Bias/ Plate dissipation question

[Captn Dave]

I am trying to adjust the bias on a 7591 push pull amp. Three of the tubes fall easliy into adjustment at 27 ma. The fourth tube will not bias to less than 36 ma at the full negative bias voltage. The problem follows the tube so I conclude that the tube has issues even though it tests good on two different testers.

My question deals with the physics of the tube. I'd like to better understand what is going on inside the envelope. What is it about the (ageing) tube that would cause it to "conduct" more current than it should at a given bias.


Notes: The 7591 is a 19 watt plate dissipation tube. The plate is operating at 460 VDC and the bias about -21.5.

[chrish]

First of all, let me apologise for not being able answer your question...

I am collecting parts for a PP 7591 amp, but have not finalised a layout/design yet. What amp are you working on and what are your impressions?

Regards,

Chris

[SY]

Several possibilities:

Cathode emission
Cathode-to-grid spacing
Gas

[refference]

Perhaps a defective cathode , with a very hot-spot point , that
damaged the control grid ???

Carlos

[Captn Dave]

OK, all good ideas.

Chris, this amp is from a Lowry Organ, I think. It's the best organ pull I have encountered. On the plus side, it has two symetrical channels, great iron, output transformers with 4,8&16 taps, quality bias pots, just above average build quality (but still south of a Conn), and a transformer supplied CRC filtered negative DC bias for the power pentodes.

On the strange & intreaguing side, it uses 7 pin pentodes for the preamp and phase splitters. Four 6AU6s. The preamp pentode has the screen, suppressor and plate straped so that it operates as a triode. The phase splitter operates as a pentode. Unusual in my experience. I would have expected something like a 6SN7 instead.

I do like the sound. It sounds about like a 6V6 but the bass seems a bit faster and tighter - but not quite as good as a 6L6. I don't think it has quite the detail of an EL84 at low power but very close.

I'm tempted to sub in a 12AT7 in the preamp socket just to see how the 6AU6 sounds in comparison. The 6AU6 in triode mode seems like a neat build trick because they are cheap.

I find it very difficult to judge a stereo amp by comparing channels, but now that it's biased (to the extent possible with the off spec tube), I don't think I can hear a difference in distortion between channels and I couldn't see anything out of the ordinary on the scope. I'm sure the "hot tube" will soon fail but it sounds ok for now.