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Sam's magical AA 151

mjalazard

mjalazard

Well-Known Member
When it comes to work on my audio, I know my limits. I think I have become quite adept at upgrading loudspeakers
http://www.audiokarma.org/forums/showthread.php?t=487987
In fact, I have a new project with a pair of Infinity Qb's underway.

When it comes to electronics, my limit is tube rolling. Fortunately, I have found skilled technicians to take care of my audio.
I have great admiration and respect for Sam Kim of Sam's Audio Labs in Canada.
http://samsaudiolabs.com

He just shipped back my Heathkit AA151 with his Platinum Triode modification and MC phono circuit.
I have it temporarily hooked up to the (pre-mod) Qb's and my trusty little Sony D-555 Discman...what a sweet sounding amp!

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I cannot believe how great it sounds!. The AA-151 drives the Infinity Qb's with ease at less than 10 watts per channel.
:D
Mike
 
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I love the sound of an AA-151. A lot of the magic comes from the 51-29 OTs. I'm restoring/modding a 151 now.

John
 
I just hooked up my Sony PSX-800 with an Ortofon Kontrapunkt B cartridge.
Sam's MC phono stage is dead quite and highly musical. I'm having lots of fun. Gotta go, my wife wants to dance!
:music:
Mike
 
I like the potential of these amps but don't think much of the original ckts aside from the output end, they really beg for a re-design in the phono and tone stages for starters.
 
mjalazard said:
When it comes to work on my audio, I know my limits. I think I have become quite adept at upgrading loudspeakers
http://www.audiokarma.org/forums/showthread.php?t=487987
In fact, I have a new project with a pair of Infinity Qb's underway.

When it comes to electronics, my limit is tube rolling. Fortunately, I have found skilled technicians to take care of my audio.
I have great admiration and respect for Sam Kim of Sam's Audio Labs in Canada.
http://samsaudiolabs.com

He just shipped back my Heathkit AA151 with his Platinum Triode modification and MC phono circuit.
I have it temporarily hooked up to the (pre-mod) Qb's and my trusty little Sony D-555 Discman...what a sweet sounding amp!

IMG_2560-XL.jpg


IMG_2564-XL.jpg


IMG_2555-XL.jpg


IMG_2544-XL.jpg


IMG_2547-XL.jpg


IMG_2546-XL.jpg


IMG_2545-XL.jpg


I cannot believe how great it sounds!. The AA-151 drives the Infinity Qb's with ease at less than 10 watts per channel.
:D
M
Click to expand...
Hi Sir do you know if somebody selling that modified heathkit aa151 triode? i love to listen that amp in triode mode.
 
lovegudmusic said:
Hi John, Do you have heathkit triode modification schemetic? i would love to do that modification on my heathkit aa151. Thanks.
Click to expand...
Triode mod is very simple. Here's a section of the AA 151 schematic:
aa151.png

In stock form the output stage is ultralinear - there are taps from the OT going to Pin 9 of the 6BQ5s which are the screens (aka G2). To change to triode just remove those connections (green and green-yellow wires) from the socket pins and shrink wrap the ends so they can't contact anything. Then just solder a 100 ohm resistor between the plate (Pin 7) and screen (Pin 9) of each of the 6BQ5 output tubes.

You could also install a switch to choose between UL and Triode operation.

I don't own one of these and I don't know if the negative feedback should be modified if you make the change. Do a search and I'm sure you'll find threads here and elsewhere about this and other mods to this model.
 
I ha
mjalazard said:
He just shipped back my Heathkit AA151 with his Platinum Triode modification and MC phono circuit.
I have it temporarily hooked up to the (pre-mod) Qb's and my trusty little Sony D-555 Discman...what a sweet sounding amp!

IMG_2560-XL.jpg


IMG_2564-XL.jpg


IMG_2555-XL.jpg


IMG_2544-XL.jpg


IMG_2547-XL.jpg


IMG_2546-XL.jpg


IMG_2545-XL.jpg


I cannot believe how great it sounds!. The AA-151 drives the Infinity Qb's with ease at less than 10 watts per channel.
:D
Mike
Click to expand...
Technically, this is no longer a HK aa151 since the preamp stage has been radically changed and apparently the output stage is also different. The original 151 used 6eu7 phono, 6au6, preamp and 6an8 for the splitter/driver for the el84s. So, with vastly different circuit the only thing left is the chassis and transformers. It would be interesting to know what circuit he copied and i hope he gave you a schematic for future reference. cheers.
 
Nice looking setup you have there. I love the look of this era of Heathkit, but not necessarily the sound. I own an aa100 and would love to make it sound as good as it looks, so your unit gives me hope. They especially look good stacked up with the matching Tuner.
 
As Primo says, this is a redesign. I too recommend that you get a schematic for future reference, even if it is supplied on condition that you do nor publish it.
 
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