Marshall 100watt Plate voltage

[flemingmras]

As PDF64 explains above "If the [screen] dissipation is below the limit, eg typical fixed bias idle, then the voltage can be above the limit. With saggy supplies, things can pan out ok. The issue is both voltage and dissipation being above their limits simultaneously."

So the 425v limit does not matter too much. But you need 1k+ screen resistors

I wish we still had access to the Metro forum. I recall several years ago being part of a thread discussion regarding this very issue. Tubes were redplating because the high voltages of yesteryear were being copied, but the regulation spec of the modern transformer builds was too tight for it to sag enough to work.

[Roe]

As PDF64 explains above "If the [screen] dissipation is below the limit, eg typical fixed bias idle, then the voltage can be above the limit. With saggy supplies, things can pan out ok. The issue is both voltage and dissipation being above their limits simultaneously."

So the 425v limit does not matter too much. But you need 1k+ screen resistors

I wish we still had access to the Metro forum. I recall several years ago being part of a thread discussion regarding this very issue. Tubes were redplating because the high voltages of yesteryear were being copied, but the regulation spec of the modern transformer builds was too tight for it to sag enough to work.

yes I started a long thread on this as well as a shorter one on this board, see https://ampgarage.com/forum/viewtopic.p ... 2&p=109098

with the 1.7k primary the plates must sag to 420-30v to avoid redplating

[flemingmras]

with the 1.7k primary the plates must sag to 420-30v to avoid redplating

The B+ must sag. The plate voltage alternates under signal, and, at the peaks, will go as low as the load line allows (down to the crossing point).

If the load were ideally matched for the voltage, you'd maximize power transfer to the load while reducing power wasted across the tube, which would also reduce the redplating.

[eddiep1966]

Update:

Ok the Hammond transformer was not at spec. So order a new tranny. I have used Hammond for years and never had an issue. Guess there quality control is going to crap to.

[martin manning]

Update:

Ok the Hammond transformer was not at spec. So order a new tranny. I have used Hammond for years and never had an issue. Guess there quality control is going to crap to.

How so? What did you measure to determine this?

[eddiep1966]

By using a variac and monitoring voltages.

[eddiep1966]

Sorry Martin guess I read your response wrong. Spec's sheet states 120v on primary places 350v on secondary loaded. lt places 388v on secondary under load. Of course when applying my full wall line voltage 125.5v when test was conducted that sent the secondary higher. To get the secondary down to 350v had to turn the variac down to 108v but then it lower heaters volts down to 5.1v.

[martin manning]

No problem, so if you correct for the wall voltage did it produce the correct no-load voltage? That would be the easiest thing to check. You could also look at the DC resistances and compare to the spec sheet.

[eddiep1966]

Not at the amp right now but I beleive it was 394v. I'll check the ohms tomorrow.

[pdf64]

lt places 388v on secondary under load.

It requires 420mA load current to bring the secondary voltage down to 350V AC.
That's an 833R load resistor dissipating 147W!
The no load secondary voltage is 370V AC +/-2%
https://www.hammfg.com/files/parts/pdf/ ... 1697661948

[eddiep1966]

Update: Well got a new tranny in it and it sounds great. Voltages are good now tubes and screens are happy. Had one custom made at Pacific Audio Magnetics. Basically it a stand up version of their 100watt plexi replacement.
Thanks to all that gave input and suggestions.
On to the next build.