Help biasing Ampeg JET J-12 with 7591s

[Enzo]

Seems unlikely that two new film caps would get leaky at the same time.

Your cathode sits at 20v, so if there is no difference in voltage between there and your grids, then yo have no bias.

Those 270k grid return resistors do not go to ground, they go to the trem circuit.


I'd bet something is open there. Power off and discharge. What resistance do you measure from each power tube grid to ground? Leo is expecting 270k there, I am expecting the 270k plus the 680k of the trem out. So for me, something just short of 1 meg. But not open, we do not want open.


Your tubes are getting hot because they are cranking out 25 watts of idle dissipation. They are doing that because they have no bias.

[angelothewolf]

th

[angelothewolf]

alright, the 680k in the tremolo was open, replace and we got 1meg from grids to ground.
Now i got -15vdc from grids to cathode and 500mvdc from grids to ground. the tubes are reaching the 200°C anyway
we got 17vdc across cathode resistor, 390vdc on plates and 390vdc on screen grids

[Leo_Gnardo]

the 1k stoppers are there and ok, now the junction of those 270k are coming from the tremolo osc that has an 680k to ground...

I was lookin' at the J12 without the A - and as always Enzo's right. All seems OK now. If there was any leakage thru the 0.1 uF that passes the vibrato signal, that would upset the bias. But at the moment it's back to measuring the bias current again, and maybe selecting yet another Rk.

Gots to get some work done for the crowd of howling customers now. I'll check back in later. Sure you'll find success with your Jet.

[angelothewolf]

Ok Leo_Gnardo thanks for the help, i will try another Rk now and see if i can cool it down a bit

[angelothewolf]

i put a 270ohm 10w resistor on the cathode, and now i got 20v across it, that gives me 74mA, 37mA per tube, times 390v = 14,4W
Now the tubes are hot, but not veery hot. 170c after 5 minutes, did you think its ok? or maybe too hot?
The amp sounds very well, only one issue> the tremolo sounds with low intensity, what could it be??

[angelothewolf]

Increasing R12 (680k) on the tremolo, will give me more intensity?

[Enzo]

More likely C9 sucks, or the trem oscillator is weak in general.

[angelothewolf]

More likely C9 sucks, or the trem oscillator is weak in general.

well seems that someone did not like the trem intensity and put a 270k resistor in the grid of V1b, i take it off and, voalį!

Intresting, looking at the schem sticked on the back of the amp, are the original voltages detailed! mine is running with 380V instead of 345V and 20v on the cathodes instead of 16, with my calculations the tubes are disipating 14,5W each reaching the 170°C. The sound is amazing, very clean and loud up to 12 o`clock then starts distorting, very nice.

Do you think the temperature could be a problem?

Thanks for the help everyone!

[Leo_Gnardo]

well seems that someone did not like the trem intensity and put a 270k resistor in the grid of V1b, i take it off and, voalį!

Intresting, looking at the schem sticked on the back of the amp, are the original voltages detailed! mine is running with 380V instead of 345V and 20v on the cathodes instead of 16, with my calculations the tubes are disipating 14,5W each reaching the 170°C. The sound is amazing, very clean and loud up to 12 o`clock then starts distorting, very nice.

Do you think the temperature could be a problem?

Thanks for the help everyone!

You've made some big steps in the right direction. And a nice find on the trem intensity. About the only thing left is possibly to increase the value of the cathode R some more, but there comes a point when it starts sounding thin and weedy due to crossover distortion. Aside from that, I'd stop working on it and start playing it.

[angelothewolf]

You've made some big steps in the right direction. And a nice find on the trem intensity. About the only thing left is possibly to increase the value of the cathode R some more, but there comes a point when it starts sounding thin and weedy due to crossover distortion. Aside from that, I'd stop working on it and start playing it.

You are right, i think the temperature is within specs, i“ll burn it a bit and see how it behaves, cheers!