[HAM] [HL] 147 "helicopter" noise at low volume *Follow Up*

[Hugh]

I can't believe what the problem was. Some former owner had removed 
the 100pF at 1000V caps from the 6550s. The 56K resistors are still 
there between pins 3 and 6 (no connection), but the caps are gone. 
I've temporarily replaced them with series pairs of ceramic 
220pF at 500V (to get 110pF at 1000V since it's all the local store had) 
and the amp is fixed.  This amp was driving a Tannoy Silver dual 
concentric in a former application.

A couple of questions:

Why would anybody do this?

I assume originally ceramic caps weren't used here. What is the 
original type of these caps?

Thanks,
Hugh

At 04:32 PM 7/11/2007, you wrote:
>I'm working on a 147 with unknown prior history that works 
>reasonably well but certain frequency pairs, especially low 
>frequencies like say G2 and B2 together from an analog synth creates 
>a "helicopter" noise about as loud as the tones. I've swapped out 
>the amp with a 125 amp and the problem disappears. I've also swapped 
>out the 12AU7 which is the only spare tube I have.
>
>Any suggestions on what to check first?  I've already recapped all 
>the electrolytics and measured the resistors for obvious 
>discrepancies and found none. I'm guessing it's an untamed 
>oscillation somewhere, but where?
>
>Thanks,
>Hugh
>
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