Tube amp power section hissing on E

[mxaniac]

I have a 69 Guild Superstar that someone added a master volume to. If I crank the power amp way down (the added knob), preamp (original volume) on max and get most of my gain through the preamp it's fine. At least that's how I assume the controls work. If I turn the preamp down and the power section up I get hissing on E notes. I swapped out the 6L6 tubes and it didn't have an effect.

Should I be chasing caps since it's frequency related? The power section has several other tubes as well. Tapping the tubes didn't cause any unanticipated sounds.

Any suggestions would be appreciated.

 

[dystrust]

Re: Tube amp power section hissing on E

It could still be a microphonic preamp tube as the tap test isn't always conclusive. Since this seems to happen most with the master volume high, try swapping out the phase inverter first.

 

[Darg1911]

Re: Tube amp power section hissing on E

Although the "hiss" description is weird, I wonder if something is vibrating. Either a tube as Dystrust suggests, or a component in the amp, part of the chassis, speaker or cabinet or something in the room. It can be really hard to pinpoint the source but it may be a funky vibration not indicative of an actual problem with the electronics of the amp.

 

[dystrust]

Re: Tube amp power section hissing on E

Although the "hiss" description is weird, I wonder if something is vibrating. Either a tube as Dystrust suggests, or a component in the amp, part of the chassis, speaker or cabinet or something in the room. It can be really hard to pinpoint the source but it may be a funky vibration not indicative of an actual problem with the electronics of the amp.

+1

An easy way to tell that you're playing too loud at rehearsal is if your amp makes the snare wires ring.

 

[Little Pigbacon]

Re: Tube amp power section hissing on E

+1

An easy way to tell that you're playing too loud at rehearsal is if your amp makes the snare wires ring.

And if it makes the bass pedal hit, you know you’re right out.

 

[Lake Placid Blues]

Re: Tube amp power section hissing on E

I wonder if this indeed aged electrolytic caps related? That seems more plausible to me than tubes in this case.

Sometimes a static sound caused by certain vibrations can be loose tube pins. I chased weird noises on an amp, but could not find it until I retensioned the female inserts for the tube pins and cleaned them with contact cleaner. That did the trick.

If the amp has a rectifier tube, that might also be an issue. Crackles and weird noises can be caused by a tube rectifier coming up the end of its life. In this case it will probably soon start to blow the mains fuse.

 

[Darg1911]

Re: Tube amp power section hissing on E

I wonder if this indeed aged electrolytic caps related? That seems more plausible to me than tubes in this case.

Sometimes a static sound caused by certain vibrations can be loose tube pins. I chased weird noises on an amp, but could not find it until I retensioned the female inserts for the tube pins and cleaned them with contact cleaner. That did the trick.

If the amp has a rectifier tube, that might also be an issue. Crackles and weird noises can be caused by a tube rectifier coming up the end of its life. In this case it will probably soon start to blow the mains fuse.

On a related note, I have an amp with two 5U4 recitifiers (Chinese), one or both of which rattle with lower frequencies. Swapped them both out with replacements as a test to confirm the source of the noise and the rattle was gone. But this is a rattle that could be described as "hiss". They sound kind of like a vibrating spring in a reverb tank.

 

[mxaniac]

Re: Tube amp power section hissing on E

Thanks everyone I'll order up some replacement tubes and go from there.

 

[mxaniac]

Re: Tube amp power section hissing on E

Well hopefully I didn't order duds but I have two Magnavox 12AX7A NOS tubes on their way. I also ordered a new Sylvania 6GW8 and a new RCA 12DW7. Not sure where the 12DW7 goes yet as I haven't had a chance to dig in but its listed as required for the amp.

 

[Gtrjunior]

Re: Tube amp power section hissing on E

Just remember to replace the tubes one at a time so you know which one the problem was occurring in. (Assuming the issue is in fact tube related)

 

[mxaniac]

Re: Tube amp power section hissing on E

Tubes have been at my local PO for a while now, they have limited hours and always find an excuse not to deliver and I haven't had a chance to retrieve them, so I got impatient and swapped the existing tubes around. It's one of my 12AX7 tubes and if I put it in a certain socket the problem happens. It's repeatable, but fortunately solvable.

Thanks everyone for your help.