EQP 1-A tube program (Analogvibes project) Help needed

[Takroc]

Hi Everyone,
I am in the process of completing two EQP 1-A Pultech-style equalizers. One seems to work ok, and the other does not. I have narrowed it down to a voltage issue on the 12ax7 tube, specifically with pin one. The reading on pins one and six should be 140 volts +/- 10%. Pin six is good, pin one isn't, and I am at a loss on where to troubleshoot next. I have changed the base, confirmed the tube is good and doubled checked the wiring against the good unit and made sure there are no shorts etc.. I have posted a pic of my readings. Any guidance would be greatly appreciated as I am on day three and at my wit's end.

 

[tony hunt]

The voltages look generally a tad low across the whole system, but not to an extent that it will not work.

What do you mean by "One seems to work ok, and the other does not." ?
How should we understand "seems"?

Compare your voltages of both units.
If you need to trouble shoot, start at the power supply end and work your way through. Compare to the good unit at each step.

 

[Takroc]

The voltages look generally a tad low across the whole system, but not to an extent that it will not work.

What do you mean by "One seems to work ok, and the other does not." ?
How should we understand "seems"?

Compare your voltages of both units.
If you need to trouble shoot, start at the power supply end and work your way through. Compare to the good unit at each step.

Hi Tony, sorry bout the confusion, the other unit has all the correct voltages but I haven’t tested it functionality yet. I have compared both units and the issue is with the pin one voltage which is reading 113 volts. I don’t understand how a tube works, I’m looking into this today, I assume that the cathode and grid voltages affect the triode, (Pin 1) output?

 

[tony hunt]

Take a look at the triode section of Navy Electricity and Electronics Training Series (NEETS). Try to understand the function of the resistor connected to pin 1 with regard to your assumption "that the cathode and grid voltages affect the triode, (Pin 1) output?"
Perhaps start at the previous chapter with the diode.
The course explains the history and physical aspects of tubes, not just the electronic side.

When trouble shooting tube circuits keep an open mind. Adjusting one thing tends to affect two other things. Ohm's law will be your guide.

Your +/- 10% for voltage tolerance is in my opinion rather optimistic for a tube circuit.
The voltages shown on the old Pultec schematics are qualified as average values.
Many of us will be happy with about 30% at first power-up.
The 113V you are concerned about is only about 19% off the stated target.

If you are happy with the power supply measurements, try swapping the 12AX7 between the units and see what happens.
Try swapping the other tubes too.

One last thing, forum contributor PRR recommended uprating the 2 Watt 470 Ohm resistor after the rectifier to at least 2x to 5x the wattage rating. I agree.

 

[Takroc]

Take a look at the triode section of Navy Electricity and Electronics Training Series (NEETS). Try to understand the function of the resistor connected to pin 1 with regard to your assumption "that the cathode and grid voltages affect the triode, (Pin 1) output?"
Perhaps start at the previous chapter with the diode.
The course explains the history and physical aspects of tubes, not just the electronic side.

When trouble shooting tube circuits keep an open mind. Adjusting one thing tends to affect two other things. Ohm's law will be your guide.

Your +/- 10% for voltage tolerance is in my opinion rather optimistic for a tube circuit.
The voltages shown on the old Pultec schematics are qualified as average values.
Many of us will be happy with about 30% at first power-up.
The 113V you are concerned about is only about 19% off the stated target.

If you are happy with the power supply measurements, try swapping the 12AX7 between the units and see what happens.
Try swapping the other tubes too.

One last thing, forum contributor PRR recommended uprating the 2 Watt 470 Ohm resistor after the rectifier to at least 2x to 5x the wattage rating. I agree.

Hi Tony,
I wanted to thank you for telling me about the voltage tolerances. I stepped back and started from the beginning. The original issue was the audio signal did not go through the unit properly, and I had lots of feedback and weird noises. So I assumed it was the low voltage reading on the tube. So I have now "assumed" the voltage was correct and took apart the capacitor cans wondering if there was an issue in that area. I still had feedback as soon as I did that, but it was very different. A pulsing tone that changed slightly when I adjusted the boost and attend potentiometers instead of a whining sound like a siren. So I will recheck all of the cap filter sections; possibly an issue in the wiring of the caps. I'll send an update if/when I find anything. Thanks again, Tony, for the fresh look......

I will look into the 470 OHM resistor, but it is working on the other unit so I will hold off on that for now.

Also, I did swap all the tubes with the other functioning unit.

 

[echoplex]

Wow, never seen such a messy built. You should have kept it clean and tidy. Makes it so much more easy to troubleshoot.
This spiderweb of wires makes it almost impossible

 

[zhuparty]

Does this include the interstage transformer? i don't see the typical transformer...