Vintage Asylum

.047 �F is definitely small for a coupler

50.106.5.182

JJ Triode

A

In Reply to: RE: Heathkit AA-121 * PT hum * Coupling cap question posted by lektrik on January 13, 2015 at 21:22:26

The coupling cap and the ground reference("grid leak") resistor on each output tube form a high-pass filter, with �3 dB frequency 1/(2πRC). For example with .047 �F and a 220K grid leak you would get �3 dB at about 17 Hz and 6 dB/octave roll off below that. There would be phase shift well above 20 Hz also. I'm not saying this is the only reason the amp sounds thin, but larger coupling cap values could be part of the solution.

Follow Ups:

RE: .047 �F is definitely small for a coupler - Sherwood Forest 09:54:57 01/23/15 (0)
Yeah, I've had to do a few tweaks to transformers and coils to make them quiet. One choke I had was really noisy, even after isolating it from the chassis with rubber grommets. The laminations were very tight and I was stumped for a while. It turned out that the coil bobbin was loose from the core, and all I had to do was shove a wooden wedge shim in between the core and coil, and it quieted right down. Usually these issues are purely mechanical, though there are some power supply conditions that could exacerbate it for sure.
RE: .047 �F is definitely small for a coupler - lektrik 05:13:15 01/14/15 (2)
How much should it be bumped up if I were to try out a larger coupling cap?

Larry D.
- RE: .047 �F is definitely small for a coupler - sony6060 06:35:33 01/14/15 (1)
  I use .15uF or .22uF on power tube grids.
  - Changed couplers.........better now. - lektrik 09:01:54 01/17/15 (0)
    I had some Cardas Golden Ratio 0.1uf 600V caps, so I put them in place of the 0.047uf coupling caps. That, along with changing out the top hat diodes with some UF5804s has given me some noticeable bass now. Its still a bit too bright on the top end, but I may just have to run it for a few weeks to see if it tames down.
    
    Larry D.

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