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Old 16th March 2004, 09:27 AM   #1
peterr is offline peterr  Netherlands
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I have recently finished this 6072 line stage. So far I like it a lot, it sounds much better than the “HarryHaller passive pre” I used until now (and even that was a big improvement on my previous SS amp).

But alas as usual there is a problem:
In one channel I hear a very high-pitched tone (my guess is above 15kHz). It isn’t always there but very irritating when it is (the other channel is alright).
It seems independent of volume or whether I have music on or not.
Does anybody have a clue?
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Old 16th March 2004, 09:30 AM   #2
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Have you swapped tubes?
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Old 16th March 2004, 09:34 AM   #3
peterr is offline peterr  Netherlands
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Not yet. They are somewhat difficult to get to so I tried to avoid this so far. But of course if neccessary I will.
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Old 17th March 2004, 08:13 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Try tapping the glass of the tubes lightly with a pencil, with the unit on (but no music playing.) You may hear a thunk sound but you should not hear a ringing sound.

Could be you have a microphonic tube.
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Old 17th March 2004, 10:37 AM   #5
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Hi,

Quote:
They are somewhat difficult to get to so I tried to avoid this so far. But of course if neccessary I will.
Peter,

I think what the other Frank means is swapping the tubes around from one channel to the other to see if the problem follows the tubes.

As I suspect a sticky filament it's quite likely the problem will track channels.
Shifty's pencil test can be used to "unstick" the filament by gently tapping the glass envelope with a pencil.
Just pull the tube from the preamp and hold it in one hand while tapping it with the other.

If that doesn't help, try running it on a 6.3 VAC heater supply for a couple of hours.

Cheers,
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Old 17th March 2004, 10:45 AM   #6
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Frank,

"I think what the other Frank means is swapping the tubes around from one channel to the other to see if the problem follows the tubes."

You are correct, Sir.
Sorry I wasn't clearer in my post.
I tend to write in "shorthand".
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Old 15th November 2004, 09:49 AM   #7
peterr is offline peterr  Netherlands
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It has been a while since I started this thread but I haven't really solved the problem yet.

I have swapped the tubes around and at first it seemed the problem moved with the tube but later I concluded that both tubes have the problem.
Then I:
Quote:
Try tapping the glass of the tubes lightly with a pencil, with the unit on (but no music playing.) You may hear a thunk sound but you should not hear a ringing sound.
The tubes seem to be VERY sensitive. If you tap them you can very easily hear it (and not just the thunk). Even when you tap the wires attached to the valve base you can hear that through the speaker.

Then I tried a new valve but is behaved exactly the same.
So far I have been using GE's. Recently I got a pair of RCA's and they behave very differently. They are far less sensitive than the GE's but they don't seem to sound as good either.

I this an experience others have had? It seems unlikely that I bought three faulty GE's. Are GE 6072's known to be very microphonic?
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Old 15th November 2004, 10:10 AM   #8
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I had a microphony problem a while back (Circlotron nearly falls of his chair when he realises it was 1974 ) and what I did was to wrap the tube in foam rubber and put a few rubber bands around it. What would probably be even better, if you can stand the poor ascetics, is to dip the tube (pins facing upward) in some sort of thick gluggy stuff like bitumen or epoxy or car underbody deadener. Anything that will damp the ringing of the glass envelope. It won't help any ringing of the internal metal structure but it could make a worthwhile improvement. Try it on an old tube frst.
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Old 15th November 2004, 12:56 PM   #9
SY is offline SY  United States
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Do check with an oscilloscope to see if you've got some oscillation going on. Microphonics may be an issue, but it may not be the issue causing the problem you're hearing.

The best solution is to get old like me so that a 15kHz tone doesn't bother you.

OT: Peterr, you would have enjoyed our dinner last night. We made four courses, all featuring porcini.
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Old 15th November 2004, 01:03 PM   #10
tiroth is offline tiroth  United States
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Quote:
Originally posted by SY
OT: Peterr, you would have enjoyed our dinner last night. We made four courses, all featuring porcini.
Yum.
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