Heathkit AA-151

[DeathRex]

I've had a AA-151 for about 10 years now, just sitting on the shelf. A friend gave it to me, it's been sitting in his barn for several decades, it was there when he moved in. It worked but is very noisy and dirty (lots of poop in it) . I cleaned it up, tried to clean the sockets, but it's still noisy. especially when you wiggle a tube around.

In the process of recapping it, I found one black cap that was making noise by itself. After replacing them I was testing it, and found that the treble tone control adjusted alot lower that it was supposed to, down to 600 Hz. A capacitor on the tone assembly between wires 1 and 3 is 0.0022mf. It should be 220pf. So I'm going to replace the whole assembly with "The Brimar Circuit" and replace the tone controls with 11 position rotary switches.



I just got in a set of sockets for it, and am installing them now. My EL84s are very aged, so I'm thinking about replacing them with Russian EL84Ms or
6P14P-EV , because they can handle the 400 volts.

I also am going to replace the 2 x 6AN8As with a 12AX7 and 12AU7 like in Shannon's ST35 mod.

[chrisby]

Terry - first time visitor to this little den of crazies (first step is admitting it ) - is that Dynamutt driver board in hot-rodded ST70 in background above?

[snitch56]

is that Dynamutt driver board in hot-rodded ST70 in background above?

It is one of these (Diytube) in the classic brown
http://diytube.com/phpBB2/viewtopic.php ... e4354109de

[TerrySmith]

snitch56 is correct, it is in fact an early run Diy Tube board! That particular board has been soldered / un soldered MANY times and still has no traces lifting.

FWIW, My iron is a Weller WTCPL with an 800'F tip.

[chrisby]

thanks guys - but to continue the question, has anyone listening experience with Geek's Dynamutt driver board in otherwise stock ST70?

The 5751 and 6CG7 are different enough from 12AX/AU that it could make for a very interesting change.

I've only experienced it in an organ-donor project, and like what I heard enough to try it an long delayed gutting / rebuild of Jolida 302B. It's a lot easier for me to decide to proceed than to actually make any of the important choices

[Linn lover]

Hello everyone,

I have acquired an AA-151. It looks in ok shape and runs ok in power amp mode. I used to own one when I was a kid - my aunt had given me the amp as it took too much room in her apartment.

The preamp tubes were shot. It has been modded to use as a power amp only; it drives some Reference 3A Decapo MM clones (8 ohm, 92 db sensitivity with a fairly flat impedance curve). The amp is dead quiet.

I have looked at some of the work you have posted here. Very impressive and I thank you all for posting pics and circuit diagrams. I'm a Newbie and the information has allowed me to better understand tube amps and what is going on underneath the metal skin so to speak.

I have ordered a full compliment of new capacitors and biasing resistor. Looking at some of the work done that was posted here, I may attempt to install a bias resistor for each tube with 400 ohm, 5 watt resistors and 220 or 330 uf electrolytic capacitor in parallel (whatever will fit).

The 6AN8s test ok. The preamp tubes were dead. The power tubes are working but old and original (made in Germany) and test good (1), fair (1) and weak (2). I may replace the tubes with Electro Harmonix EL 84 ($24 a matched pair here in Canada). NOS siemens are available for $140 a matched pair (yikes!).

1) I have installed a pair of cardas RCA input connectors feeding the 6AN8 directly. This works well receiving a signal from my preamp. I removed the old RCA connectors and phono input section.

2) Next step will be to replace all Electrolytic caps in the power supply. First cap of the RAR4 will be 50 uf and 500 Volt rating (it's the closest I could get to in uf speak). The original rectifier RAR4 tube, made in Great Britain, test strong and good.

3) After the power supply caps will be replaced, I will then be replacing the Electrolytic cap in the bias resistor network. When I get there, I may install 1 resistor (400 ohm) and 1 capacitor (220 uf) per cathode to bias each tube individually. Comments?

4) If all is working good at that point and time, I may replace all the remaining capacitors in the power amp section (they are ordered).

5) The final modification may be the installation of a choke in the power supply. Can someone tell me if this will provide a noticeable improvement in performance? I know the amp won't run as hot, but I think this may be a good thing, correct?

PS: I am a Corvair fan. My folks owned one when I was a kid. It was a fun car to seat in the back and feel the purr of the flat six. Love the pic of the blue Corvair convertible. A very nice car indeed.

Thanks for reading guys....and I will appreciate the help if you offer some.

Cheers!

[TerrySmith]

Your AA-151 will be a great sounding amp when finished! You will NEED to replace the coupling caps, the four .05uf that feed the EL84's also. These are usually leaky and will cause the EL84's to redplate!

[rmyauck]

Best to use JJEL84 or Russian mil. EL84 as EH will red plate easily. This was found out in diyaudio in a amp running the tubes much the same as the AA-151.

On coupling caps it might be better to use .1 instead of .05. Looks like other modders have done this.

The ST-70 diode mod posted on tubes4hifi using UF4007 or better as quieter than IN4007 would be a good mod too. Helps tube life.

Use a choke with a low DCR (20 ohms)

Randy

[Linn lover]

Rmyauck and Terry,

Noted about the caps and the tubes. Thanks.

I ordered the exact same values as required in the original design. I will have to order additional 0.1 uf coupling caps if you think that is best.

As for tubes, they are not ordered yet. I just checked The Tube Store (Canada) and they offer the JJ EL84 for $22 a matched pair...actually cheaper than Electro Harmonics. They also offer a Russian Premium Military build EL84 for $49 a pair. Link on the next line.

http://thetubestore.com/preferred7189.html

In any case, I am having fun with this amp. I do a bit of modding and then power it up to see if it still works well. One step at a time.

If I can figure out how to post pics, I would. I will try again at the lowest resolution I can set my camera at and snap away. If it works, I'll post pics of my red Corvair!

[Linn lover]

As I plan to only use this Heathkit as a power amp, I guess I can remove the second series of power supply caps and resistors as I will only need the 360V and 285V taps, correct?

So I can remove R64, R63, C2A, C2B, R62, C36, R61.

I am also thinking of remove the hum pots.

Am I on the right track, or, heading for a disaster.

At this rate, when I start to recap, I may re-wire altogether.

Still not sure if I will install individual bias resistor and bypass caps. Maybe keep that as a plan B after I recap the complete amp.

The wire, at some parts, seems brittle. Can't wait to see the improve after the recap.

[Geek]

http://thetubestore.com/preferred7189.html

Watch any new designed "7189" tubes..... we used several (including the supposed EL84M equivalents) and you drop them in certain amps that run 475V on the screens and plate and they make for a pretty spectacular light show

Cheers!

[TerrySmith]

I've had good luck with the Russian old stock plain old 6P14P tubes, even in the hot rodded AA-151.

One of my other AA-151 amps is setup as our main amp in the living room and it has a set of NOS RCA 8BQ5 tubes, seem to work just fine on 6.3v.

[Linn lover]

Hello again,

Jules here from Ottawa Canada. I have re-capped the amp; this said, I have not touched the power supply cans yet or the bias cap and resistor.

I have the parts to redo the power supply and install individual bias resistors for each tube. Also got a full compliment of JJ EL84 tubes.

I have re-powered the amp up and it's running. But (and there is always a but....), I forget how to hookup the speakers on the right channel. I have done research, and I see the impedance selector hooked up to:

1) Impedance selector cable hooked up to C, and the speakers hook to C (black wire -, double up) and 8 ohm tap (red wire +)

2) Impedance selector to the 8 ohm tap, and the speakers hooked to C (black wire -) and the red 8 ohm tap (double up).

3) Impedance selector to the 8 ohm tap, and the speakers hooked up to C (black wire -) and the red wire (+) to S

So, which one is it. Any comments...suggestions. Can't wait to finally get it going on my good speakers. No sparks yet!

Thanks.

[dcgillespie]

Linn -- Third time is charm! Your third scenario represents the proper connections. The whole point of those connections (versus the simple connections on the left channel) is to allow the phase reverse switch to work properly when it is operated.

Good luck with your amp!

Dave

[Linn lover]

Dave,

Thanks. Will hook it up Thursday!

Then I will let the caps break in a bit.

After that, and one at a time:

1) Individual bias resistors and caps

2) Screen resistors

3) Power supply caps

4) Maybe a choke on the power supply

5) Since I am using it as a power amp only, I may install a volume control.

This will take time...but that's the fun part.

Cheers!