Pultec clone - grounding problems

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morls

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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?

Yes, that's right.

I've checked on the other unit I have, and this is also around 45V low on the HT side.
 

morls

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Looking at the original Pulse Technologies schematic, the output from the 6x4 (pin 7) is marked as +355V, and I'm getting 315V. Maybe the 6x4 needs replacing? Output from the PT secondary HT is 259VAC on both leads.
 

abbey road d enfer

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Looking at the original Pulse Technologies schematic, the output from the 6x4 (pin 7) is marked as +355V, and I'm getting 315V. Maybe the 6x4 needs replacing? Output from the PT secondary HT is 259VAC on both leads.
It is quite possible. The normal sag is supposed to be about 20-25V, it looks like it's actually about 50.
However I wouldn't worry too much about it; the impact on performance is not enormous.
 

morls

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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.

Here's the result, looks much better:

Pultec 1 DC Heater no input EQ bypass.PNG

And a comparison of 1kHz sine test before installing the DC Heater and fixing wiring (white), and after (green):

Pultec 1 DC Heater.PNG
 

Winston OBoogie

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I can't see what the vertical scale divisions indicate but, sure does look better.

Besides your fundamental 50Hz AC line frequency and its F2, F3 etc. harmonics, there's a whole bunch less hash up right around your test signal frequency of 1kHz.
OK, good.

I think some of that lower frequency stuff at 50Hz and 100Hz etc., might be improved with cable placement/management or transformer orientation.. But you're in a much better place to be negotiating things now so, well done.

Is this result with or without the EQ section hooked up yet or?
 

Winston OBoogie

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OK, nice. Looking/comparing to your 1K reference that's pretty decent.
So, time to button it up and call it a day? Or?
 

morls

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I'm very happy with these results. The only thing I'm wondering about is the low HT voltage. Aside from that, I'm inclined to do the fixes on the second unit and move on.

Thanks Winston, Abbey and Bo for putting time into helping me here, there's no way I would have gotten this far without your guidance. Thank you as well to all who contributed. This is a great forum!

Cheers
Stephen
 

Winston OBoogie

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I'm very happy with these results. The only thing I'm wondering about is the low HT voltage. Aside from that, I'm inclined to do the fixes on the second unit and move on.

OK great.

Well, if you get the main HT feed up by 45V then there will be a marginal improvement in headroom.

It's also possible the operating points of the tubes will be more "optimum" as far as was originally intended.

But tubes are remarkably tolerant of actual HT supply voltage and differences in ideal operating points can tend to even/smooth themselves out when the HT is a few volts from the design centre.

Maybe, if you can see that more than a fair share of volts is being dropped in the filtering then, sure, get the HT higher.

If you have to halve a resistor value in the filtering network to drop less voltage then, within reason, you can increase the following cap value in proportion (double the cap value in the case of a halved value resistor) to stay in the same ballpark of HT ripple level.


You could draw out Drip's power supply HT section and post it if you need help with messing about with it

👍
 

morls

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Ok, you've convinced me 😁

I might as well get this HT voltage sorted out while I've got the unit on the bench. I'll get a drawing of the Drip power supply HT section happening.
 

morls

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Here is the Drip schematic, and the original Pulse Technologies Pultec. I've included a sketch of the original on the Drip version, it's a lot simpler! It stops at R14, which refers to the corresponding point on the Drip schematic.

After R14 in the Drip version, power goes to the amp section. I haven't gone through in detail but it looks to be very very close, if not identical to the original from this point on.
 

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Winston OBoogie

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I don't see why you need all the extra RC filtering in the Drip HT line tbh. The topology of the amplifier is such that PSRR is good.
As Abbey indicated, actual voltage measurements at each point would be good to see...
Are you sure that the feed to the 12AX7 first stage comes after your R14 and not before? I'm questioning the placement of C9 too,

I would say that you could probably just short out resistor "D", the 1K 2W. You shouldn't need it. Have C4 & C5 just be in parallel.
You should pick up about 19 volts DC from eliminating that resistor.
 
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morls

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I'm wondering if the fact that both units show the lower voltage (-45V) rules out a problem with transformer?

I got this from another forum, regarding the Tektronix 2235:
The scope has a 400Vdc/pk max rating, but does appear to allow 800Vpp for an AC signal

I think I fried the oscilloscope probes on the 258VAC from the secondary. I'll need to get some x100 probes to check that voltage out.

The AC input voltage to pins 1 and 6 of the 6X4 is 258VAC to each. The output at pin 7 is 310VDC, which is minus 45V from the value given in the schematic. This minus 45V carries through to all test points.

This is consistent across both units, which suggests to me an issue with the HT circuit implementation rather than a failure of one or more components. Does this sound reasonable?
 

morls

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Are you sure that the feed to the 12AX7 first stage comes after your R14 and not before? I'm questioning the placement of C9 too

Sorry, I made a mistake with the circuit. You're right about the feed to the 12AX7. I also made an error before that point, C3 has been corrected.

I'll get the actual voltages and post them.
 

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Winston OBoogie

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That looks more like it. OK so, going by your last schematic, short resistor "D" the 1K, which puts C5 & C9 in parallel.
 

morls

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Ok, here are the voltages.

Pin 7 6X4: 319V (Pulse schematic 355V)
C2 - 303V
C4 - 299V
C5 - 296V
R14 - 280V (Pulse and Drip markings 325V)
Output transformer - 248V (Pulse and Drip markings 295V)

After jumping "D" resistor:
R14 - 297V (+17V)
Output transformer - 263V (+15V)
 

Khron

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Here's a thought - how about measuring your mains (with a multimeter)? Maybe that's a few percent low in your area, and since the secondary voltage is a greater-than-1 ratio to the mains, that could result in (or contribute to) the secondary voltages being lower than expected.
 
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