Heathkit rebuild or...?

[motherone]

I am in possession of a pair of Heathkit A-9 amps (a heathkit A-9 and an A-9c respectively). I am unsure of their condition. One was rescued from a local radioshack (long story, but they were just going to throw it out, and the other was picked up off of ebay.

I'm unsure of either of their working conditions. I still need to pick up a variac and power them on slowly to check them out.

That said, I'd like to see what the general opinion on these amps are. Is it worth doing a complete tear-down and rebuild with quality modern components? Or would I be better off just repairing any faults and using them as-is?

Both amps have a little bit of rust and some paint chips. I figured that if I did a complete tear down, I could also restore the chassis and transformer housings. I can take pics if anyone is interested.

Sorry if this seems like a strange request. I haven't found much info on how these amps sound.. I've only built a few tube projects, so tearing these down will be a major undertaking.. But if the results are worth it, I'm willing to do it!

Thanks,

Mike

[Tom Bavis]

Replace the bad caps - that's ALL of them. Measure the resistors and replace any that are off-value. Add a FUSE to this sucker! a 3-wire line cord is a REAL good idea too.

I have a schematic for the A-9A if it's any help...

[amperex]

I had a pair of those. Mine had an EF86 preamp tube. Not a good sounding amp, but the audio transformer is excellent.

They are a great platform to modify & will save ya a bunch of money regarding the existing good parts- chassis, iron, etc.

BTW- I have a close to mint A-9 face plate. Need it to have matching pair? I will mail it to you.

[Joel]

...but the audio transformer is excellent.

Really? The 10k potted output transformer on the A-7 I own is the worst measuring unit I've ever tested. Different manufacturer maybe?

Joel

[SY]

John Atwood shows some pretty good measurements on the A9 tranny.

http://www.one-electron.com/trans_p1.pdf

[motherone]

Originally posted by Tom Bavis
Replace the bad caps - that's ALL of them. Measure the resistors and replace any that are off-value. Add a FUSE to this sucker! a 3-wire line cord is a REAL good idea too.

I have a schematic for the A-9A if it's any help...

I'd planned on a full cap replacement. I think most of the resistors were 10% carbon comps.. I thought that maybe swapping in 1% riken's or something along those lines might improve things a bit. After all, if I'm going to tear the caps out, I'm probably going to have to tear most of the resistors out.

I have the manual that came with one of them, which includes all the build instructions (near mint! pretty neat to see how they did it back then). But if you have a PDF of the A-9A, it'd be nice to have on hand for comparison sake.

Originally posted by amperex
I had a pair of those. Mine had an EF86 preamp tube. Not a good sounding amp, but the audio transformer is excellent.

They are a great platform to modify & will save ya a bunch of money regarding the existing good parts- chassis, iron, etc.

BTW- I have a close to mint A-9 face plate. Need it to have matching pair? I will mail it to you.

Thanks Amperex! That'd be great.. As far as i can tell, the only difference between the A-9 and the A-9C are the fact that the C has a light on it.

If the transformers and output tubes are a decent enough, it sounds like replacing the input stage would improve the sound quality?

Originally posted by SY
John Atwood shows some pretty good measurements on the A9 tranny.

http://www.one-electron.com/trans_p1.pdf

Are those measurements actually good? Based on that page, it looks like the transformer doesn't fair so well on the 10khz square wave test?

[Tom Bavis]

Those aren't "grades" for square wave response - they're a list of waveshapes. read it all

[motherone]

Thanks for pointing that out. I was poking around, trying to find the page that referenced that PDF, but gave up after 10-15 minutes and thought I'd tackle it again in the morning.