LK800A PROBLEM

[ws4bs]

After the soft start times out, I am showing 450 ma of plate current at idle. No drive at all. Tubes get warm really fast like the amp is transmitting, but it is not keyed. No grid current. What might be causing this to happen?

[w4bfs]

please refresh my memory .... what tubes does it use ....

[w8khk]

Perhaps a problem with the relay that shorts out R1 on transmit.  Maybe the relay contacts are shorted....  Or possibly a problem with the Zener diode between R1 and the tube cathodes.  This sets operating bias when the relay contacts close, and idle bias is provided by R1 when the relay is open.

I would suggest adding a wire to the cathode of one of the three tubes, then closing everything up such that it is safe to apply power.  The wire connected to the cathode could be connected to a voltmeter to inspect the cathode bias when you apply plate voltage (after the soft-start period has elapsed).

If no problem is identified here. I would suggest trying to power up with only one tube installed.  Try three times, testing with each tube individually.  It is possible a defective tube is causing the excessive plate current, assuming no problem is found with the cathode bias components.

Make sure the plate supply is completely discharged before touching anything inside between each test!

Hope this helps.   GL OM!

[ws4bs]

Yes. I looked there first, but no problems found in that area and as stated, no grid current at all. I will do as you suggest and remove the plate supply from two of the 3cx800's at a time and see if the problem persists. This rig is nice in that it has a separate 240 vac power for the HV and 115 vac power for the low voltage side and makes it much easier to troubleshoot the main unit when the HV is not present. Thank you for your help

[w8khk]

It is possible to have a cathode to grid short in one of the tubes.  That is why I suggested testing with only one tube in the amplifier at a time.  Removing just the plate supply from two tubes, and leaving the other tubes in the amplifier, will provide inconclusive results.  That method of testing will also NOT isolate which, if any, tubes are defective. 

Testing with only one tube at  a time is the reliable way to isolate tube problems in an amplifier with this circuit.  Hope this additional information proves helpful to you.

[ws4bs]

Each tube draws about 170 ma when connected alone. I am not sure as to how to proceed at this point. I know the zener in the cathode circuit is supposed to serve a purpose, but I am not sure what. Still need help. There is about 1.2 volts on the cathode at idle. I believe it should be higher, but I am not sure of what it should be.

[ka1bwo]

Each tube draws about 170 ma when connected alone. I am not sure as to how to proceed at this point. I know the zener in the cathode circuit is supposed to serve a purpose, but I am not sure what. Still need help. There is about 1.2 volts on the cathode at idle. I believe it should be higher, but I am not sure of what it should be.

It appears that zener D3 is shorted.

[WBear2GCR]

Each tube draws about 170 ma when connected alone. I am not sure as to how to proceed at this point. I know the zener in the cathode circuit is supposed to serve a purpose, but I am not sure what. Still need help. There is about 1.2 volts on the cathode at idle. I believe it should be higher, but I am not sure of what it should be.

If the grid is on ground = 0 volts, when you lift the cathode UP from ground by "N" volts, that makes
the grid look negative when compared to the cathode! Aka negative bias.

Reducing the voltage the cathode is lifted UP from ground, that causes less negative bias, and more
current to flow.

                            _-_-

[DMOD]

Each tube draws about 170 ma when connected alone. I am not sure as to how to proceed at this point. I know the zener in the cathode circuit is supposed to serve a purpose, but I am not sure what. Still need help. There is about 1.2 volts on the cathode at idle. I believe it should be higher, but I am not sure of what it should be.

It appears that zener D3 is shorted.

I think Rick and others most likely identified the culprit, since no cathode bias voltage results in Max plate current.

Zener D3 is a 9.1 Volts Zener diode (1N3308, Stud Mount) that biases the cathode to a positive 9.1 volts on Keydown, which biases the grids at -9.1 Volts.

It sounds to me like the zener is either shorted or someone has reversed the leads.

There are power Zener diodes out there that are manufactured with the Anode and Cathode leads reversed. For a typical 1N3308, the stud should be at ground.

Phil