New owner to an old Heathkit AA-121

[klm1]

but if I remember correctly, as long as the mechanical bolt or mechanism holding the plates together is tight, NOT LOOSE, you are ok for that moment...
you takes your chances etc.
The rectification is based on sucessive passes thru each plate; loose connection equals hi temp, therefore toxic/heat failure...PLEASE be careful.

can't stress that enough...:bigey es:

[kmcarpenter]

The selenium rectifier in this amp does not use any type of fastener to hold the plated together. This component is the size of a tootsie roll, and looks like an electrolytic capacitor, except for the fact that it has a large diode symbol on its case.

ANother note, I am having issues with the amp going red plate on a few of the tubes.

I have turned the bias voltage down on that particular channel, switched the tubes around, and the same tubes go red plate regardless of what channel they are in.

When I say red plate, the red on the plate is very dim (with the room lights on, you cannot see the red glow), but I hear that anyh plate color at all is not a good thing.

The red color gets brighter if I flog the amp.

The cathode resistors are ok, values right on the money.

My tube amp guy says that there isnt enough negative voltage on the control grid, makes sense.

At any rate, I havent had a chance to measure the voltage against the voltage indicated on the schematic, because I've been working with a cheap analog meter, and "somewhere between 25 and 50" doesnt cut it for precision.

What can I do to increase the negative voltage on the control grid?

I also was told that a screen resistor can be installed too.

Any ideas?

[danFrank]

Changing out the selinium rectifier for a silicon diode will give you more bias voltage. In fact, you will probably have to insert a resistor in series with the silicon diode in order to drop the voltage to acceptable levels.
Also, if only one tube is glowing red, I would suspect that tube as being bad or the coupling cap attached to the grid of this tube. In fact you should change all 4 coupling caps to the power tubes as they are probably leaky. You really need to find out what the plate current is going through each output tube. Easiest way to do this is insert a 1 ohm resister between each output tube's cathode and ground.

[kmcarpenter]

Something new tonight.

I played around with my hunch some more tonight.

I tested some things:

In the bias supply loop from the power xfrmr, I am supposed to have 45 volts, I have almost 50.

In the control grid, (pin 5 on an EL34) I am supposed to have -40 volts. I have -31 volts. (across all tubes.)

I have 499V on the plate, and 499v on the suppressor suppressor grid, I am supposed to have 490, and 495 respectively.

I moved the tubes around a bunch, suspecting that it was a tube related problem, and I have since ruled out the tubes. I have Isolated the issue to the A channel.

Only the tubes in the A channel go red plate now, and it happens very quickly after the tubes come up to temp, they will not hold the bias. The B channel is happy now, no red plate at all. With the Bias voltage turned all the way down (with the factory adjustment knobs) it still runs away in that channel.

Now, are you saying that by replacing the selenium diode with a silicon diode, I would have more negative voltage on the control grid?

If so, I think that is my answer.


Now, the amp already has cathode resistors. I tested those and they are all right on the money.


Also- by coupling cap, are you referring to the caps on the control grid?? El34's have 3 grids.

If so, I have already replaced them.

[klm1]

http://www.freewebs.com/toddm/aa121001.pdf

[kmcarpenter]

^^Yeah, thats the one.

[Bob, W7ETA]

Old amp-red plating-replace the coupling caps, at least that what I remember on this site or another. I looked at a schematic to verify that if the all of the voltages to the EL84 or El34 are close to spec, a bad coupling cap makes the most sense. Besides, Russina PIO K40Ys or Sprague "Orange-drop" seem to commonly reported to dramatically improve the sound over those old coupling caps. I can't comment on orange drops for coupling caps; the Russian K40-Y make a huge improvement when I install them.

If you have the schematic, measure the resistors to see if you should replace any while in there.

I played around a lot with HeathKit integrated PPEL84 and found that the RCA white lettering 6AN8 sounded the best, May be it was Eli that suggested them?

Best from Tucson
Bob

[Bob, W7ETA]

I see your coupling caps are 600V .047uF.

The old notation of uuF, micro, micro is now call picoFarad

I recently bought some HeathKit monoblock using 6L6GCs and 6SN7G and discovered I drive them with an MP3 and my Dynaco FM-3 stereo tuner that has a volume control--no preamp necessary!

Best from Tucson
Bob

[kmcarpenter]

I replaced the bias rectifier today with a new one. I have the proper voltage on the control grids now.

I've been playing it for some time now and the tubes seem to be under control.

I'll get a set of coupling caps anyway.

I am going to ditch the "surgister" and replace it with a standby switch on the center taps for the OPTxfmrs.

That'll remove the issue there.

I now need to find a switch that can handle >500VDC.

[Bob, W7ETA]

I use Mouser and Digi.

I can't help with the value of the coupling caps since yours are quite different from the .1 to .3uF I've encountered, seen as suggestions.

While in there, I would replace the resistor between the coupling cap and the El34 grid and the cap paralleling it.

Best
Bob