Pultec clone - grounding problems

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Winston OBoogie

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It was me who suggested that Stephen's first take on a DC supply could probably be bettered in terms of getting lower residual AC noise. When PSUD indicated he'd gotten that residual down much further I said "build it".
If there's any blame, throw it my way folks.
 

morls

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Not sure why the word blame is floating around? Sure it's clunky and overkill, but it's also my very first attempt at:

- measurement and analysis of build
- use of PSU design software
- design and build of rectifier/filter
- using KiCad to generate 3D model and prototype PCB
- contacting PCB manufacturers to build prototype

Not to mention learning a lot of basic tube circuit and power supply design principles.

Thank you to all contributors here. I would never have leant so much without your help. (y)

Stephen
 

morls

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I've got the work done. It's late here so I'm going to go over it in the morning with fresh eyes before I power it up. I've re-positioned the big caps, installed the rectifier, moved the power switch across, run the AC along the chassis and in the corners, tidied up the transformer and inductor wirings, installed a ribbon cable between the 2 control boards, followed the grounding layout suggested by Bo.

If anyone notices an issue please let me know.
 

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morls

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Ok, so I've had it up and running. I haven't tested any further than the power supply, here are some measurements:

PT secondary high voltage - 257VAC / 257VAV

PT Heater voltage at connection to rectifier: 6.45VAC +/- 30mV

Voltage across C6 (last cap of rectifier filter): 5.81VDC +/- 10mV

Voltage of C6+ with reference chassis ground: +2.21VDC +/- 10mV
Voltage of C6- with reference chassis ground: -3.35VDC +/- 10mV

The centre tap of the heater is grounded to PS star ground.

The output of the high voltage PS is down by around 45VDC. There is a test point after the last resistor in this section, which is marked 325V. Measurement here is 276V.
Another test point is the centre tap of the output transformer primary. This is marked 290V, and measures 245V.

I'm thinking that these should be treated separately? Should the heater DC 0V reference be at least level with chassis ground? If so, maybe grounding at the -ve leg of C6 instead of centre tap of supply would bring this up? I imagine an adjustment to the filter resistor values would take care of the 0.5V that I need to get to 6.3VDC.
 

abbey road d enfer

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I'm thinking that these should be treated separately?
Yes. The HT issue is not complicated; you need to measure the voltages along the RC filters.
Should the heater DC 0V reference be at least level with chassis ground? If so, maybe grounding at the -ve leg of C6 instead of centre tap of supply would bring this up?
I don't see how that would change since these points are ar the same zero voltage. And what's wrong with the measured values? that was expected.
I imagine an adjustment to the filter resistor values would take care of the 0.5V that I need to get to 6.3VDC.
Yes. Be aware that iy will probably result in slightly higher ripple.
 

morls

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Thanks Abbey. I’ll adjust the resistors and go from there. What is a ballpark ripple figure for good performance?
 

Winston OBoogie

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Not been on here that much in the last few days, sorry.

I say that that's a good start for your first build based on using PSUD and coming to grips with what a DC heater entails and what needs to be done etc. Good progress :)

If you have extra resistors, try tacking on some parallel values so that every resistor is about half of what it was.

As for where the ground connection goes (transformer centre tap or C6) it won't change the total DC output voltage of your circuit, but it may make a difference when it comes to noise of the whole circuit once you're at 6V3 or so.
You can try either way once you have the main amplifier passing signal.
 
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Winston OBoogie

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That looks good to me Stephen, leave it be at 6.35V.

Next, I would hook up the output from the make-up gain amplifier, without the eq/filter network in front etc, and check the noise, or lack of noise as the case may be.
 

morls

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I've got an old oscilloscope (Tek 2235) so I thought I'd get a trace of the output from C6 of the rectifier filter. Probe was on +ve leg, ground clip of probe on -ve leg. They're not expensive probes and the scope may be way out of calibration, so this might be inaccurate. Vertical divisions are 0.5VDC.

Scope 1.jpg Scope 2.jpg

Edit...
Just read up on the *enormous* ground loop I had for this trace between the probe and ground clip. I'm going to work on getting a better test setup...
 
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morls

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That looks good to me Stephen, leave it be at 6.35V.
(y)
Next, I would hook up the output from the make-up gain amplifier, without the eq/filter network in front etc, and check the noise, or lack of noise as the case may be.
Would this be the XLR output, with EQ on bypass?

Should I get the HT voltage up to the right value before doing the output test?
 

Winston OBoogie

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(y)

Would this be the XLR output, with EQ on bypass?

XLR output. You don't need to connect any of the eq and filter stuff just yet so whether it's EQ bypassed or not, won't matter.



Should I get the HT voltage up to the right value before doing the output test?

So you're saying that, with your DC heater now at 6.35V, your HT voltage is not the same as it was when you ran on AC heater?


Also, if you have the centre-tap of your heater winding still grounded, then by hooking your probe's negative lead to the bottom of C6, you are most likely double grounding the circuit through attaching the 'scope, so bets are off as far as what the trace shows being meaningful.


For simplicity's sake, maybe remove the centre-tap ground connection and attach the circuit ground to the negative of C6 too.
 

morls

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So you're saying that, with your DC heater now at 6.35V, your HT voltage is not the same as it was when you ran on AC heater?
I can't answer that, as I didn't get a HT voltage reading before installing the rectifier. The readings I'm getting now are down about 45V at the PS output. I might have to replace the motor run caps so I can get to the resistors in the PS.

For simplicity's sake, maybe remove the centre-tap ground connection and attach the circuit ground to the negative of C6 too.
Sounds good, thanks.
 

Winston OBoogie

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One thing, measuring the noise on a power supply with a scope might best be done using 2 channels in differential mode.
Otherwise, an isolated scope might be used where it's powered by an isolation transformer.
What I'm saying, I guess, is that it's very easy to actually be coupling noise into the supply by your measurement technique so, caveat emptor.

On the HT voltages you're getting: so you're saying that, compared to what's indicated in the schematic/printed on the pcb etc., your voltages are 45V lower?
But that they may have been 45V lower than specified all along, when the unit was AC heater?

Or?

Not quite sure what you mean on that aspect.
 
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