Swoosh after turning on amp

[markusw]

When I turn on the standby switch of my 15 year old 50W Dumble style amp (see https://ampgarage.com/forum/viewtopic.php?f=4&t=7506) after having it powered up for 1-2 min I get this swoosh sound in the attached mp3 for a few seconds. Then the amp is quiet and works nicely.
The swoosh is gone if I remove the PI tube.
With the PI tube in and in clean channel with gain at 0 (i.e. only V1b active) the swoosh gets slightly louder when I turn the master to 5 but it is not gone with the master at 0. It is also not gone when I remove V1 tube.
I thought that it is the PI tube but I went through several tubes in the PI position but it didn't help. In fact with some new tubes the swoosh even got slightly louder.
When I keep the amp running for 15 min and switch to standby, wait a few seconds, and turn on again the swoosh is gone.
Overall I'm not too worried because after the initial few seconds the amp is quiet.
I just would like to understand what's going on. My 100W Dumble style amp does not do the swoosh.
Any ideas?

Best,
Markus

[martin manning]

Tired filter caps would be my first guess.

[markusw]

Tired filter caps would be my first guess.

How exactly do tired filter caps produce the sound?
When the amp has warmed up for say 30 min, I can even power it completely off for 1-2 min, turn it on again and there is no swoosh.
Weird..

[Stevem]

That’s the sound of a weak preamp tube(s) building up to the level of gain there capable of when the B+ voltage is applied.

If you leave your standby switch in the on position and turn the amp on and off with the power switch you can remove each preamp tube one by one starting with V1 and find out which tube is coming on the slowest.

As tubes get weaker there rise time gets longer and as you might surmise this can effect play feel if you play the amp loud enough that the power supply sag’s as notes are hit.

[markusw]

That’s the sound of a weak preamp tube(s) building up to the level of gain there capable of when the B+ voltage is applied.

I even get the swoosh sound also with only the PI tube in and with brand new tubes in the PI position.

[martin manning]

Ok, after listening to your mp3 that is not the sound I was thinking of. I would describe what you hear as static noise that quiets down in a few seconds.

[xtian]

Eliminate the standby switch as the noise generator.

[markusw]

Eliminate the standby switch as the noise generator.

How can the standby switch generate this sound when the amp is cold but not once the amp was running for a while?
I tried to turn on standby before turning on the main power and I also get the ~5sec swoosh after something like 15 sec.

[xtian]

Pitted and oxidized stand by switches make INTERMITTENT noises. It's easy to test, just use an alligator clip to connect the switch's two terminals.

[markusw]

Pitted and oxidized stand by switches make INTERMITTENT noises. It's easy to test, just use an alligator clip to connect the switch's two terminals.

I checked the standby switch by bridging it with an alligator clip but it didn't help.
I then started to re-flow the solder joints in the power supply (B+1, B+2, B+3) but it didn't help either.
When I knocking on the main board with a wooden stick I noticed some microphonics.
Knocking at the resistors of the PI there was a clear noise.
After I re-flowed the joints of the PI resistors the swoosh was gone.
I hope it doesn't come back.
Any idea how a "not so great" solder joint might have produced the swoosh?

[xtian]

Any idea how a "not so great" solder joint might have produced the swoosh?

Good job. Chopsticking is an important talent.

When connections are BARELY connected (like broken solder joint) you can get all kinds of mysterious noises, like icebergs slowly breaking apart.

[markusw]

Good job. Chopsticking is an important talent.

Thanks for your support!

When connections are BARELY connected (like broken solder joint) you can get all kinds of mysterious noises, like icebergs slowing breaking apart.

[Stevem]

Yes, a poor connection will impede current flow which in turn will slow the voltage rise to that tube and make for the same tube reaction that I posted about earlier.

I you have a variac at your disposal you can simulate that same issue by bringing up the voltage to the amp slowly.

[markusw]

Yes, a poor connection will impede current flow which in turn will slow the voltage rise to that tube and make for the same tube reaction that I posted about earlier.

I you have a veriac at your disposal you can simulate that same issue by bringing up the voltage to the amp slowly.

Unfortunately, I don't have a Variac.
What I do not understand why the swoosh was gone when I kept the amp running for 15-30 min, turned it off and then back on....
In my amp the tubes are pointing upwards. Therefore, heat should not have an impact on the weak solder joint....