opinions of the heathkit w-A9C iron

[Jcricket]

Hey Folks,
Does anyone here have experience with the heathkit w-A9 amps.
I have a set of iron I am going to build into an amp. I was planning on using the heathkit circuit too. Skip all of the controls and make it a straight amp.

I'll be using an Antek torrodial PS. Probably a CLC power supply. Maybe rectified filaments, I haven't haven't decided yet.

Any thoughts?
TIA,
Mark

 

[jaymanaa]

Alright, here's what my research has revealed in the last week or so. (There's been a lot of this iron floating around as you may know). It's my understanding that the "A" units were primarily built from US Military surplus stuff, and the transformers were closer to PA quality than HiFi. They are supposedly no where near the W stuff, at least that's what I read Mark. This is all hear say, as I have never used this iron, so I'm just as interested as you to hear from some folks with first hand experience. :yes:

 

[jaymanaa]

I wish the hell we lived closer together so we could do some building together. I think we would both benefit, not to mention all the fun it would be.:yes:

 

[Jcricket]

I wish the hell we lived closer together so we could do some building together. I think we would both benefit, not to mention all the fun it would be.:yes:

Heck we are only about 10-11 hours drive apart.

Yeah no kidding. I would drive you nuts though with the barrage of questions I would have.

According to the brochure HeathKit put out these amps were 20-20k+/- 1db with 20 watts output.

 

[Jcricket]

My title is wrong, I didn't realize it. It is a heathkit A9c not a w-a9c. OOps!

still, heathkits brochure says 20-20k:scratch2:

 

[Jcricket]

I had all 0f the stuff I needed to build this amp as stereo. I am going to go ahead and give it a whirl. I'll see if I can post the schematics in a bit. Maybe there are things that could be done to improve the performance of the amp.

Also, the original amp used 6l6 tubes. I have 6l6, 6l6gc, and kt88, 6v6gt's and 6v6g's tubes. I believe the schematic I am using will put about 360v to b+, 340v on the plates. I can adjust the voltages to the screens. My PS transformer can put out 375v.

What of those tubes, if any would you use with this set up? Pics to follow soon too.
TIA,
Mark

 

[s-petersen]

I would go with 6L6GC, and if sold, put in 6N3C or 6N3C-e tubes because they are cheap and work well.

 

[jaymanaa]

6l6 for me too.

 

[s-petersen]

I have the full resolution scan if it helps (have the whole catalog)

 

[Jcricket]

Thanks for the offer.
I got a really good copy of the schematics from Hi-fi engine.
I machined and painted the chassis yesterday. I have started the assembly. I am kind of stuck right now because I haven't been able to paint the transformers yet. If I had them painted I would likely have this amp completed tomorrow sometime. Maybe by Monday

pics to follow soon.
mark

 

[crispycircuit]

The power xmer's onW-A9, W4,W5 and tend to burn out. They're cathode biased, so they run hot. Glad you have new pwr xmers. Keep us informed and take some pics...

 

[Jcricket]

here is the initial pic. Transformers are not mounted, just sitting in place to show how it will look.

I am going to start building tonight. Maybe tomorrow I will be able to paint and then Sunday install the transformers. More to follow..................
mark

 

[Dandy]

Watching this build too. I have one of these output transformers because I bought the A9B while on holiday overseas. I just took the opt home in my suitcase, as the pt was 120V only. The opt works just fine with other 6L6 amps, but I've never used it long enough to listen to it critically. My A9B did not have the 500ohm output tap.

 

[jaymanaa]

Wow, a 500 ohm tap, handy for some Senns I know. Looks great Mark, as always. I think I'd definitely put a headphone jack on that, with some appropriate attenuation of course.

 

[s-petersen]

Thanks for the offer.
I got a really good copy of the schematics from Hi-fi engine.
I machined and painted the chassis yesterday. I have started the assembly. I am kind of stuck right now because I haven't been able to paint the transformers yet. If I had them painted I would likely have this amp completed tomorrow sometime. Maybe by Monday

pics to follow soon.
mark

The main reason I posted was for the specs and writeup

 

[Jcricket]

I got side tracked but I am now back onto this build. I have discovered one error in my design. I have an Antek torrodial power supply. It has plenty of current, but it is just a bit shy on voltage. It is has a dual 360v output at .55 amps each. I need 405v at b+. Plates need to be 360v. Also, the Heathkit A-9c is a cathode bias circuit. So what would you do? Build a voltage doubling circuit, get a different transformer, live with it, change tubes?t of the circuit will be fine as designed, I will just need to change the PS section.

I may just live with it and see what happens.

Sorry for the micro schematics. I'll see if I can get a better copy and post it. My current copy is a word doc pic, it is great, but way to big a file size to post.

 

[Jcricket]

Here are a few pics of her current status. She is fully assembled with the exception of the 125vac line and power switch. I'll have that done after lunch today. Then come the fire up with the variac, troubleshooting if need be, checking voltages, and possibly some testing. She is getting very close now.

 

[Jcricket]

I am done for the day. I have it all assembled but it won't fire up. Something is wrong in the PS I think. It will not get to the right voltage and draws massive current, then pops the fuse. I am too tired to work on it more today, time for a break.

My variac show the voltage at as high as 80vac, the b+ out is only 20vdc, I can't seem to get it higher than that. :scratch2:

till later
mark

 

[Jcricket]

I made several dumb mistakes. The cause of the above problem was me using one filament voltage tap for one leg, and the other for the second leg.

I am not thinking well apparently!!

With that said I have been able to power it up, bit no sound, dead quiet. I checked the source, it is good. I checked the b+, it is good.
I did find a very unusual voltage on the 12au7. Pin one on the 12au7 is supposed to be something like 68v, I am getting over 300v.
Can anyone here help shed some light on what is wrong, give me a place to look.

Posted below is a good copy of the schematics and an actual drawing of my PS. Is there something wrong with my PS? I don't see it if there is.

 

[s-petersen]

if the filament is not lit it would make sense