Allen Amps power tranny hum

[HiGain]

Hi.


A couple of years ago I purchased a Allen Amps deluxe power transformer. Since installation I have not been Able to get rid of hum, even with all controls at zero (normal operating voltages with 6V6s).

The transformer itself produces a fairly loud hum (coming form the unit itself even in standby). Can anyone recommend anything to reduce this, such as not using the heater center tap and using a 250hm hum balance pot?

Thanks in advance.

Jake

[Phil_S]

Is this a mechanical hum from the PT? That's what you seem to be describing. If you remove the PT from the chassis and apply power to the primary, if it's the same hum, that's mechanical and not circuit related.

You probably can't cure the mechanical hum but you can minimize the impact on the amplifier circuit by insulating it from the chassis. Try using some sort of rubber stand off -- maybe an o-ring or something like that, between the PT and the chassis. If you think the lams are loose, you might see if tightening the lam retaining bolts helps.

[Firestorm]

Phil is likely right, but before you go removing the PT, etc., see what happens by removing tubes: if you pull the PI and it still hums, it's pure power amp (and may be mechanical), but if not it may be something else. There are a lot of amps with hummy PTs (actually, ALL of them -- the degree varies) that don't hum audibly. It could also be a bad PT, but Allen's vendors are all top-flight, so less likely.

[HiGain]

Thanks, guys. I will try these things and report back.

[HiGain]

Well, I pulled the power tubes and could still hear the hum from the xformer. It's fairly loud, and the noise is picked up and (I think) amplified by the power tubes when they are back in.

The xformer also vibrates very noticeably, so much you can feel the vibration when holding the far end of the chassis.

Other than this issue the amp plays and sounds great.

Btw, The heater leads measures at 3.25 to ground from both sides.

I wil still pull all the preamp tubes to see what that does.

Any other thoughts?

[Structo]

Jake, what amp is this PT in?

Does it have SS rectifiers?

Do you happen to have ceramic snubber caps on the diodes?

This is just part of my research into what Aleksander said about snubber caps in power supplies.

[pula58]

Jake, what amp is this PT in?

Does it have SS rectifiers?

Do you happen to have ceramic snubber caps on the diodes?

This is just part of my research into what Aleksander said about snubber caps in power supplies.

I myself have, just this week, learned about snubber caps across the rectifier diodes! Buzzzzzzzzzzzzz.

[HiGain]

Hi Tom. It's in a 77 duluxe (now in head form) with a gz34 rectifier.

I usually can't mess around during the week so I will poke around more over the weekend, but I'm starting to think its just a noisey transformer.

Jake

[guitarmike2107]

Sometimes Pt's just hum, it not great and I usually end up replacing them, Marshall replaced one for this reason in my DSL under warranty years back.

if you can live with it move on, if not then try the isolation methods above or see if there is a tranny guy locally that can stick it in the vaccum bath to try and get the thing settled down again, that probbaly wont work though but might be worth a shot.

[jazjamr]

Have you called Dave Allen? He is a great guy,and should be able to help.

[Firestorm]

Have you called Dave Allen? He is a great guy,and should be able to help.

I second that. Maybe you got a bad one (happens in the best of families). Dave is a great guy (although he won't give me the name of his cabinet builder ).