Grid voltage Pultec 1A

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,
First off, im really a noob on this subject, so hope someone can clear this a little for me.

Got a Pultec 1A up and running, and have made some measurements
On the 12AX7, 12AU7 tubes from ground, to pins with tubes in.
I just want to be sure everything is looking ok, so i dont waste my hard earned cash, and get a bad surprise.
Im very confused when it comes to the grid voltage on the 12AU7,
should it be like this 27-39 = -12VDC

12AX7 VDC

Pin 1 = 145
2 = 0.0
3 = 1.2
4 = 57
5 = 57
6 = 149
7 = 0.0
8 = 1.2
9 = 57

12AU7 VDC

Pin 1 = 286
2 = 27
3 = 39
4 = 57
5 = 57
6 = 286
7 = 27
8 = 40
9 = 57

eqp-1a_schem.jpg


Help is much appreciated!

Cheers
 
Last edited:

PRR

Member
Joined 2003
Paid Member
> simulations I am getting about 3 volts grid to cathode on a 12au7.

Reasonable for small work.

Here the AU is used as a POWER amplifier. Program EQs were sometimes asked to drive long lines or multi-recorder buses at levels to 100mW. There is little NFB. Level meters were primitive (even VU was just starting to come in). The Power stage must deliver most of a Watt so that at typical 10mW-100mW levels the distortion would be small, and it would "never" hard-overload and go splatt.

So it runs a very hefty 250Vp-k and 8mA cathode current. 2 Watts per plate. Yes, 12AU7 is rated 5.5W for both plates. However the Pultec was used in studios where good life and extreme reliability was wanted. (You don't make movie stars or orchestras sit idle while you find the failure in the recording chain.)

Small parameter shifts in the tube as it ages or is replaced could put you much closer to over-heat than you want in this service.

So the 470K+62K divider from 250V to 0V establishes a +29V source. (Says +31V on the drawing. All these old numbers were rough.)

The bogie number on the Tung-Sol sheet for 250V and 8mA is about 10Vg-k.

(3V bias at 250Vp-k puts a 12AU7 at 27mA and 7 Watts, not a happy point.)

Taking say +30V bias and 10Vg-k in the tube, we expect cathode to sit at +40V above ground. In 4700 Ohms, passes 8.5mA. With 290V-40V= 250Vp-k and 8.5mA in the tube, 2.12 Watts in the plate. OK.

With the 4700r dropping 40V and the tube dropping 10V-12V, even quite large changes in Vg-k amount to quite small changes in tube current and dissipation.
 
Thanks for clearing this up for me.
Please if you guys spot anything that needs to be tweaked, or looked further into please let me know.
I did some tests this morning, and measured black probe to cathode
And red to grid on the 12AU7. i will now get -9.28 this way (voltage in reference to ground looks to be the same as yesterday.
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.