Buzz at output until tubes conduct

[Hearinspace]

I've been running a 417a>IT>VT25 amp on a scrap of plywood for about a year now and it has been just about perfect. So feeling it's worthy of more listening I decided to clean it up, bringing the IT, filament supply reg on heatsink, and current source boards onto the plywood from where they had been (sitting off board on the carpet).
This also allowed me to shorten the lead lengths to them a little.
The strange part is that the amp now buzzes full bore into the outputs for the five or so seconds it takes the tubes to conduct , then it goes quiet. I tried a strip of shielding around all the parts that came on board to no effect. I'm sure somebody here knows what this is without having to think twice about it. Can you help me out?
Thanks!

[Matt.B.H.]

You moved it.

Really though it could be anything. has a lead moved.........anything. Thats the trouble with the good old bread board. It is a prototype.

Cheers Matt.

[tubelab.com]

I'm sure somebody here knows what this is without having to think twice about it.

I had an amp do this before. In my case it was magnetic coupling from the power transformer into the output transformers. I had them all lined up in a row. Rotating the OPT 90 degrees fixed the problem. An SE amp is the worst case. The current through the OPT reduces its ability to pick up stray fields.

[tubelab.com]

The clue here is that the buzz apears before the output tubes are conducting and goes away when the amp goes live. Try removing the output tubes and see if the amp still buzzes. If so you can rule out most of the complicated stuff. In my case the buzz remained when the output transformer primaries were totally disconnected. The buzz went down, but did not go completely away when all the tubes were removed because the power transformer produces less of a field when it has no load.

[Hearinspace]

Thanks for getting me back on the trail.
No Buzz with the VT25 pulled. The output circuit is parafeed so OT current is not the issue I think, but hunting around I was reminded that the parafeed cap is on the ground side of the primary and returned to the VT25 cathode and wonder if that is helping start-up ripple on the fil supply (LC>lt108_ ) get onto the OT primary.

[Hearinspace]

Got it!
After moving things around and trying various different iron orientations resulted in no change, I started looking at the serving tray with ~2" raised sides for handles that I had put the breadboard on to make it easier move around the bench.
It was made of . . . . .uhhhhhhh. . . . .sheet steel. Clip leading it to circuit ground stopped the buzz entirely.
Thanks tubelab and sorry for the false alarm. It amazes me a little that I had taken so long to notice it because it was "a tray" and not "a chassis".

[richwalters]

Originally Posted by Hearinspace

sorry for the false alarm. It amazes me a little that I had taken so long to notice it because it was "a tray" and not "a chassis".

Not always. Those of us using HP p-p stuff with high VA transformers in close physical proximity with the output trannies will notice hum on switch on. Reason, a short period delay arises from not all tubes conducting at the same time, so at this moment the global nfb loop is not effective in reducing the output impedance. Give it time.

richy

[DF96]

Hum can also be caused by phase splitter or output stage being temporarily unbalanced while warming up. Some circuits rely on balance to reduce hum in normal operation.