Heathkit AA-151

the thermionic watercooler

My amp with Zeners and choke installed

Postby Linn lover » Sun Oct 14, 2012 7:50 pm

Hello Dave (Gillespie), DR, Terry, Randy et al,

Rainy day up here in Canada so I dusted off the AA-151 and got her done.

Well to a point. Zener are in to drop the voltage and the hammond choke has been installed.

I powered it up and it sounds fine. Critical listening with good speaker is next. I will report back.

It's not optimum, but she's stable and running quite equal on both channels.

Here is the voltage report:

AC in at Pin #8 of the rectifier: 405 VDC
After the Zeners & choke: 381 volts
After the 1k resistor in the power supply: 300 volts

The Bias is at 14.5 volts over a 120 ohm resistor. (I may put back in the 100 ohm resistor in the short term. In time I would like to bias all tubes independently).

The 6AN8 is equal (within 1 volt between each channel) on both channel and close to each other.
Pin #1 is at 234 volts
Pin #2 = 53 volts
Pin # 3 = 65 volts
Pin #4 = 48 volts
Pin #5 = 48 volts
Pin #6 = 55 volts
Pin #7 = 64 volts
Pin # 8 = o volts
Pin # 9 = 1.45 volts

I am still off, but I have come a long way. The amp runs quiet and stable and seems to sound clear.

Do I increase the voltage to the EL84 by +5 volts...or do I leave it as such.

I can easily change the bias resistor for a bit more power (from 10.5 watts to 12.9 watts per channel if I install a 100 ohm resistor).

Comments?

Again, thanks for all your help (Especially Dave G.) and your comments (Death Rex, Terry).

PS: If I find the sound is lacking, I will order the proper choke with a lower DCR.
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Heathkit AA-151 breaking in

Postby Linn lover » Tue Oct 16, 2012 8:32 pm

For information purposes only.

I hooked up my amp and let it run with music for a couple of hours yesterday and today.

It was sounding a little bit nasal at first. I guess with all the new parts some warming up was in order. It got better after about 2 to 3 hours, but was still not sounding it's best (still pronounced in the mid-range)

I had been running the amp cool out of caution (lower voltage than necessary on the plates and screens, and, lower bias voltage and current).

So onto tweaking: Tonight I changed the bias resistor from 120 ohm to 100 ohm as per the original design. I then put the amp back into service. It was immediately sounding better. A warmer sound and the highs were more extended and smoother at the same time. At least now the amp was enjoyable to listen to.

Dave G: the zener (a 10 and a 5 volt) diodes are doing the trick to lower the voltage and I put in the 1n4007s to protect the zeners along with the bypass cap as per your instructions. Amp is running very quiet with the choke.

DeathRex: The choke in the circuit is a Hammond 156R....has 56 ohm of DCR. More to come.....stay tuned.

The next step will be to remove one of the zeners to increase the voltage a bit to the plates and screen. They are at 374 and 378 volts respectively. I will try to get them closer to 390 volts as per the original heath design. I will then take notes to see how this will improve the sound overall. The bass seems a little shy at this time.

My final tweak will involve work around the choke to see how it is affecting the sound overall.

The final modification will be individual bias resistors - but that will come later.
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Postby DeathRex » Tue Oct 16, 2012 8:49 pm

How many watts are the EL84s running? If it sounds better with more current, might just push them up to 80-90% of their limit, as long as they sound better. If you're up to it, you can install Dave's EFB circuit, it will get the most out of the EL84s and run them cooler.

That choke is fine, .5 henries is better than a resistor. If the EL84s are running low still (below 10 watts) remove the zeners and see how it sounds with a bit more voltage.

What kind of EL84s are you using?
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Message for Death Rex

Postby Linn lover » Tue Oct 16, 2012 9:11 pm

Hi Dr,

My last calculation had them running at about 10.9 watts (Total bias current divided by 4). Do I have to remove some current for the screens too? I forget. If I do, the power would be less than 10.9 watts.

The amp was originally rated at 14 watts per channel.

Tonight I decreased the bias resistor to 100 ohm. I did not do any measurements (bias voltage or current calculations). I just put the amp back into service. It sounded better with more current.

I did read about Dave's EFB circuit. He did mention in his article that because of the topology of the transformers that the AA-151 will not greatly benefit from this modification. The Dynaco ST-35 will! So I pushed that mod out until later after I get the amp singing at the top of it's game.

The Choke is actually a 1.5 henries (400 Volt) unit with a DCR of 56 ohms. I was told a higher DCR will affect the sound.

I do plan to bypass the choke to see how that will affect the sound & how much noisier the amp will be. Edcor do offer lower DCR chokes (17 or 21 ohms), so I may consider that option as well. The advantage of the Hammond unit is that it is small and fits nicely the body of the AA-151. From the material I read, the Edcor units will be bigger and heavier. Not a big issue if it all fits nicely in the body of the amp.

To continue answering your questions, I was planning to remove one of the zeners, and or the choke, to raise the voltage to the plates and screens and see how that affects the sound. One step at a time (first one zener, then test - then the choke and test again).

I want to document and share what components do to the sound in the amplifier.

The EL84s I am using are JJ EL 84 that I bought from the tube store here in Canada:http://thetubestore.com/teslael84.html

They also offer the following:
Sovtek
Electro-Harmonix
Mullard re-issue
TAD
Preferred series 7189 premium EL84
Tungsram
Siemens NOS E84L (can take more of a beating they claim)

Ciao
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Postby DeathRex » Wed Oct 17, 2012 8:57 am

A choke from 0.5 to 10 henries would be good. I usually use one from 0.5 to 2 henries, because they are smaller and cheaper. Alot of people like the 10 henries, I don't know why it's a magical number. Chokes are used to get rid of AC while passing DC, so any choke more than 0.5 henries is gonna be better than a resistor. That choke seems to be a copy of the Dynaco choke used on the ST-70. So it'll work great on the AA-151, where Heathkit didn't even have a resistor to filter the AC and must have had some 3-5 volts of ripple going to the EL84s.

I need to see a schematic of what you have so far. I was asking about the brand of EL84 because EL84s are supposed to have a max plate voltage of 300 VDC. Heath and others pushed it past 300 (some up to 400) to get more out. 7189s and Russian 6P14P-EVs have a max of 400VDC, JJ says their max is 300.

What is the filament voltage? What is the B+ at the rectifier, and after the choke?
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Message for DR and DG

Postby Linn lover » Thu Oct 18, 2012 7:51 pm

Dr,

I am going to have to get back to you with the latest measurements. Tonight I removed one zener to get the voltage up to 388 volts on the screen and 384 on the plates.

Powered up the amp and it is sounding just peachy now.

So my question is: does the choke help or not? It obviously filters and lowers my DC a bit (which I needed), but does it contract the sound?

DG - don't know if you are still around but the amp is sounding mighty fine. Voices, guitar strings are top notch if not beyond belief. The bass is a bit shy, but that is to be expected with only 14 watts on tap. But all the information is there and the imaging is very good.

Who would of thought that an old amp like the Heathkit AA-151 has so much potential.

My next step will be to install individual bias resistor for each tube. Not in a hurry and will enjoy the amp for awhile. But that would be my next step.

FYI - I drive Reference 3A Decapo MM with the amp. They seem to be a great match. A fairly sensitive speaker + stable load into 8 ohms.

Over and out for tonight.
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Re: Heathkit AA-151

Postby micajah » Sat Aug 01, 2015 8:00 pm

Resurecting an old post - Having read this and just finished a rebuild on an AA-151. I've got a few questions.
I've replaced the Cap Can with a 60/40 and the second can with a 30/30. All .5 and .01, .02 caps have been replaced with mallory. Fresh NOS 6AN8A's and 6EU7's were installed though it seems to want to minor redplate. I've been waiting to replace the EL84's until it's closer to being finalized. 3 of the 4 EL84 tubes tested good on my tester and I had a JJ to sub in to get it running.
The EL84 50uf /100uf 7W bias cap and resistor have been replaced with 6uf/400v cap and a 100uf/10W resistor
The 470K resistors on the EL84's were all drifted up and were replaced.
The 50uf caps and 2.7k resistors on the 6EU7's were replaced.
#1 Having a low 120hz hum and some crackling even with volume down.
#2 At volumes over 3/4 dial on low transients I'm getting a oscilllation in the right channel that wont stop unless I turn the volume down all the way.
I thought the feedbackloop resistor might be bad but test well under the 10% drift.
Compared to the voltages in the previous posts, mine (listed below) seem to be comparable EXCEPT for the 6AN8A's which seem low.

Transformer voltages R-RY-R 320v both legs, G&G 6.4v, B&B 6.4v
Rectifier GZ34 pin 8 - 381v
Filter caps B+ (60) a 381v, (40) b 315v, cap2 30a - 307.5v, 30b - 228v
EL84s
V4 2-53mv, 3-13.3v, Heater 6.1v, 7 - 377v, 9 - 380v
V5 2-54mv, 3-13.4v, Heater 6.0v, 7 - 378v, 9 - 330v
V9 2-60mv, 3-13.4v, Heater 6.4v, 7- 379v, 9 - 380v
V10 2-64mv, 3-13.4v, Heater 6.4v, 7 - 375v, 9 - 379v

6AN8As
V3 1- 268v, 2 -32.6v, 3 - 47v, Heater 6.4v, 6 - 32.7v, 7 - 54v, 9 - 1.5v
V8 1- 260v, 2- 41.0v, 3 - 54v, Heater 6.2v, 6 - 41.4v, 7 - 61v, 9 - 1.5v

Will new 84s clean up the hum and crackle? also whould I replace them with El84M Sovteks?
Thanks ahead of time for any insight or help.
Micajah
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Re: Heathkit AA-151

Postby DeathRex » Sat Aug 01, 2015 10:00 pm

micajah wrote:The EL84 50uf /100uf 7W bias cap and resistor have been replaced with 6uf/400v cap and a 100uf/10W resistor

What? Do you mean you replaced the cap and resistor with a 6uf/400V cap and 100ohm/10W resistor?
The resistor is good, but the cap should be 50uf or higher at 25 volts. Using only 6uf will give you less bass.

micajah wrote:#1 Having a low 120hz hum and some crackling even with volume down.

Could very well be a tube, or in my case a socket. I had to replace all of them.

micajah wrote:#2 At volumes over 3/4 dial on low transients I'm getting a oscilllation in the right channel that wont stop unless I turn the volume down all the way.
I thought the feedbackloop resistor might be bad but test well under the 10% drift.

Move tubes, one at a time from the right channel to left to see if it follows a tube. If it doesn't, compare levels between the two channels, making sure they are the same.

micajah wrote:Compared to the voltages in the previous posts, mine (listed below) seem to be comparable EXCEPT for the 6AN8A's which seem low.

Transformer voltages R-RY-R 320v both legs, G&G 6.4v, B&B 6.4v
Rectifier GZ34 pin 8 - 381v
Filter caps B+ (60) a 381v, (40) b 315v, cap2 30a - 307.5v, 30b - 228v
EL84s
V4 2-53mv, 3-13.3v, Heater 6.1v, 7 - 377v, 9 - 380v
V5 2-54mv, 3-13.4v, Heater 6.0v, 7 - 378v, 9 - 330v
V9 2-60mv, 3-13.4v, Heater 6.4v, 7- 379v, 9 - 380v
V10 2-64mv, 3-13.4v, Heater 6.4v, 7 - 375v, 9 - 379v

6AN8As
V3 1- 268v, 2 -32.6v, 3 - 47v, Heater 6.4v, 6 - 32.7v, 7 - 54v, 9 - 1.5v
V8 1- 260v, 2- 41.0v, 3 - 54v, Heater 6.2v, 6 - 41.4v, 7 - 61v, 9 - 1.5v

Will new 84s clean up the hum and crackle? also whould I replace them with El84M Sovteks?
Thanks ahead of time for any insight or help.
Micajah

6AN8's triodes are barely working. The voltage on pin 6 should be near 90 to bias the triode section. Check or substitute R20 220Kohm. You should have about 300 on one side and 90 on the other. The 330 volts on the second EL84 has to be a misprint, it's impossible, impossible I tell you. It just can't happen that way. Stop looking at me, stop looking at me. You're all fools. Aaaaaaaaaaaa.
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Re: Heathkit AA-151

Postby micajah » Sat Aug 01, 2015 10:59 pm

Thanks for the response
DeathRex wrote:eplaced the cap and resistor with a 6uf/400V cap and 100ohm/10W resistor?

Typo Typo Typo it is a 60uf/400v cap.
DeathRex wrote:Could very well be a tube, or in my case a socket. I had to replace all of them

Already re-tensioned the El84's sockets. And already had to replace one of the 6AN8's socket because of a brittle solder connector.
DeathRex wrote:Move tubes, one at a time from the right channel to left to see if it follows a tube. If it doesn't, compare levels between the two channels, making sure they are the same.

Did that already to see if it was a tube - but it stayed on the right side. Are you talking voltage levels at the speaker connects?
DeathRex wrote:The voltage on pin 6 should be near 90 to bias the triode section. Check or substitute R20 220Kohm.

Will try that.
DeathRex wrote:The 330 volts on the second EL84 has to be a misprint, it's impossible, impossible I tell you.

I'll re-check that one.
DeathRex wrote:stop looking at me.

I'm not looking.
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Re: Heathkit AA-151

Postby DeathRex » Sun Aug 02, 2015 9:31 am

micajah wrote:Typo Typo Typo it is a 60uf/400v cap.

Is that a film cap? I believe Ed says it's best to use a electrolytic cap at 80% of it's rated voltage. It might sound better if using a 25 volt cap, I don't know. I would get a Nichicon Muse cap for the job. Like a KZ or KA series.
micajah wrote:Did that already to see if it was a tube - but it stayed on the right side. Are you talking voltage levels at the speaker connects?

Yep. Just put in a 1000 Hz sinewave and measure AC on the speaker terminals. Best to have a 8ohm 25 watt resistor across speaker terminals, instead of speakers. Don't leave terminals disconnected.
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Re: Heathkit AA-151

Postby micajah » Mon Aug 03, 2015 10:28 am

It is an electrolytic - I'm actually going to split the bias resistor/cap into the 2 sections mentioned in some of the earlier posts and I'll try the reduced voltage on the caps.
And I understand about the speaker outputs - I have a dummy load for tests.
Thanks
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