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

[David Anderson]

Hugh,

Yes, that would make the amplifier unstable, leading to oscillation.

Ceramic capacitors were used there originally; they are common in  
this value range and reliable.

For replacement, choose a C0G ceramic (that's a 'zero' in the  
middle). NP0 and Class 1 are other terms for this dielectric type.  
Don't get the X7R or Z5U types.

Here's an example:

http://www.mouser.com/search/ProductDetail.aspx? 
R=C317C101KDG5TAvirtualkey64600000virtualkey80-C317C101KDG

If you want to go all-out, buy a few and match a pair for your amp.

David



On Jul 14, 2007, at 12:48 AM, Hugh wrote:

> 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