Hissing sound from a Fender Twin

[zeebra]

I have a 1976 Fender Twin. Got it completely gone over, new caps, etc. about 1 year ago.
All of a sudden I am getting a strong hiss from the amp. After it gets warmed up the hiss starts. Sometimes it will stop and start up again.
Also after use the power seems to drop, start volume at 3 at home and can go as high as 6 at times.
Any ideas?

[Groovey Records]

Sounds like the power tubes

they do go bad over time

[zeebra]

Tubes replaced 1 year ago. Do you think they could go bad that quick?

[Structo]

Tubes or could be the carbon comp resistors.
Everybody wants those for m0jo but if you like hiss and crackles you will love CC resistors.
They absorb moisture over the years and also tend to drift in value quite a bit with heat and age.

[Groovey Records]

First swap the tubes

Yes absolutely a year is not long for Power tubes

Start with the easy stuff

if that doesn't fix it it is always good to have a spare pair

[slider313]

Sounds like it could be a bad preamp tube. Change one at a time.

[zeebra]

Thank yo very much, I'll get some tubes tomorrow and try one at a time, I appreciate all your help!

[Rosewood]

If you're using the reverb channel pull v2 and see if that kills the hiss, if so it's probably the tube. Also don't forget to turn the other channel volume off if you're not using it.

[donnyjaguar]

Pretty common fault with dirty tube pins - but could be whipped plate resistor too. Generally you can measure the resistance of the plate resistor(s) with the amp powered down and discharged. Failure mode is normally higher than expected resistance. ie: 100kΩ plate resistor will measure 140kΩ etc.

[zzmoore]

If you still have hiss, get rid of the 100k plate resistors. Don't bother to measure anything, doesn't matter, just get rid of them. I will happily cut against the grain here and say replace them with 1 watt carbon film. JMHO
Good Luck

[Kmaz]

I actually heard the sound this amp is making. Has a cracking/sizzling character to it. Doesn't interfere with the sound coming through the speakers.

[Rosewood]

Originally Posted by Kmaz

I actually heard the sound this amp is making. Has a cracking/sizzling character to it. Doesn't interfere with the sound coming through the speakers.

That may be plate resistors with the crackling/sizzling sound, tubes are easy enough to eliminate.