What are those diodes for..????

[ODwan]

Hi, Folks!

Looking at the Kelly'90 schematic I wonder what those diodes from ground to the plates of the powertubes do. Never seen anything like this before!
If anyone knows please fill me in.

Timo

[toddyjoe]

The surge-protector diodes will absorb large transient voltages across the primary of the output transformer. This is a protection for the transformer and tubes when problems occur in the amp, including when the amp is being operated with no load or an open speaker load. There is an article by Ken Fischer included in Aspen Pittman's book that recommends adding them. Some amps, including some Traynors, used them stock from the factory. I first encountered them in a Traynor YBA-1A. My Traynor kept blowing these diodes, which was an indication of a much bigger problem with the amp. If those protection diodes had been removed, I might have been sticking big money into new transformers.

[ODwan]

ToddyJoe: Thanks for the info!

Seems like a good idea to me! I might add them to my builds. Is there any perceived soundaltering?

Timo

[mhuss]

The older Mesa Boogies (Mark II era) used these as well. There *shouldn't* be any audible effect during normal operation (i.e., they should be in the nonconducting state 100% of the time), but I still prefer to just put an Ampeg-style 250 ohm resistor across the output.

--mark h

[Bear]

Ken's comment on it (which may be in the Pitman book as well, but which I know is in the first Gerald Weber book) mentioned that it's a limited protection, but a more complete protection messed with the tone. I doubt I would notice any audible effect to the diodes.

Bear