Heathkit A-9B schematic

[Juanbanzai]

Good evening all.
I just became the proud owner of a gorgeous Heathkit A-9B mono block I was a little reluctant to pull the trigger on this one because of the potential issues that can be had with a Heathkit that wasn't put together properly. At first glance, everything looked as if it had been assembled by someone who really knew what they were doing. However, a closer look revealed that some modifications were done - especially around the 40,30,20,10 can cap.

I've been searching the web for days trying to find some information about the A-9B but it's as if it never existed. Not a single write-up other than "had one...sounded great" could I find. Trying to locate a schematic was impossible. However, there's a ton of information about the A-9C. So my question is... what's the difference?? And does anyone have a hi-res scan of the schematic for the A-9C they'd be willing to share? Of course, the schematic for the A-9B would be fantastic, but if they're basically the same amp then the A-9C schematic would be most appreciated. The web is loaded with the same lo-res gif file of the schematic... but it's completely illegible to these tired old eyes.

Thanks in advance for any replies.

Aaron

 

[chinacave]

Hi -

I have the orig assembly manual for the A9-C, I'd be happy to hi-res scan it for you, but it will have to wait until Friday, if that is OK -

 

[Juanbanzai]

That would be fantastic! At first glance, there doesn't appear to be any difference between the two other than chassis color. I'm also getting some troubling readings from testing the output transformer so I don't want to put any money into this amp if it's a goner. Hopefully the manual will help me bring this thing back to life.
Thank you so much for your offer. I really appreciate it.
Aaron

 

[Tom Bavis]

I have an A-9A schematic - 12AX7, 2x 12AU7, 2X 6L6... PM with an email if you want a copy.

In the case of the A-7, the B and D versions were line-level only, C and E versions had mag phono preamp - so they were selling two versions at the same time... but maybe all the A-9s included the phono preamp.

 

[Juanbanzai]

Thanks for the reply, Tom. I was able to find the schematic for the A-9A on the web. The trouble is that the 9A uses a 5U4 rectifier and the 9B and 9C do not. There's also a slightly different resistor network around the 40,30,20, 10 electrolytic cap. I was getting lost trying to use the 9A schematic especially since a modification had been done to my 9B. In looking at chassis pictures I found on the Web of the 9C, it appears to be a dead match for component placement of the 9B.

 

[wildone]

I just finished a A-9 C last night to have a pair. Even though my set are not the same color.

Here is a couple pics.

WO

 

[Juanbanzai]

That's fantastic! Looks like I hit on a good amp to set up as a mono/Hi-Fi listening station. I restore antique jukeboxes as my main hobby so I know full well the proper procedure with restoring these old tube amps. However, after not finding a schematic for the 9B, I decided to take a chance and bring the amp up slow on a variac to see what I was in for. All NOS tubes are in the thing and all test good or better. After about an hour of bringing the power up.. and nothing exploding... I got absolutely nothing out of the amp other than a faint hum. Tried an input signal of a turntable and iPod. Not even a pop as I was unplugging sources. I realize that any number of things could be wrong...such as the coupling caps and filter caps... but testing out the output transformer didn't give me any encouraging feelings.

 

[Thatch_Ear]

I wouldn't worry to much about the rectifier tube. Maybe it shows a 5AR4 which has the advantage of a softer start and many consider that the mullards have a better sound. But any rectifier tube that can handle the voltage and amps of the power transformer will work, the largest difference would be in how much of a power drop it causes for the tube to be used, very easy to find out.
5U4G and GA have a 44 volt drop at 225 ma the GB drops 50.
5AR4 isn't in my 1950 RCA book and the GE doesn't show it the same way the 5U4s work. Seems to like a bit more ma at 250 but has higher peak voltages and currents. The 5AR4 has a different basing diagram but that is just the soft start. For any 1950s 6L6 design I would fel safe with either of those as well as quite a few others. One of my favorites is the 5V4 but I don't think it can handle PP 6L6s especially if it calls for 5881s. I don't think the GLCs were around in those days and the highest voltage etc would be a 5881. If you were planning on using 6L6Gs PP Class AB in the G, GA and the GB are all 360 volts and if you are building it you wil know what the voltage is 6L6GC is 450 still shows 360 tied in triode 250 which is lower than one in SE. 5R4s would be good, some of those that are WGA or close to it look like potato mashers are military 5R4s Chatham, I think, those and the 5AR4s will take on any kind of 6L6 and 5U4s too. Just another chance to roll tubes is the way I look at it. 5U4 were used in so much gear that a good ued one, well 2 shouldn't be too much, 5AR4s are going to cost a lot more.
I have a A9C book around here somewhere, but it seems like that is all covered. Get the resister values you need and get new ones because the cans aught to be new while you are at it. Those are some old amps. I have a built one that is rough and a complete except for tubes, unbuilt kit and I would never use the caps in the kit. Some might be good for crossovers but not a high voltage amp like this.
I got a W-5 book too in case I wanted to try and pull that off, still out in the garage with about 7 years of good intentions.

Thatch

 

[chinacave]

If you DO have a bad OT - I may have a spare......

 

[Juanbanzai]

Thanks very much for the offer. Perhaps I'm not testing the OT properly. Normally, what I would do is is connect an analog meter across chassis ground and the primary tap of the OT. With the meter set to measure resistance, I watch to see if I get a full scale reading, then a slow dip. I've been told that any reading less than 10,000 ohm could indicate a problem. I also measure the resistance between ground and the plates of the 6L6 tubes to see if I get a reading that's similar on each plate. If I don't, that could be a cause for concern. Am I on the right track with this test or is there a better way to test the OT on these Heathkit amps? Obviously, the amp has NOT had any rebuilding done to it. I want to confirm that the OT is in usable shape before spending any money on caps and resistors even though I stock every value needed to rebuild the 9B.
Thanks again for all the replies.

 

[Juanbanzai]

Wildone...
Your rebuild looks great. But did you replace the electrolytics (cans) with new cans or just the coupling caps and tone control caps? I see the new 40/450 electrolytic in your picture, but it's attached to the original terminal of the 40/450 in the can. This is one of the issues I have in my 9B. That electrolytic was completely rerouted in some convoluted way that without a schematic, I can't figure out what the heck the guy was trying to accomplish. There's all kinds of weird jumpers and small pieces of wire running from that cap to an extra terminal strip to choke a rat.
Aaron

 

[ab0ez]

You really need to test the primary tap of the transformer out of circuit to really tell if it's bad or not.

 

[Thatch_Ear]

Somebody told me there were more than average problems with the A9 C outputs, one of the reasons I haven't been too hot on getting the transformers tested on the built but beat up one. The others have never been used and should be OK, but the A9s have never been popular, but then again I know they are out there and haven't heard of a blown transformer yet.

Thatch

 

[Juanbanzai]

Need some more advice while waiting for the schematic, folks. Changing out the coupling caps is pretty much a no brainer, but like I said in my first post, someone had done a modification to this amp and I'm trying to figure out what was done.

Can anyone tell me why a 200 ohm power resistor - I'm going to guess rated at 5 watts - would be connected between the 40/450 electrolytic cap and pin 8 of the 5V4G? Actually, the original 40, 30, 20, 10 electrolytic can is still in the circuit, but the 40 uF section has been cut free and a substitute electrolytic about as old as the amp was put in its place - very similar if not exactly like in the second picture of Wildone's post. The + side of the new cap is going to a terminal strip, and it's here that the yellow wire from the output transformer is also attached in addition to that 200 ohm resistor. Also on this same terminal is a wire that connects to pin 7 of the 5V4G, and running between pin 7 and the 20/450 electrolytic cap is the 3750 ohm power resistor.

My problem is I don't see anything connected to pin 7 of the 5V4G in the schematic I have for the A-9A and the impossible to read schematic for the A-9C. So can anyone tell me what this is all about and what the heck that 200 ohm power resistor is doing in the circuit?

Thanks very much.

Aaron

 

[JoeDotCom]

Heathkit A-9C schematic

Hey Gang,

Sorry to necro / crash this thread.... but a couple of you mentioned you own the Heathkit A-9C. I found a low resolution copy of the schematic here: http://www.heathkit-museum.com/hifi/hvma-9c.shtml But I can not read the small details on it. I have a friend who has been unsuccessfully looking for this schematic and I would love to help him out. Any chance anyone has a better copy than the Heathkit Museum version?

Please and thank you.


Oh, and how did your A-9B turn out Juanbanzai?

-Joe

 

[chinacave]

Hope This helps!

 

[JoeDotCom]

Hope This helps!

Awesome, thank you!!