THE end-all FIX for the Heathkit AA-100 integrated amp.

Ok. But just to clarify, the goal is to have them all at or around 30ma ? Because all 4 of the 7591s are coming up as much less than that
 
One thing I wonder about is when changing those resistors from 470K down to 220K the cap value of the associated coupling cap should have been raise by at least double... The combination of both create the frequency cut and leaving at the original value could end up with the coupling cap saturating on heavy bass notes and limited bass response of the amp.

I didn't read the entire thread so maybe I'm repeating someone...
 
Ok. But just to clarify, the goal is to have them all at or around 30ma ? Because all 4 of the 7591s are coming up as much less than that

This is one of the problems of these amps when stock, as Gordon explains in the thread.

The other problems are that the boards are brittle and many of the resistors and all the caps need to be replaced (at least that was my experience). If you don't want to do the fixed bias mod, and even if you do, please check all the caps and resistors on the boards. A lot of mine were way out.

At the same time, get your 7591s tested, or at least check that all are good in another amp.

NOS: I don't think your point is raised in the thread. So, up those to 0.22?
 
One thing I wonder about is when changing those resistors from 470K down to 220K the cap value of the associated coupling cap should have been raise by at least double... The combination of both create the frequency cut and leaving at the original value could end up with the coupling cap saturating on heavy bass notes and limited bass response of the amp.

I didn't read the entire thread so maybe I'm repeating someone...

When I did this the first time, I ran the numbers on the coupling caps- even with the change in bias resistors to 220K, the rolloff point of the transfer function of the original coupling caps and bias resistors is about 7 Hz. That's far below the power bandwidth limit of the output transformers (in fact, close to two octaves below).

One could, of course, replace the coupling caps with .15uf or .22uf. In my tests, it didn't make any audible difference.

Regards,
Gordon.
 
Thanks for covering this Gordon. After all, this thread is 'the end-all FIX' for these amps. :)
 
Heathkit AA-100 rebuild and Mods

I'm a newbe. This site seems terrific and there seems to be a lot of knowledge here!!

Some of this I get like putting in a 100 ohm 50w resister to drop the voltage to spec supplied in the 1960s.
I get replacing the caps and testing the resisters.

I'm reading a total of 4 12AX7 tubes 2 on the left channel and 2 on the right. Would you replace all 4 with 12AT7's or just 1 in each channel?
if so witch one?
I take it this swap doesn't require any other changes to the circuitry just popping in the tubes?

The 470k resisters you are changing to 220k on the output tubes. Are they R85 R86 R91 and R92 on the schematic?
Are these the same resisters you recommend cutting the leads on and then connecting in the to the 10K pots for individual biasing?

Is there a newer schematic than the one I uploaded on the individual biasing from Gordon W's post? I thought I heard tell of an updated one but may have been confusing it with a different post.


On to a couple tube questions.
I understand that vintage tubes are better. I the spirit of trying to keep the cost down on the tubes knowing they are easy to replace, are Electro-Harmonix 7591A's an acceptable substation for the 7591's?

With the 7199 tubes specked in this Amp look to be out of production. Has any one tried substituting 6GH8A Instead of 7199 Tube, with a plugin tube adaptor? Good results? bad results? Loss of sound quality?


Sorry for all the beginner questions. I watched my dad built Heathkits before he passed in 1972. This looks like a fun thought challenging project made easer with the knowledgeable posts from this site. I have the manual, schematics and a AA-100 that needs some TLC.



Well, I'm officially done.

Some of you are aware of the trouble some folks have encountered, trying to get Heathkit AA-100 integrated amps to sound good.

It's a real puzzle, on the face of it. Good stuff in there- Mullard 5AR4/GZ34, 12AX7s and EF86s, Amperex/Mullard 7199s and Westinghouse 7591 PP outputs. One would think the amp should sound good, right?

Well... not on the face of it. At least, not sometimes. In fact, these amps have inspired a whole web-page, from a poor soul who has gone around-and-around with his... doing all sorts of stuff, to "un-constrict" the sound. Distorted at anything over medium-low levels, with questionable sonic balance too (midrangy)...

My amp fell into this category, too... compounded by the fact that obviously, something had caused some really serious trauma to one or more of the output tubes (the unmistakable signs of burnt paint on the top cover!). So, some rather serious consideration and detective work was definitely in order.

First off, a COMPLETE recap. I mean, EVERYTHING, other than a few caps in the tone controls (which never see much voltage) and one of the two can caps. Note that I HIGHLY recommend replacing the first can cap (the 60/25 @ 500v)... mine actually was ARCING internally (you could HEAR it popping!). I used a 50/50 J&J cap... seems to work just fine. Alternatively, you could use a 40/20/20 wired as 60/20 (or a 40/20/20/20 wired as 60/40) as well...

As for coupling caps- stuck with basics- Illinois poly caps from AES. Bumped everything that was 400v rated up to 630v- it's good insurance!

After reading the aforementioned web treatise, I came to the conclusion that the first agenda was output bias... or the lack of control, thereof, in the amp in stock trim. Most of that could be laid fault at the use of 470K bias-feed resistors to the output tube grids- where 300K is the maximum rated value for the 7591! So, that was immediately fixed- 220K resistors installed, check.

Next, was going around looking for any other resistors either out-of-spec or spec-out-of reality. Found that the anode resistors on the first stage of the 7199 output driver tubes were WAY out of spec- should have been 220K- one of the 220K resistors read about 800K, the other FOUR MEGAOHMS. Needless to say, those got swapped out, posthaste. Now, at least we had STABLE output, the same from both channels.

Did correct one other minor problem- some mis-matched 7199 tubes. One was exhibiting significantly higher transconductance than the other- which lead to drastically different plate voltages under load. This is a problem... due to this stage being DIRECTLY COUPLED to the grid of the phase inverter (the second side/triode of the 7199)... this leads to the phase inverter halves (the top and bottom of the triode) being biased wrong- which loses quite a bit of headroom! Juggled a couple of 7199s around- found two that read within 5% of each other on plate voltage, and close to the stock plate voltage value too... problem solved.

Well, now the amp is sounding fine... now, onto why the preamp seems to clip so much. Did some investigation- Hmmm. Plate resistors of only 100K on 12AX7s? That's fine for guitar amps... not much good for hi-fi. Also, these were set up for very little bias voltage- so, not much headroom either.

Started thinking- what's like a 12AX7, but would like a 100K plate resistor? Answer: 12AT7. Popped a pair of 6201s in there (military 12AT7s)... PROBLEM SOLVED. Without question. Twice the bias voltage- 6 dB more headroom. And only lost 1/2 dB of gain, to boot!

Now, we're getting somewhere. It sounds like a REAL amp now! But, I could still hear a little distortion on heavy bass. Time to pay more attention to balancing plate currents on the output tubes... they were a bit out of whack, with around 35ma on some tubes, while others were having a hard time hitting 25ma...

NOTE: The following includes graphic accounts of amp surgery- those with weak constitutions might want to look away... :D

Basically, I converted the thing over to INDIVIDUAL bias controls for each output tube. To do this, first I installed bias sense resistors on each cathode- 10 ohm 1/4 watt resistors. To do this, I mounted a little 7-pin terminal strip between the two output transformers, and mounted the four resistors on it. Then, I CUT the PC board traces to each output tube cathode, and ran individual leads to the resistors. Then, I ran a common ground lead from all four resistors, back to the power supply ground (where the cathodes grounded to before).

Next, the individual bias pots. I used 10K, 15-turn pots from Radio Shack... easy to find, worked fine. Mounted them on another 7-pin terminal board... the input and output leads to the terminal board, and the "grounds" (the leads going to the resistor to ground, that is) connected in common with an "overhead" bus wire. Here's the circuit diagram I used:

attachment.php


Again, I cut the PC board traces, right at the 220K resistors for each tube, and ran wires to the pots, as shown in the diagram.

Once this was done, I set all the tubes for 30ma bias. The 15-turn pots make dialing it in EASY... you can get it within a tenth-of-a-milliamp or so, pretty quickly...

WOW... what a difference. It's CLEAN. It's balanced between channels! And it sounds REALLY dynamic and neutral. In fact... dare I say it... it actually sounds like a MODERN hi-end tube amp.

Yeah, it's a bit of work... but it's the difference between an also-ran, and something pretty DAMN IMPRESSIVE, IME and IMHO...

Oh, BTW: One optional mod- I installed a 100 ohm 50 watt resistor between the output of the 5AR4 and the first can cap. This may or may not be necessary in your amp... it helped me get the voltage down to around 460V... close to stock. But, if you bias your outputs harder, you may want a smaller value (say 50 ohms) or none at all...

In all... I'm pretty darn happy now. Now, to just get a few replacement knobs, and this thing will be GOOD TO GO... maybe for another 45 years!

PS: To boot, the accompanying AJ-41 tuner is up and running too! Man, this tuner sounds GOOD! I think we may have a new contender to challenge the Eico ST97 here...

Regards,
Gordon.
 

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Better coipy of Schematic

A member asked me to put up a higher-resolution copy of the AA100 schematic, showing the bias mods that I did.

The jpg file that I have is too big to post (it automatically gets truncated/shrunk down to 900 pixels wide) when posted as a .jpg file (the native format of the file). To try and address that, I'm posting it as a two-piece .gif file (.gif files apparently don't have the same size restrictions as .jpg files)... hopefully, this will allow reading of the file in the original size, for everyone...

attachment.php


attachment.php


There's about 40 pixels of overlap between the two files (in the middle). If you are reasonably clever, you should be able to fit them back together as a single file in a paint program...

NOTE: On the earlier diagram I posted of the replacement bias circuit, a few post above this, the resistor values are different than that on the full schematic in this post. EITHER set of resistor values should get you within the spec. I would recommend using the filter cap scheme (one 100u cap on the top side of the pots, to ground, and one 100u cap on the bottom side of the pots, also to ground), since it gives better hum filtering than the one cap in the diagram from the earlier post.

Regards,
Gordon.
 

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Hey Gordon. I have an AA100 that I want to use the from to make a PP 807 amp with. Do you think the parts will work well in this configuration? I already made a sculpture chassis for it. What do you think?
 
Hey Gordon. I have an AA100 that I want to use the from to make a PP 807 amp with. Do you think the parts will work well in this configuration? I already made a sculpture chassis for it. What do you think?

I see no reason why not. I used the iron from a derelict AA100 (bought off of that auction place) to retrofit a dead Harman Kardon A500. I used 5881s as output tubes- it sounded great.

As 807s are pretty much "parametrically equivalent" to the 5881/6L6 from all practical angles, I see no reason it won't work. They're about 7500 ohms primary impedance- a little high for 6L6s or 807s, but do-able (you just need more B+ and less screen voltage, a condition which suits the 807)...

Regards,
Gordon.
 
Thanks Gordon. So when you say I may need more B+ does that mean the power supply the AA100 has now is not enough?
 
I'm trying to find a top cover for the heathkit AA-100,I am also looking for what they refer to as wipers and rivets for the rotary mode selector switch sections. I need to remake 2 of these sections.
 
Thanks Gordon. So when you say I may need more B+ does that mean the power supply the AA100 has now is not enough?

It would work, but you can probably get a little more dynamic range from the 807, using SS rectifiers to generate a slightly higher B+. Also, seemingly paradoxically, the higher B+ will take a little bit of the load off of the screens (as screen voltage stays constant, increasing plate voltage decreases screen current on average).

Regards,
Gordon.
 
A thousand thanks To Gordon W. With the post and the revised schematic I was able to rebuild one of these. It now plays warm sweet tubular goodness!!!
When I was rebuilding it I found a series of breaks in the Power amp board. I fixed them as I was going along. I did the recommended full recap, tested all the resisters many were far out of spec and I replaced them too.
I built the bias circuit to replace the existing bias circuit, I put in the recommended preamp tubes, and replaced the out of spec 470 K ohm resisters on the power tubes.
I did run into 2 issues when I powered it up. Half the heaters on the preamp tubes ( RT channel) were not working. It wasn't until I realized V12 didn't have heater power that I found the break just before the last power tube in the heater circuit.
The other problem I ran into was V11 wouldn't bias. It was reading 80 V instead of .35V. Im a novis or I probably would have realized right off the ground was missing for this part of the bias circuit. As soon as I soldered in a ground every thing checked out. Again many thanks to Gordin W!
 
I'm thinking about bringing this project off the shelf. I am only building the amp section without all the other stuff. Where in the schematic does the amp section start? Is the 7199 part of the amp or the "other stuff"?
 
I'm thinking about bringing this project off the shelf. I am only building the amp section without all the other stuff. Where in the schematic does the amp section start? Is the 7199 part of the amp or the "other stuff"?
The 7199 part.
 
The two 7199s combined with the 7591 output tubes form the power amp section of the amp, the power supply is of course common to all sections.
 
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