Heathkit AA-151 Complete Rebuild

A customer asked me to rebuild a Heathkit AA-151. We normally do AA-100 rebuilds but I'm always up for saving a Heathkit, no matter what model.
The AA-151 is a delightful sounding amp!

[@daniannaci3258]

I enjoyed (and felt your pain) your video as I recently restored my own beautiful Heathkit AA-151. The power amp section in these things is truly glorious, likely due to the huge power supply as that 5AR4 rectifier can easily supply amps with three times the power and the transformers used are oversized and of the highest quality. This power amp circuit can even be operated with no feedback at all and still remain stable indicating excellent engineering. This power amp circuit dates back to some of Heathkit’s earliest EL84 amps such as the UA-1 and UA-2. Although, I’m not at all fond of the shared bias supply for all four output tubes. This demands a matched quad of output tubes, something that was cheap in 1962, but not so much now. The 6AN8 triode/pentode tube used as the input/phase splitter/driver in the power amp section can be prickly. No longer made, even a NOS 6AN8 can be leaky, microphonic, or just plain noisy. (By the way, the 6AN8 was used on Dynaco Mark III’s and several other Heathkits). Buy a bunch - don’t be afraid of forgotten brand names like Silvertone or Channel Master or Zenith - and try several in the amp. The preamp section is another story. The preamp is compromised from the beginning by using AC on the filaments, including the phono section. And the line level inputs are padded way down right at the input jacks (those 470k resistors) and now needs big-time amplification by being fed to the second half of the phono section, right after the RIAA equalization network. Then everything needs to go through two more tubes, the second section of triode 6EU7 and the 6AU6 pentode, before the signal is squeezed through the encapsulated tone control circuit before it finally reaches the power amp. Ugh. Way too much trouble to redesign and rewire the preamp. So I yanked the 6AU6 tubes, put a little tag board with a pair of jacks that lead to the input of the power amp, and use a vintage Sony TA-E80ES preamp to drive that wonderful power amp into a pair of Martin-Logan Aerius (yes, they love that electrostatic speaker). Now it’s a great amp!

[@peterhutchinson3424]

Very well done. 👍👍 I have two of these.

[@pcbgraphix1903]

They are sweet amps.

[@daniannaci3258]

And all vintage amplifiers need a Bucking Transformer in series with the AC power cords. These old amps were designed for 115-117 VAC, not the 123-126 VAC I have at my house. The voltages on the plates of the output tubes will be at or beyond indicated design limits and that’s NOT good - for safety, for reliability, and yes, for sound. A Bucking Transformer safely lowers the voltage at the input of the amp, no mods to the amp are necessary. Though I know of no one who sells one, it’s a very simple circuit, can be easily built, and plans for them are widely available online and on KZbin. I run my AA-151 at 113-114 VAC using a Bucking Transformer and it couldn’t be happier.

[@HFX1955]

Thanks for the comprehensive rebuild video. As you have mentioned in response to some comments, close up detailing of some operations will assist in clarifying these replacement steps. I will go through all of your videos before I do my AA-100. It was really interesting to note that when you put the meter probe onto the high voltage points, after installing the rectifier that you could hear a little glitch in the audio. This may have been due to EMI produced when the probe touched the high voltage point on the tube socket and the test lead acting as an antenna coupled that into the microphone.

[@pcbgraphix1903]

I thought that it was interesting to hear the "spark gap" transmissions from touching the meter probes! I didn't notice it until I went to edit the video. Please consider replacing your AA-100 output board with one of my brand new ones. You can get them here: www.ebay.com/itm/333954566862

[@6StringPassion.]

This is GREAT! I have an AA-100 I will be rebuilding soon enough.

[@pcbgraphix1903]

Awesome! Do you have one of our Output Board Kits?

[@T_Burd_75]

My fiancee has ordered me a rebuild kit for my AA-151 from a seller on ebay. I'm looking forward to having that as a winter project. ;-)

[@T_Burd_75]

Okay, so I'm in the process of replacing the capacitors and I'm replacing the carbon resistors with metal film resistors. But I'm concerned about how much smaller the wire gauge is on the resistor leads, even though the new ones are still rated 1/2 watt (I'm also replacing the appropriate 1-watt and 2-watt resistors). Is the smaller gauge leads on the new resistors normal?

[@T_Burd_75]

Okay, so the restoration is completed. I was brave enough today to plug this amp in and turn on the power switch. It took a couple of seconds before the pilot light came on. I have 6.6 volts on the hum pots and 5.4 volts on the yellow leads going to the rectifier socket. Everything seemed fine. I plugged in the rectifier tube and turned on the power, and the filaments are glowing. I went to check the high voltages. I didn't install the ground lug on the chassis like you did yours, since I'm not planning on running a turntable on this at the moment. I grounded the negative lead to the chassis. and when I touched the red lead of the voltmeter to the rectifier lead, it was showing around 450, then shot up over 1000, then the 2-watt 4700 ohm resistor on the capacitor started smoking. The capacitors are still nice and cool, and nothing else seems to be out of the ordinary. What would cause this? Would it be what I used for the ground?

[@pcbgraphix1903]

Sorry for the long delay, I just found this. Yes, the metal film resistors will be smaller than the carbon ones with the same wattage rating. I though the same thing the first time I saw them too! As for the power supply, I recommend going over it with a fine tooth comb against the schematic to be sure it's wired correctly. I didn't think that it was capable of producing that many volts. Check your transformer wiring carefully. If you have any more problems you can reach me at {name of this channel}@gmail.com

[@patlyle2621]

Very nice! U have inspired me to go back into my AA-151 and finish replacing all the resisters. I had replaced the caps as you have here. I only replaced the resistors out of spec. Great job! Still not clear on exactly how to do the death cap replacement. Could you do a closeup of the safety cap install process. John Ocala fl

[@pcbgraphix1903]

Yes, that's a great idea. I'll try to shoot some detailed video on the AC power mod next time I rebuild one.

[@GTCSRobert]

Can you tell me where you got that cable and strain relief please? I've been looking around, and could use a hint.

[@pjimmbojimmbo1990]

I built a couple of Heathkits in the mid 70s, they were Solid State. I still have one and it worked when I hooked it up a few yrs ago. It is an AD-27A, a Compact Stereo System, it used the AR-14 redesigned to fit in the Cabinet. I convinced my Dad I needed it as a Project for Grade 10 Electronics Class. Even though it was one of Heathkit's cheaper units, it always impressed people with it's sound Quality, and of course I was very Proud to have built it. Do you ever work on the later Solid State Models?

[@pcbgraphix1903]

Unfortunately we both missed out on the tube era! I specialize in tube units but can work on anything that needs to be worked on, even if it doesn't have glass. ;-)

[@gastontaylor6594]

hI, I am in the process of rebuilding my aa-151. I am not sure where exactly the 3 prong wire is connected in the circuit. Do you have a close up video or instructions to do. Secondly, what is the dimension of the 60/25 elect cap. Can't find a part # 25-101 cap dimension. gaston taylor

[@pcbgraphix1903]

I didn't do a good job of showing the power cord conversion. I'll try to get a better video in the future.

[@gastontaylor6594]

Just wondering if you could pin point the connection on a schematic. At least the blk/white connection.

[@pcbgraphix1903]

The hot lead should go to the center pin of the fuse. The outer fuse pin should go to the power switch. The power switch return should go to the system. The outlets should be wired to follow their function. Ground should tie to ground.

[@gastontaylor6594]

@@pcbgraphix1903 Thanks very much.

[@TuanTran-io6pm]

Day nguồn mình thay vào có kích cở bao nhiêu Xin cam ơn

[@pcbgraphix1903]

What source are you asking about?

[@victortrinidad5588]

I have done a recap on my AA 151 and have tried plugging it in without the Rectifier it blows the fuse what could be wrong,I have replaced the power transformer and one of the output transformer.

[@pcbgraphix1903]

For troubleshooting help, please send mail to [e-mail].

[@MichaelBeeny]

That turntable has dreadful wow on it. Don't get that issue with digital.

[@pcbgraphix1903]

I have a different turntable now. I don't know if it was the table or the record that was causing it.

[@MichaelBeeny]

@@pcbgraphix1903 Having used digital music since 1982 I find almost all analogue music played on disc, tape etc often suffer from wow and flutter to some degree. I don't know why I have become so conscious of it. Good video, however.