Heathkit AA-151

the thermionic watercooler

Postby DeathRex » Thu Mar 22, 2012 3:32 pm

Or a 5C4M short Russian version of the 5Z4. It'll drop about double what a 5AR4 will, although the current capacity will be close, and you can only use a smallish capacitor after it, 10-15uf.
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Message for DR, DC and Terry

Postby Linn lover » Thu Mar 22, 2012 6:37 pm

DR, Terry,

Thanks for the all the ideas guys. Gonna wait for the choke to be installed before I make any further adjustments to the power supply. Once the supply is figured out, I will install new catalytic caps.

The Amp is sounding really good and it will only get better from here on out I can imagine. I would like to get another unit so I can play around with it and learn more. I think I will focus on the AA-151 for now. Maybe play around with a Dynaco unit in the future as they seem pentiful.

Dave, DR,

Below is the latest voltage report. Like I stated in the previous paragraph, I will not make further changes until the choke arrives. After the choke is installed, I will remove the 220 ohm resistor but will (must I guess) leave the 30 micro farad cap between Pin #8 of rectifier and the future choke.

I did tonight replace the feedback resistors. They had also drifted high in value. I replaced them with 28k (R29 and R51) and 1k for R23 and R50. 28K is bigger than the original 27k value, but is much closer than 32k and 34k respectively for the old resistors that had drifted. The 1k units were around 1.15 k. So now, all new resistors and caps! I measure all resistors prior to installation. I picked ones that were just below 27K in value.

Voltage report:

Voltage in = 407 volts
Pin 8 of rectifier tube = 378 volts
After R65 in the power supply = 302 v
Pin 9 of EL 84 (screen I guess) = 376
Pin 7 of EL84 (plate) = 372
Bias = 14.41 volts over 120 ohm resistor. ** I will eventually replace these with 4 x 400 ohm with individual electrolytic bypass caps

6AN8

Right tube left tube

Pin 1 226V 233V
Pin 2 63.2v 56.8V
pin 3 74.3 67.4V
Pin 6 63.8 57.7
Pin 7 64.9 62.7
Pin 8 0 0
Pin 9 1.4 1.42

Not perfect, but much better than before. And the amp sounds darn well. I am surprised how much bass this little amp produces.

I cannot imagine, with the addition of a choke in the power supply (and removal of the 220 ohm series resistor), that the bass response will further improve. Impressive little amp!
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Message for DC (Dave)

Postby Linn lover » Thu Mar 22, 2012 6:41 pm

Dave,

Here in Ottawa (yes Canada), we have a person who has built 30,000 tube amplifiers. His name is H.H. Bloom and built soundmaster amplifiers. Not sure you ever heard of him or his amps, but here is an article if you want to read it (see link below).

He is 90 years old and still goes to his shop daily. I have met him a couple of times. Interesting fellow. FYI if you want to know about a local tube maker of our area.

http://www2.canada.com/life/sound/52177 ... id=5217771
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Postby dcgillespie » Thu Mar 22, 2012 8:13 pm

Jules --Thanks so much for passing that story along! I can relate with so much of what he did.

I am always so mindful of Canada's contribution to technology and the arts among other things. I think the one that still blows me away to this day is the story of the Avro Arrow interceptor. That aircraft still holds records that have been unmatched to this day. That such an incredible design fell victim to such self serving politicians is almost criminal to me. But it did, and the rest is history -- but what a contribution to mankind it was.....

Dave
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Postby TerrySmith » Thu Mar 22, 2012 9:11 pm

I would like to get another unit so I can play around with it and learn more. I think I will focus on the AA-151 for now.


You can't go wrong with an AA-151. The transformers are very high quality and sound excellent. There are other Heathkits also, all well designed units EXCEPT the AA-100! Well, the iron is really good on a '100, that's about it.
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Message for Terry and DC

Postby Linn lover » Fri Mar 23, 2012 8:48 pm

Terry,

I had a AA-151 when I was very young and after thinking that tubes were passé, my folks sold it in a garage sale. I always had a sweet spot for it and now I know why. It sounds so smooth. And now I have one again.

Dave,

The cold war made people scared and when in such a state, make decisions that are not well reflected. They destroyed the planes, but what I don't understand, they also destroyed the blue prints. Why?

But the fall out of all that genius was a benefit to the Lunar program as well as the supersonic Concorde. I always think that the Concorde is but a stretch version of the Avro Arrow.

RE: amp - sounds good, but the bass has a bit too much bloom to it. Like you mentioned in a previous post, I may be lacking the bass detail due to the series resistors used to lower the input voltage. Now that I know that voltages can rise to 390 VDC, I will be removing one of the resistors (there is a 100 and 120 ohm in series) and see where I stand. And of course, the choke will help a bit once it is installed. But this said, the choke is rated for 400 VDC and I got 407 volts at the input! Oh what to do..oh yeah, the zener diodes! Ok, under control!
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Message for DC (Dave)

Postby Linn lover » Mon Apr 09, 2012 7:26 pm

Dave,

I don't yet have the choke, but have 10 and 5 volt zener diodes. Choke should be in within 1 week.

Until the choke arrives, I am thinking of installing the diodes to help me get the voltages to levels that were of the original Heathkit design without affecting the sound too much by limiting current.

I have installed new Electrolytic can caps...a pair of 50 uf off the rectifier (as per the original design, but 50 instead of 60) and 32 uf after that. The physical install went well with the new caps.

I want to remove the 30 uf cap I previously installed with the 220 ohm series resistors (they were installed prior to the old 60 uf cap as a way to drop voltage in the amp) and replacing them with the zener diodes as required. I wish to install the 10 volt one first and then progress from there. That should get me from 407-409 volts down to 397 volts...and then lower to 390-385 volts at the tube screens and plates.

Any issues installing the diodes right off the rectifier output? Or, should I leave the 30 uf cap currently in place off the rectifier and put the diodes in between the 30 and 50 uf cap.

Can't wait to fire it up again. With all new components, and with the proper current flow, I am sure it will sound as good as new!
Cheers from Ottawa
Jules.
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Re: Message for DC (Dave)

Postby DeathRex » Mon Apr 09, 2012 8:39 pm

Linn lover wrote:Until the choke arrives, I am thinking of installing the diodes to help me get the voltages to levels that were of the original Heathkit design without affecting the sound too much by limiting current.

Don't normally use zeners because of the noise they generate. And the have a tendency to short or open, if too small of wattage is used.

Linn lover wrote:I have installed new Electrolytic can caps...a pair of 50 uf off the rectifier.
pair?

Seems like zeners will filter the AC even worse than resistors. After they drop the 10 or 5 volts the AC will just ride on top (5 volts ripple before, 5 volts ripple after). I might just be pulling this out of my ass, but it does sound correct. A choke will drop less DC, but more AC (better filter) than a resistor.

Don't overcomplicate it (like an engineer would). The AA-151 is simple, just put in a couple of components, better than when it was new, and you're done.
Last edited by DeathRex on Mon Apr 09, 2012 8:40 pm, edited 1 time in total.
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Re: Message for DC (Dave)

Postby DeathRex » Mon Apr 09, 2012 8:40 pm

What kind of output tubes are you using, EL84s, 6P14Ps, 7189s?
The last 2 can take 400V B+. If using real EL84s, I would lower the B+ to somewhere around 350-360.
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Postby dcgillespie » Mon Apr 09, 2012 9:34 pm

Death -- the Zeners were never meant to filter, because like you basically said, they can't filter anyway. But they are a very low impedance way to drop the B+ without adding any significant impedance in the B+ circuit. The choke he is installing will handle the filtering, while the Zeners will do the dropping. The Zeners can be bypassed for noise (and make far less than the PS ripple!), and used in small voltage values, their individual dissipation will be rather small.

Linn -- I would still have the cap directly on the 5AR4, then go through the Zeners, then the choke, and on to the output tubes. Put a small cap (to ground) at the junction between the Zeners and the choke to contain any kick back from the choke. What wattage rating Zeners did you get?

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Message for DR and DC

Postby Linn lover » Tue Apr 10, 2012 6:10 am

DR,

As per the original design of the AA-151, the output tubes are EL84s. As per the original design, they are to operate at 390 volts. Hot running it is!

DC,

I'll leave the 30 uf cap in place, install a zener diode, then the choke, and onto the remainder of the power supply circuit as per the original design.

The Zeners are 5 watt units as per your recommendation.

Dave, when you say to install a small cap to ground at the junction between the zeners and the choke, my question is, what value should I be looking for when you reference a small cap, and, must it be an electrolytic cap as well? All the caps in the power supply are electrolytics.

Cheers.
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Postby dcgillespie » Tue Apr 10, 2012 12:00 pm

Linn -- I'm sorry, I should have been more specific.

Placing a cap from the junction of the Zeners and choke to ground will help squash any Zener noise, and help absorb any kickback the choke might create. This cap need not be an electrolytic, and frankly, a good film cap between .47 and 1 uF should be just fine.

The zeners can be further protected by making a common ordinary 1N4007/UF4007 diode part of the Zener string (place at either end of the string with the arrow facing away from the rectifier), and another such diode reverse connected across the entire Zener string (arrow towards the rectifier), which will then protect the Zeners from virtually all transients, positive or negative, and make the Zeners basically bullet proof. Finally, another small cap (say .1 uF) placed across the Zener string will finish off any remaining possible noise from the Zeners. Due to their position in the power supply however, any noise they might develop will be extremely small to begin with.

I have used this very approach to minimize the voltage drop usually associated with common screen dropping resistors in pentode amplifiers, and it works exceedingly well. In my case, the choke and Zeners were much smaller since the total current flow in this case is much less. But the principle is the same. This approach provides much better filtering due to the choke, and a very low impedance way to drop the B+ as required, resulting in much better regulation, all versus that of the original resistor.

Keep us posted on your results!

Dave
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Message for DC (Dave)

Postby Linn lover » Tue Apr 10, 2012 2:07 pm

Dave,

I was thinking for sure I would need an electrolytic cap after the zeners, but happy to see a relatively small film cap will do the job.

I'll get some 1n4007 and wire them in and will wait for the choke to arrive before firing it up again. I'll report back at that time.
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Dynaco ST-70 - all original

Postby Linn lover » Tue Aug 21, 2012 6:59 pm

Hello DC et al,

Just a quick update: I have not touched my Heatkit AA-151 as we are having a great summer. I plan to return and finish it in the fall. All that is left is the zener diodes to drop the line voltage and the choke to install. The final mod will be the individual biasing resistors and caps.

But a question: A friend of mine is looking at acquiring a Dynaco ST-70 that is all original. Can you advise me what quirks or things to look out for on one of these units. Thank you.

I realize I should post in the Dynaco area, but just thought I would ask here quickly.

Cheers Dave....and fellow tubers.
Jules (from Ottawa).
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Postby DeathRex » Wed Aug 22, 2012 11:46 am

Are the transformers good.
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