Spitfire Clone Noise

[mbratch]

I have a Spitfire clone without any special mods. It really sounds great tone-wise but it does have a little bit of a buzzy hum. It's not a strong 60 cycle hum, but something a bit higher pitched. Maybe there's a hint of 60 cycle in it. Here are some facts surrounding the hum:

• It gets louder as the tone control is turned up, and is gone if the tone control is all the way down.
• The noise is greater with greater volume or gain.
• I rewired the input jacks so that the two input resistors are direct connect to the jacks, and those are wired with a shielded wire to V1 input.
• I've tried different tubes in V1 and have noticed that the noise varies, but no tube is quiet. (A NOS RCA seems best.)

Given the response to the tone control, I'm supposing the noise must be between V1 and that pot, but there's not much there to inject noise except the tube itself.

Frankly, I don't think the noise is that great, but it's just enough that our sound man wants to switch on a noise gate. I was trying to avoid that if possible.

Can this circuit be made quieter, or is it just the nature of it? Perhaps I'll play with the grounding a bit...

Thanks

[Chuck H]

Hiss is a common effect of high gain (ala, cranking the amp) and if you compare most amps and pedals on a strictly "volume/gain" basis I think you'll find that hiss is an unavoidable fact of life. That said I do think if your main problem is buzz or hum then you should be able to reduce it through grounding scheme and layout. Since I don't have any specifics about your build, that's all I can offer. But, FWIW, hiss CAN be reduced if you are using CC resistors in the plate supply. Just change the plate R's to metal film R's. And hum can often be traced to grounding (often the input and/or the heater wiring. Buzz is likely a lead dress issue.

HTH

Chuck