Heathkit AA-13 integrated tube amp

[Tom_r]

I recently purchased a Heathkit AA-13 integrated mono tube amp. For those more familiar with it's predecessor the model AA-161, it's basically the same except power supply caps are bigger and the schematic appears to have correct values for the voltage measurements. I've attached schematics of both.

I'm looking at re-capping the amp so I'll be replacing the film caps and p/supply can with separate e-caps under the chassis. I can check the resistors for out of spec values ... or are there some that should be changed to film types anyway.

I have no problem performing the work and am getting better at reading schematics (I have 30+ years experience reading AND interpreting Architectural blue prints), but design is another story so here are some things I was pondering:

1) I saw mods to the phono section of a Heathkit AA-151:

http://audiokarma.org/forums/showthread.php?t=432956

I'm wondering if someone could suggest the same mod modded to suit the AA-13?

2) What do I need to do to use the Xtal input as a line input?

3) I will be connecting a tuner to the tuner input, should that circuit be left as-is?

4) This post mentions changing the 6AN8 to a 6u8:

http://www.audiokarma.org/forums/showthread.php?t=355525

Is it just a matter of changing some of the wire connections around on the socket or should some caps/resistors also be changed? I have numerous 6U8/6GH8 tubes and their brothers.

5) Are there any other mods or updates that should be performed (other than a new power cord)?

Thanks.

 

[dcgillespie]

The AA-161 and AA-13 are nearly direct copies of one channel of the AA-151 stereo amplifier (except for the power supply), so whatever mods are appropriate for the stereo version are applicable to these two models as well.

1. To change the xtal input into into a standard line level input, simply change the two resistors and capacitor that make up the input attenuation network for the xtal input to those used for the tuner input.

2. Leave the tuner input as is. It is considered the same as an Aux input.

3. The 6AN8 socket can be rewired for the other tubes, but to do the job properly would require a scope and square wave generator to make sure the HF response of the circuit remains the same (and stable) with the different tubes. It would probably be best to just use the 6AN8. It is not worked hard at all in this design, easily available, and will have a very long life.

4. One of the best mods is to remove the attenuation networks and place the selector switch directly ahead of the volume control, allowing both stages of the 6EU7 to function only in the phono preamp circuit. This is not a hard modification to perform, but it is fairly involved to make the necessary changes. If you are not comfortable with the work required, I suggest that you first get the unit thoroughly brought up to date (cleaned, recapped, tubes checked, etc.), and then consider the input modification if you are comfortable with the required work at that point. There are a number of folks here who can help you with that modification.

Good luck with your amp!

Dave

 

[Tom_r]

Thx Dave. Just a few clarifications referring to your numbers:

1) Change R2 (and R33) to 470k metal film, change C31 (and C30) to 33pF polystyrene film, and change R36 (and R1) to 33k metal film. But what about the connection between R36 and R7?

2) OK. I saw some mods that removed the two resistors and cap.

3) Easier to leave it. Among other things I have a within calibration spec Tek 2246 scope and Heathkit AG-10 Sine/Square Signal Generator (along with an HD-1 Harmonic Distortion Meter and IG-42 RF Signal Generator). I picked up the Heathkit gear at a yard sale a few weeks ago on my way to a Hamfest to buy some test gear to align my FM tube tuners. The Heathkit stuff is in really nice shape - it was well looked after. I'm just in the process of rebuilding and calibrating it all. I have the parts but not the time. I know the Heathkit is not lab grade equipment, but I'm sure it'll get me by for stereo gear from the same era.

4) That sounds like a winter project ... I just want to get the amp going now. I have also rebuilt and calibrated an RCA WV-77E vtvm, Stark VMK-4 vtvm, Sprague TO-4 cap/resistor tester, Sencore TC-136 tube tester, and Eico 666 tube tester. I have a few other bits and pcs of test gear to go through. I checked the AA-13 tubes before I bought it and the 6AN8 was bad, and has now been replaced.

I'm really enjoying working on vintage tube gear after buying my first pc last August and never having performed any type of work on audio gear nor having any audio test gear other than an analogue VOM that I use for working on my old cars. That first pc was a Dynaco FM-3 which I rebuilt and modded, and sounds great. Since then I've bought about a dozen more various brands of tube tuners and a project Sherwood S-5000 integrated amp. Public forums and members very generous with their time and knowledge make this all possible. Thank you everyone for your help!

The AA-161 and AA-13 are nearly direct copies of one channel of the AA-151 stereo amplifier (except for the power supply), so whatever mods are appropriate for the stereo version are applicable to these two models as well.

1. To change the xtal input into into a standard line level input, simply change the two resistors and capacitor that make up the input attenuation network for the xtal input to those used for the tuner input.

2. Leave the tuner input as is. It is considered the same as an Aux input.

3. The 6AN8 socket can be rewired for the other tubes, but to do the job properly would require a scope and square wave generator to make sure the HF response of the circuit remains the same (and stable) with the different tubes. It would probably be best to just use the 6AN8. It is not worked hard at all in this design, easily available, and will have a very long life.

4. One of the best mods is to remove the attenuation networks and place the selector switch directly ahead of the volume control, allowing both stages of the 6EU7 to function only in the phono preamp circuit. This is not a hard modification to perform, but it is fairly involved to make the necessary changes. If you are not comfortable with the work required, I suggest that you first get the unit thoroughly brought up to date (cleaned, recapped, tubes checked, etc.), and then consider the input modification if you are comfortable with the required work at that point. There are a number of folks here who can help you with that modification.

Good luck with your amp!

Dave

 

[dcgillespie]

Tom -- Welcome to the vintage audio bug! I haven't found a cure for it since I was about 5 years old (or, back when vintage wasn't so, er, vintage!).

Regarding your question for the connection between R36 and R7, it may not be easily discernible at a quick glance, but that connection is really a ground buss, as brought over from the buss running between the three input jacks on the schematic. Therefore, these two resistors do not interact together, but merely both have one end that connects to ground. Otherwise, you've got the component changes for making the Xtal input a regular Aux input down pat.

I hope that helps. Good luck getting it going!

Dave

 

[Tom_r]

Thx Dave.

When I get to the below mod will we be rebuilding, modifying or removing the PEC module?

...

4. One of the best mods is to remove the attenuation networks and place the selector switch directly ahead of the volume control, allowing both stages of the 6EU7 to function only in the phono preamp circuit. This is not a hard modification to perform, but it is fairly involved to make the necessary changes. If you are not comfortable with the work required, I suggest that you first get the unit thoroughly brought up to date (cleaned, recapped, tubes checked, etc.), and then consider the input modification if you are comfortable with the required work at that point. There are a number of folks here who can help you with that modification.

Good luck with your amp!

Dave

 

[dcgillespie]

Hi Tom -- No. The PEC makes up the frequency sensitive networks for the tone controls. The PEC can be replaced with discrete components if desired, but doing so is not part of the mod in question with the input attenuation networks and the selector switch.

Dave

 

[Tom_r]

Do R31 and C17 provide NFB? Can I move them to the 4ohm speaker connection since that is what I'll be using? Thx.

 

[dcgillespie]

Tom -- You can try it, but I wouldn't recommend it for two reasons: (1) It will reduce the feedback to 1/2 of the original amount which will alter the damping, distortion, and frequency response characteristics of the design, and (2) The HF stability components (made up of C17, R37, R34, and C22) were all carefully chosen based on the composite response characteristics of the output transformer that are presented at the 16 ohm tap. Using a different tap to take the feedback from invariably requires at least an alteration of these components, or even the use of a different HF stability scheme to achieve the stability margins of the original design. This is one of those areas where indiscriminately changing components and connections around can really invite troubles.

Regarding this issue, you might check out the recent thread on the Fisher forum where Derekva was attempting to clone an Fisher 30A amplifier with an exact build of the original 30A audio circuit, except he was using a different Fisher EL84 OPT that was "basically the same" as the originally one. It is very enlightening regarding this topic, and shows how the HF stability network topology and component values are typically very targeted to a given design, and specific components and connections that are used.

I hope this helps!

Dave

 

[Tom_r]

Thx Dave, it was a thought as I read about someone moving the components from the 16 ohm tap to the 8 ohm tap (on an Eico ST-70 IIRC). I'll just leave well enough alone.

I guess those EE's really did know what they were doing all those decades ago! :thmbsp:

 

[Tom_r]

Well, I finished replacing all the film caps, e-caps, power supply caps, and a few resistors that were in the replacement path. I attached a speaker and my rebuilt Dynaco FM-3 tuner, turned on the power and the result ... hum and a glowing bright red V4 (6BQ5) and no audio what so ever. The filament adjustment did nothing to reduce the hum. I opened the chassis and nothing looks or smells burnt.

The amp was humming when I got it but I figured that was because someone had removed two of the three replacement power supply caps. I installed three new e-caps to replace the power supply can. I also installed new correct value 1 watt metal oxide resistors across the replacement PS caps.

All of the tubes test good with my calibrated Eico 666 so I don't think that's a cause.

I'm wondering if anyone can suggest a plan of attack to diagnose the problem(s). Could there be a bad resistor? Can I pull all the tubes except the rectifier and check the voltage at the PS caps? I have a variety of test gear including VTVM and Tek 2246 scope but don't relish turning on the power to check anything lest I melt V4.

Thanks.

 

[dcgillespie]

Hi Tom -- before turning it back on again, check that you are reading 210 ohms from pin #3 of each 6BQ5s to ground. Then check that you are reading 470K from pin #2 of each 6BQ5 to ground. Also, make sure the polarity is correct on C32 if you replaced it, or that it is not shorted if you have not. Finally, measure the the resistance from the red lead of the OPT to pins 7&9 of both 6BQ5 tubes. At pin #9, it should read less than 100 ohms, and pin #7, closer to 200 ohms, but the readings will not be exactly equal between the tubes. If any of your readings do not match this information (within reason), then stop and investigate at that point, as any of these items can cause the condition you are having. Of course, also check for any obvious shorts, solder bridges, etc.

If all is good with the previous readings, then turn the unit back on with the output tubes reversed from the position they were in when one of them red plated. If the problem stays with the tube, then one of the tubes is at fault -- but it may NOT be the one that is red plating. If the the tubes are seriously mismatched, or the tube that is not red plating is seriously weak, then enough bias will not be developed across the common cathode bias resistor, causing the one good tube to red plate in either socket. Of course, a tube can develop defects that will cause itself to red plate as well.

On the other hand, if both tubes will red plate in the same socket but not the other, then there is a problem with that socket, or the wiring associated with it.

I hope this helps. Let us know what you find!

Dave

 

[Tom_r]

----> thank you Dave :thmbsp:

From Ground to pin 3 measured 210 ohms at both 6BQ5's.
From Ground to pin 2 measured 470k ohms at both 6BQ5's.
From red lead of OPT to pin 7 measured 185 ohms at V5 6BQ5, and 150 ohms at V4 6BQ5.
From red lead of OPT to pin 9 measured 68 ohms at both 6BQ5's.

Replacement C32 installed correctly.

One problem found -> one of the 6BQ5 tubes was bad as it red plated when switched to the other socket.

Install another pair of 6BQ5's ... no red plating, but still no audio.

Second problem found -> pins were dirty on my replacement NOS 6AN8A.

Now we have audio!!! :yippy:

Right now I'm using a Monitor Audio Silver 10i centre channel speaker connected to the 4 ohm tap. After a few hours the OPT and PT are fairly warm so things seem to be OK.

The audio is detailed with no audible hum, however the treble is a little rolled off and the bass more rolled off ... using various brands of vintage US and European tubes (and the RCA's which were red plating LOL). I don't use tone controls. Maybe I'm just used to the (blasphemy coming ... please close your eyes if you sensitive to SS amp talk) bass slam and detail of my Krell amps, but the Dynaco FM-3 tuner lacks nothing as it sounds astounding in my main system. There are still more parts I have to install including changing the input and PEC caps/resistors to film types. And that will be followed with the attenuation mod. However, overall I'm impressed with the sound quality vs $$ spent and I like how it attacks the music rather than being laid back. I plan on using the Heathkit in a mono system with the coolest tuner I've seen: a Bogen R660.

Just a few more questions:

1) Is there a way to test for the condition which causes the 6BQ5 tube to red plate.
2) Do you make house calls? I have a few beers for you!! LOL Years ago I was in Atlanta at this time of year and it was pretty toasty for someone from up north. I had a great time in Buckhead!
3) Are the resistance values you provided common for PP 6BQ5 amps? I'm just wondering as that could be a simple check when buying an amp.
4) Are there perhaps 6BQ5 friendly speakers? I'm listening to two systems concurrently which are in separate rooms outside of this room:
-20 feet to the west is my second system with a rebuilt HH Scott LT-110 tuner that audio-wise isn't in the same league as the FM-3.
-15 feet to the north is the AA-13 (third system) with FM-3.

The AA-13/MA 10i combo definitely leaves some of the instruments at the door vs the Krell/B&W Matrix 804 second system combo. This is my first (working) tube amp and from what I've read tube amps should blow the doors off SS amps but that doesn't seem to be the case. Not complaining, just wondering as there's always room for improvement.

Thanks.

 

[dcgillespie]

Hi Tom --

1. Other than trying the defective tube with other tubes and seeing if it is always the culprit, that's about the best that most folks can do. Most tube testers do not test power tubes under conditions of actual use, so their results can be misleading with these types of problems.

The real way to test a power tube is with a power output type tube tester that provides conditions where a power tube can develop its maximum possible current flow (within rated limits) under dynamic conditions that mimic actual conditions of use. Under those type of test conditions, a tube that will pass a simple emission or transconductance test in a commercial tester will either sink or swim real quick to cull it out.

After years of frustration with commercial testers, I finally built my own such device. It is not hard on a tube (always operating it within its design limits), but the results are extremely accurate and brutally honest. Your experience with tubes that test "good" but fail in application are hardly unusual. Welcome to the limitations of tube testers!

2. Atlanta can get pretty toasty for sure! I live out in the country about an hour north of Buckhead in the foothills of the mountains, so I am pretty isolated from all of the Atlanta "stuff" which is just fine with me. If your ever back this way again, you're always welcome for a visit. Or, I do respond to airline tickets! (hee, hee, hee....) Actually , I surely hope to take a trip across your fine country in the next few years, so who knows?

3. The resistance values I quoted for the output transformer readings would certainly be good, which is THE most important part (discard the pin 9 readings if it is not an Ultra-Linear transformer, which the Heath transformer is). The output and power transformers, and various switches and pots are the usual items that are not commonly available anymore, meaning that if they are bad in a given unit, then finding a good junker to donate parts from becomes the order of the day. Most other parts can all be replaced with modern components.

4. Efficient speakers are tube friendly speakers and will work best with the AA-13. Speakers of at least 95db/meter/watt will work best.

If the amplifier lacks expected frequency response, first make sure that the tone controls are accurately centered. If you want to bypass them, it is a rather simple matter in the AA-13, which will also eliminate V2 in the process. If you are interested in this modification, let me know and I'll provide the steps. The losser type tone control design used in this unit have only so-so on center accuracy, so this may just be the ticket for you.

Good luck with it!

Dave

 

[BrokenHill]

Hi Tom, when you get to changing or upgrading the other components it's very likely
you will get improvements in the treble and bass.
When I rebuilt my AA-13 amps I did one at a time in little steps and then compared
it against the other amp, these amps responded very good to the new components and
progressively got better sounding.

BTW. that's a cool looking Bogen tuner you have.:thmbsp:

 

[Tom_r]

....

4. Efficient speakers are tube friendly speakers and will work best with the AA-13. Speakers of at least 95db/meter/watt will work best.

If the amplifier lacks expected frequency response, first make sure that the tone controls are accurately centered. If you want to bypass them, it is a rather simple matter in the AA-13, which will also eliminate V2 in the process. If you are interested in this modification, let me know and I'll provide the steps. The losser type tone control design used in this unit have only so-so on center accuracy, so this may just be the ticket for you.

Good luck with it!

Dave

Thx Dave.

4) Ah huh, there's trouble. All my small speakers are 88-89db/m/w. I'll keep my eyes open for a single more efficient speaker.

I would be interested in the tone control mods ... at your convenience.

Hi Tom, when you get to changing or upgrading the other components it's very likely
you will get improvements in the treble and bass.
When I rebuilt my AA-13 amps I did one at a time in little steps and then compared
it against the other amp, these amps responded very good to the new components and
progressively got better sounding.

BTW. that's a cool looking Bogen tuner you have.:thmbsp:

Thx for the support Brokenhill.

I was going to use either a Sherwood S-2000 or S-3000 III mono tuner I have on the 'to be rebuilt shelf', but I really like the look of the Bogen even more so it moved to the front of the to be rebuilt line. If it doesn't sound very good when done I'll put it on the 'to be looked at' shelf and use a Sherwood. I don't need to worry about tuning distant stations so that's not a factor for me with FM tuners.

BTW, there was a matching (integrated?) amp for the Bogen tuner. The tuner dates from around 1957 and is in pretty good shape. It must have spent most of it's life in some sort of cabinet as there isn't much dust in the nooks and crannies. I've attached a few more pics if you're interested.

 

[dcgillespie]

Tom -- to bypass the tone controls in your amp, simply disconnect the shielded cable that connects to the wiper of the treble control, and the lead that connects to the wiper of the volume control. Then connect the shielded cable that was connected to the treble control wiper to the wiper of the volume control. Ground the lead that was originally connected to the wiper of the volume control. And that's it.

If the voltage rating of C2, C3, and C4 is sufficient, the V2 can simply be removed with this modification, as it is no longer in the signal chain. The B+ voltages to V1 will rise some, but this is not a bad thing at all.

The perceived gain of the unit will likely change somewhat, but I doubt that it will amount to anything significant (assuming that you were operating the unit with the tone controls set at "flat"). The gain that V2 provided is largely canceled out by the loss of the tone control network, so removing both from the circuit should produce only minor changes in gain. If the gain is altered significantly, it can be compensated for by altering the input reduction networks for the high level inputs.

This modification will allow the unit to operate completely flat. If the gain changes are minimal, and you then do the input modifications mentioned earlier, the high level inputs will basically then be going through the level control, and straight into the power amp section, meaning that the unit is basically then operating like a basic power amplifier with a gain control added.

I hope this helps!

Dave

 

[BrokenHill]

I really like the Sherwood S-2000 tuner and some of the other AK members seem to like
them as well although I do not own one, after I saw the last two pics of the top of the
Bogen I see why you like it so much, that tube tuner is awesome looking and I hope it
sounds as good as it looks, thanks for sharing those pics.

The AA-13 amps I have are in such excellent condition I am going to leave these units
full functioning, the tone controls do work very nicely and I do use them a little, they
don't need much turning of the knob to get what you like to hear.

The removal of the tone circuit sounds very easy to do from Dave's instructions and I
think that is very nice of him to take the time to explain how that can be archived for
anyone who would like to bypass the tone controls, actually I will copy that post and
save it in my HeathKit AA-13 folder just in case I change my mind in the future,
thanks Dave.:thmbsp:

BTW. the speakers I am using are 94db Realistic Mach two's, I know a lot of people
don't like those speakers but they sound very good with these AA-13 amps of course
the crossovers been have upgraded with better caps, maybe that's one of the reasons
I get such good bass, the AA-13 amps have no trouble at all driving the 15 inch woofers
at all, I do have a Fisher 400 and it never had the synergy with those speakers like these
AA-13 amps have, actually I had two Fisher 400 receivers and both of them did not
sound all that great on the Mach two's although they did sound very good on other
speakers I have.

Robert

 

[Tom_r]

Tom -- to bypass the tone controls in your amp, simply disconnect the shielded cable that connects to the wiper of the treble control, and the lead that connects to the wiper of the volume control. Then connect the shielded cable that was connected to the treble control wiper to the wiper of the volume control. Ground the lead that was originally connected to the wiper of the volume control. And that's it.

If the voltage rating of C2, C3, and C4 is sufficient, the V2 can simply be removed with this modification, as it is no longer in the signal chain. The B+ voltages to V1 will rise some, but this is not a bad thing at all.

The perceived gain of the unit will likely change somewhat, but I doubt that it will amount to anything significant (assuming that you were operating the unit with the tone controls set at "flat"). The gain that V2 provided is largely canceled out by the loss of the tone control network, so removing both from the circuit should produce only minor changes in gain. If the gain is altered significantly, it can be compensated for by altering the input reduction networks for the high level inputs.

This modification will allow the unit to operate completely flat. If the gain changes are minimal, and you then do the input modifications mentioned earlier, the high level inputs will basically then be going through the level control, and straight into the power amp section, meaning that the unit is basically then operating like a basic power amplifier with a gain control added.

I hope this helps!

Dave

Thx Dave. Sorry to be a pest but just to clarify in my mind, now the PEC is no longer part of the tone control circuit so if the voltages you mention are are OK I don't need to update the PEC unless I want the practice.

I'm not too worried about change in gain. I did always have the tone controls set flat ... but that doesn't mean that's where they actually were. I prefer to control tone with tube rolling.

Are there any other mods out there. I don't think much left that we haven't touched, but just wondering.

BTW, I was looking at a Fisher 400 before I bought this but the unit was incomplete and missing all tubes and the seller still wanted an arm and a leg so I didn't pursue that any further.

 

[dcgillespie]

Hi Tom -- You've got it correct. The PEC is still part of the tone circuit, but since the modification completely bypasses the tone circuits, updating it would simply be a practice procedure only. With the bypass modification in place, the PEC is no longer part of the signal path to affect anything, updated or not.

There are some other mods for the power supply and output stages sections, but I would address those already on your plate before attempting any more so you'll be able to keep track of what mod has produced what effect!

Within its power range, the AA-13 has greater performance potential than does the Fisher 400. That is hardly to disparage the Fisher (I have one), but is simply based on the design goals that were set for each unit.

Dave

 

[BrokenHill]

I think the Fisher 400 is a very nice performer and that was why I had a hard time
believing how these AA-13 amps sounded so good when compared to the more
powerful classic 400.
With the Heathkit AA-13 it's the sound quality that I definitely prefer over the 400
I find the stereo imaging and bass to be a lot better then the 400, the vocals and
instruments just stand out in their own space.
Good Stuff.

Robert