New owner to an old Heathkit AA-121

[kmcarpenter]

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

[rknize]

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?

[kmcarpenter]

I have a really blurry PDF of the original manual.

I'll see if I can get a better copy scanned.

[Hi-Q]

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

[kmcarpenter]

^^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.

[Hi-Q]

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

[rknize]

Mouser has them.

[kmcarpenter]

Off to the tube doctor this friday, I'm stoked.

[kmcarpenter]

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.

[Hi-Q]

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.