Rectifier Hum

I have a Dennis Had Inspire Fire Bottle SEP HO amp that supposedly will work with just about any rectifier. I was using a new Gold Lion GZ34/U77 for a while and recently noticed a low hum from my subs. Took everything apart, checked cables, etc., and finally got to the tubes and discovered that swapping out the GZ34 with a Shuguang 274B fixed the issue. Assuming the GL had gone bad, I bought another GZ34, a JJ this time, stuck it in and there's the hum again. I also tried the originally supplied JJ 5Y3S and it hums also, but less than the Gold Lion or the new GZ34…why? I don't want to send the amp back to Dennis as it does work fine with the 274b, but it's on my mind so I seek counsel. Help me…help me…Atmasphere? 


Why not E-mail Dennis Had and get his opinion. He is a talented designer who is also approachable. I hope you will share his opinion with the Audiogon community.

David Pritchard

I did and haven't heard anything yet.
If I recall correctly, the tube rectifier in your amp is bypassed with a solid state device as well. This is making me wonder if its snubbed properly. If not, it could certainly contribute to hum. Tube rectifiers on their own don't need to be snubbed.

The term 'snub' refers to a network, usually a resistor and capacitor, that prevents the capacitance of the rectifier from interacting with the inductance of the power transformer in such a way as to form a tuned circuit with a resonant peak. If that peak occurs, the rectifier will make noise. Its a bit more complicated than that but that's it in a nutshell.
I'd be surprised if it has a solid state device anywhere, but that's interesting!…and the issue is a new one for this amp so it remains a mystery...I did send off for another 274b to have as a backup. 
The most mystifying thing is the fact that the 274b tube is dead quiet (no low hum) relative to the other tubes mentioned. But I soldier on…I'll report whatever Had says when and if he responds.

I think this is more than a rectifier tube issue.  The filter capacitor may be almost gone.  The 274b tube does better probably because the impedance is different than the others and the capacitor does not need to work that hard for that tube.

^^ I'm doubting the filter cap failure theory, but you might be on to something; on reading the specs on the 274b one of the claims is lower hum, provided the power supply is built in a certain way, which is to say a fairly low filter cap value for the first filter cap.

If that's true it might explain this.

By the way, this is a new issue as previously with any of the rectifiers mentioned the amp was utterly quiet with zero pesky extra low bass noise. So something wearing out makes sense, and note the 274b remains utterly silent…the plot thickens!
Can you be sure this just started happening or you just noticed it?
When the amp showed up with the JJ 5Y3S (10 months or so ago) I thought it was surprisingly quiet, and I had tested the subsonic noise by simply touching one of my sub's driver cones…then more recently I noticed the audible low noise and sure enough, the sub drivers had a vibe goin' on. That may have been coming on gradually…not sure to what degree as I hadn't notice it recently until it became audible. 
@wolf_garcia most probably unrelated to the different voltage drop of the 3 rectifiers as the 5Y3 should have the largest, but just have to ask...

You recently bought a pair of Klipsch Heresy III, with (I think, but could well be mistaken) significantly higher sensitivity than what they replaced. Sometimes, we don’t immediately notice such an impact, but any potential correlation?
The hum is very low, inaudible mostly but I can feel it when I touch the sub driver…with its permission of course.
I forgot to follow up with the solution...the two subs had opposite grounding (one was lifted one not). Lifted the ground on the other sub and hum is now gone.