Pultec clone - grounding problems

[scott2000]

that could result in (or contribute to) the secondary voltages being lower than expected.

And the PT itself. Which one is being used? I have missed it.

 

[morls]

Interesting thought...

Mains is 237VAC, around 1.25% down from 240V.

I'm glad to have gotten the extra 15-17V following Winston's suggestion.

 

[morls]

Using the Hammond 369JX

369JX

 

[solkatten]

Have you dubble checked primary PT connection? There are many ways to hook up the primarys?

 

[Winston OBoogie]

While it isn't the pinnacle of efficient HT supply designs, it's probably OK for the job. Gaining that 17V is almost halfway to where you want it to be. You could just let it be and move on.

Or put the circuit in PSUD and re-jig it. The info from the Hammond spec sheet tells you the DCR of the windings so you have enough info for that. Your load is 17 - 18mA so put that in at the junction where C5, C9, R13, & R14 meet. No need to include R13, & R14 and their associated subsequent caps in the PSUD schematic for now.

Caveat: Looks like the largest cap value you can use as the first reservoir cap after the 6X4 is 40uF. However, you'd probably get away with using a 47uF in this circuit. But don't go bigger in that spot.
Cap values downstream from the 6X4 can be larger in value so your C5 & C9 in parallel would be OK.

 

[radardoug]

Lose the 6X4.

 

[abbey road d enfer]

Again I think the secondary voltage should be checked with an oscilloscoe.
The simulation I made shows tha tmainly because of the DCR of tehe HT secondary, the sinewave is drastically clipped at about 260-270V.

 

[morls]

Or put the circuit in PSUD and re-jig it. The info from the Hammond spec sheet tells you the DCR of the windings so you have enough info for that. Your load is 17 - 18mA so put that in at the junction where C5, C9, R13, & R14 meet. No need to include R13, & R14 and their associated subsequent caps in the PSUD schematic for now.

Caveat: Looks like the largest cap value you can use as the first reservoir cap after the 6X4 is 40uF. However, you'd probably get away with using a 47uF in this circuit. But don't go bigger in that spot.
Cap values downstream from the 6X4 can be larger in value so your C5 & C9 in parallel would be OK.

I'll have a look at it in PSUD, thanks for the suggestions.

Again I think the secondary voltage should be checked with an oscilloscoe.
The simulation I made shows tha tmainly because of the DCR of tehe HT secondary, the sinewave is drastically clipped at about 260-270V.

Although I couldn't see the whole waveform at once, I was able to see the peaks on my scope by moving the trace. I could see clipping, although not the voltage at which it occured. It looked like the waveform on the right:

 

[morls]

The simulation I made shows tha tmainly because of the DCR of tehe HT secondary, the sinewave is drastically clipped at about 260-270V.

Following on from my previous post, rather than building a Kelvin sensing path so I can measure the HT secondary DCR I'm going to make the assumption that the clipping shown in your simulation is what I observed with the scope.

So my question is, what can I do to fix this issue? Would a different power transformer solve the problem? Another possibility is that there is a note on the PCB indicating that resistor "A" between the big caps can be replaced with a 10H choke.

 

[abbey road d enfer]

So my question is, what can I do to fix this issue? Would a different power transformer solve the problem?

Of course a xfmr with a lower DCR would probably help.

Another possibility is that there is a note on the PCB indicating that resistor "A" between the big caps can be replaced with a 10H choke.

A choke is also a possibility, but solutions are costly, much more than shorting a resistors. And what do you care if your B+ is a little low? Do you want +24 into 600r?

 

[morls]

Yep, very good points. It makes sense to tweak what I can on the existing board.
It’s already a good improvement. I’ll spend some time playing around in PSUD.

 

[morls]

Do you want +24 into 600r?

It would be very nice…

I found the typical DCR on the spec sheet, HT secondary is 354.4 Ohm +/- 20%

 

[Winston OBoogie]

If you enter the hv winding dcr along with the other circuit details, PSUD should show you the issue that Abbey found.

So, for a possible fix - the other obvious places where you could pick up a few volts are your R14 & R15.

I think the filtering beforehand is good enough, and the amplifier's psrr high enough, that coming down to 30K or so on R15, and to 1K1 on R14 would be fine.
That should get you within spitting distance of the design centre voltages.

Other than that, I don't know that buying another transformer or a choke is worth the expense or potential issues you'd encounter when integrating them.

Thanks for doing the sim Abbey

 

[morls]

Yes, thanks Abbey!

I worked out the specs for PSUD as best I could, and when I ran the sim I got an error message (attached). The sim was set to begin after 17s, so the 6X4 was quickly overloaded. Is this the source of the clipping you found Abbey?

Thank you for those resistor values Winston. I think I’ll follow the advice, make these changes and leave it at that. I’ll post the results once done.

 

[abbey road d enfer]

I found the typical DCR on the spec sheet, HT secondary is 354.4 Ohm +/- 20%

That's the value I entered in my simulation, 177 ohms each side, along with 40 ohms for the whole primary.

 

[abbey road d enfer]

I worked out the specs for PSUD as best I could, and when I ran the sim I got an error message (attached). The sim was set to begin after 17s, so the 6X4 was quickly overloaded. Is this the source of the clipping you found Abbey?

I didn't use PSUD (I hate the HUI); I made it with LTspice.
I made a crude approximation of the 6x4 (with a resistor and zener), but anyway it shows the dominant sag source is the DCR of the sec. The actual leakage inductance is also a probable but unknown factor.

 

[Winston OBoogie]

One thing to add is that, since the dcr of your secondary is limiting peak current into the 6X4, you'll be fine upping that first 10uF filter cap (or paralleling a 47uF with it, same as you have with the second 10uF cap) if you wish.

The new values I gave you for R14 and R15 were just fingers and thumbs sums but you can "adjust on test" if necessary... Should be close enough for tooobz and rock 'n' roll though.

Edit: The first 1K resistor marked "A" on your most recent circuit - may as well add another 1K in parallel to make it 500r. Or just make it another 220r like the succeeding values. This will get your pre R14 & R15 voltage a bit closer to spec.
I might be tempted to make it 220R which would pick up probably another 10V, then just leave R14 and R15 alone. You're only 20V shy of spec then, not enough to care about.
Your old 45V shy isn't really that much to care about either, although I do understand the temptation to make it as close as possible.

 

[morls]

Once again, thank you Winston and Abbey, there is no way I could get this sorted out on my own. I'll go ahead and make these final changes, and post the results to tie this thread up.
Stephen

 

[morls]

Ok, I've finally had a chance to finish up on this one (moving house is a biiiig job...)

After changing resistor "A" to 220R and installing some decent 40uF caps the voltages are much better:

Test point after resistor "D" is marked 325V. Now measuring 317V, up from 276V.
Centre tap of output transformer primary, marked 290V. Measuring 279V, up from 245V.
Pin 3 of 12AU7, marked 40V. Measuring 39V.

Very happy with these, big thanks again to all contributors.
Cheers
Stephen

 

[Winston OBoogie]

Hi Stephen,
thanks for the update. Good stuff, well done.
Use and enjoy them in good health. The same goes for the new house
Cheers