Plate bypass capacitor; 500pf too much?

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jarrodthebobo
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Plate bypass capacitor; 500pf too much?

Post by jarrodthebobo » Mon Feb 05, 2018 1:22 pm

Hey guys!

Over the past month I've been building a really interesting Single Ended marshall-esque amp which was strongly influenced and based upon the SIR #34/36/39 (combination of all of these really), but has slowly grown into something much more; with 7 switches for multiple different tonal possibilities, a tone slope pot, an 'effects loop' (its pretty basic), effects send volume (which acts almost as a prepiMM), and a master volume before the power tubes. The amp sounds really awesome, and I currently have a WGS Veteran 30 running as the main speaker.

Now my issue here is this; I have switches that can put the amp into the specs of a Mod 5 (ala Rockstah); however, it lacks that very bitey presence that the Van Halen tone gets. It gets closed with presence and treble maxed, but its not nearly 'brittle' enough, or the highs don't extend high enough to get 'that' sound.

Initially I thought it was just my speaker choice and maybe that sound came with Greenbacks; but people with similar builds have clips with v30s and they have 'that sound' so maybe its not so dependent on the speaker.

Could it be the plate bypass cap, perhaps? I have a bit of an issue with oscillation in this amp; and it appears as a high pitched whine whenever A: When the gain is up too high B: Treble is too high C: Presence is too high. I can counteract the whine by turning down Gain 2 (a 1 meg pot off of the cold clipper stage; it replaces the voltage divider. However I still have a 470k acting as a grid stopper). Typically I have gain 2 set around where 500k would be on the rotation; but if I back it off a bit, the oscillation goes away. I can turn gain 2 up if the preamp gain 1 is down a bit, or treble is backed off. It's an odd issue for sure.

Due to this oscialltion, I have a 500pf cap bypassing the plate to bleed off some highs. I've always had this cap in; and the whine has always been present (so I'm not 1000% certain this cap is truly accomplishing what I want it to be anyways).

Could 500pf be too high and thusly removing those presence frequencies I'm craving?

I also have a 220pf cap between plate and cathode of the 3rd stage (stage before the cathode follower), but honestly havn't noticed a drop in treble with that cap there, nor much of a change in oscillation.

This has been long winded, so I'll leave more information about the amp in another thread, as I'm going to be posting some clips and such with it soon!

Thanks for the help!
"I hear you knocking but you can't come in-
I know you've been drinking Gin." -Louis Jordan

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