New owner to an old Heathkit AA-121 - Page 2

kmcarpenter · 11th April 2010, 01:41 PM

Yeah, I'm sorry about the lack of a diagram. I just graduated from college, and they revoked my webspace. The diagram I have is attached to the rest of the manual, and is 7+ megs.

Sound like a good idea with the heaters and stand by switch?

Eli Duttman · 11th April 2010, 08:46 PM

Quote:

Originally Posted by kmcarpenter

Yeah, I'm sorry about the lack of a diagram. I just graduated from college, and they revoked my webspace. The diagram I have is attached to the rest of the manual, and is 7+ megs.

Sound like a good idea with the heaters and stand by switch?

Why bother with switches, when Jim McShane will sell you a CL150 and Teflon "spaghetti" tubing, at a fair price?

kmcarpenter · 24th April 2010, 03:38 AM

Well, I have some good news, the resistors I thought were popped are actually OK. I guess they just got hot. They test out at the values they are supposed to be right on the money.

I think it is because of a bias problem. The amp has a hodgepodge of tubes in it, and I dont think it was ever biased after the day it was built.

The small tubes (6AN8) are not lighting up because of a heater circuit problem.

We'll see. I hope to be hearing music from it in about a week.

kmcarpenter · 24th April 2010, 03:43 AM

Oh- one more thing-

What would your opinion be about losing the solid state (selenium) rectifier and installing a tube rectifier be?

I dont care that the rectifier is made from selenium, if that is the only reason to get rid of it.

I know I'll need a filament xfmr, but I'm willing to install one if anyone thinks it would perform better than the solid state one.

Eli Duttman · 24th April 2010, 04:16 AM

The Selenium rectifier is probably for the O/P tube bias supply. In any event, it's a ticking toxic time bomb and MUST be replaced. One or more UF4007s, as is necessary, will do the job well. Forward drop in Silicon is less than that in Selenium. So, a series dropping resistor is necessary.

klm1 · 04:47 AM

I have a W3M ; be careful with that selenium! Eli is absolutely correct, it will eventually corrode and burn; due to the lower loss, you have to use another resistor to bring the voltage back in line w/ the original supply voltages.
might i suggest you read the safety section on tubes; also, Tubelab has a very good section on his site; as well, the tube learning for newbies section.
Careful with the wire on that socket and connector; Heathkit passes hi volt thru
some wires from the power supply; take your time, do some research, get and find a schematic you trust, think twice solder once...

kmcarpenter · 24th April 2010, 06:10 AM

I am not attempting these repairs myself.

I have a friend who is the lead guitarist from a local band who is a fire bottle junkie looking at it. He has tube testing and diagnostic equipment that I do not have.

I have the (original) factory electronic diagrams for this amp from the original owner.

RE: the selenium rectifier being a time bomb, what can I expect from it? An actual catastrophic failure?

You make it sound like chernobyl.

kmcarpenter · 24th April 2010, 06:15 AM

LOL @ me. I was looking at some pictures of other types of selenium rectifiers in battery chargers etc... I have lots of antique equipment with this type of device. I have a Kohler generator from the 40's that has one of these rectifiers that is enormous.

kmcarpenter · 9th May 2010, 12:37 AM

I got the amp running today.

The official Parts list:

5 capacitors in the bias supply
4 Power tubes, and 2 pre tubes.

I replaced the factory electrolytics with Sprague-Atom electrolytics of a slightly higher WVDC.

The power tubes are now EL34 SED Svetlana Winged C's and the pre tubes are GE 6AN8A's made in Gt. Britain.

The resistors I thought were blown are ok, they just got hot from a bias problem before the amp was mothballed.

The sound is crystal clear. I had the pleasure of playing it through a pair of spendy Infinity floor standing speakers. I can crank it so high and it has never sounded so good.

Eli Duttman · 9th May 2010, 01:15 AM

Quote:

RE: the selenium rectifier being a time bomb, what can I expect from it? An actual catastrophic failure?

Selenium rectifiers go bad over time. Failure is inevitable. When they fail, a cloud of toxic vapor, along with a bunch of toxic dust, is generated.

24th April 2010, 04:16 AM	#15
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	The Selenium rectifier is probably for the O/P tube bias supply. In any event, it's a ticking toxic time bomb and MUST be replaced. One or more UF4007s, as is necessary, will do the job well. Forward drop in Silicon is less than that in Selenium. So, a series dropping resistor is necessary. __________________ Eli D.

24th April 2010, 04:44 AM	#16
klm1 diyAudio Member Join Date: Oct 2004 Location: Highland, MI	Heathkit... I have a W3M ; be careful with that selenium! Eli is absolutely correct, it will eventually corrode and burn; due to the lower loss, you have to use another resistor to bring the voltage back in line w/ the original supply voltages. might i suggest you read the safety section on tubes; also, Tubelab has a very good section on his site; as well, the tube learning for newbies section. Careful with the wire on that socket and connector; Heathkit passes hi volt thru some wires from the power supply; take your time, do some research, get and find a schematic you trust, think twice solder once... __________________ some like it hot;"the biggest mistakes are made by those who make assumptions!" Graham Maynard Last edited by klm1; 24th April 2010 at 04:47 AM. Reason: poor thinking(typing?)

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11th April 2010, 01:41 PM	#11
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	Yeah, I'm sorry about the lack of a diagram. I just graduated from college, and they revoked my webspace. The diagram I have is attached to the rest of the manual, and is 7+ megs. Sound like a good idea with the heaters and stand by switch?

24th April 2010, 03:38 AM	#13
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	Well, I have some good news, the resistors I thought were popped are actually OK. I guess they just got hot. They test out at the values they are supposed to be right on the money. I think it is because of a bias problem. The amp has a hodgepodge of tubes in it, and I dont think it was ever biased after the day it was built. The small tubes (6AN8) are not lighting up because of a heater circuit problem. We'll see. I hope to be hearing music from it in about a week.

24th April 2010, 03:43 AM	#14
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	Oh- one more thing- What would your opinion be about losing the solid state (selenium) rectifier and installing a tube rectifier be? I dont care that the rectifier is made from selenium, if that is the only reason to get rid of it. I know I'll need a filament xfmr, but I'm willing to install one if anyone thinks it would perform better than the solid state one.

24th April 2010, 06:10 AM	#17
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	I am not attempting these repairs myself. I have a friend who is the lead guitarist from a local band who is a fire bottle junkie looking at it. He has tube testing and diagnostic equipment that I do not have. I have the (original) factory electronic diagrams for this amp from the original owner. RE: the selenium rectifier being a time bomb, what can I expect from it? An actual catastrophic failure? You make it sound like chernobyl.

24th April 2010, 06:15 AM	#18
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	LOL @ me. I was looking at some pictures of other types of selenium rectifiers in battery chargers etc... I have lots of antique equipment with this type of device. I have a Kohler generator from the 40's that has one of these rectifiers that is enormous.

9th May 2010, 12:37 AM	#19
kmcarpenter diyAudio Member Join Date: Apr 2010 Location: New England	I got the amp running today. The official Parts list: 5 capacitors in the bias supply 4 Power tubes, and 2 pre tubes. I replaced the factory electrolytics with Sprague-Atom electrolytics of a slightly higher WVDC. The power tubes are now EL34 SED Svetlana Winged C's and the pre tubes are GE 6AN8A's made in Gt. Britain. The resistors I thought were blown are ok, they just got hot from a bias problem before the amp was mothballed. The sound is crystal clear. I had the pleasure of playing it through a pair of spendy Infinity floor standing speakers. I can crank it so high and it has never sounded so good.

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