Heathkit AA-151

[Linn lover]

Dear DR,

Just back from some X-skiing. We got some nasty weather this weekend which translates into an extension of winter, so took advantage of the snow to cross country around Gatineau Park...but I digress....

Pin 9 is sitting at 1.5 volts. I will double check the measurements, but I think I was fairly accurate. Something is amiss!

In my parts bin, I have found a 30 uf, 450V electrolytic cap (new) and a 10 watt, 100 ohm resistor. I will try adding that to reduce the PS voltage down from 390v...to hopefully...360V. That is the goal.

For DC, I found an inter-connect I am not using, so I will try using the cable from that to see if things improve. If not, I may consider drilling some holes in the base plate and mount the inter-connect points close to the driver tubes. I will report back on progress.

After I have this baby under control and running fine, I may re-wire it altogether with high quality wiring, new sockets...everything. But for now, I need to better understand what I am doing and getting this amp working as it should.

I think the biggest challenge will be to get the 6AN8s to behave...at least get the voltages under control.

Update you later. Cheers from Canada (Ottawa).

[dcgillespie]

You said earlier that "prior to the mods, the amp was working better at high frequencies". If you will, exactly what mods have been done between that time and now? The answer must be in there somewhere!

Dave

[Linn lover]

DC,

Tonight I tried to shoehorn a better cable from the RCA inputs to pin #8 of the 6AN8 tube. The results were not good. I must of done something wrong.

I will go to the local parts shop tomorrow and pick up some shielded cable and resistors. If they don't have what I need, I will order this type of cable from mogami: http://www.takefiveaudio.com/mall/shopexd.asp?id=1020

As to your question with regards to what I changed, I removed all the old RCA connectors and the preamp tubes (they were dead). All connections to the bass, volume, treble and selector, have also been removed. Since it was running as a power amp only (modded by the prior owner), and I wanted to explore what was inside, I decided to to away with what I thought was superfluous. javascript:emoticon(':(')

And now I have problems. I think the next thing I will do is put back the coupling caps to the EL 84s to 0.05 uf and wire in RCA inputs with quality wiring with the 470 Kohm and 10Kohm resistors as recommend above.

Now where would be the best place for these resistors. Right at the input, or, near the ground bus bar? Any suggestions?

PS: I will also order a choke tomorrow. I have a 450 V, 30 uf cap (electrolytic) on hand (it's new). I guess I could use that as suggested by yourself.

I will keep you posted with my progress.

[Linn lover]

DC,

If I reference the diagram at this link:

http://mcnally.cc/pics/aa151l.jpg

The aux input has a 33mmf cap in parallel with the input 470kohm resistor.

If I am correct, I also see the input to the preamp has a .1 uf cap with a 1 meg resistors connected to ground.

I know it is a different tube, but should I also be adding caps to the input of the 6AN8s?

[Linn lover]

DC,

I put the amp away yesterday so I could reflect on what changes have occurred.

This morning I looked at the amp and found this: A newer resistor, value of 440k ohm, connected between pin #2 and ground. This is obviously not as per the original design.

I will remove it tonight.

[Linn lover]

With regards to the 440k ohm resistor, I could leave it in, and from pin #2, run a 10k ohm resistor in series to the hot input and properly ground the - side and all should be good (fingers crossed).

When I had the 470k ohm and 10K ohm wired as per your recommendations, I was measuring approximately 220k ohm. Now I know why. 470K was in parallel with 440k and series with the 10k ohm.

Back to the bench tonight and I will also change the coupling caps back to 0.05 mF.

[DeathRex]

It sounds like the wiring around the 6AN8 was changed sometime in the past. You would need the 440K to ground if there is a coupling cap between pins 2 and 6, if there is one, the bias on the triode side is gravely off. Plus you don't need the coupling cap, and you don't want a coupling cap there. With a 6AN8, it has to be pretty much wired exactly as in the schematic as a cathodyne.

With your increase in B+ the voltages on the 6AN8 should be:
Pin 1 - 210
pin 2 - 95
pin 3 - 100
pin 4 - 6.3 to 6.8AC
pin 5 - 6.3 to 6.8AC
pin 6 - 95 same as 2
pin 7 - 55
pin 8 - 0
pin 9 - 1.7

You should also have the same voltage drop across R25 and R26, about 95 to 100 volts.

If you can, take a very close up picture of the 6AN8 wiring, it might help us.

[Linn lover]

DC,

I meant to say...pin #8 on the driver tube.

[DeathRex]

The 440K is on pin 8 to ground? If that's right then that's good. Can you check if you have about 150K ohms from pin 2 to pin 6. Make sure the AA-151 is unpowered and pull the 6AN8 to check.

[Linn lover]

Hi DC and DR,

I wired in a 120 ohm bias resistor today (10 watts).

I did nothing about the power supply yet, but I ordered a choke for it (Hammond 156R).
I did pick up a 47 ohm, 10 watt resistor that I may install with a 30 uf, 450V cap to reduce voltage inside the unit.

DR - yes, there is 150k resistor from pin 2 to pin 6. I checked it tonight. I did not measure voltages but the value is ok - on both sides.

Input: I did wire in a 10k resistor in series with pin 8 of the 6AN8. I left the 440K resistor in place at pin #8 to ground. Does it matter if it is before or after the 10k series resistor?

I buttoned everything up and powered her up. It's better. Not great, but better...but it still needs work - it sounds veiled. Oh what to do next.

Next I will install 0.05 uf coupling caps and do some voltage measurements around the 6AN8 to share with you all.

At least it's working again. I could replace the 440Kohm with some 470k I bought today. I guess that increases the input resistance a bit. I am still on the quest to get the high frequencies to perform like they once did. Question is: why did result at this time? Maybe some of the caps I installed are just to darn....modern?

[dcgillespie]

Some of these new coupling caps are so physically big that they can represent a significant capacitance to ground for the circuit they are coupling. That could be a possibility as well.

Ideally, the 440K resistors should be removed, so that only the 10K resistors and the 470K resistors form the input network. The 470K resistors should be located on the input jack side of the 10K resistors, and can be located at the actual input jacks. The 10K resistors should be located very near the 6AN8 grid terminals.

Dave

[Linn lover]

Dave,

The 440 will be removed (easy to do) and I will install the 470k at the input terminals (easy to do as well).

The 10k is right at the input of the driver tube.

The 0.05 uf will be much smaller than the 0.1 uf I have installed; this said, the 0.1uf were smaller than the ones I replaced that were in the amp originally.

I will keep you posted on my progress. Will be silent for a couple of days.
Signed: Linn lover (AKA - Jules).
Cheers!

[Linn lover]

A colleague just dropped off a Dynaco ST-35 amp schematic. Very interesting.

The input section is somewhat similar in concept. It has that 470 K resistor that you suggest I install (and now understand why!) across the input to gnd, but with a 33 pf capacitor in parallel to it. I also noticed that the heathkit input sections have that cap as well.

I also notice in the ST-35 that they include a 0.1 uf cap in series with the hot input. I guess that is there to filter out any DC component.

Should I be installing any of these on the input stage of the AA-151?

The input tube of the ST-35 is a 7247. The series resistor they use is a 47k. I guess different values for different tube types.

[dcgillespie]

Linn -- The network on the input of the ST-35 is there as part of the HF stability networks install in the amplifier. It functions by limiting the HF response above 40 kHz, so that signals above this frequency will not cause the amplifier to "ring" at those frequencies. Since it has no impact within the accepted audio spectrum, it (theoretically) has no impact sonically.

The AA-151 does not need this type of network to maintain HF stability, so adding such a network to it would not produce any significant change in the overall stability of the amplifier.

Dave

[Linn lover]

Dave,

I have the amp running all night with the mods (470k between hot and gnd at the input, 10k in series with the hot at the input of the tube). I also installed the 0.05 uf coupling caps.

Amp is working better. High frequencies are better. Not like they were originally, but much improved.

This said, the bass response seems too much and out of proportion with the high frequencies. Also, if I turn the volume up to high, the distortion in the woofer returns. The amp plays plenty loud. At lower volume, it plays fine and sounds good with classical music. With rock, with heavy bass guitars and drums kick, it's a bit too much.

I am running the old power tubes until I get the choke installed when I get it. And I have not done a complete detailed voltage check, but will do so this weekend.

So things have improved. Should I try changing any value of resistor components to fine tune the input to the 6AN8?

Again, thanks and have appreciated your help. But the amp is not 100% yet....but I am confident I can get there.

Cheers.