Heathkit AA-151

[Sal Brisindi]

Tom,
That's a really nice Heathkit amp you have next to that car.....

Is that your daily driver.... the amp I mean....

Sal

[TomMcNally]

I did retrieve it from the attic after seeing all the great info in
this thread ... time to work on it. It "works" but needs some TLC.
Maybe a rebuild, not sure.

[ecir38]

Also did away with the tone PEC's and used discreet parts, I'm going to re-do that part to make it look neater.

Terry can you elaborate a little more about this. Is it a known mod or did you come up with it? I thinking that it is a Baxendall circuit.
http://www.duncanamps.com/technical/tonestack.html
Throwing the old tech tone controls out is a good idea. If a more modern replacement to work with the speakers we use these days is used I can see the reason to leave it in place. A artical in the June 09 issue of audioXpress discusses this briefly in a preamp build.

All awesome cars guys!!! A freind recently purchased one of those Maserati's, he is still single in his mid 40's so needless to say he has been having allot of fun with it.

Brad

[TerrySmith]

The tone control circuit is the same as the PEC's, just duplicated with individual parts. Sound quality is noticeably improved!

[TerrySmith]

NEXT!

Here is another AA-151 project. I "borrowed" the power transformer two years ago for my get*set*go amp but the insulation on the leads crumbled too much to try to salvage. I do have an extra AA-100 power trans, 375-0-375v with a bias tap which should net around 500v at the EL84 plates. My plan is to build it similar to the one pictured earlier except maybe a SRPP preamp section with 12AU7's in place of the 6AU6's. Fixed bias instead of cathode bias.

Before pics:

[ecir38]

Gotcha, didn't know what a PEC was but now do after some research. I like your approach of eliminating the PEC's. Maybe the Baxendall circuit used can be tweaked using the software in the link I provided above as a guide.

Cool that you have plans to do another one up. The cool thing about this intergrated is you can use any pre or phono you want with plenty of room to work.

Brad

[sorenj07]

I do have an extra AA-100 power trans, 375-0-375v with a bias tap which should net around 500v at the EL84 plates.

I'm not sure those are safe voltages for an EL84. Why not go choke input for around 330V? That should get you a lot more life, assuming the tubes last for longer than even a moment with 500V a-k.

[Ty_Bower]

I'm not sure those are safe voltages for an EL84.

Don't tell that to Roger Modjeski. He runs his at 700 volts. The key is to keep the screens pentode connected at 350 volts. I seem to recall the AA-151 output transformers have no UL taps, so maybe a similar strategy is appropriate.

Also note that Terry specifically mentioned he intends to run this amp fixed bias. I believe he also holds favorable opinions of Roger's work. Perhaps there is a little inspiration here?

[TerrySmith]

The AA-151 outputs have UL taps and is 10,000 ohm p to p. The AA-100 (7591) does not have UL taps.

I'm planning on running the plates around 500v and screens at 330v to 350v. The UL taps will be disconnected.

[ecir38]

Finally got some parts in. Here is how I reworked the PS.



Bias for the EL84 is in pairs with a 200 ohm resistor and 150 uF cap. With the B+ being a little lower with the 150 ohm choke the cathodes are at a conservative 32 mA. I have a 156r choke if I want to raise the B+ higher.

Will work on the the 6AN8 tommorrow.

Brad

[nyazzip]

terry what is that car? a ragtop corvair? i can't tell by the thumbnail
sorry to get off subject guys

[TerrySmith]

Yes, it's a 1965 Corvair Corsa convertible 140hp/4speed. I have owned it 25 years!

[ecir38]

Think I will settle on this till I find some 7247's. Probably will just buy two of the JJ's from Jim Mcshane to work with.



Thanks for all of your help.

Brad

[Greg Smith]

The AA-151 is a great project amp. I resurrected one about ten years ago. One thing I noticed was that hum was reduced significantly after I relocated the power switch to the back of the chassis (in place of one of the convenience outlets).

Greg

[TerrySmith]

Here's the progress I've made on the second AA-151:

The EL84's are seeing 460v on the plates, 330v on the screens, fixed bias at 25 ma per tube, no red spots, no weird stuff and noticeably more power. I do need to work on a better way to control screen voltage as it varies greatly depending on bias current. I might try to cram in a regulator circuit or maybe an independent power supply.

For the linestage section I used tubebuilder.com 6SN7 circuit and wired it for 9A tubes. Worked out very well. The tone controls are like the other AA-151 except I mounted the parts on a Rat-Shack perf board.

The phono section is like the other one also except LED bias. I'm going to yank the LED's as it cuts the gain somewhat.

I cut the chassis brace down as you can see and added an angle reinforcement underneath. The input wiring is routed through a piece of 5/16 steel fuel line with mounting tabs soldered to it.