[Amps] Amp failure advice please.

[g0jhc at blueyonder.co.uk]

 have an amp problem anyone any advice what to do next please?

Completed it last week,  tuned up and worked great to begin with, but after a little “soak testing” something has gone wrong, I have lost my output!

The amplifier still gives an indication on both the PSU meters and amp meters that it is drawing current. As an example plate voltage no load is still 4500v, plate voltage at ZSAC (350mA) 4200V, plate voltage at 2kw 4000v, and plate current 1 amp.

As I said ZSAC is still there at 350ma when I go to TX ( no drive)

Heater volts remain fine.


	The tube still blows off warm air.
	
	Everything is there to indicate that RF is being produced but no longer finding its way to the output, as my meters no longer show any grid current, very little power output or reflected power.
	
	When I adjust the load ( a 10kv vacuum variable) and plate capacitors ( a flapper) little if anything happens, although I can sometimes “peak” it to 250w before it goes back to zero. None of the trips, grid or plate kick in just the tune trip! I can not then just reduce the drive and bring it back up to the failure point, I have to re-tune to a completely different set of c1-c2 values.
	
	If I vary the power from my drive rig, I vary the plate voltage and plate current to the same levels as when I had output.
	
	I can still tune for a 1.1 match doing the standard cold test with my analyser and load resistor. When it was working I had only minimum adjustments to make the first time I powered up, suggesting my spreadsheet analysis is close.
	
	I have removed the anode choke and checked it, nothing arcing or burnt out there, the  decoupling cap which brings the HV into the RF cabinet seems to cold test ok for SC/OC.
	
	I have removed the 3 200pf HT57 blocking caps, and each test cold as you would expect, reading 200pf on my capacitance meter.
	
	I have removed the Z50 on the output as a last resort.
	
	 
	All tests conducted directly from the rf output socket, removing the directional coupler or relays from the system and into a dummy load.
	
	
	What would be my next move to finding the problem. Could it still be the blocking caps? There is no obvious arcing visible.
	
	Unfortunately I don’t have any spare vacuum caps or blocking caps to substitute and they are expensive items, and don’t want expenditure if its not the problem.
	
	How would I progress with any fault analyses under these circumstances please?
	
	Hope the experts can advise.
	
	
	
	73 Neil G0JHC