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In Reply to: RE: Heathkit AA-121 * PT hum * Coupling cap question posted by lektrik on January 14, 2015 at 16:36:10
"I think part of the problem might be with the tiny yellow IC film caps since the roll-off with a .047uf cap is where you say it is."
The IC MPW caps are good caps, the bass cutoff is an issue of cap value. So I don't think that's the issue. I use the IC MPW polypropylenes in some locations where I'm replacing mylar/polyester caps to retain some "snap" in the voicing.
"It shouldn't be reaching up into the mid-bass frequencies as I am hearing."
Keep in mind the low frequency rolloff begins at a higher frequency than the cutoff. It can be affecting the bass to some extent an octave or two above the -3db point.
"I never use the yellow caps, but I left them in to find out if the amp worked. They might take longer to burn-in and mellow out than I'm used to with the K-40Y PIOs."
What you are hearing is not just the caps, it's a fairly high frequency bass rolloff. Don't judge those caps by what you hear. They'd sound much better if they were .1 or .15 uf.
Follow Ups:
Thanks for clearing that up for me Jim. I'm letting the amp play for a while tonight to put some more time on the caps to hear if there's any difference. I will probably change them out this weekend and put in 0.1uf caps. I need to change out the old top hat diodes too. Again, thanks for the insightful explanation.
Larry D.
If the top hats are still in then you should also look at the bias supply voltage and what current the outputs (cathodes)are running at.
I'm thinking that thin,clear but lacking in bass could also be a symptom of running the outputs too cool.
I made that mistake with my Scott 299A and when I got the outputs to run hotter but still within the safe zone for the 7189A outputs, the bass appeared and the thin sound became full.
Well I put in four UF5804s (3A 1000V, I believe) and there is a bit more bass now. I also found some badly soldered connections in the power supply and fixed those too. Still a bit bright on the top end, but it may tame down after a few weeks of play.
Larry D.
Wanted to make sure the transformers were good before I changed out the diodes. Probably get to replace the top hats this Friday. Hadn't looked at the diode specs yet, but I'm sure I have something that will work in there. Should have a matched quad of EL34s to install by then too.
Larry D.
Hi Letrik, I have a Dyna ST-35 amp all done up right that was putting out a hum when I got about 3 or 4 feet from it. I could not hear it at my normal listening position,the Archie Bunker chair. Mike Samra had a good idea about tightening the bells on the transformers, and the mounting screws them selves. Anyway, I tried putting a 5 pound cast iron dumbell plate,York,or whatever on top of the Dyna amps cage and the hum went away. Why,I'm not sure. Could it be some sort of inductance like an iron core choke that the 5 pound plate creates? Anyway, there used to be something for tube amps called the magic brick that had to do with what your searching for....I think it was the same idea as I am using.....Mark Korda
Tried tightening the bells, but they're already tight. The slight buzz is not that bad, but I do like a dead quiet amp if possible. I have one of those dumbbell plates and I'll try it. If it works I'll post you back, if not, I can live with it.
I had a buzzing power transformer problem once before, so I had a local machine shop cut me a 12" X 15" piece of 1/4" plate steel to mount on the underside of my power supply chassis and that was the end of the noise from that transformer once I got everything re-mounted.
Thanks for the info!
Larry D.
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