Heathkit AA-151

[Linn lover]

I hooked up my amp tonight after changing caps and installing new RCA inputs.

The amp powers up and no smoke. ! Ok. Good so far.

But the sound is terrible. Not as loud as before, and when I crank up the volume the bass is badly distorted. The highs seem ok.

I did note on the diagram that there is Chassis GND and GND. From my input RCA connectors (left and right), I ran my + (I am using red wire) input signal to the inputs of the 6An8A as before. I then used the - (I am using black) to the main GND bus bar underneath. My way of thinking is that GND is GND. Should I run this to the chassis instead? Would it make a difference? Am I missing something here.

The only other thing I will check is the coupling caps. I changed them from 0.05 uf to 0.1 uf. Maybe go back to 0.05 uf caps (I have extras).

Both channels behave exactly the same way. Highs are ok, lows are distorted - both channels. And I have to crank the volume higher than usual on my preamp to get decent volume out of the amp.

I have not changed the bias resistor or it's bypass cap yet.

Any suggestions where I can start looking for trouble? I will start with the input at the RCA and work my way through it...or...

If someone has a better idea where I can start looking?

Thanks people.

[DeathRex]

First off I would check the voltages on the 6AN8 and EL84s. All in reference to the ground buss bar.

EL84:
Pin 2 - 0
3 - ~15
7 - 350 to 380
9 - 355 to 390

6AN8
Pin 1 - 190
2 - 85
3 - 90
6 - 85
7 - 50
8 - 0
9 - 1.5

I then used the - (I am using black) to the main GND bus bar underneath. My way of thinking is that GND is GND. Should I run this to the chassis instead? Would it make a difference? Am I missing something here.

Should goto the bus bar.

Oscope would be good to check distortion. Output transformers hooked up right? Feedback hooked up right?

[Linn lover]

No scope at this time.

But I will start with the basics and check voltages.

Thanks.

[Linn lover]

Deathrex,

I did some preliminary measurements tonight.

Bias voltage is running about 15.6 volts

Pin 9 on the power tubes are at 390 + volts.

399 volts from the power supply.

Where I should be getting 285V from the power supply to feed the 6AN8, I am getting about 330 V. Too high. Pin 7 is about 57 volts...and others are lower. Can't remember from memory, but will take accurate measurements tomorrow.

Looks like I have too many high volts flying around in the amp + weird volts on the 6An8.

May have to modify the power supply to lower voltages...one step at a time I guess.

FYI - I have not replaced any power supply caps at this time.

[TerrySmith]

I like to add a 33uf @450v capacitor at the rectifier then a 1.5h choke, then connect that to the original circuit. The choke is a Dynaco part found in the ST-70, it will knock down the voltage a little and add some filtering.

[Linn lover]

Terry,

I had seen that in prior postings on this site. I will see if I can source the part locally. I think I need to focus on getting the voltage down a little prior to fixing what is going on with the 6AN8s.

Thanks for your input.

[DeathRex]

Those EL84s are doing 14.6 watts each , they aint gonna last long. Need to get the B+ down to 360-370 or increase the cathode resistor a bit, or both. I put in 2 cathode resistors, one for each pair in mine.

If you can't find that choke that Terry talked about, then you can use a resistor, but it will add heat inside. A 33 to 47 ohm 5-10 watt will drop somewhere around 12-20 volts. For better filtering add a 10 to 33 uf cap in front, but the resistor will drop less.

A capacitor and resistor won't filter as good as a capacitor and choke, but a resistor will drop DC better, at the expence of heat.

[dcgillespie]

Lynn -- I assume from your comments that you are trying to use the AA-151 as a power amplifier with an external preamp.

If so, and if you disconnected all connections from the input grid of the 6AN8 and simply hooked that grid to your new input jacks, the your problem might be that your external preamp is not providing any DC return path for the grid of the 6AN8 pentode section. If that is the case, that would explain all the distortion you are hearing.

If this is the way you have modified your amp, then all you are missing is some input resistors. Ideally, when you do a modification such as this, you should install 470K resistors across the input jacks (between the hot and ground), and then connect the hot of the input jack to the 6AN8 through a 10K resistor. This will isolate the effects of any cable capacitance on the grid of the 6AN8.

Your ground connection is perfect, although ideally the input jack ground should be connected to the buss at a point where the cathode resistors of the 6AN8s connect to the ground buss. Doing this will ensure that no ground loops are formed by connecting the input ground in a different place on the buss.

Because this amp uses NFB in the power amplifier, increasing the value of the coupling caps will do little to improve LF performance, other than potentially decrease the LF stability of the design. It would be best to use the original value if possible. They were plenty big as originally designed.

Good luck with your amp!

Dave

[Linn lover]

Thanks.

I indeed removed all the preamp stuff, and yes, I am feeding the input directly to the 6AN8s.

I will try your modification as recommended for the input.

Others have installed 0.1 uf cap and recommended I try that and I have installed those. I do have some 0.05 uf that I originally ordered.

For now, I want to get the amp working again. I will measure all voltages later today and see what parts I have (470k and 10k ohm resistors). If not, I' ll go buy resistors next week and modify the amp accordingly.

Thanks for your input. This is good information.

[Linn lover]

Death Rex,

I installed the 470K and 10K ohm resistors. All is working ok. Amp is dead quiet, no distortion with the bass - it works.

But, the high frequencies are no longer present as they once were. The bass seems more powerful and tighter, but the highs are not as bright as before.

I have not changed the coupling caps back to 0.05 uf farad yet.

Anything you can suggest to improve the high frequency detail would be appreciated.

I made good progress tonight, but still more work to do.

Appreciate your help. You got me back on track.

[Linn lover]

DC,

I installed the 470k and 10k resistors and the amps works fine.

Now it's just that high frequency performance has suffered.

More work needed.

[Linn lover]

DeathRex,

Here is the voltage report. It's not perfect!

6AN8 (average of both tubes)

Pin 1 = 260V
Pin 2 = 145V
Pin 3 = 135V
Pin 4 = 58V
Pin 5 = 58V
Pin 6 = 50V
Pin 7 = 46V
Pin 8 = 0 V

EL 84

Pin 7 = 385V
Pin 9 = 390V
Pin 3 = 15.5 (with new tubes) to 16.4 V (with old tubes)
Pin 2 = 0 V

Guess I need to check components (mostly resistors) around the 6AN8.

Guess I need to drop the voltage as you recommended with a 47 ohm, 10 watt resistor until my choke comes in...or park the amp.

Amp is working now and is quiet, but the high frequency performance is not up to par. It sounds veiled at this point and time.

[dcgillespie]

Lynn -- With the input connections corrected, the unit is now properly configured for the way in which you wish to use it, resolving your initial distortion problem. The resistors allowed the bias for the pentode section of the 6AN8 to be restored, which resolved the issue.

Regarding your loss of HF reproduction, your attention must now focus on the preamplifier you are using to drive the AA-151's power amplifiers with, and how these two components are interconnected.

Within the AA-151, the only thing that could really affect reproduction at high frequencies -- and this is a very long shot -- is how your new input jacks are installed. Ideally, the "run" from the input jacks should be short, and if shielded cable is used, it should be of the low capacity type to minimize any loss of high frequency information. But honestly, a properly designed preamp will have a low enough output impedance to negate the effects of any cable capacitance -- either from the interconnects used, or due to the internal wiring within the power amplifier proper. That is precisely why stand alone preamps must have a low output impedance. It protects the performance of high frequency information when capacitance is introduced into the circuit from the effects of cable and wiring.

If you also installed "preamp output" jacks on the AA-151 so that the unit can still potentially be driven from its own internal preamp, then the external interconnects used for this purpose must be extremely short in length -- no more than say a few inches at best. This is because the output impedance of the internal preamps within the AA-151 is very high. This is not a problem for the unit as originally designed however, as the internal connections between the preamp and power amp sections are (relatively) very short in length, so there are no effects of cable capacitance to consider. However, if you now have the unit configured in this manner, and are driving the unit from its own internal preamps through typical length external interconnects, then there will be a significant loss of high frequency information, no doubt. With the modifications you have made to provide power amplifier input jacks, they must be driven from a properly designed external preamp that has a low output impedance by design, or drive the unit from its own internal preamps through very short interconnects to prevent the loss of high frequency information.

If you will describe how and what you are using to drive the AA-151's power amps with, then we can get an idea where the high frequency information is being lost. Due to the basic design of the AA-151's power amplifiers proper however, it is extremely doubtful that they are the problem.

I hope this helps!

Dave

[Linn lover]

DC,

I have now configured the AA-151 strictly as a power amp. When I received it, the preamp was shot and I was using it as a power amp only (it was configured that way by the previous owner). I removed anything relative to the preamp section in the amp to simplify the circuit and allow me to work more easily inside the unit.

There are two cardas input connectors at the rear that I have installed, and yes, the run between these connectors and the input of the 6AN8 is long. Further to your comment about cabling, and I was worried that this could happen sooner or later, I am using plane jane aluminium wiring I had lying around (should I actually be saying this here the board?).

I have two preamps and they are both working well with low impedance outputs. An old NAD 20 and a new Rogue Perseus Magnum. Prior to the mods, the amp was working better at high frequencies. My preamps also work well with my main tube amp (a Rogue Atlas).

I guess the next step is to get some decent wiring, double shielded if possible, and shorten the run if I can...but difficult to do with the inputs at the back and the driver tubes at the other end of the enclosure.

The quest continues. Many thanks.

[DeathRex]

6AN8 (average of both tubes)

Pin 1 = 260V
Pin 2 = 145V
Pin 3 = 135V
Pin 4 = 58V
Pin 5 = 58V
Pin 6 = 50V
Pin 7 = 46V
Pin 8 = 0 V

Here's the schematic courtesy of Tom of the AA-151's 6AN8.

The voltage on Pins 2 and 3 hopefully are reversed; 2 should be lower than 3. And the voltage on pins 2 and 6 should be the same. Also what is pin 9? Recheck the wiring to see if it matches the schematic.