Odd sound when taking off standby...normal?

[greiswig]

I asked this on the tech forum, but got lots of blank stares.

I'm just wondering if this is normal. When I have had my D'Lite on standby (no high voltage at filters) for more than a second or two, then flip the switch to the "play" position, I hear an odd sound. It's pretty faint, but...

In the space of about 1 second, there's a whistling noise very high in frequency that drops in frequency quickly down to pretty low (under 1kHz), then rises back up and goes away or goes up so high that I can't hear it anymore.

I assume it has something to do with capacitors charging, because if I have a voltmeter hooked up to the bias taps, the sound is gone about the same time that the voltage plateaus.

Is this normal? Could it be a precursor to or symptom of a problem like oscillation? Part of why I'm wondering is that I still have an issue with one of the KT-66s in this am having red plated twice, both after the amp had been on for over an hour, but that issue seems to be so intermittent that I literally haven't been able to force it again. This sound happens no matter what the output tubes are, though.

Thanks as usual!

[ampdork]

What's the status of the filters? Did you put a snubber across the switch?
Symptoms sound like bad caps to me.
Did you test each filter with voltages Drained. Meter across each caps should emasure very high resistance many megs. Anything less and thereis your bad cap. This is not a definitive test. You could read a high resistance and still have a bad cap. However you could not have a good cap that measures low.
Do you have a scope or can borrow one? Looking at the nodes can often tell you if something is awry.
From the symptoms you describe it really sounds like a cap issue almost certainly.
If need be I can get you some caps at cost or if you suspect a certain one I may just have it here...
On a secondary note if there is ever a get together thing locally you know I am all for it.

[greiswig]

What's the status of the filters? Did you put a snubber across the switch?
Symptoms sound like bad caps to me.
Did you test each filter with voltages Drained. Meter across each caps should emasure very high resistance many megs. Anything less and thereis your bad cap. This is not a definitive test. You could read a high resistance and still have a bad cap. However you could not have a good cap that measures low.
Do you have a scope or can borrow one? Looking at the nodes can often tell you if something is awry.
From the symptoms you describe it really sounds like a cap issue almost certainly.
If need be I can get you some caps at cost or if you suspect a certain one I may just have it here...
On a secondary note if there is ever a get together thing locally you know I am all for it.

Hey, Shad. Are you saying that this sound is something that you only hear if you've got a bad cap, or are you extrapolating from my previous post? The red plating tube I've narrowed down to a bad tube.

At this point I've been laboring under the assumption that the filter caps are all okay. When I disonnected them from the circuit (dropping string in place except for the 150k FET simulator) and applied voltage to them through a 50k resistor, the voltages were consistent across all caps: about 260V from a 435V supply. So it seemed to me that either this was normal or *all* the caps were bad to an equal degree. I haven't measured them.

I'd ordered mouser caps that I thought would fit, but they won't go in without a fight.

[JimiB]

F&T's will fit. Are you sure your plate leads aren't reversed? Does the amp go nuts if you disconnect the NFB?

[greiswig]

F&T's will fit. Are you sure your plate leads aren't reversed? Does the amp go nuts if you disconnect the NFB?

Yes, F&T's are what are in there currently. Disconnecting NFB has no effect other than making the amp louder and brighter, as you'd expect. Why would you think the plate leads are reversed, and in what tube or section? (Thoroughly confused now...)

[JimiB]

I had an amp that I build once that I had the plate leads reversed and I made a sound like you describe when I threw the standby switch. Thought you might have your plate leads backwards. If you did, lifting the NFB would probably have made it occillate. Does it occillate if the presence is turned all the way up? Or if you crank the gain?

[greiswig]

Do you have a scope or can borrow one? Looking at the nodes can often tell you if something is awry.

I put a scope on each of the nodes. Measuring on the input side to the 500ohm resistor, I measure a sawtooth wave of 5V peak to peak. The other nodes all measure well under a volt. Is that about right?