High Pitched Sizzle

[Structo]

Hey guys, I'm getting my D'Lite 44 (6L6's) about where I want it tone wise thanks to this forum .
Yesterday I changed the V2 plate resistors to 220K/150K values and the bypass resistors to 3K3/2K with 4.7 uf caps on both halves of V2.

So now both V1 and V2 have identical Rp and Rk.
Cl1 has a 4.7uf and Cl2 has 10uf caps.

I now hear a very high pitched sizzle type noise along the lines of how a snare drum rattles when you play next to one, although this noise is higher pitched and faint but enough to be bothersome.
This is when playing my strats through it.
I checked the tubes for microphonics by tapping on them with the volume up and they are quiet.

Presence control doesn't seem to affect it.
I have a 330pf treble cap which isn't the best quality one.
It's one of those super tiny ones rated at 1kv (if I recall).

No scope here so I can't look at it like that.

Any ideas about the cause or solution?

I thought about maybe a small value ceramic (15pf?) across the bypass caps of V2?

Seems like I have seen a schematic with these before but now I can't remember where they were put exactly.
Thanks, Tom


I

[Funkalicousgroove]

is it mechanical rattle or coming through the speaker? If you used solid core wire it can sometimes rattle, you can bend it or put a teeny bit of silicone on it to stop that-

Could also be internal structure of one of your power tubes-

[tubedogsmith]

This is probably obvious but have you ruled out flourecent lights and is there a ceiling fan in the room.

[Structo]

I suppose it could be the power tubes.
They are JJ 6L6.
Could it be filament rattle?
It is apparent even when the amp is turned down low.

There is no interference from lighting, etc.

The thing is I didn't notice it until the last tweaks that I mentioned above.

Any ideas about filtering it out?

[tubedogsmith]

If it's the tubes rattling you could hear it by flicking them with a finger.

[Fischerman]

If it's the tubes rattling you could hear it by flicking them with a finger.

I flicked a JJ KT77 the finger once and it immediately went micro on me. Be careful...they can be thin-skinned sometimes.

[Pete]

Is the speaker cab isolated from the amp (head) or is this a combo?
Does separating the two do anything?

[greiswig]

I suppose it could be the power tubes.
They are JJ 6L6.
Could it be filament rattle?
It is apparent even when the amp is turned down low.

There is no interference from lighting, etc.

The thing is I didn't notice it until the last tweaks that I mentioned above.

Any ideas about filtering it out?

I wouldn't concern myself with filtering it out until you're more certain of where it is coming from. Otherwise you'll just end up chasing your tail. My fizz (posted elsewhere here, and it sounded a bit like a torn speaker) actually went away when I changed OT's. Eliminate all the easy things first (tubes, cold solder joints, etc.) Is it possible that the tweaks you just did simply made something more audible than before?

[Structo]

Yeah separate head cab and extension 2x12.

I've tapped on the tubes, no joy there.

I really don't think it is anything mechanical as in vibrating parts or wires.

[Structo]

I guess that I was hoping that someone else had run into this type of problem and could recommend a solution.
Seems to have reared it's head after I changed V2 Rp and Rk values.

[kleinm]

Did you accidentally swap the Rk/Rp's for the two triodes? i.e. - did you accidentally combine 3k3 and 150k?

Doing so could throw off the D-prefered bias for the triode. A cold bias could impart an unpleasant sizzle to your OD tone.

[Structo]

Thanks I'll double check that but I don't think so.
If I recall when I was probing voltages the cathode voltage on the preamp tubes was in the 1.7-1.9v range.
The more I read and think about it, it could be an ultrasonic oscillation or some kind of parasitic oscillation. Like a ringing oscillation.
Need to get in there and chopstick some leads.
Maybe I I'll recheck my power tube grid leads.

[Structo]

You guys ever put a small ceramic cap (47pf) across the plates of the PI to curb oscillations?

[greiswig]

You guys ever put a small ceramic cap (47pf) across the plates of the PI to curb oscillations?

I did in my Bluesmaster PI.