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yestertech · 18th Dec 2009, 5:02 pm

From your description and the valve types, I'm guessing the 75 is the audio amp. A screwdriver placed on the top of this should yield a healthy "buzz" to show that the audio is fine. If this test still works when the signal fades, then the fault lies somewhere in the 78/6A7 HF stages.
You should get some sort of response from touching the top caps of the other valves. Again, if this is missing when the fault occurs, then you may be able to tie it down to one or the other, although a failure in the 78 IF stage will silence any preceding 6a7 response also !
As has been suggested, compare voltage readings on the 6a7/78 valves before and after the fault develops.

Andy

18th Dec 2009, 5:02 pm	#4
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Re: Newb Stumped on a Derwent From your description and the valve types, I'm guessing the 75 is the audio amp. A screwdriver placed on the top of this should yield a healthy "buzz" to show that the audio is fine. If this test still works when the signal fades, then the fault lies somewhere in the 78/6A7 HF stages. You should get some sort of response from touching the top caps of the other valves. Again, if this is missing when the fault occurs, then you may be able to tie it down to one or the other, although a failure in the 78 IF stage will silence any preceding 6a7 response also ! As has been suggested, compare voltage readings on the 6a7/78 valves before and after the fault develops. Andy