Schemo is on post #105
Yes overkill, in order to solve a problem that comes from a different issue.
He's learning...
Last edited:
Pultec clone - grounding problems
- Thread starter morls
- Start date
Help Support GroupDIY Audio Forum:
Yes overkill, in order to solve a problem that comes from a different issue.
He's learning...
Electrotanya is it?
Winston OBoogie
Well-known member
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.
If there's any blame, throw it my way folks.
No blame on my part. I believe he will learn something in the process.
Winston OBoogie
Well-known member
For sure.
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.
Stephen
- 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.
Stephen
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.
If anyone notices an issue please let me know.
Attachments
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.
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.
Yes. The HT issue is not complicated; you need to measure the voltages along the RC filters.
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.
Yes. Be aware that iy will probably result in slightly higher ripple.
Winston OBoogie
Well-known member
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.
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.
Last edited:
Winston OBoogie
Well-known member
Shooting from the hip: 6mV or less
Thanks, this has gotten me much closer. Now getting 6.35V +/- 0.02V at the filter output. Is this ok, or should I get closer to 6.30?
Winston OBoogie
Well-known member
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.
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.
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.
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...
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...
Last edited:
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
Well-known member
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.
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.
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.
Sounds good, thanks.
Winston OBoogie
Well-known member
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.
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.
