New owner to an old Heathkit AA-121 - Page 4

Eli Duttman · 17th May 2010, 04:04 AM

Quote:

Originally Posted by 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.

IIRC, Heath's power trafo can't take surges. That's exactly what will happen, when you apply power. Put a NTC inrush current limiting thermistor in, as a replacement for the surgistor. It is probably a good idea to put a pair of CL90s on the rectifier winding too, just like McShane puts on the Cit. 2's rectifier winding.

kmcarpenter · 01:56 AM

I am still having some issues darnit.

One tube in my A channel is still going a very dim red plate.

I retested all of my control grid voltages, and they are right on the money.

I am about to order a set of coupling caps (if I can find them)...

Would leaky coupling caps also cause a 60 cycle hum?

Iunno... I ran the amp for a couple of hours tonight, and the 60hz started out pretty quiet, almost silent. After the TV show/concert we were watching was over and the sound went quiet, the 60hz sound sounded like a guitar amp.

I am going to replace the coupling caps and see where I get with that i guess....

Damn, I thought I had this thing nailed down.

ANother thing:

The coupling caps I currently have are electrolytics. The only ones I can find at the proper values are poly pros (they look like orange drops). Is a cap a cap in this case? Do I have to be picky about what kind it is as long as the values are kosher?

azkid · 19th May 2010, 02:11 PM

I would think you'd want to switch over to polypro where possible... ?

kmcarpenter · 19th May 2010, 11:24 PM

Upon further research, the supplier that heath used for coupling caps was kind of an oddball.

They are in fact polypros (but look like electrolytics?).

I ordered the caps today.

danFrank · 19th May 2010, 11:36 PM

If all 4 of your control grid voltages are right on the money then you have a bad output tube or that one tube is way out of spec (bad). Switch the 2 "A" chanel output tubes and see if the same tube glows red.
Do you have a tube tester? Don't assume that new tubes bought from a dealer are automatically good. Things happen...
You really need to test for current going through each output tube. There should be no more than 0.3 volts going through each 6 ohm resistor on the cathodes of the output tubes. 0.3 volts = 50ma = 25 watts per output tube. Measure the voltage at each 6 ohm resistor and see what it is.

kmcarpenter · 20th May 2010, 02:40 AM

If I were to measure current directly, would I not put the electrodes from my DVM across the center tap (red) and the plate?

I played with it some more tonight. I removed the resistors in the bias supply by the rectifier diode to give me a few more negative volts on the grid. The spec says I should have -40 on the grid and 490 on the plate, well I have 499 on the plate and had just above -40 on the grid(-38 to -39.5), now I have -45 and it seems to have helped it stabilize. I put the tube that was glowing in the B channel and that was the only tube that was mis behaving tonight.

I have an RA in with the tube supplier and a new set of tubes on the way, along with a set of coupling caps.

Eli Duttman · 20th May 2010, 03:15 AM

Quote:

If I were to measure current directly, would I not put the electrodes from my DVM across the center tap (red) and the plate?

NEGATIVE! A current measuring device gets inserted in series with the line. Do you really want to break the wire between the plate and the O/P trafo?

A voltage measuring device gets connecting in parallel with the line. Measuring the voltage drop across the cathode resistor and using Ohm's Law to compute the current is THE way to go.

moonbird · 4th September 2010, 12:26 AM

Hello all --

Saw this thread on a web search -- Do any of you have a copy of the AA-141 manual/schematic by any chance?? thx much!!

kmcarpenter · 6th September 2010, 02:27 AM

^^^

I do have the 141 schematics, but not in a digital format.

Perhaps I can fax them to you?

A new issue- my filter caps are starting to leak white stuff from the bottom of them onto the floor of the under chassis.

Time to get new ones. But--- I cant find them.

I need Multiple anode Single cathode single section caps.

2 each of 150uf at 300V and 200uf at 300v

They say "type EY" on the caps, made by Astron.

They also say B-25-50 on the 150uf caps and B-25-51 on the 200uf caps.

19th May 2010, 01:42 AM	#32
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	I am still having some issues darnit. One tube in my A channel is still going a very dim red plate. I retested all of my control grid voltages, and they are right on the money. I am about to order a set of coupling caps (if I can find them)... Would leaky coupling caps also cause a 60 cycle hum? Iunno... I ran the amp for a couple of hours tonight, and the 60hz started out pretty quiet, almost silent. After the TV show/concert we were watching was over and the sound went quiet, the 60hz sound sounded like a guitar amp. I am going to replace the coupling caps and see where I get with that i guess.... Damn, I thought I had this thing nailed down. ANother thing: The coupling caps I currently have are electrolytics. The only ones I can find at the proper values are poly pros (they look like orange drops). Is a cap a cap in this case? Do I have to be picky about what kind it is as long as the values are kosher? Last edited by kmcarpenter; 19th May 2010 at 01:56 AM.

4th September 2010, 12:26 AM	#38
moonbird diyAudio Member Join Date: Sep 2008	Heathkit AA-141 schematic or manual?? Hello all -- Saw this thread on a web search -- Do any of you have a copy of the AA-141 manual/schematic by any chance?? thx much!! __________________ "Life is hard -- afterall it kills you ..." Katharine Hepburn

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19th May 2010, 02:11 PM	#33
azkid diyAudio Member Join Date: Oct 2009 Location: Colorado	I would think you'd want to switch over to polypro where possible... ?

19th May 2010, 11:24 PM	#34
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	Upon further research, the supplier that heath used for coupling caps was kind of an oddball. They are in fact polypros (but look like electrolytics?). I ordered the caps today.

19th May 2010, 11:36 PM	#35
danFrank diyAudio Member Join Date: May 2005 Location: California	If all 4 of your control grid voltages are right on the money then you have a bad output tube or that one tube is way out of spec (bad). Switch the 2 "A" chanel output tubes and see if the same tube glows red. Do you have a tube tester? Don't assume that new tubes bought from a dealer are automatically good. Things happen... You really need to test for current going through each output tube. There should be no more than 0.3 volts going through each 6 ohm resistor on the cathodes of the output tubes. 0.3 volts = 50ma = 25 watts per output tube. Measure the voltage at each 6 ohm resistor and see what it is.

20th May 2010, 02:40 AM	#36
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	If I were to measure current directly, would I not put the electrodes from my DVM across the center tap (red) and the plate? I played with it some more tonight. I removed the resistors in the bias supply by the rectifier diode to give me a few more negative volts on the grid. The spec says I should have -40 on the grid and 490 on the plate, well I have 499 on the plate and had just above -40 on the grid(-38 to -39.5), now I have -45 and it seems to have helped it stabilize. I put the tube that was glowing in the B channel and that was the only tube that was mis behaving tonight. I have an RA in with the tube supplier and a new set of tubes on the way, along with a set of coupling caps.

6th September 2010, 02:27 AM	#39
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	^^^ I do have the 141 schematics, but not in a digital format. Perhaps I can fax them to you? A new issue- my filter caps are starting to leak white stuff from the bottom of them onto the floor of the under chassis. Time to get new ones. But--- I cant find them. I need Multiple anode Single cathode single section caps. 2 each of 150uf at 300V and 200uf at 300v They say "type EY" on the caps, made by Astron. They also say B-25-50 on the 150uf caps and B-25-51 on the 200uf caps.

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