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Heathkit AA-121 * PT hum * Coupling cap question

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Posted on January 13, 2015 at 21:22:26
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 Got my re-capped Heathkit AA-121 up and running and it does sound clean and clear. I did notice at idle that there is an audible power transformer hum within four feet of the amp. Is this normal for this model? I know how to damp the transformer mechanically. Is there a way to electrically reduce this hum/vibration without dulling the amps performance?

I also thought the sound that the re-capped amp puts out is kinda thin, lacking bass and mid-bass. Could that be the small yellow Mallory or Illinois brand film caps the previous owner installed or could it be the .047uf value of the coupling caps?

I would like to get this amp sounding a bit better. Any help would be appreciated.

Larry D.

 

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.047 µF is definitely small for a coupler, posted on January 13, 2015 at 22:29:55
JJ Triode
Audiophile

Posts: 716
Location: Northwest
Joined: December 2, 2004
 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 µF 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.

 

RE: .047 µF is definitely small for a coupler, posted on January 14, 2015 at 05:13:15
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 How much should it be bumped up if I were to try out a larger coupling cap?

Larry D.

 

RE: .047 µF is definitely small for a coupler, posted on January 14, 2015 at 06:35:33
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 I use .15uF or .22uF on power tube grids.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 08:17:10
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Larry
The power transformer hum may just need the end caps on the transformer tightened up...As far getting more refined and detailed bass,what value caps are you using in the power supply?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 08:28:27
Crazy Dave
Audiophile

Posts: 14371
Location: East Coast
Joined: October 4, 2001
 I remember, back in the day, that some guys wood isolate the transformer with this wood blocks between it and the chassis. I don't think it is that dangerous, but safety could be an issue. What do you think, Mike?

Dave

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 09:02:03
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 I'm using the stock values. I'm not at home or I'd look them up.
The output transformers are pretty beefy so I was expecting a bit more dynamics than what I'm hearing.

Larry

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 12:52:13
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 Since a .047 uf cap produces a -3db point of 17 Hz you can change to a .1 and the -3db point becomes about 1/2 of 17 or 8.5 Hz. If that was the source of the bass issue the 8.5 Hz point will help a lot.

.15 uf will give you 6.8 Hz or so which is a good -3db point as well.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 16:36:10
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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. It shouldn't be reaching up into the mid-bass frequencies as I am hearing. 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.

Since I can use .1uf then I may eventually use a quad of copper foil caps in it. That should warm it up quite a bit. I am quite impressed with the clarity and power the AA-121 puts out. Could be a keeper.

Larry D.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 17:36:50
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 "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.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 18:45:25
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 19:47:59
DaveV
Audiophile

Posts: 513
Location: SC
Joined: December 26, 2009
 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.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 21:51:53
mark.korda@myfairpoint.net
Audiophile

Posts: 1253
Location: Maine
Joined: August 16, 2011
 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

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 22:08:42
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 14, 2015 at 22:20:21
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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.

 

RE: Heathkit AA-121 * PT hum * Coupling cap question, posted on January 17, 2015 at 08:54:15
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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.

 

Changed couplers.........better now., posted on January 17, 2015 at 09:01:54
lektrik
Audiophile

Posts: 447
Joined: May 31, 2002
 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.

 

RE: .047 µF is definitely small for a coupler, posted on January 23, 2015 at 09:54:57
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 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.

 

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