diodes on output tubes - Francesca

[rockstah]

...are these 3 diodes on each output tube a tonal thing or simple circuit protection of some kind?

Mark

[paulster]

Mark

The diodes are to protect both the tubes and the transformer against flyback transients.

A 1N4007 has a reverse breakdown voltage of something like 1000V, after which it will conduct in both directions. In the Express example, with three connected in series between the plate and ground, they won't conduct until there is a voltage of above 3000V at the plate, and this will then be dumped to ground rather than into the output tube.

It also prevents the inductive nature of the transformer from attempting to raise the voltage higher in order to maintain the magnetic flux (as inductors do), which would ultimately result in arcing across the windings and permanent damage to the transformer.

Paul

[rockstah]

cool deal. thanks for that.
so is it something about this amp that needs them? its not a mod i have seen before.

good idea to put this mod in my Marshall's to protect the transformers?

what about the two 1k's on v4? hum balance mod?

[paulster]

I use them on all my builds to help protect the OTs. Larry mentioned that he uses a different diode with a higher reverse voltage on his Marshall-style builds so he can get away with a single diode, but I can't remember what type it was.

The 1Ks should be 100Rs I think, and are to create a virtual heater CT since this transformer doesn't have one.

[rockstah]

yeah i thought should be 100 ohm as well - thanks man.

now i wonder if the diodes still serve their purpose if one has the 1 ohm resistors in place to bias the amp.

[paulster]

Mark

Even with the 1R resistors (which I put in as well) the diodes will do their job. Basically, once you get a plate voltage of 3000V or so the diodes will conduct and dump the current to ground, hopefully preventing arc-over within the transformer primary windings.

This will happily coexist with the bias resistors.

Paul

[rockstah]

thanks Paul.

[M Fowler]

July Premier Guitar Mag had an article by R.G. Keen noting using IN4007 for up to 450V which is rated at 1000V and 1 amp.

Use St Semi Conductor STTH112U rated at 1A and 1200V from Mouser Electronics for 450V to 550V B+ voltage.


Mark

[novosibir]

I use them on all my builds to help protect the OTs. Larry mentioned that he uses a different diode with a higher reverse voltage on his Marshall-style builds so he can get away with a single diode, but I can't remember what type it was.

BY509 - as small as a 1/4W resistor and good against spikes up to 4,000V

Larry

[jaysg]

Some Ampegs like the V4-B, used this circuit. I use UF4007's as they are a bit faster than the 1N part.

[paulster]

Some Ampegs like the V4-B, used this circuit.

And, considering who Ken used to work for, it's not surprising to see it in the Express then!

[UR12]

Here is what Ken had to say regarding the diodes.YMMV

[img:400:233]http://ampgarage.com/forum/files/thumbs ... on_113.jpg[/img]

[stoo]

so..umm.. where do these 3000 volt spikes come from.?????
Stew

[jaysg]

Here is what Ken had to say regarding the diodes.

Dana, do you know when he wrote that? Rick Erickson (former Heart tech) went through an analysis of the 1N vs. UF and felt that the 1N's weren't fast enough to do much of anything. That was a year before Ampage converted to MEF.

so..umm.. where do these 3000 volt spikes come from.?????
Stew

They come from distorting the power amp. No spikes if you play clean.

[UR12]

Here is what Ken had to say regarding the diodes.

Dana, do you know when he wrote that? Rick Erickson (former Heart tech) went through an analysis of the 1N vs. UF and felt that the 1N's weren't fast enough to do much of anything. That was a year before Ampage converted to MEF.

I really don't know when he wrote it but it was probably over 20 years ago. For all I know it might rate right up there with the color of wire that he used. I just thought I would throw that out there as it was Ken's take on the diodes. I think if a person spent enough time and money we could probably prove that steril water is bad for our health.