Loud Hum

[birdseye]

Hi All I have just finished a stereo power amp based on two pairs of 6L6GC push pulls. I have biased to 35ma. When i switch it on there is a loud hum from the speakers gets louder as it warms up.I strummed a guitar no audio signal is present. When I turn off the standby it sounds excellent very clean until the caps drain. I wondered if any of you experts have any ideas. I have a mains switch & the standby has a resistor across it which drops the mains 230v to half of the secondary until the stand by is switched on.( both switches on mains side)The PT is from a Matamp 120 & gives 510v with no tubes.   426v at the power tubes with tubes in. the OTs are two Matamp 120s. I am off on holiday soon so can't get to stuck in to find the fault. Look forward to your thought. Regards Dave

[shooter]

I strummed a guitar no audio signal is present

That's a good indicator that something is probably wired wrong.  schematic and or pics will help.  If both L&R are doing it, find the "common" points. PS, input jacks, system grounds.

[jjasilli]

Use documented troubleshooting procedures, e.g.:


http://geofex.com/ > tube amp debug page > hum


Ampeg troubleshootin flowchart in the Archives Section of this Forum.

[birdseye]

Hi Shooter & Jjasilli thanks for your replies. I know I have to go through the fault finding stage. I just thought the fact that it works on powering down sounds real good might be a clue to those who know. I live in hope. Regards Dave

[jjasilli]

Ok,a quick diagnosis is sometimes possible.  You should state the frequency of the hum:  60 Hz (suspect filament supply); 120 Hz (suspect B+ rail).  In your location that may be 50 / 100 Hz but that's the idea. That's no guarantee, but sometimes leads to a quick solution. 


Bias circuit issue?


Also I don't understand the standby circuit.  Could you be leaking mains hum to the chassis?  If so your chassis might be live & dangerous.  Maybe temporarily remove the standby circuit.

[birdseye]

Hi Jjasilli The switch set up was from Merlin's book a mains switch followed by a  switch with a parallel resistor across it when switched on the resistor is bypassed. It was to reduce in rush current. Regards Dave

[jjasilli]

I have a mains switch & the standby has a resistor across it which drops the mains 230v to half of the secondary until the stand by is switched on.( both switches on mains side)The PT

??? Merlin's online info does not show a standby on the mains side:  http://www.valvewizard.co.uk/standby.html

[shooter]

go through the fault finding stage

Yup, the fun stuff

easy check of B+ hummm, make SURE your meter can handle the B+, but set to VAC, measure each rail taps(red at tap, black at ground).  Look for <1VAC at 1st tap, decreasing each tap.

[PRR]

> something is probably wired wrong. 

Agree.

> i switch it on there is a loud hum
> When I turn off ...it sounds excellent

Suspect that your rectifier return is going to your input jack before the First Filter Cap.

> I am off on holiday soon so can't get to stuck

Is the amp getting to go on holiday? If not, it is sure to misbehave until you get back, and longer until it gets over its sulk.

[birdseye]

Hi PRR, Jjasilli, Shooter. Thanks for all of your replies. Will be off for a couple of weeks. No the amp weighs a ton. It stays at home. I will reflect on it while I'm away. Perhaps the amp will pine and pull it's self together. It did sound real pretty when switched off. Thanks again. I will post the fix when I find it. Regards Dave

[jjasilli]

Yes, but we may have gone in a circle.  You now have enough replies that if you were try them all, it may have been more efficient to use a troubleshooting flowchart at the outset! 


BTW. I'm thinking that when you say the standby is on the "mains" side, you probably mean the standby is on the secondary's side of the rectifier???  A "standby" that was actually on the mains (primary) side would turn-off the filaments, thereby defeating its purpose.