Dual Pultec Build Ground Noise

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I want to believe it is something with this transformer that was sold to me with the enclosure. If it were a proximity thing then I should hear it more in channel A. I moved the cables around and the secondary wires somewhat and moved the board a little further holding it in place. It does seem that a soon as 2 boards are powered it is inducing something to happen

For example, even though it says 230 V on it after rectification, I am getting 317 V at the tube. I’m imagining that toroidal transformer quality can vary.

And by independently I mean there are 3 ground wires coming from each srpp board ground point and going to star ground. There are a 2 ground points on each filter board both with their own wire going to star ground. So that makes 10 wires going to star ground total.
 
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The transformer specs printed on it say 230v 0.1A red and 12v 2a blue
50VA
I presume you’re referring to the dual secondary ratings here. It seems they are in spec compared to the Don Audio one.

Have you checked the AC voltages at the board?

12V 2A
Are the current secondaries. My transformer doesn’t have 3 secondaries as it probably should. And isn’t hooked up like this.
I believe the 5V secondary on the Don audio one is not used in the audio circuit but can be used for an LED.

IMG_2795.jpeg

Really need a schematic to see what is going on.
It’s the Gyraf Pultec which can be found here https://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif

And you’re correct that one of those transformers marked ‘output’ is actually being used as an input.

The term used to describe the incorrect connection is often "pin 1 problem," so if you search that term you will find a lot of material (quite a bit even on gdiy forums).
Thanks, that’s exactly what I stumbled across.
 
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I want to believe it is something with this transformer that was sold to me with the enclosure. If it were a proximity thing then I should hear it more in channel A. I moved the cables around and the secondary wires somewhat and moved the board a little further holding it in place. It does seem that a soon as 2 boards are powered it is inducing something to happen

For example, even though it says 230 V on it after rectification, I am getting 317 V at the tube. I’m imagining that toroidal transformer quality can vary.

And by independently I mean there are 3 ground wires coming from each srpp board ground point and going to star ground. There are a 2 ground points on each filter board both with their own wire going to star ground. So that makes 10 wires going to star ground total.
What is the AC voltage coming out of the transformer for each winding set?
 
I presume you’re referring to the dual secondary ratings here. It seems they are in spec compared to the Don Audio one.

Have you checked the AC voltages at the board?


I believe the 5V secondary on the Don audio one is not used in the audio circuit but can be used for an LED.

View attachment 110780


It’s the Gyraf Pultec which can be found here https://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif

And you’re correct that one of those transformers marked ‘output’ is actually being used as an input.


Thanks, that’s exactly what I stumbled across.
230v @ 0.1A = 23VA + 12V @ 2A = 24VA sum = 47VA but if the input rating is only 23VA (230V @ 0.1A) something is wrong with the spec. However the tube heaters only draw marginal current and the HT supply is drawing nothing much at quiescent idle stage so the transformer can’t be overloaded.
 
230v @ 0.1A = 23VA + 12V @ 2A = 24VA sum = 47VA but if the input rating is only 23VA (230V @ 0.1A) something is wrong with the spec. However the tube heaters only draw marginal current and the HT supply is drawing nothing much at quiescent idle stage so the transformer can’t be overloaded.
I suspect the input rating is higher (~4.6A) and that 230v @ 0.1A is the rating of one of the secondaries.

(I did edit my initial comment as I misinterpreted, in case I caused any confusion there)
 
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I suspect the input rating is higher (~4.6A) and that 230v @ 0.1A is the rating of one of the secondaries.

(I did edit my initial comment as I misinterpreted, in case I caused any confusion there)
Ok but either way you have a massive overvoltage on your HT rail - what’s the actual AC voltage of the supposed 230V secondary?
And do the two DC HT voltages differ?
 
Ok but either way you have a massive overvoltage on your HT rail - what’s the actual AC voltage of the supposed 230V secondary?
And do the two DC HT voltages differ?
I’m not the original poster, but the advice on the G-Pultec support thread is to increase the value of the 3K3 HT resistor until the voltage falls within 200-250v. I increased mine to 15k.
 
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I will check the AC voltages shortly. Might as well also try this rectifier board I built to see what happens if the long crossing power rail is no longer carrying AC. I believe i can quickly wire this up if I don’t have to remove and jump the pcb bridge rectifiers. I believe DC can be fed into a bridge rectifier just for testing..? Aaron at Don Audio said an external bridge rectifier made a difference awhile back for him. How much remains to be seen.

https://electronics.stackexchange.c...appens-if-you-feed-dc-into-a-bridge-rectifier
Also yes I increased the power resistor on one of the boards to 15k and that tube is now seeing about 255V. Haven’t changed it on the other board yet cause it’s still a little high. Need to go up a little more in resistance.
 

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Also yes I increased the power resistor on one of the boards to 15k and that tube is now seeing about 255V. Haven’t changed it on the other board yet cause it’s still a little high. Need to go up a little more in resistance.
Yes probably a little hot still but likely fine for testing. I think mine ended up around 225v.
 
Both boards need to come down in HT maybe try 18K. Are both HT DC voltages matching with the original resistors or are they different - you may have a dodgy rectifier if they are different.
 
Secondaries read 13.5v and 255v. The dc reading on the tubes were very similar originally at 317v or so until I changed the resistor recently. Can I send dc into the rectifiers on the boards for testing? After my little rectifier board I’m getting 250v dc out. Should I just remove the bridge rectifiers on the boards and jump them or can’t hurt to try as is with them in place?
 
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even though it says 230 V on it after rectification, I am getting 317 V at the tube

Transformers are rated by RMS AC output. A standard rectifier into large capacitors will have close to the peak voltage, not the RMS voltage, so approximately 1.4x the RMS rating. A 230V labeled transformer secondary would be expected to be around 320V-325V at the first filter cap.
If that first filter cap is rated for 350V as shown in the Gyraf schematics it is a little under-rated. Traditionally you would design for +10% line voltage over nominal, so design for at least 357V, and then derate from there for increased reliability and lifespan. Not a short term concern to get your build working, just something to note for the future.

by independently I mean there are 3 ground wires coming from each srpp board ground point and going to star ground. There are a 2 ground points on each filter board both with their own wire going to star ground. So that makes 10 wires going to star ground total.

That makes no sense electrically and is asking for problems. It might not cause problems, but it makes a big physical mess and makes problems harder to troubleshoot, so why do it?

It’s the Gyraf Pultec

That schematic really encourages wiring in a pin 1 problem and is guaranteed to be noisy with at least some equipment you connect to the input and output.
I hope Gyraf has learned better in the intervening 20 years.
It also is a little incomplete, because it does not show the interconnection points between the different PCBs. Or was the original Gyraf design all on one large PCB? These PCBs are apparently from DIY-Racked, but DR no longer makes that project, so hard to tell.

Might as well also try this rectifier board I built to see what happens if the long crossing power rail is no longer carrying AC

You only have a rectifier, no capacitors. I guess that technically is not AC, but it also isn't what most people would call DC. You are going to have full wave rectified pulses on the wiring between rectifier board that you built and the power supply board. If you just move the transformer outside the chassis you will have sinusoidal-ish voltage on the wires, but full wave rectified current pulses, which isn't much of a difference if you twist the wire pair and have enough physical separation that you don't get any capacitive coupling to speak of.
In other words any benefit would be primarily because of moving the transformer out of the chassis, not because of changing where the rectifiers are located.
The picture you attached still had the transformer bolted into the case, and only the rectifier sitting outside the chassis, and I would not expect that to make any relevant difference. If it does I am not sure what that might imply, probably that the problem is very sensitive to the specific power wire routing path.
You could test that hypothesis by moving the wires into the same location but jumper around the external bridge rectifier.
I am having trouble imagining what problem a different rectifier is trying to address, or why that would even be suggested. The only thing I can think is that there was a misunderstanding and what was proposed was an external power supply, but that is just guessing on my part without seeing the original reference.
 
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Thank for the response. Yeah at this point I am sort of grasping at straws. I understand that what is coming out of my rectifier is not true DC but again I’m trying everything in this recipe below from the one person who posted describing an operating hum free unit. Here is a pic of someone else who did something similar with external rectifier

I have yet to demonstrate that the proximity of the toroidal is causing this as the noise is the same in both channels. I will however move it out of the chassis in the next experiment. I need to prove that helps before building a complete external PS. The pic below didn’t have to who had it horizontal and the poster below rotated it to vertical with success.

In terms of the grounding again I’m following the post below and may have misinterpreted. What seemed to make a big difference in noise was grounding the filter boards and SRPP (separately i presume)



-63,4 db
normal connection and 0v to star point ground
------------------------------------------

extern rectiviering (only with the 350v wires)
- 63,6 fb

grounding the eq filters to starpoint
-71.4

grounding the eq filter to starpoint AND Starpoint the psu "Filter Ground"
-75db

Distancing the 0v StarGround from the toroid...
-75,7

0v StarGround cable under the potentiometer - front section.
-76,5

Rotating the toroid vertical to the right site of the inside chassi.
-81 db
Hum is gone!

- Rotating The toroid did a hum difference of -76 to -95db on one channel. This was the most hum.

———————————————

The build always had 2 pcbs per channel. See here

https://www.gyraf.dk/gy_pd/pultec/gy_pd.pdf
 

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I think I’ve done that sufficiently evidenced by quiet single channels. What electrical changes are happening when 2 channels fed power?
 
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Secondaries read 13.5v and 255v. The dc reading on the tubes were very similar originally at 317v or so until I changed the resistor recently. Can I send dc into the rectifiers on the boards for testing? After my little rectifier board I’m getting 250v dc out. Should I just remove the bridge rectifiers on the boards and jump them or can’t hurt to try as is with them in place?
As long as there are no breaking down diodes in either of the rectifiers - I’d remove them anyway to get them out of the equation. The odd thing is you get 250V with this rectifier and get over 300 with the on-board ones.

I presume you will run 2 wires each for HT+ and ground from your new rectifier to the two boards and not loop across from one board to the next?

Make sure each board has the same HT feed resistor value.
 
I have one pair of AC wires coming into the board directly off the transformer and two pairs of “DC“ wires coming out of the board going to each PCB. These were wired in parallel on the rectifier board. No extra ground wires. we’ll just go ahead and remove the onboard rectifiers as well as give the transformer more slack to move outside the chassis freely for further experimentation
 
What electrical changes are happening when 2 channels fed power?

Increased current draw (so slightly lower output voltage at the transformer)
Higher magnetic field strength from the transformer because of increased current draw
Current flowing in additional wiring pair running from first SRPP board to second SRPP board.
Potentially current flowing in unexpected places from the 10 additional wires connected to the PCBs
 
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