Heathkit AA-151

[DeathRex]

Congrats on getting it working nice!
Actually that first cap 60 uF should be close to 30 uF . Some manufactures just used what they had without a care for tube life. Caps down the line however can be increased.

Maximum capacitor for GZ34 is 60uf. 50 would be OK, 30 would increase hum, unless you add the choke. With the choke in, 30uf would lower your B+ a little (a good thing).

You might want to check the filaments to see if they are running high.

[Linn lover]

Hi guys,

When I wrote the last message, I forgot to tell you that I indeed have a 30 uf cap right off the rectifier with 220 ohm series resistor to lower the B+ inside the unit.

And yes, their is a bit more background noise at idle, but nothing I can't live with.

Choke is on order and until it arrives, I am going to replace the electrolytics. Since I can live with the noise a bit for now, I'll go ahead and leave it in and start with the 50 uf cans and so on.

So a lower capacitance will be easier on the tube rectifier, but at the expense of noise (hum). Interesting.

[rmyauck]

Jim McShane who sells tubes and HK Citation parts recommended close to 30 uF and that's what Dynaco spec'd on their amps back in the day.

Look for a choke with a low ohm measurement, 21 ohms for a lower DCR power supply. The AA-151 doesn't have any resistors or choke in the stock PS before the B+ so it actually has a really low DCR PS. Maybe 60 uf was used there to compensate a bit, but the Eico HF-81 is a similar PS design with no resistors. It's supposed to keep the sound open open and dynamic if you do this.
There are at least 2 advocates of this on the forms that say to keep the use of resistors or chokes below 21 ohms before the B+. Edcor sells a new line of affordable chokes with low ohms for a low DCR. Try it to see the difference as I'm sure you will. You can always add more resistance for the cathode bias to compensate for the higher B+.

Randy

[Linn lover]

I currently have a 30 uf cap and a 220 ohm resistor before the power supply. I did this to reduce the EL84s screen and plate voltages. It is working, but like you say, and others have said (like DC), this may affect the sound.

I have a hammond choke 156R on order. 1.5 henries, 200 ma, 56 ohms, 400 VDC.

After installing new power tubes, here is my voltage report:

Voltage in: 409 volts
EL 84 = 377 screen
El 84 = 375 plate
input voltage for pin #1 of the 6AN8 = 303 volts
Bias = 14.45 v (over a 120 ohm resistor for the four power tubes).

6AN8 (voltage report)

Pin 1 = 243 and 214 (why are they so different)
Pin 2 = 46.8 and 79.1
Pin 3 = 58.1 and 87.2
Pin 6 = 47.5 and 80.1
Pin 7 = 72 and 52
Pin 8 = 0 and 0
Pin 9 = 1.6 and 1.4

I have ordered a pair of new 6AN8s. I will put them in to see if it makes a
difference. And waiting for the choke.

The next step is the power supply with new caps and the choke (when it comes in).

It's the high voltages for the EL84s that have me concerned.

Dave (DC), we were talking volts around the 6AN8. Can you let me know what the screen and plate voltage should be on the EL84s. I am still referencing the schematic from the Tom McNally web site. They have them listed at 355v for the plate and 360v for the screen.

Cheers.

[dcgillespie]

Linn -- Heath lists the plate and screen voltage as 385 vdc and 390 vdc respectively.

To verify that the varying voltages between the two 6AN8s is in fact due to the tubes themselves, swap them between the channels and remeasure the voltages. If the actual components are fairly well matched between the channels, the it is most likely that the voltages measured for a specific 6AN8 tube, will stay with that tube -- regardless of which channel it is installed in.

Dave

[Linn lover]

Hi Dave,

Gee, why did I not think of that! Will do and report tomorrow.

And thanks for the Heathkit specified voltages. They are higher than what I am measuring now, so I guess I am safe for now. I will fine tune everything once I get the new Power Supply caps in with the choke for the EL84 and the 6AN8s.

Cheers from Ottawa (where it is 25 sunny celsius degrees today = 77 Fahrenheit!)
Jules (AKA Linn lover)

[Linn lover]

Dave,

I swapped the 6AN8 tubes today and did more measurements.

You are correct: the problem with the voltage levels follows the tube.

I would say that this is true to 90% of the voltage values measured. For example, where I was measuring 243 and 214 volts on pin #1; when swapped, the new readings are 240 and 216. Same thing with all the other measurements.

So swapping them actually makes things a little better, but nowhere near what it should be. When I get the new tubes, I will re-measure and report again.

New 6AN8 and a choke on the way. As I fine tube this amp, I'll order appropriate parts (low DCR choke as per Randy) and learn along the way. What fun!

PS: I found a website with your name on it. You are famous!

[dcgillespie]

Jules -- Glad you've about got this one all chased down.

The only web site I am associated with is the Tronola site: www.tronola.com which is operated by my good friend Steve Lafferty. On that site, I have my own section entitled Dave's Lab. It is not a site I manage on a regular basis, but as I put articles and material together that I think would be helpful to folks, I publish them there.

Your final plans sound great. I am sure you will be quite pleased with your amp when you are all done!

Dave

[Linn lover]

Dave,

As you, and many others, have great knowledge on tube amps, which is a rare breed I guess, I would like to know more about your background? Are you an electrical engineer or technician, or, you have done this as a hobby over the years. Just curious.

[Linn lover]

Dave,

I looked at the Tronola site. Screen resistors? Would 200 ohms be too much to add? Seems like a worthwhile thing to do.

[dcgillespie]

Jules -- When output tubes are operated under certain conditions, the use of screen stability resistors are mandatory. This primarily includes the use of fixed bias operation, and very low impedance screen grid power sources. Under those conditions, EL84 class tubes require screen stability resistors of at least 47 ohms, with 68-100 ohms being optimum. However, for the Heath amplifier, those conditions do not exist, so no screen stability resistors are really required.

As for my background, I fell intensely in love with vacuum tube amplifiers when I was about 6 years old back in the 50s. It is a hobby I have never deviated from, even when virtually everybody had abandoned them. My training has been from the giants of golden era: I've read and reread just about everything every published by the likes of Crowhurst, Hafler, Langford Smith, Tremaine, Keros, McIntosh, Williamson, and so many others I could name, until I thoroughly understood the material, and basically the covers fell off. Along the way, I have built and designed more amplifiers than I really can remember, but still find this hobby as enjoyable today as I first did so many years ago.

I certainly remember what it was like when I had more questions than answers, but unlike today's new crop of diyers, I had no Internet to help get the answers. As a result, part of my library now includes shelves of journals where I've done countless experiments to find my own answers. Now I simply enjoy the fruits of my own labor, and try to help others as I can to help them get the most from their equipment, and keep the hobby alive.

Thanks for the interest!

Dave

[Linn lover]

Dave,

Quite the journey! Thank you for sharing. I love hearing stories like that. Sometimes it's more than about the equipment: it is also about the people.

Thanks for being generous with your knowledge and time.

Jules (AKA Linn lover - Ottawa).

[Linn lover]

Dave, DR,

I have received some NOS 6AN8s yesterday. I installed them late last night in the amp and did quickly verify voltages around said tube.

The voltages are good all around and quite close to each other as well. So any serious voltage discrepancies surrounding the 6AN8s have been resolved.

Still waiting for the choke (The hammond one). I will also order the Edcor unit with the lower DCR and compare once I get them. Spare parts will now be the norm around my workbench.

I will let you know how the amp improves when I get the new parts in.

[DeathRex]

The amp might not improve with the added choke. But once installed, it will give you another tool to fiddle with the B+. Now with 375 on the plate
and 30 ma, the EL84s are doing 11.3 watts. You can slightly increase the B+ to get a bit more power out, probably staying below 400V.

The other thing you can try is Dave's EFB circuit. You probably won't get anymore power out of a AA-151, but you can extend the life of the tubes.

[TerrySmith]

Another trick to lower the B+ is replace the 5AR4/GZ34 with a 5R4, it will drop the voltage somewhat. However, it won't fit if you plan to use the cover.