• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Kofi Annan in: "Oscillation Blues"

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So, I've been experimenting with the concept of ignoring a problem and having it go away. So far, no soap.

About a month ago, I noticed that the right channel on my 300b DRD monoblocks was kinda noisy. The chassis was buzzing, but that buzzing wasn't making it through to the speakers.

[REFRAIN] So I just... kinda... let it go...

Fast forward a few weeks and it's just kinda getting Kofi down overall, so I did some topside experimentation. The buzzing sounded like it was coming from the rear of the amplifier near the PSU section and I thought it might have been the rectifier tube, so I swapped rectifiers between the monoblocks (benefit of monoblock design) and there was no change.

[SECOND REFRAIN] So I just.. kinda... let it go... again...

Well, then a couple of weeks ago, I noticed during a personal journey through the Miles Davis Live at the Blackhawk record that a very high-pitched and steady squeeeeeeeeeeeeeeeeeee was coming from the right speaker. Shutting off the record but keeping the monoblock on, the squeeeeeeeeeeeeeeeeeee was still there. I shut it off and back on in a few minutes and everything was again kosher, like it had never happened.


Over the last couple of weeks, it's happened a few more times and it seems to be contingent on the record played. High-pitched trumpeting seems to set it off, but this could be total coincidence.

So, Kofi will be looking over the guts today and he'd like to know what to look for here. It may very well be a chipmunk using an electric razor nestled next to the filter caps, which I'd probably spot right away. Otherwise, pointers would be much appreciated.

My chassis is rattlin'
[Ba NAOW Now now..]
And it's troubling me
[Ba NAOW Now now..]
Miles Davis' trumpet makes my Fostex speakers squeeeeeeeeeeeeee
'Cuz I got dem blues...

I got 'dem Oscillation Blues

Where it comes from I don't know
Cuz I just kinda... let... it... go...

[BRANG Braaaaaaaaanngg!!!]

Thanks for coming out tonight! Schematic attached!

Blind-Melon Kofi


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My first guess would be the obvious -- try a larger grid stopper on the d3a -- like 470R or so -- and make sure it is right on the pins with the body of the resistor touching the pin, and attached at the point closest to the tube pin. If that doesn't help, perhaps try tying G3 to K instead of P without a resistor. Finally, be sure there are no long leads from the D3a -- such as from the cathode.

Edit: maybe that 2.2uF cap bypassing the heaters to ground could be 100uF? That's what I always use, and I've never had trouble with a D3a, though I don't know why that's what I always use ...
Hi Kofi,

How's the UN pension? ;)

IMHO, it sounds like 2 separate problems.

Is the buzzing/rattling at 60/120Hz?

Are the power transformers fitted to the monoblocks, or do you have separate power supplies?

I suspect that there is an AC joint or transformer vibratng somewhere.

The oscillation from the speakers doesn't sound as though it is related.

I could well be wrong.

Kofi Annan said:

Is it possible that the oscillation from the D3a would be so severe that it would cause the chassis to vibrate? I'm not challenging, I'm really just curious to know how serious oscillation can get.


Hard to imagine. But, it could be the other direction. Triode strapped pentodes, especially these high Gm tubes, can ring at high frequency due to vibration in the various little grid wires. So, maybe some other vibration either from the speakers or the power transformer is setting them off and you have a mechanical oscillation in the tube.
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