Deco-Doctor wrote:Have you tried running them a bit cooler, ie backing off the current to below 50ma? Then see what happens.
Hi ya Martin, yep, tried that. They just seem to have a mind of their own.
Deco-Doctor wrote:As you say they are getting on a bit and may be slightly gassy and have a bit of grid emission. Does the anode current remain stable if you put them in a cathode biased amp?
I haven't got a cathode bias amp to hand at the mo so TBH I don't know the answer that yet.
Mike H wrote:
I presume the valve using the IT is OK with a 12k load?
Yup, it's a chunky little beast the 6C45p, our AndrewL has a spud amp using one. He drove the massiff Sachiko (sp?) speakers with it once at an Eggy-bash.
Mike H wrote:That implies a current is flowing through the IT secondary and with resistor making a Voltage drop, wouldn't have thought it should be that much to make that sort of difference.
Yup, I'd have thought that too Mike but the effect on the waveform is very pronounced. Here's a 120Khz sinewave 'getting tidied up'... click pic for mini movie...
It was just an experiment really to see 'what happens if....' but after re-jigging the (fixed) bias back correctly to -45 on the grid (after the introduction of the 12K resistor threw it right out) the amp sounds pretty good. I'd imagine it's not conventional to have a 12K grid leak pulling 3mA through it to ground on the -45 grid bias feed but the amp does seemed to have gained some 'grunt'. I'm guessing the 6C45p is seeing a 12K load though the 1:1 ITx?
Bias on the ShuGuang 2A3
C's are steady, usual 250V @ 50mA or so OP, no drift using these.
The best part of a whole weekend 'playing' with 'scope and sig gen and amps.
DTB