6550 madness

[Phil Y]

Hello all, I have a weird situation and would like ideas/suggestions.

My Kat88 won't work with the set of 6550s I have. It works fine with JJ KT88s and also with a set of Golden Dragon 88s I have here.
What is more odd is that I have here a borrowed current model WDKT88 amp and the 6550s work fine in that. The 6550 also test fine on my AVO valve tester. What makes this even more odd is that the 6550s were the ones that came with the amp when I bought it from Greg of this forum. At the time, I decided that I marginally preferred the JJs so the 6550s have been in a drawer since then, but having the borrowed amp here I took the opportunity to dig them out and have a play with them.

The amp is circuit wise(other than servicing) as I received it.
With the 6550s when the amp is switched on, as valves warm up, there is a hum, not super loud but very obvious, and I noticed a blue glow around the bottom mica of the 6550s.

Anyone any ideas?

Phil.

[John Caswell]

Hi Phil,
Start by measuring the voltages on the 6550 especially the cathode (pin8) and the grid (pin5) to give you an idea if the valves are drawing any current.

John

[Phil Y]

Thanks John, and yes, I'll have to get the amp on the bench.
Realistically, I won't have time for this before the weekend.

Phil.