Help biasing Ampeg JET J-12 with 7591s

[angelothewolf]

Hi! im working on this old Ampeg JET J-12A. just install a new PT from Heyboer, exact clone of the original transformer.I change electrolitics, a new set of high quality tubes, etc.
The amplifier has a 5Y3, a pair of 6SL7 and a pair of 7591. The thing is that the voltages with the new PT are a littlebit higher than the original PT, so i think i must tweak the bias setting a little. The 7591 share the cathode resistor (140R) with a 22uf paralel cap. im getting around 380Vdc of plate voltage and 20vdc across cathode resistor, that gives me around 68mA on each tube, right??
The 7591 are getting very hot, around 200°C, so the idea is to cool it down, its just matter of increase the cathode resistor?

[angelothewolf]

anybody??

[Enzo]

Well geez, at least give us a day. I intend to go back to sleep in a minute.

I=V/R so I get I = 20/140 = 143ma or 71ma per tube.

So 360v across the tube and 71ma through it P = VxI = 360 x 0.071 = 25.6 watts. The tube is rated for 19 watts , so I would cool it down some, yes.

[angelothewolf]

Right! my doubt is how i do that, just rising the cathode resistor?

[g1]

25W is way too hot for those tubes as Enzo mentioned. It makes me wonder if there is a fault that is making them run so hot. Lift one end of the 22uf cathode cap, if the bias changes replace the cap. Also check if there is any DC on the 7591 grids which would indicate that C14 or C15 coupling caps are leaky. Make sure the tremolo is turned off completely.
If it all seems ok, then increase the value of the cathode resistor. For a 19W tube like this you probably want to be idling somewhere around 13 to 15W.

[angelothewolf]

Alright! the cathode cap is brand new, but i just measured 20VDC on the grids ...orange drop bastards! will replace those and see what happens, thanks!

[angelothewolf]

do you think will make big diference if i put some .033 instead of .022 in the grids?

[g1]

Try measuring from grid to cathode (instead of grid to ground), what do you get?

[angelothewolf]

Alright i think the caps are ok, From grid to cathode i get from 0 to -1VDC, i cant turn it on for a long period because the tubes are geting very hot! already try increase the cathode resistor but no change, it gets very hot anyway

im measuring 6,7VAC on the heaters 370VDC on plate and 330VDC on screen grid, is that ok?

[Leo_Gnardo]

do you think will make big diference if i put some .033 instead of .022 in the grids?

You get a pinch more bass. If .033 is what you have on hand, put 'em to work.

Unusual for OD's to go bad but . . . you never know. Control grids should be sitting at zero volts DC. As-is the old Ampegs have raging high bias current for 7591's with those 140 ohm stock cathode resistors. I often wind up with 180, 200, 220 Rk's after trying a few options. I've found sometimes I have to settle for a little more crossover distortion than I'd like to see at clip. For those who play loud and brash, that's not much of a worry. OTOH not so good for those rare clean jazz players who love their old Ampegs.

Another thought is, which 7591's are you using? Not much trouble with the original item. However the emission figures varied widely from tube to tube.

EH has a current reissue which looks nearly identical to the Sovtek 5881 wafer base, and acts reasonably close to a 7591. Now here's the potential "ringer": about 20 years ago EH sold a tube they called 7591XYZ which were simply Sovtek 5881 with a couple of pin reassignments to make them work - sort of - in a socket wired for 7591. BUT they biased like 6L6 and it's up to the tech to alter the bias accordingly. If you have these, you'll have to up the Rk by quite a bit, probably to around 400 ohms.

[angelothewolf]

alright, im using electro harmonix 7591AEH, the ODs are not the problem. i cant figure out why they are getting so hot! more than 200c if i leaveit on for a while. i try a 270ohm in the cathode r but still getting hot... any thoughts?

[g1]

Your measurement of 0V from grids to cathode indicates a problem There should be 20V difference. So the 20V you measured from grids to ground is really there. Try lifting the coupling caps. C14 and C15 and see if there is still 20V on the grids.
You want 0V at the grids.

[Leo_Gnardo]

alright, im using electro harmonix 7591AEH, the ODs are not the problem. i cant figure out why they are getting so hot! more than 200c if i leaveit on for a while. i try a 270ohm in the cathode r but still getting hot... any thoughts?

Control grids "should" be @ -20V relative to cathodes. I'm at a loss to explain why they're nearly at the same voltage. Let's try anyway...

Had a blink at a J-12 schemo. You should have a couple of "stopper" resistors between the caps and tube control grids, plus a pair of 270K to ground. So - are the 270K's there, and with one end of each at ground?

Don't discount the possibility of a "factory wiring error." If they put the junction of those two 270K ground reference R's at Rk's + end instead of ground, that would explain why things are goofy in that amp.

Any possibility someone has altered the wiring of the output tube sockets to accomodate 6V6/6L6 ?

[angelothewolf]

ok, when i change that caps the 20v went away. Now i got like 1vdc on one grid to ground and 0vdc on the other. Grid to cathode is 0vdc on both.

[Leo_Gnardo]

ok, when i change that caps the 20v went away. Now i got like 1vdc on one grid to ground and 0vdc on the other. Grid to cathode is 0vdc on both.

Well I'll be ..... OD's almost never fail. I think you'll get your Jet dialed in now that's out of the way. Still odd that grid-to-cathode is 0vdc. Are you sure?