7591A fixed bias , no reading on cathode..

[azatplayer]

I been hankering for some Ampeg love lately. Got meselfs some 7591A's to try out. Just rewired some sockets on one of my mules to take em.
Left the fixed bias circuit as is, setup for 6L6's, just wanted a quick setup to try em out.
Pin 1 has no connection, theres no pin on the tube, so i used that to tie the screen resistor to. From there to pin 4(screen) and a wire back to pin 8 from the screen. Pin 3 is plate. Pins 2 and 7 heaters.
Pin 6 is control grids, and pin 5 is connected to ground thru a 1ohm bias checker resistor.
So, fired up and i only get a shitty blatty distorted lo output noise.
BUT no reading across the bias checker. They read 1 ohm give or take, but no voltage there at all.
Question. Is there an alternate wiring for the cathode, pin 5, fixed bias as opposed to cathode biased?
Had a quick look at some ampeg stuff and they have some fixed bias amps that still use the pins as i have assumed.
Tubes are new EH.
Got 405V on the plate 400V on the screen(1K in series with 1K screen R's)
-32V on the grids, zero reading on the cathode.
Without knowing anything about these tubes, im gonna have to try it cathode biased and see what happens., cos i think the wiring must be different for fixed, somehow.

[azatplayer]

In looking around, i see that there are amps with a low value resistor from the cathode to ground, like 10R to 70R, thats hifi stuff. Also saw some fixed bias ampegs with the cathodes linked with a 100R resistor then to ground somehow.

[azatplayer]

OK, set it up with cathode bias and its workin fine. Very chimey tubes!

Can someone explain how to get it fixed bias properly?

Right, some volts first.
WIth a 180R cathode R bypassed with 47uf i have 16.5V across it. Grids have about 16mv on them.
400V plates, 389V screens. I work out its dissipating around 35watts for the pair. No redplates. Seems a little high for a 19watt tube tho?

Its a mule chassis, kinda cross between dc30 and rocket, using a 5K2 OT as well. Will try and get the layoit tuned in to something more familiar, maybe stock rocket so i can gauge things a little better.

[martin manning]

Looking at the data sheet, -32V on g1 has these in cut-off; Ia is zero. See if you can raise the Vbias and bring the idle current up to get 60-70% of max plate dissipation. At 19W max Pa and 400V you need about 33mA. The load should be something like 4k3 a-a or greater.

[martin manning]

double

[tubeswell]

I use 1% 1R (0.6W) cathode resistors in fixed bias amps. It makes virtually no difference in the ground potential at the cathode, but allows you to read off the current in mV equivalents by measuring the voltage drop across the 1R resistor. (1mV/1R=1mA)

[azatplayer]

Yep had it setup with the 1R bias readers. Zero reading. Bit of an unknown tube for me. Cathode bias was straight forward. But for some reason fixed won't work. It's like the cathodes have been lifted.

[tubeswell]

check your wiring, the 1R need to be in series with the cathodes and the ground (not in parallel). If they are in series, then the tube current will flow through them, and you will get a (milli)voltage drop across them.

[azatplayer]

Ahh Martin, youre correct mate. It was in cutoff. I found an old Gibson schemo using them in fixed bias. It stated 18.4V bias voltage, i had 34 or so. I dropped the resistor values and got a usable range that gives me that 18.4V, which allowed me to bias it to 33ma. And it sounds soooo much better than cathiode biased. Very cool, cheers!
I have them in a rocket kinda thing right now, got too much bottom end, but as a mule, its a pretty good initail result!

[martin manning]

Great, sounds like you'll have some fun there! Note that when you had it working with cathode bias there was 16.5V at the cathode and the grid was referenced to ground. That tells you that you need about -16.5V on the grid with the cathode grounded to hit the same operating point in fixed bias.

[azatplayer]

Ahh good one, cheers mate!
Had an hour or so with it tonite with different guitars, its really really nice.
Has a lovely bloom and rising sustain that ive not had in a rocket build before.
Using a liverpool OT 5K2, spec is around 6K6 so its ballpark.
Bypassed the second stage cathode with 22uf, (rocket) and am getting some very sweet slightly driven tones. These tubes have a top end presence that sits almost above whats going on everywhere else. And when riffing, its kinda like a slapback when you hit the higher notes, its very cool.
Cant wait to throw some gain in front of it, see what comes out.
Lovin it!