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-   -   Wad 2a3 pse hum problem (http://www.world-designs.co.uk/forum/showthread.php?t=6595)

Paulrr 1st July 2012 06:52 PM

Wad 2a3 pse hum problem
 
I have had a WAD 2A3 PSE amplifier for a number of years, and had a problem with motor boating on one channe so I swapped out the capacitors between each stage and that disappeared only to have motor boating on both channels. Looked at the DC levels with a simple multimeter and swapped out all of the the main Power supply 100uF 450V capacitors, one of the new ones blew, fuse too, both replaced, but still have more hum than normal, have tried adjusting the 2A3 pots, still residual hum, I assume I still have a power supply problem, any ideas, still a capacitor I think, any advice greatly appreciated.

Fab amp otherwise.

Paul

Richard 2nd July 2012 08:46 AM

Re: Wad 2a3 pse hum problem
 
Hi Paul,

Motorboating and hum are 2 different things though they may both be caused by the power supply.

Has the motorboating (a steady phut phut at very low f, c. 1 or 2 Hz) now gone? Are you just left chasing hum?

Hum may be the power supply or may be heaters. Check all the heaters for correct voltages and preferably for AC level on the DC ones.

Paulrr 2nd July 2012 10:24 PM

Re: Wad 2a3 pse hum problem
 
The motorboating has gone after changing the decoupling capacitors in one channel.mthen the capacitors in the HT.

I will try to change the capacitors in the 6.3 heater supply to,the 6SN7GT valves as the hum is on both channels equally and the 2A3s each have their own heater supply.

Many Thanks

Paul

Richard 3rd July 2012 09:59 AM

Re: Wad 2a3 pse hum problem
 
Quote:

Originally Posted by Paulrr (Post 77676)
The motorboating has gone after changing the decoupling capacitors in one channel.mthen the capacitors in the HT.

I will try to change the capacitors in the 6.3 heater supply to,the 6SN7GT valves as the hum is on both channels equally and the 2A3s each have their own heater supply.

Many Thanks

Paul

Yes, I suggested checking heater voltages in case the 6SN7 regulator or its ground ref diodes has gone which might then show as high V. If your meter has AC mV you can check for ripple on there too.


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