May 19, 2000

Revisions since September 9, 1999

OK, A semi-breakthrough (finally). I received an email from a gentleman in which he theorized that Heathkit's approach to the AA-100 was to bias it so strongly towards Class A that its (as he descibed it) power supply's poor regulation wouldn't be as great a contributor to intermodulation distortion as it would in a less linear class of operation. He also mentioned that the 470K grid resistor value for the 7591A's that Heathlit used was too high for use with a higher bias voltage. I started to think--maybe this is a biasing/grid resistor problem after all.

A trip to the tube manual, and I started to hit myself in the head--the maximum grid resistor value for fixed-bias operation is 330K! (Cathode biasiong allows a value of 1M.) I quickly checked schematics of EICO, Scott and Fisher amps I have and it's true--they're all a lot lower than 470K. So I swapped in 270K resistors. Result? I can actually drive the amp to a decent volume now. I'm right at -17vdc on the 7591A grids, but with the tubes that are in there, that only equates to 27mA idling plate current, which is fairly low. I prefer to run 7591A's there as they're expensive, and besides, they sound fine that way in my other amps that use them.

Is this THE FIX for the AA-100? Nah, but it gets me a lot closer than I have been.

September 9, 1999

Revisions since August 3, 1999

Forget 7247/12DW7's altogether--this amp should have 12AU7's in the linestage.  I have to say I'm about this close to hacksawing off the preamp and making a power amp out of the damned thing.  Other than that, the only thing I can think of at this point is to mimic a Scott linestage from a 299D or something in the preamp and let it go at that.  There is something very, very wrong with this integrated amp. No wonder you see them all the time stripped of their tubes. Eat me Heathkit!

August 3, 1999

Revisions since May 13, 1999

This is the single most difficult amp to get sounding right of all the amps I've worked on.  I keep dragging it out when I have nothing else to do and it keeps eating up the hours, getting a little closer each time, but I see in the not too near future a dumpster for this thing after I strip out all the parts I've stuck in it.  Maybe I'll get lucky one day, but it doesn't look like it.  Ah well, I digress.

1. Some backtracking here: I replaced the series 100K resistors that I had removed from the plate circuits of V3A and V9A.

2. I lowered the values of the series cathode resistors in both V3B and V9B from 2.2K by paralleling a 1K across each (two per tube section).  This yields approximately 3.6vdc at the cathode, which prior to this change was sitting at 9vdc (oops). It also increases the dissipation to 1 watt per resistor, which the 12AU7 side of the 12DW7's now in there need.

3. I increased the feedback resistors in the output stage from 10K to 15K, thus reducing negative feedback.

4. I installed Frank Van Alstine's Stereo 70 bandpass filter between the preamp and the power amp by replacing the 4.7K series grid resistor going to the grid of the pentode section of each 7199 with a .02 in series with a 10K, and installed a 470K in parallel with 1000pf from that grid to ground.

5. I changed the grid resistors of V3B and V9B from 470K back to the original 1meg, and restored the coupling caps feeding each to .025 (actually they were .02's in the original design).

I think that's it.  I stopped there because a Scott 340B showed up and I was sick of looking at the AA-100. How did Heathkit screw up this design so thoroughly? The entire problem is what sounds like intermodulation distortion and a distinct lack of highs, but I think the amp portion itself has possibilities. Maybe it's time to bypass the preamp entirely and try it as a standalone power amp! Some day.

May 13, 1999

Revisions since May 8, 1999

Forget DONE. Thought I was done--wasn't done. Now I'm done. Hang on to your seat--pictures and more changes, but I'm stopping here. Whew--what a mess this turned out to be. Again, we'll be backing out some former changes, and adding new ones. Bottom line is that I'm now able to generate a full 15 volts at the input to the amp board with no distortion, so the limiting factor is now the amplifier section, and in particular the condition of the 7199 driver tubes, from what I can determine. Isn't that always the way? Here goes:

1. Return all negative feedback circuit components back to their original values (10K in parallel with 68pf). Use new components or measure the old ones, as typically the carbon composition resistors in this thing are way high. Again, I use 5% carbon film resistors from Radio Shack--their bargain assortment is big in this house.

2. Remove the 1Meg swamping resistor at the input to each 7199--no longer necessary.

Note: By the way, there is a big-time problem in the difference between the tube assignments on the PC Board and those used in the Sam's PhotoFact that I have--go figure. I have been referring to V3 (marked V5 on the PC board) and V9 (marked V6 on the PC board). I will continue with these assignments. I know it's confusing, but no more so than if I were to try to go back and change them all. And no, I'm not using a word processor here.

3. Change the cathode resistors in V3A and V9A from the stock 1K to 2.2K.

4. Change the plate load resistors in V3A and V9A from the stock 100K to 220K.

5. Change the two series cathode resistors in each circuit of both V3B and V9B from the stock value of 1.5K (i had switched them to 1K) to 2.2K (four resistors total). Keep the new value of the bottom bypass cap at .0047mfd.

6. Change the cathode resistor of each channel's 7199 pentode section from the stock value of 1K--read this one when you take it out, as it will probably be around 1100 ohms!--to 820 ohms (I used a 680 and a 150 in series=830 ohms).

7. Here's a cool one I borrowed from the EICO ST-40 and ST-70 7247 circuit--add a 330K (local) negative feedback resistor from the output side of the .1mfd coupling cap coming off pin 6 of both V3A and V9A back to the grid (pin 7) of the same tube. Be sure you connect it to the OUTPUT side of the coupling cap, not directly to the plate (remember, we eliminated the 100K resistor that was in series between the plate and the coupling cap). I mounted the resistor next to the .1 coupling cap in each case with two small holes I drilled in the pc board, and ran one side to the output of the coupling cap and the other through a 22-ga. insulated jumper wire back to the grid, which is a jumper wire on top of the board as it turns out. Long leads, but no instability. You may not like negative feedback in general--live with it, I say--but it lowers impedance and smooths out the sound, and I like the way it sounds in this application.

8. Lastly, with the unit powered up and at idle, measure the plate voltage at pin 2 of each 7199 and swap the two 7199's around until you get them as even as possible and as close to 95-100vdc value as possible.

9. By the way, I get 28mA resting plate current at 475vdc plate voltage using -21vdc bias. The resistor values I used to achieve this were a 22K in place of the stock 8.2K, and a 10K in place of the stock 3.3K in the bias resistor divider network. With this setup, I'm now seeing well over 15vac RMS across 8ohms before clipping, which is around 28watts per channel RMS with both channels driven. I'll take it. I run all the input level controls full up now, and there's comfortable headroom and more volume control range than before. Remember--use 7247's in place of the linestage tubes (V3 and V9)--12AX7's just never worked for me in this amp. (Ei makes great ones--order fast!)

10. Important--read all of this page and remember that we changed the grid resistor for V3B and V9B to 470K, the coupling cap that feeds that 470K from .022 to .1mfd, and bypassed the amp input level controls. Other than that, I think all the changes are reflected in this revision summary.

Ok, that's the deal for now. Enjoy the pictures--I'll sprinkle them around so you don't think I'm making this stuff up.

May 8, 1999

Revisions since May 7, 1999

Well, I'm done--DONE, as in finished. After lots of bench time, and then verifying what I heard with an audio oscillator, some dummy loads and a scope, the AA-100 shown above is now putting out in excess of 26 watts RMS per channel of clean, sonically balanced power (both channels driven into 8 ohms). I had to backtrack on some of my former bouts of stupidity but that's nothing new. Basically, here is how it's done (working with Sam's PhotoFact schematic parts designations here):

1. Go back to the stock plate load resistor values of 100K for V3A, V3B, V9A, and V9B (talking 12DW7/7247's here instead of the stock 12AX7's, remember).

2. Change the cathode resistor values in V3B and V9B from the stock 1.5K in series with.another 1.5K to 1K in series with another 1K.

3. Change the bypass cap across the bottom 1K resistor on both V3B and V9B from the stock value of .01 to .0047mfd (I used a 716P 400vdc Orange Drop).

4. Add a 1meg swamping resistor at the input of the amp board from the input side of the 47K series resistor going to the grid of the first stage of the 7199 to ground on both channels (this requires bypassing the input level controls to the amp as described below).

5. Importantly, set all (AUX1, AUX2, and TUNER) line-level input level controls to the 9 O'Clock position (12 O'Clock being full-on), or at about 80% of maximum! For some reason, these level settings seem to set up the crossover distortion point for the entire amp, so the first stage needs more work, but as long as you set them for 9 O'Clock, it can (just barely) take a 2V input signal without distorting.

6. Keep the following changes as described below, i.e., replace the series 100K resistors from the plates of V3A and V9A with jumper wires, replace the stock .022 coupling caps from the plates of V3A and V9A with .1mfd film caps, change the output stage negative feedback network in each channel from the stock 10K in parallel with 68pf to 5.1K in parallel with 82pf, bypass the chassis-mounted amplifier board input level controls, replace V3 and V9 with 12DW7/7247's, and change the grid resistors on V3B and V9B to 470K.

That's it--this thing now cooks. The tone controls are kind of overkill, although somewhat useful for simulating a loudness control at lower volumes, but overall the amp is set up properly and sounds it. I still have to get the bias resistor values I used for the bias resistor divider network (forgot), but basically aim for around -22.5vdc--should have said -21vdc (sorry)--on the grids of the 7591's and you won't be too far from wrong--I'm seeing 28mA idling plate current per tube.

As you read the following daily reports, you'll see the path I took to get here, and though it may be confusing, I'm leaving it in as a history of the changes tried and the difficulty it took to get to this point. Contributions and comments readily accepted.

The bottom line is that the iron in the AA-100 is not the limiting factor, and in fact, the amp has quite a bit going for it--too bad Heath did such a lousy job with the preamp section. They created the only bad-sounding 7591-based amp I've ever heard, but it's fixed now. You can get these things for maybe $45-50 stripped of tubes, and as my next experiment will be to try the cheap Sovtek 7591XYZ's, with any luck, this could end up being one of the sleepers of all time. (I just hate the PC board in the amp section, though--lousy pink crapboard that it is.)

May 7, 1999

Revisions since May 5, 1999:

1. Well, the more you look, the more you see. Can you believe a 100K resistor in series with a .1mfd coupling cap to reduce gain?  I'm talking about the first stage of the linestage I'm working on, and as I discovered this today it occurred to me that the AA-100's problems are more than just the wrong tube in the wrong position, and that more sweeping changes would be necessary than just cutting out the .01 bypass on the second stage cathode as I imagined below. So onward and upward, as they say. Note--these are preliminary changes, and so far, so good, but some may be backed out. Too bad this page reads in reverse--most recent first--maybe you should try reading it backwards!

2. I backtracked a little in the cathode circuits of V3B and V9B (now 12DW7/7247's), and simply changed the two 1.5K series cathode resistors in each of those stages to 2.2K, leaving in the .01 bypass caps. The grid resistors remain 470K and the coupling caps leading into these sections remain at .1mfd (both modified from stock values of .022mfd and 1Meg, respectively).

3. I changed the plate resistors in V3B and V9B from 100K to 47K, attempting to reduce gain (accent on attempting).

4. I also attempted to reduce the gain of V3A and V9A (the preceding stage), by changing the plate resistors from 100K to 47K.

5. I replaced the 100K resistors in the plate circuits of V3A and V9A that are in series with each of their .1mfd coupling caps with a jumper wire, thus eliminating them (and increasing gain).

6. I increased (nearly doubling) the negative feedback in the output stage by changing the stock circuit from 10K in parallel with 68pf to 5.1K in parallel with 82pf. I tried higher capacitor values with no effect--almost all change was effected with adjustment of the resistor value. This is a critical value--I wish I had some more small resistor values on hand to try, as my feeling is that this resistor should be on the order of around 6K or so (identical to that used in the EICO ST-40, by the way, which parallels it with 125pf). Looking at this and other 7591-based amplifier designs, feedback circuits vary widely, but clearly it seems that this amp can handle more feedback and it helps mitigate the too-high gain of the driving circuit.

7. Tomorrow I try changing the plate resistor on V3A and V9A back to 100K now that I've significantly increased negative feedback, thus reducing overall gain of the amp. Putting 12AX7's back in these positions after these changes, by the way, yielded no improvement, and in fact, more than ever convinced me that this is a clear application for a 12DW7/7247.

8. Also, as this has taken on a life of its own far beyond my expectations, it's time to get Medieval and start putting sine waves through the amp and watching them on the scope at various places in the curcuit--duh. Bear with me--we're getting close.

May 6, 1999

Revisions since May 5, 1999:

1. Well, I took a long look at what is now effectively a 12AU7 triode section (the second half of the 7247 I put in the linestage of the preamp that drives the tone controls), and decided it was time to change some circuit values. I think I'm now about 96% of the way home. As tube designations seem to vary from schematic to schematic, even within the Sam's PhotoFact, let's just say we're talking about the circuitry associated with pins 1, 2, and 3 of the (formerly) 12AX7, now a 7247 (V3B and V9B on the Sam's).

2. First up is to change the cathode circuit (pin 3). I started by just shorting out the bottom 1.5K resistor and its associated .01 bypass, and the improvement was so dramatic that I knew more could be done. (If you want, you can just do that as a top-of-the-board fix to start.) Pull out the bottom 1.5K resistor and paralleled .01mfd bypass cap that was in there on the second section of the cathode circuit and put in a single wire jumper in place of the resistor. Then replace the remaining 1.5K resistor with a 4.7K bypassed by a .01mfd film cap--I had to tie it to the resistor at the top of the PC board. Note--the cap may get pulled out of there anyway on the next trip in, as the amp is a tad bright, which I think will be the last 4%. So the now the cathode circuit consists of a single 4.7K cathode resistor bypassed with a .01mfd both going to ground. I used a half-watt carbon film from--you guessed it--Radio Shack.

The main reason I upped the cathode resistor so much is that the AA-100 runs this section at a pretty high plate voltage, compared to say, the EICO ST-series. So, using the EICO design as my guide, I compensated by upping it from what is 1.5K in the EICO to 4.7K, as the EICO is running at 195vdc as opposed to 320vdc, and uses a 33K plate resistor as opposed to the 100K plate resistor for this stage in the Heathkit.

3. Next, change the grid resistor (pin 2) from a 1Meg to a 470K to ground. Again, I used the EICO design as a guide.

4. Then change the .022@400vdc coupling cap leading into that grid resistor to a .1@400vdc film cap. I tried it and liked it, basically, so it stayed.

5. Be sure that if you're doing these things, you're running 7247/12DW7's instead of 12AX7's in the linestage positions (V3 and V9).

6. The last thing I did was pull out the 1Meg resistors that yesterday I put in parallel with both sections of the volume control as detailed in step 2 below, so now the front panel 1Meg volume control feeds the 7199 inputs directly. Bottom line is that the amp is putting out more--much more--and cleaner--much cleaner--power than ever before. The treble is, however, just a tad hot--not grainy or harsh, but just a bit too preemphasized--so I think that with the .01 bypass on the cathode circuit detailed above cut out, it will fall right in line. I never thought this amp would even come close to where it is right now. Shame, Heathkit!

May 5, 1999

Revisions since May 4, 1999:

1. Replaced original output transformer secondary "snubbing" networks, consisting of .02mfd ceramic cap in series with 100 ohm 1 watt carbon comp resistor located across each OPT secondary. Doesn't seem to hurt anything, and probably helps stabilize the amp. Who knows?

2. Bypassed amplifier board input level controls, running twisted-pair leads directly from preamp board outputs to amp board inputs, with ground leads terminated on preamp board only. This requires shunting each section of the the existing 1 Megohm front-panel dual volume control with a 1 Megohm 1/2-watt resistor (I used carbon comp resistors under the preamp board), effectively making it a 500K dual-section volume control. (Normally with the now-bypassed.level controls full on, it would see a 1 Megohm resistance in parallel with its wiper to ground (as I recall) anyway, so after some testing I decided to put the resistors across the entire pot, as the 1 Megohm control alone was too high--also see typical uses of 7199 in back of RCA Receiving tube manual). Less things to worry about and fewer things in the signal path can't be all bad, I say.

3. Replaced used Mullard 7247's discussed below with new EI 12DW7/7247's from Ned--excellent.

May 4, 1999

Welcome to my mod page for the Heathkit AA-100 amplifier. The AA-100 is a not-so-popular 25 watt-per-channel integrated amp manufactured by the Heathkit Company of Ann Arbor, Michigan (a division of Daystrom Industries if memory serves). Like the Mark III, although very attractive from a purely price/performance basis, it presents the modern user with more than a few challenges, especially in the overall gain and bias supply area. However, once refurbished, it sounds sturdy, clean and powerful (seriously--I know, I didn't believe it either). It consists of dual printed-circuit boards--one for the entire preamp and one for the entire power amp. And this is the tipoff that all is not right with the stock design: it has level controls every frickin' place--inputs (all of them), amplifier inputs, and so on. Obviously (to me, anyway), Heath threw two existing separates together in a single cabinet and Presto!--an integrated amp with minimal development cost. Well, they should have spent a little moire time on it, but we'll get to that. Basically, the circuit consists of an EF-86/12AX7-based phono stage driving a 12AX7-based line amp into a 7199 driving a pair of 7591A's in push-pull using fixed bias. The power supply uses a 5AR4/GZ-34 rectifier tube in a full-wave configuration and a half-wave selenium rectifier in the bias supply. Output taps are provided for 4, 8, and 16-ohm speaker impedances, along with a speaker phase reversal switch and a center channel speaker output. It has an aluminum chassis and painted steel cage.

The transformers are beefy mothers, which was the first thing I noticed about it after the 7591A's, which is really why I bought the thing, but we'll get to that. One nice and unexpected feature, aside from originally having all Mullard tubes (except for the outputs--duh), is that it has cathode-follower buffered tape outputs--hard to believe, but remember, this was a preamp that they dropped into an enclosure as is. The other neat thing is that it is enclosed in a fako-jako vinyl leather-covered steel cabinet with a black front panel illuminated by a full-width top-mounted light bar--seriously, just like your bathroom in the 1950's! Very exciting in a retro-kitsch kind of way.

The other wierd and irritating design feature is that all inputs are made on the underside of the amp. Can you say BLIVET? Get used to it, although it helps to clean the existing feet with Naptha (lighter fluid) and add some Radio Shack stick-on guys to raise the unit, or replace the feet with taller ones entirely. You'll have to if you use hot-shit interconnects. Finally, it has a strange balance control configuration, along with a variable separation control for use with the center channel output (who cares?).

Of course, a schematic is always helpful. Unfortunately, I don't think there is one up on the Web, but you can order a Xeroxed copy from Heathkit--don't, as it's expensive and I did and I've never used it. Also, you can still get a Sam's Photo Fact Sheet on it from A.G. Tannenbaum, which is the way to go. Cheap and all you need to know (got a few errors in it, though).

My Personal AA-100 Story

I used to look at amps and receivers based on the 7591A and think--what is the deal, here? An unfindable tube, and fairly high prices. But then I got all wrapped up in an auction up on eBay for a Fisjher 800C, and having won it, got involved in rebuilding it, which was in itself a month-long (plus) project. But that sound! So I started looking around for spare tubes, as I figured I'd be hanging on to the Fisher 800C for a long, long time. Looking around, it looked to me that you could either pay unacceptably high prices for NOS tubes, which are in who-knows-what kind of condition, (forget matching), or actually buy equipment that nobody wanted that has the tubes in it. I think that maybe my EICO ST-40 came first, and it came relatively cheap ($127) and wonder of wonders, it had all its original tubes! Mullards everywhere you looked, and the 7591A's all tested fresh (on my TV-7D/U). I thought this is too good to be true. But then I got all wrapped up in rebuilding the ST-40, as it sounds so GOOD. So that was out as a tube source (until later when I did a 7591XYZ conversion--to be documented). I then happened on an auction of this thing--the AA-100. Here was an amp nobody ever talked about (the AA-50 is another--same amp, different cabinet), and it had all the essentials (described above). I got it for an insanely low price ($72), and also copped the matching FM stereo tuner, the AJ-12 (although UPS had its way with it, but let's not talk about that). The AA-100 came in semi-decent shape, and before you know it, I was rebuilding the damned thing instead of robbing it for tubes (I hate when that happens).

Anyway, this first treatment explores the vastly untapped potential of the AA-100 with its basically stock circuit configuration but with a trick tube replacement that absolutely transforms it into a real ass-kicker that you can still pick up for a song--just make sure the tubes are still in it. Future updates will explore other tweaks, but this first iteration will be  a basic rebuild with some essential minor redesigns. Prepare to be amazed.

Check the Circuit Boards

Here's a real issue--this thing used a lot of stand-up caps that with any vibration at all tear up the pads to which they're soldered, and to make matters worse, the PC boards are of such el shitto quality that you're bound to find problems, so your first job is to do a complete assessment of the physical condition of the amp. You'll be pulling out almost all of the caps anyway, so don't get too into repairing the board just yet, but check it carefully. You'll have to disassemble the underside input bank to get access to the preamp PC board, but you'll have to do that anyway, so just do it. Also, tighten all hardware, and make sure it's all in place.

Replace All of the Caps

Here again, just assume that all the caps in the amp are bad and you'll be a lot happier about going out of the house to get the mail, for example, with the amp still playing and coming back to an intact house. Consider the age here--we're talking on the order of 30 years. For coupling caps, I used a little of what I had, but mostly 716P Orange Drops and ECI yellow polyester film jobs from AES. The stand-up caps are easily replaced with .1mfd@400vdc Xicon jobs, which are small and can be mounted horizontally in the existing holes with a little bending of the leads. The can caps in mine are still OK (knock on wood), but the good deal there is that they're FP mounts in bakelite wafer rings, so LCR jobs with clamps will drop right in--I can't wait! Just replace all of them with original values, with some exceptions--the cathode follower tape output stage got .1mfd@630vdc film caps (from Radio Shack--they're marked 200vdc on the package, but look IN the package--the caps say 630vdc!), and for the output caps of the tape outputs I used 1.0mfd@400vdc ECI film caps from AES. And of course, the selenium bias rectifier has to go--it's a cleverly disguised bastard, though--it looks like a silicon diode from years past, but in reality it's a selenium diode in a wierd plastic package. I used a 1N4004, bypassed by a .01@500vdc ceramic cap (from Radio Shack). Remember to orient the diode properly--band end to the transfomer wire--this is a negative voltage supply. I bypass all my silicon diodes with .01mfd ceramic caps--keeps down the digital nastiness. There is more to come on the bias supply side--you have to up the negative bias so as to reduce the idling plate current on the output stage to 28mA per tube or so, but that comes later (when I can get a look at the values I had to use to do it).

While you're at it, check all critical resistor values--you know, like the output tube grid resistors, and the phase inverter plate and cathode resistors. I use hefty carbon comps if I have them, or Radio Shack carbon films if I don't--they're fine as far as I can tell. Measure every component before you put it in--a good rule to follow, whether the part is new or used. Take my word for it.

You also want to clean all the pots, switch sections and tube sockets with DeOxit5 deoxidation spray solution (another must-have).

Replace the Line Cord

This is so important, I'm going to give it a section all its own (I ALWAYS do this and nobody ever mentions it). Replace the line cord with a 3-wire, 16-gauge line cord (or larger--I used Radio Shack), bypassed with .01@2KV (line-rated) caps from each side to chassis ground. You can use Panasonic X-Y Series (best) or Radio Shack ceramic caps (also fine). This is a must-do, in my opinion. The IEC says green is chassis ground, so attach that to the chassis (e.g., transformer mounting bolts).  White is neutral, and goes to the unswitched side of the power transformer primary winding (in this case, the fuse holder).  Black is HOT, so this goes to the AC power (on/off) switch.  Connect a .01@2KV from black to chassis and another .01mfd@2KV from white to chassis at the point of entry of the line cord, and you're done.  Not more shocks, no more hum, no more electrical code violations. With the AA-100, you'll be somewhat challenged, however, as the line cord enters the chassis up and almost under the power transformer. Be careful and you'll get it, though.

Redesign the Gain of the Amp (My Secret Fix)

OK, here's the secret to this entire rebuild. A little history. I just could not get this amp to sound right. No matter what I did--cap changes, tube changes, level adjustments, level control bypassing, nothing helped. The amp just sounded bad, and distorted at what should have been an easy level for it to obtain, given the size of the iron and power supply components. Very mystifying. I did make some headway by taking out the speaker damping/snubbing circuit on the output of each channel by removing the (I think it was) .02mfd ceramic cap in series with a (I think) 100ohm resistor on the ouput of each output transformer secondary. But it was hardly enough (I'm thinking of putting it back, now that I think about it). Also, by reducing the input level controls about 25% and the amp input level controls about 20%, I could reduce the amount of overall distortion, but in doing so I also reduced the overall power output of the amp, so something was clearly wrong.

So last night while brushing my teeth, I had a brainstorm. I was studying the schematic all day, trying to figure out what exactly was wrong, and the only thing I could come up with was that there was simply too much gain in the amp. So while brushing, I was thinking of what stages I would bypass. Bypassing the tone controls would increase the gain, unfortunately, so it would have to be something else. Maybe make the pentode stage of the 7199 a triode, I thought, but that wouldn't address the input level control reduction required to get the distortion down. Then I thought of it--in the EICO ST-40 and the ST-70, they use 7247's (also known as 12DW7's) to drive the tone control circuits--one side looks like a hi-mu 12AX7 and one side looks like a low-mu 12AU7. The question was, what was the pin-out of the 12AX7 driving the tone controls in the AA-100--would it be OK to drop in a 7247? I had a spare pair of Mullards with some EI versions due in soon, so it would be easy enough to just try them, but I looked at the schematic first when I got down to the shop this afternoon.

I pulled out the GE Tube Characteristics book to be sure which side was which, and checked an EICO ST-40 schematic to see how they did it and I couldn't believe my eyes. Just about right down to the cathode resistors, it looked like an absolute natural. I threw in the used Mullards and tried it. Game, Set, and Match. This is The Fix--Period Point Blank.

Tubes--What To Use?

This is the last decision point of the whole deal. As far as preamp tubes go, Mullards were original, so if you've got 'em, use 'em. If you aren't going to use the phono stage, you can eliminate the EF-86's, although the phono stage isn't too bad. If you're not going to use the phono stage OR the tape outs, you can even eliminate another pair of 12AX7's (see the schematic--the other side of each of the 12AX7's is part of the phono stage).

As for output tubes, chances are you're going to be using used tubes. Who can afford "new" 7591A's? Actually, now that the amp works properly, I may try 7591XYZ's (if they'll fit--with some changes the bias supply appears to have enough available negative voltage). One thing I would advise is when you test the 7591A's on a tube tester, do what I've been doing lately--measure the plate current drawn by each tube as it is tested and write that down along with the tube tester rating number and try to match up those numbers--they're actually more important than the tester rating. I use the Allessandro dual analog bias meter with one plug-in plugged into the TV-7D/U octal socket into which I plug the tube under test, which works well. The AA-100 doesn't have adjustable bias, and I don't recommend putting it in--there isn't enough room to do it right, and if the pot gets bumped, there go those expensive tubes. Set it up and nail it down with fixed resistor values and then shuttle around the tubes until you get two more-or-less matched pairs according to the bias meter. There is no provision for measuring bias across cathode resistors either, and as the output tube sockets are PC-board mounted, it isn't an easy proposition to add them either. Break down--buy a bias meter.

Sit Back and Let 'er Rip (first on a Variac):

Bring it up on a Variac, if you have one. Otherwise, pull the rectifier, and check for smoke, observing all the other first-time turn-on precautions and then successively plug in the driver tubes, then the rectifier and then the output tubes (I think I have that right--just get a Variac and you don't have to worry about the sequence all that much). When you think you're ready, bring the amp up and make sure the idling plate current doesn't exceed (my preference) 28mA per output tube. They'll take more, but why push it? The amp runs a pretty high plate voltage (got to check that one), and 7591A's fortunately sound just fine biased down. Save the tubes! I'm telling you--this is an ugly duckling waiting to become a swan, sound-wise. There really is a decent amp underneath all that fake vinyl leather, but if you don't swap the 12AX7 linestage tubes for 7247's, you'll never know it.

Have fun and let me know how you make out.

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