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You're not going to like the answer. Your amplifier is oscillating. That is a typical problem with smaller Texas Star amplifiers, due to their bias circuit design, sharing an RF contact on the relay.

No, in fact I've seen one brand like RCA, produce both the single and double versions.

Those 3 amps of filament current, also give the 6KN6, a whooping 1.5 amps of peak cathode current! It has the widest oxide coated cathode of any sweep tube. I bought sleeves of these tubes years ago for $4.99 per tube. I've been using them ever since in every application from a single tube grid...

I think the problem has to do with the DC biasing on your first single-ended transistor driver. Since it is single-ended and NPN, I don't think it should be powered from the same split rail Supply that is feeding the complementary symmetry outputs. If I suspect what is happening is correct, your...

Not a good idea to crack that BEO ceramic insulator. Any dust particles from the cracked insulator, are highly toxic if inhaled.

With sockets changed and bias reset, it's not hard to get this amp to run inexpensive 4X150A / 7034 tubes. About $50 each for NOS. Even though this version of the tube has 150 watts less dissipation than the 8122, the anode radiator is the same size and there is no noticeable drop in output...

You'll need a real peek reading watt meter in order to see the 45 watts PEP. Output power is not the problem with that radio. Being able to receive anything through the noise in the mobile environment, is the problem. The radio has no noise blanker, just a mislabeled automatic noise limiter.

Use a volt and ohm meter, to start checking for filament voltage and open filaments in the tubes.

I've been testing everything from 4CX250B, to 8877 on and old Precision tube tester, with an external socket board and a 15 amp filament Transformer. With a bigger filament Transformer, you can test a 10,000 tube. All that's required is a known good tube, to determine the correct load value. If...

Nomad has you on the right track. This has nothing to do with the switch, it is the relay this switch is operating. This relay may come back to life with use or flipping the switch a bunch of times. They are difficult to remove the cover and clean without damaging.

I'm glad everything worked out as expected, and everyone is pleased. Nice work on the restoration.

This is not a correct diagnosis. When bias does not turn on with a transistor, it works under less stress in class C.

My pleasure Ranch. Glad to see it go to someone who appreciates its 1990, all original factory work. Please let me know once you've had a chance to test it. I want to insure it arrived in good operating condition, as well as physical condition.

The top section of your DC plate choke, has arced and and fused together. It is visible, with black carbon deposits on the porcelain insulater, in the middle picture of the post I responded to, when you blow it up.

I wanted to be an optimist, before having to consider problems under the board.

This rules out the relay and makes the circuit suspect. I remember something about if L1 or L2 were overheated, it reduces the sensitivity of the RF keying circuit.

The capacitor I was thinking about will look more like C1 C2 or C3. It's the one that feeds our RF into the keying transistor. Also check L1 and L2 to make sure they are not burnt looking. In this case, D1 will not be burnt out but it is the diode you could measure across to check the coil voltage.

More than likely, the capacitor, RF diode or transistor in the keying circuit. I doubt it's the relay. You could measure the voltage being applied to the relay coil, by checking it across the black diode that is in parallel with that coil. If it's under 10.5 volts with the microphone keyed, and...

If we just consider input power for simplicity, it only takes half the current, at double the voltage, to produce the same power. Half of the current, means that the output load impedance, is now.... Well, as I think about this more, for current to drop in half with a doubling of voltage...

If you are going to apply 28 volts, without increasing the impedance in the output section, you are going to blow those transistors up, long before it makes the output it did when it was stock. The output impedance should be double what it is on 12 volts, to make the same output on 28 volts...