Problem with P1800 kit built amp

AustrianJohn

I built an Amp Maker P1800 amp a couple of years ago. It worked fine back then, but hasn't been used for a while. I fired it up recently and it was very noisy. I've checked all of the solder joints (some of which needed re-doing which improved the sound quality significantly), and all of the measurements are within bounds of the Amp Maker manual.

The outstanding problem is a sound I'd describe as an intermittent whoosh or rustle. It is louder when the power is turned up, but is not affected by the volume control. It is worst when PP is selected, but is still there (but quieter) with PP+ or SE. Playing a guitar through the amp is much louder than this sound, but it still affects the guitar sound.

When I turn the amp off (power and volume at mid-point) it makes a sound like a wave breaking on the shore. 

Any ideas what the problem might be?

robinbowes

Not affected by volume control -> some part of the circuit after the volume control

Quieter with PP+ + SE -> makes sense - SE is inherently quieter (just one valve), PP+ adds a filter across the phase inverter output.

I'd check your Phase inverter, components R10-R15, C10-C12

R.

AustrianJohn

robinbowes said:

Not affected by volume control -> some part of the circuit after the volume control

Quieter with PP+ + SE -> makes sense - SE is inherently quieter (just one valve), PP+ adds a filter across the phase inverter output.

I'd check your Phase inverter, components R10-R15, C10-C12

R.

Thanks - I'll look at that tomorrow.

Modulus_Amps

microphonic valve or filter caps may need replaced would be my guess

AustrianJohn

Thanks to @robinbowes and @Modulus_Amps ;

I re-soldered all the joints in the phase inverter late yesterday evening, too late to do a proper test.

Went to  check the valves for microphonic activity this morning to find the whooshing noise has completely gone and the amp is now perfectly behaved. 

Modulus_Amps

great stuff, enjoy.

If you are mothballing guitar amps it is always a good idea to turn them on every few months, give the pots a good couple of turns etc