another noisy Silvertone 1484

[duffy878]

Hey guys, I got a bit of hiss going on in my twin-twelve that I know atleast one of you can help me narrow down.

For starters, I've already replaced the electrolytics, the diodes, the coupling caps, the power supply resistors, and any other bad resistors (I think I got all of them). The amp sounds good as it is right now, but the hiss is starting to drive me crazy.

The hiss is not related to volume, tone control changes, channel selection, tremolo, or reverb (even though there is no reverb 'pan' in it).

The hiss disappears when I pull the 12AX7 before the PI (the one connected to the volume pot wipers) and the 12AX7 in the reverb section (the one connected to the reverb pot wiper)

the schematic is here: http://www.schematicheaven.com/barga...ertone1484.pdf

I don't want to start replacing everything in the amp, so I guess I'm looking for some educated guesses about what specific parts it could be. Any help at all would be greatly appreciated.

[Amp Kat]

You either have a wire going somewhere it's not supposed too or a wire not going somewhere it's supposed too or a cold solder joint. Check you're wiring good !

[guitician]

R28 or R31 may be bad and getting noisy. Or that 12ax7 your pulling is.

[melvin]

what about R24, R25 & R27? Or an actual tube?

[duffy878]

I've switched my tubes around quite a few times and the hiss is consistent.

The wiring is original, I only replaced faulty parts and I'm rather confident in my soldering abilities. This is not a modded amp.

I'm thinking like it is very probable it is R28 and/or R31.

If R24, R25, and/or R27 drifted, the voltage at the tubes plates would have decreased but they're rather close to those listed on the schematic. Carbon comp resistors get noisy when they drift, or can this happen even if they stay within the 20% tolerance?

The hiss is there if one of those two tubes is in the circuit.

If they are BOTH pulled then it is gone.

I Don't know if this helps, but there is no reverb pan hooked up to this amp.

I really appreciate the fast response guys, I'll check it out when I get out of work.

[Jazz P Bass]

Silvertone hiss

What happens if you ground the reverb return wire?

[duffy878]

grounding the reverb return wire didn't change anything.

I just did some poking around and R24 and R25 are both reading around 79.5K which is pretty far off. I don't have any 1/2 watt 68Ks on hand so I'll pick some up tomorrow.

[Amp Kat]

Can't see where old carbon comp values would cause hiss as much as the carbon comp composition itself. If you can try locating which part of the circuit it's coming from. Disconnect right after the first tube after the coupling cap and send it to the output grid or PI. Still hissing ? If it's hissing there it will hiss everywhere
downstream. If it's good there then move downstream.

[duffy878]

I was under the impression that resistors get noisy only when they drift, but I guess thats not 100% correct.

I just replaced R24, R25, & R28 with 1/2 watt metal films and I still got the hiss....

R27 and R31 are next.

[duffy878]

I replaced R27 and R31 and now this thing is as quiet as it probably ever will be.


Thanks for the help guys.