the max voltage rating on the 7199 as well.

The (300V) 7199 won't feel the full B+ voltage. Probably less than half. Indeed the schematic shows 162V and 130V on 7199 sections when raw B+ is nominal 500V.

[Jack1962]

Sorry , guy's my mistake they are 7027's , Yes PRR , it is approximatly half of the B+ supply on the 7199 , but at the increased voltage , that this amp has will it if not damage the tubes , or at least wear them out faster?

[PRR]

> half of the B+ supply on the 7199 , but at the increased voltage , that this amp has will it if not damage the tubes , or at least wear them out faster?

Everything will rise very nearly in proportion.

Schematic for 500V shows 162V from plate to cathode on 7199 pentode.

At 574V this will be 574V/500V or 1.15 times higher, 186V.

You would have to take the raw "500V" up above 900V to put the 7199 near its ratings.

At 900V, the 7199 will be the least of your worries.

IMHO, the Ampeg 7027 amps really should be run with 6550. Ampeg shaved the LOUDNESS-per-dollar thing to get "market life".... they out-lived their warranty. But when a 7027 gets tired of the abuse it may short-out. This then blows the PT. Which is not a trivial fix. The 6550 will stand abuse better.

[Jack1962]

Thanks PRR , I was thinking of replacing the 7027a's with 6550's . I would also like to thank everyone else that has chimed in on this post .

                                                       Rock On

[basajauna]

I'm myself repairing an old B25b too, sounds awfull, it had been modified, I tried to get it back to stock but without layout availiable...I Tried it with the schems. I changed the filtering caps with new ones from TAD (best tube online store in europe), all tubes are new; I also put a solidstate rectifier (the ones that you can put int he octal socket from the tube rectifier). I checked voltages:
With solidstate rectifier: 425V
With tube rectifier: 378V
What could be the problem? 
(As i'm in europe, my amp is a 110V model, nor european, i have a 220/110 transformer that gives me 115V output.) May it be the amp's intput transformer or a rebuilt error?
Thanks for helping!

[ernie_jr]

I have built a scratch built B25 with 6550's. They wound great and can handle the high voltages without any problems
Ernie