Hum and volume drop on 300B PSE - World-Designs-Forum

Guy Pettigrew · #1 16th March 2012, 10:24 PM

One of my WAD 300B PSEs is suffering from a bit of hum and is slightly lower volume than the other one.

I've swapped the valves between the two amps, and the problem's still there, so it isn't the valves.

Twiddling the hum bucker seems to make no difference whatsoever. Perhaps the problem is in some part of the humbucker circuit?

The amps run with no feedback.

All suggestions gratefully receved.

Oh, it also makes a horrid "crack" on turn on, then crackles away for a minute or two before going relatively quiet--apart from the hum!

Thanks.

Guy

Richard · #2 17th March 2012, 02:33 AM

Hi Guys,
Did you swap the rects too?
Check voltages around the circuit.
Check cathode bypass caps on 6au6 and 300B see if any are split, or remove and use a meter, if one is starting to fail it may be noisy and would cause a loss of gain.
Do a visual on all the other components.

John Caswell · #3 17th March 2012, 03:15 PM

Hi Guy,
Adding to Richard's comments, look at the anode and screen resistors of the 6AU6 and the cathode resistors of the ECC82 as they were close to max ratings. Also hve a look at the main smoothing and negative bias capacitors.

John

Guy Pettigrew · #4 17th March 2012, 04:41 PM

Thanks for the advice. Much appreciated.

All the valves have been swapped, Richard. This is OK, though, as it means the problem is inside the amp and fixing it should be no real problem.

Tomorrow's looking good for a poke around inside the amp.

The cathode bypass caps on the 300Bs were changed for some NOS Black Gates a couple of months ago, so they should be OK. But who knows?!

This problem has been with me for a very long time, I've just lived with it by turning the volume control on one channel up by a notch.

Having just radically rebuilt the 'speaker crossovers, though, it's more intrusive. The "new" 'speakers are a bit more revealing.

A report back will follow in due course.

Guy

Guy Pettigrew · #5 31st March 2012, 02:09 PM

Hi Guys

Oh dear!

The problem with the horrid cracking and background hum in one of my 300bs has been identified. I don't know how I missed it first time round, but it's an ECC82.

Ah well, easy to sort out, at least.

Thanks again for the advice---sorry I sent you off on the wrong track.

Guy