Pultec P2P - test point with less current than expected

Hi,
I would like to obtain the right measure (+-15%), at those spot :
12 au7 pin 3 and pin 8

I shoud obtain 40v regarding the schematic, but i have 16.5v here.. Can somebody look at this ? I do not know where to look to solve that issue. The eq sounds great but you know, we always want to fit the specs..

Thank you,

Hugo

Looks like your tube have lower cathode emission. 40v/4700R = 0,0085A, 16,5v/4700R = 0,0035A. Replace this tube, for new, and check  again. Chack anode voltage to(must be some 290v).

lukas said:

Looks like your tube have lower cathode emission. 40v/4700R = 0,0085A, 16,5v/4700R = 0,0035A. Replace this tube, for new, and check  again. Chack anode voltage to(must be some 290v).

Click to expand...

thank you

Personally I do not believe the 40V figure. Looking at the 12AU7 curves a -40V grid bias is at cut off. The 16.5 volt measured implies a plate current of 3.5mA and that is spot on the 12AU7 datasheet curve. So I think there is nothing wrong with the tube.

Cheers

Ian

ruffrecords said:

Personally I do not believe the 40V figure. Looking at the 12AU7 curves a -40V grid bias is at cut off. The 16.5 volt measured implies a plate current of 3.5mA and that is spot on the 12AU7 datasheet curve. So I think there is nothing wrong with the tube.

Cheers

Ian

Click to expand...

Hi, i made a mistake during my build, swaping 2 resistors...after checking i corrected it and i have 36v here now, so it is very fine with my CC resistors (all are 10% too much resistive).. Sounds great ! Thank you Ian

I just realised the bias circuit is a bit more complex than normal. As well as the bias due to the current through the 4700 cathode resistor, there is a positive voltage applied to the grid due to the 68K and 470K resistors connected across the 250 HT line. This puts about +30V on the grid. The bias is this less the nominal 40V at the cathode i.e. -10V. From the data sheet this gives a quiescent current of about 7.5mA which is safely below the maximum dissipation curve. I have no idea whay they chose to bias it this way. The only thing I can think of is it allows you to use a larger value cathode resistor so you can get away with a smaller value cathode bypass capacitor.

Cheers

Ian

I know this is an old post, but I'm having a problem in the same area of the circuit and thought it'd be best to post here rather than starting another thread about this eq.

I'm getting only millivolts on the cathodes  of the 12au7. I do get 31v at the junction of the 62k and 470k, but nothing on the grids.

Everything else checks out fine. I'm a bit baffled. I've checked and re-checked everything around the area and slept on it, but can't find the fault.

Thanks for any help.

I figured it out. I had the wires wrapped around the turret where B+ goes to the center tap of the transformer and on to the anodes of the 12au7, but not soldered on that one turret. Connection was intermittent. So... there probably was B+ when I first measured at that test point, but then wasn't immediately afterward...
Some of these earlier projects of mine developed problems with intermittent noise from things like this: slightly loose ground wires, etc.
Lesson: be thorough.