33pF cap at C18, C19?

[Ty_Bower]

The ST35 Rev D has a 33pF cap at C18 and C19 (C22 and C23 for you Rev B owners). They look to me like they basically just short high frequency content from the inputs to ground.

What do they do? I don't see it there on the Eiclone, Poseidon, or ST70 driver boards. They all have a 12AX7 on the front end as well...

[paart]

You are on the right track! It's purpose is to limit the high frequencies (RF) into the first stage, which can really cause problems in a power amplifier with wide frequency response such as the ST-35. Since this sort of problem can be difficult to detect without using a 'scope, and can be intermittent, I choose to leave this cap in, since it's doubtful that it degrades sound quality otherwise. If you are absolutely certain that you have no RF coming in (even from your preamp or line stage-which can be far more susceptible than power amps to the problem), than you could eliminate it, or reduce it's value.

[erichayes]

The DIY ST35 is a verbatim ripoff of the original Hafler design, except for the output tube biasing. Hafler found that RF from all the downtown Philly radio stations was goofing up the front end of the ST35, so he put the caps in. Apparently there wasn't a problem with the amplifiers using a pentode front end.

It's conceivable that Shannon's modded driver boards for the other amps could have a problem in an area with high RF density, but I'd think it would have come up by now.

[TomMcNally]

My original factory wired ST-35 used to live on TOP of one of
those Philadelphia FM transmitters ... at WDVR 101.1 - and it
worked just fine as a monitor amp.