nacho459

Pultec voltage problem
« on: August 14, 2004, 08:08:24 PM »
On the Gyraf SRPP Pultec schematic, the output cap coming off the ECC88 plate before the output xfmr is labeled as a 4.7mF / 250v poly.
Is this the correct value? Aren?t they usually in the .22mF range?
I ask this because I am getting 80v on the output side of the 4.7mF cap.
Any ideas?


PRR

Pultec voltage problem
« Reply #1 on: August 15, 2004, 01:31:43 AM »
> the output cap coming off the ECC88 plate before the output xfmr is labeled as a 4.7mF / 250v poly.
Is this the correct value? Aren?t they usually in the .22mF range?


I don't have the Pultec schematic memorized.

If it is driving a transformer, 5uFd is much more likely than 0.2uFd. Sub-microfard caps are adequate for driving most grid resistors, but transformers are lower impedance and need bigger caps.

> I ask this because I am getting 80v on the output side of the 4.7mF cap.

Get a fast voltmeter and watch it start up. Betcha it goes nearly to the power supply voltage. You have to design for the worst case, which is often not the normal case.

gyraf

Pultec voltage problem
« Reply #2 on: August 15, 2004, 07:51:05 AM »
Nacho,

PRR pretty much summed it up:

Yes, there is a 4.7uF/250V polyester cap driving the output transformer. The output is somewhat low impedance, so with a lower value capacitor you'll loose low-end.

If you have 80V at the output side of the capacitor, then either something is wrong with the capacitor, or you haven't mounted the transformer yet (meaning that DC transferred to the output end of the cap at start-up has nowhere to go)

If you want to use the circuit without the output transformer (i.e. unbalanced), mount a 10K resistor from output to ground to shunt the DC to ground.

Jakob E.
..note to self: don't let Harman run your company..

nacho459

Pultec voltage problem
« Reply #3 on: August 15, 2004, 07:56:45 PM »
OK, I'm going to install transformers on the I/O but I haven?t yet. I just like to test as I go to eliminate potential problems along the way. So far I have the power supply and the amp built, and tested the eq section. I should be getting the panels this week, and then the real fun will begin.

I?ll try to get some pictures up soon.

I?m building point to point, AKA, the Right way. haha
 :razz:

CJ

Pultec voltage problem
« Reply #4 on: August 16, 2004, 06:01:56 PM »
you don't want 80 volts on your transformer, so do Jakes test first.
If I can't fix it, I can fix it so nobody else can!
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