Peavey Ultra 60

[chunkitup]

I would like to know what certain diodes are used for in this amp. First pair is found right after the third stage coupling cap and the diodes are in parallel but in opposite directions(as far as flow is concerned) then fed into the voltage divider for the next stage.
The second pair are going from the plates of the power tubes to ground.
Are these some mods I can try in a build of mine? I have the schematic for this amp but no way of posting for everyone to see.

[Steve Conner]

The first pair produce a kind of noise gate effect that reduces noise on high-gain sounds and tightens them up somewhat. I guess it's really for the "Ultra" channel. This mod is well worth trying on a high-gain amp: I tried something similar on a homebrew and liked it a lot. It's no use for clean or slightly crunchy sounds though.

The second set of diodes are to help protect the OT from damage by mismatched speaker loads. They shouldn't make any significant difference to the tone.

[chunkitup]

There is also two diodes before the grid resistors of the power tubes that go to ground. What do they help or do? They are each paralleled by 100pf caps.

[Enzo]

Essentially they are there to prevent the tube grids from drawing current. Though the grid sits at the bias voltage of about -45 or -50 or whatever, if the signal coming into that grid gets a positive voltage more than that, the peaks over the 50 volts will turn the grid positive. A positive grid will atttract electrons - current - from the cathode.

With the diode, any voltage over about +0.5v will turn the diode on and shunt to ground.

The caps add stability.

[chunkitup]

Nice!! That is worth a quick print so I may keep for my records. Never read that in my "Beginners Guide to Tube Audio Design" book.
Thanks