Strange 300b Push Pull Noise

[david]

Hi,
My WAD 300b push pull clone has developed a strange noise. I would describe it as a fairly regular click, at 1 second intervals. And no it doesn't chime once an hour

It reminds me of the static noise some of us got before we earthed the metal covers of mains txs on Kat 6550s and 34s. But it's not that as earthing the mains tx or metal caps or output txs makes no difference. It occurs on both channels, independent of volume of course as it's a power amp. I might have disturbed something when I improved my speaker wiring last week but I thought it was OK after that.

It will be a few days before I can lift the bonnet, so thought I'd get some other fine, analytical minds working on the problem in case anyone has any bright ideas!
Thanks

[Richard]

Hi David, is it through the speakers or mechanical? If through the speakers it sounds like motorboating, slow phut phut sound. Is your circuit standard spec and are you using feedback?

[bearded fiddler]

Hi David,

I had the same problem when my amp was settling in. It was a sparking noise, but without actual sparks being seen. It disappeared around the time I removed battery bias, but that could have been a coincidence of course.

Jan

[John Caswell]

Hi David,
Following on from Richard's comment, if you are not using feed back is the L/S secondary of the optx earthed? Apart from that it does sound like static discharge.

John

[david]

Hi,
It comes out through the speakers. The circuit is absolutely standard, apart from increased capacitors on the Schottky high voltage diodes, so the speaker secondary negative is earthed.

I'm sure it's not motorboating. It really does sound too regular /more like static. One interesting thing is that it stops instantly when you turn the power switch off, it doesn't fade away.

Up until yesterday evening the amp was very silent over 20-30 hours of use, with very low hum /hiss, and it was not picking up any extraneous electrical noises either. The sound as Jan says is like sparking, and very similar to the self curing noise of polyprops, but pretty regular.
David

[david]

Ok problem solved! It's the noise you get when you don't solder one end of a high voltage Schottky into the psu! I missed it in bad light and it's a little tight in there. So back to audio bliss!

[John Caswell]

Hi David,
That's very interesting. I am surprised that you didn't get hum or lower voltages etc.

John

[david]

Hi John,
I guess there was a good enough mechanical join, and the other leg was soldered so the component was held static.

But there's still something not quite right. Now I get a few of the clicks staright after turn on, with a pop from one channel, and a brief period of 50hz buzz on one channel before everything settles as the heaters come on and the voltages come up.

I wonder if it's the 5687 playing up but don't have a spare to check at the moment.
David
PS good luck with the hernia op. They are pretty straightforward these days. I had one a few years ago and the surgeon said I could play tennis staright after without harm. Of course, I didn't want to, but ten days later I was playing again with only minimal discomfort which went very quickly as he had said. It was also supposed to be keyhole surgery, so I was disappointed to find out that the scar was the size of a key to the Tower of London. But, exactly as the surgeon said, thescar disappeared after 6 months and now you really have to look for it!

[david]

Well the amp is settling down and sounding better and better all the time! I still get that slight sparking noise on switch on which disappears as the heaters /voltages come on.

As Jan got it too, and he seems to be using the same ASC oil filled psu caps and Mundorf Surpremes, I wonder if it could be related to either of these settling in?

All voltages and performance remain stable and the amp remains silent after these few switch on sparking sounds. It's not the valves, as I've subbed both input and driver valves with no difference and swapped around the 300bs too.

I did discover tho that a 5687wa sounds sweeter and more detailed than the 5687wb previously in there.
David

[david]

Hi All,
My mains was up to 253 volts at the weekend-a bit of a joke for 230v nominal! That pushed the ht on my 300b push pull and I did see a small red spot on one 300b, so I've taken out the 27k parallel resistors to reduce dissipation a little.

My HT is 595v, so I've ordered some resistors to drop the HT down a bit. I will fit these before the GZ37, so it will be running balanced and with the lower HT too.
David