Hello there,
I am lucky to have been given a pair of wonderful amps.
I have a matching AA-141A with its power amp, a AA-121.
The owner built them in about 1960 or so.
He gave me the manuals for them both as well.
The 141 has six 6EU7 tubes on the chassis, and they all light up when switched on. The 141 was wired with a special plug to power the 121 with activation of one single switch.
I tested for power at the plug and it is there. However, when I plug the 121 into the receptacle, and switch it on, the voltage goes to zero.
I dont know why that is, but never the less, it is one more issue I have to fix.
I also intend to replace the special plug with a standard plug to go into a standard wall outlet on the power amp.
Now, onto the power amp. It has a quad of EL34 output tubes and a pair of 6AN8 voltage amplifier tubes as well.
At any rate, I powered the amp on, and the El34's all light up and get nice and warm. The 6AN8's do not light up.
In order to investigate this, I pulled the bottom cover off and started looking for broken parts. I found 2 blown resistors attached to one of the EL34's. This does not exactly explain to me why the 6AN8's do not light up.
It also has what the manual calls a "surgistor." This is essentially a time delay resistor (100OHM @ 300W, looks like a toaster oven heating element) that acts on a bimetallic blade that closes after about 60secs and shorts out the resistor. The problem is that it wont stay closed.
Parts express does not offer "surgistors" what should I replace it with?
I am also looking into re-capping this amp, as well as getting a new set of matched SED =C= El34's.
Any insight would be helpful, TIA.
-KMC
I am lucky to have been given a pair of wonderful amps.
I have a matching AA-141A with its power amp, a AA-121.
The owner built them in about 1960 or so.
He gave me the manuals for them both as well.
The 141 has six 6EU7 tubes on the chassis, and they all light up when switched on. The 141 was wired with a special plug to power the 121 with activation of one single switch.
I tested for power at the plug and it is there. However, when I plug the 121 into the receptacle, and switch it on, the voltage goes to zero.
I dont know why that is, but never the less, it is one more issue I have to fix.
I also intend to replace the special plug with a standard plug to go into a standard wall outlet on the power amp.
Now, onto the power amp. It has a quad of EL34 output tubes and a pair of 6AN8 voltage amplifier tubes as well.
At any rate, I powered the amp on, and the El34's all light up and get nice and warm. The 6AN8's do not light up.
In order to investigate this, I pulled the bottom cover off and started looking for broken parts. I found 2 blown resistors attached to one of the EL34's. This does not exactly explain to me why the 6AN8's do not light up.
It also has what the manual calls a "surgistor." This is essentially a time delay resistor (100OHM @ 300W, looks like a toaster oven heating element) that acts on a bimetallic blade that closes after about 60secs and shorts out the resistor. The problem is that it wont stay closed.
Parts express does not offer "surgistors" what should I replace it with?
I am also looking into re-capping this amp, as well as getting a new set of matched SED =C= El34's.
Any insight would be helpful, TIA.
-KMC
It's possible that the missing current draw from the unlit 6AN8s and probably little draw on B+ is not enough to keep it closed. For the heaters...perhaps a wiring issue or bad winding on the transformer if they have their own.
Link to a schematic?
Link to a schematic?
Some info here, particularly about the surgistor
Vintage Asylum - Heathkit AA-121 amp - under chassis picture - PeterI - December 13, 2007 at 13:55:17
Les
Vintage Asylum - Heathkit AA-121 amp - under chassis picture - PeterI - December 13, 2007 at 13:55:17
Les
^^Excellent info. What would you suggest I use? A CL150 or a CL90? A link to one for sale?
I am sorry to appear such a noob, I promise that I learn quickly.
I am sorry to appear such a noob, I promise that I learn quickly.
Probably the CL150. I cannot see its low resistance when hot being a problem as the original surgistor became a short circuit when hot.
Needless to say, I have no experience myself with the CL150 but looking around the net gives the impression that it is a good device for surge limiting.
As I am in the UK it is difficult for me to suggest a supplier in the States but I found this one:-
CURRENT LIMITER INRUSH - CL-150
It would be nice to have a glimpse of the circuit but so far I have not been able to locate one. It would make giving suggestions a little easier to do.
The main components to check in a vintage amplifier are the coupling capacitors feeding the control grids of the output tubes and the B+ smoothing components. If the amp has not been used for a long-while then ideally the smoothers should be reformed by applying low voltage at first, slowly increasing whilst watching the leakage current flowing. Eventually finishing with the capacitors normal working voltage with hopefully a very low leakage current. Reforming does not necessary save a capacitor if it is too far gone but at least one has tried even if you have to replace at the end of the day. Blown cathode resistors suggest excess currents through the EL34's which could be caused by a) leaky coupling capacitors as previously mentioned, b) failure of the fixed bias supply or c) short circuit in one or more EL34 tubes. If possible, try to find someone who can verify that the EL34's are ok on a tube tester as this may prevent repeated failures while you try to get the amp up and running. It sure looks like a lovely amplifier as seen here:- Heathkit Virtual Museum | AA-121
Well worth a little TLC.
Les
Needless to say, I have no experience myself with the CL150 but looking around the net gives the impression that it is a good device for surge limiting.
As I am in the UK it is difficult for me to suggest a supplier in the States but I found this one:-
CURRENT LIMITER INRUSH - CL-150
It would be nice to have a glimpse of the circuit but so far I have not been able to locate one. It would make giving suggestions a little easier to do.
The main components to check in a vintage amplifier are the coupling capacitors feeding the control grids of the output tubes and the B+ smoothing components. If the amp has not been used for a long-while then ideally the smoothers should be reformed by applying low voltage at first, slowly increasing whilst watching the leakage current flowing. Eventually finishing with the capacitors normal working voltage with hopefully a very low leakage current. Reforming does not necessary save a capacitor if it is too far gone but at least one has tried even if you have to replace at the end of the day. Blown cathode resistors suggest excess currents through the EL34's which could be caused by a) leaky coupling capacitors as previously mentioned, b) failure of the fixed bias supply or c) short circuit in one or more EL34 tubes. If possible, try to find someone who can verify that the EL34's are ok on a tube tester as this may prevent repeated failures while you try to get the amp up and running. It sure looks like a lovely amplifier as seen here:- Heathkit Virtual Museum | AA-121
Well worth a little TLC.
Les
Mouser is ******* me off. Perhaps I am not understanding the website properly.
I need to get 2 resistors so far.
6 Ohm, .5 W 1%
and
12 Ohm, .5W 1%
I cant get how their inventory search engine works.
The resistors that are blown are between the cathode and the ground bus, and are there to allow an adjustable bias.
I am going to eliminate the Surgistor and install something like a standby switch to fire up the heaters with the power switch and then to flip the standby switch and put current on the plates manually.
I need to get 2 resistors so far.
6 Ohm, .5 W 1%
and
12 Ohm, .5W 1%
I cant get how their inventory search engine works.
The resistors that are blown are between the cathode and the ground bus, and are there to allow an adjustable bias.
I am going to eliminate the Surgistor and install something like a standby switch to fire up the heaters with the power switch and then to flip the standby switch and put current on the plates manually.
In the spec sheet for this amplifier it is mentioned that there are four controls for setting the bias, 1) A Channel Bias Balance, 2) A Channel Bias Level, 3) B Channel Bias Balance and 4) B Channel Bias Level. The resistors that you mention, being 1% tolerance, are probably used to show the bias through the output valves by measuring the voltage across them.
With the EL34's removed, may I suggest that you verify presence of a negative voltage on the control grid pins of the tube sockets, this being pin 5 which, if you are looking at the top of the tube holder, will be the 5th pin ANTI-clockwise from the location spigot. You should see something like -35 to -45 Volts. Make sure this is present before replacing the tubes but take care as there will be very high voltages on some of the other tube base connections! If you have low voltages on the Pin5's then suspect a problem with the Bias supply, also a positive voltage (WHICH YOU DO NOT WANT!) may be due to a leaky coupling capacitor from the phase splitter stage.
I cannot help you much further as I do not have a schematic to advise on the voltages you should be setting to across those 6/12 Ohm resistors.
I am hoping that someone else on this excellent Forum who has a schematic can advise you better on this point.
However, it is normal to attach a multimeter between both pin 5's of each channel and setting the Bias Balance to give zero volts, i.e. both pin 5's have the same bias voltage. Then set the Bias level to give the correct current through one of the tubes, you can then slightly re-adjust the Bias Balance so both valves have the same current flow, I would guess at something like 35mA.
With the EL34's removed, may I suggest that you verify presence of a negative voltage on the control grid pins of the tube sockets, this being pin 5 which, if you are looking at the top of the tube holder, will be the 5th pin ANTI-clockwise from the location spigot. You should see something like -35 to -45 Volts. Make sure this is present before replacing the tubes but take care as there will be very high voltages on some of the other tube base connections! If you have low voltages on the Pin5's then suspect a problem with the Bias supply, also a positive voltage (WHICH YOU DO NOT WANT!) may be due to a leaky coupling capacitor from the phase splitter stage.
I cannot help you much further as I do not have a schematic to advise on the voltages you should be setting to across those 6/12 Ohm resistors.
I am hoping that someone else on this excellent Forum who has a schematic can advise you better on this point.
However, it is normal to attach a multimeter between both pin 5's of each channel and setting the Bias Balance to give zero volts, i.e. both pin 5's have the same bias voltage. Then set the Bias level to give the correct current through one of the tubes, you can then slightly re-adjust the Bias Balance so both valves have the same current flow, I would guess at something like 35mA.
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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?
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?
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.
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.
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.
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.
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.
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...
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...
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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.
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.
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.
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.
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.
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.
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