View Full Version : Heathkit A-7 Help

10-29-2011, 11:33 PM
I picked up one of these cheapos at the dayton hamvention last year, just now finally starting to get around to messing with it.

Going to make it as my PC headphone amplifier, of course its mono but i dont care.

Well, apart that it has quite a bit of 60hz hum, it looks like its been recapped.

The power transformer gets really hot after running for 20 minutes or so. sounds fine, works fine, but its almost too hot to the touch. The transformer also isnt a square one. its a regular transformer that says Grand Transformers Inc.

maybe a replacement... It also has 12V6 tubes inside.

any ideas? thanks.

10-30-2011, 10:31 AM
I have a matched set of this but have not done anything with them. I have the last model they sold which has the phone input AND a line input. Should be good for 8wpc or so. Cute little OPTs on them. Looking forward to reading what people say about them.

10-30-2011, 11:08 AM
well i do wish i had another one to make my headphones at least stereo, BUT mono is fine. But before i decide to pick up another one, i need to fix this one as the powertransformer seems to overheat and theres 60hz hum.

10-30-2011, 03:19 PM
Finding the 60hz hum should be easy. That is the hz of your incoming 110 so just track your power supply before it converts to DC. As far as the PT getting hot, that seems to be common with a lot of amp of that era. Get a touch-less thermometer from RatShack and let us know how hot it is getting.

10-30-2011, 03:44 PM
the 60hz isnt from a capacitor i dont think, because when i unplug the amplifier, hum goes silent of course, and itll play for a few seconds before it fades out.

10-30-2011, 05:18 PM
actually, nevermind, its 120hz hum. becuase i listened to some 60hz and 120hz tone sounds.

60hz is like holding your finger on an RCA input. 120hz is like a bad filter cap hum.

i have a 120hz hum.

10-30-2011, 10:12 PM
Update: I removed rectifier, and left the thing on for about a couple hours. the power transformer barely got luke warm, due to it still powering the filaments of the other tubes.

Also, the buzz is definitely 120hz, not 60hz. i thought it was 60hz, but a 60hz tone sounds lower (like your touching a mic jack). its 120hz (like a bad filter).

But i dont think its the filter capacitors, because when you unplug the amp from the wall while its playing, the hum goes away of course, but the amp stays playing for a few seconds before it starts to fade out (caps holding charge).

I'm lost. ive restored my fair share of tube radios, TVs, etc, even several amps. But this one is throwing me in a limbo. the B+ shoots up to 500v when warming up, and settles back down to about 388Vdc.

Checked the tubes for any possible H-K shorts, everything is good.