G-PULTEC problems, please help!

[paolomunarini]

I just found out that there's no input transformer inside my gPultec, as it's "optional" -(the guy who built this was on a save-spree).
obviously its working.. but what am I missing out on, without it?
And what will happen if I install one?

Input is unbalanced (even if it is an XLR).
With the unbalanced input:

• A few more dB (cannot quantify, cable length dependant) of common mode noise getting in from the unbalanced input
• 6 dB of volume less due to the absence of the 1:2 step-up
• Input impedance is approx 10k instead of 2.5k (reflected impedance through the transformer gets lower)
• Less THD from transformers if you drive the unit hot

All in all I'd say "whatever"... Only thing that would bother me a little bit would be those 6 dB...

Edit: I checked the schematic and the trafo is wired in a 1:1 configuration. So none of the turn-ratio dependent consideration is valid.

 

[MiamiBoy]

What you just described, is that how my unit is NOW, without the transformer..?

 

[paolomunarini]

What you just described, is that how my unit is NOW, without the transformer..?

Exactly. But check my edit. I just went through the schematic and I found out that the trafo is wired as a 1:1 (primaries are in parallel and secondaries are in series)

 

[MiamiBoy]

Alright.. sound like I need to install the missing transformer asap... I can only benefit from it, right?
Can you describe how sonically the unit will change, once I install it?

 

[paolomunarini]

Alright.. sound like I need to install the missing transformer asap... I can only benefit from it, right?
Can you describe how sonically the unit will change, once I install it?

Actually it's up to your taste.
I'd suggest you try it out with and without (the unbalanced input is still available after you mount it) and decide by ear what you like the most.

It's just a little bit more distortion from the transformer...

And a 5402 is in the end not that expensive for line balancing transformers... I found two at a very good price here in the black market category...

 

[MiamiBoy]

Ok cool, i'll try it out then. They're like 50 bucks at Don Audio
Thanks for everything

 

[paolomunarini]

Ok cool, i'll try it out then. They're like 50 bucks at Don Audio
Thanks for everything

Check on the black market. I got 2 for 60€ a few months ago.

 

[calvinjosephalex]

Hey!
I'm doing my bachelor's project and currently building a vaccum tube amplifier. I planned to do the g pultec one as it's much simple. Is there any way I can alter the gain of the circuit. Can anyone help please! .

 

[gyraf]

Basically no, the SRPP circuit is fixed gain only - look up the article in an old issue of "glass audio" explaining the SRPP in detail. Also "Audio Xpress" has a good foundation on the topic: Fig. 1: The humble SRPP seems to inspire and confuse in equal measure. - PDF Free Download

But as the SRPP stage is an inverting amplifier, you can reduce it's gain some by means of feedback.

/Jakob E.

 

[paolomunarini]

Hi everyone, I need a quick advice.

I am finishing my build. Everything works but the two following:

• Low atten kicks in all together in the first 5-10% of the pot. I swapped for a 100K log and the response is better. But it still appears to be attenuating too much at the beginning of the pot.
• High boost does not work at all. No change in bandwidth and high frequency gain at any position of the two pots.

I started analyzing the DC resistance of the high boost signal path. I suppose that the inductors that I bought have a too high DC resistance. The 4 inductors in series sum up to almost 1K. Could this be the reason why the high boost is totally inactive?

Edit: build is based on creative clowns PCBs. Bandwidth pot is 2K5 log, high boost is 10K log, HF capacitor network is styroflex. Inductors are CTCs found on banzai

Edit 2: Low boost is also working strange. It boosts only when Low atten is active.

 

[BGroen]

Naa, I wouldnt worry, 98 is close enough for jazz...

How about 85V? Too low or OK?


For anyone using the boards from Analog Classics. Read the posts from Blissy on page 42.
Hope it saves you the time reading trough 41 pages!

 

[gyraf]

if you think it sounds fine, it is fine - But a bit low for my taste

 

[BGroen]

Also, the 125V was down around 98V or so. I did some testing with a pot, and it looks like about 470R instead of the 1K resistor beside the 4.7uF cap should get it close to 125V.

I haven't heard it yet haha. But I'd like to build it as it is meant to be.

Why is it lower in my case? And what is the best way to increase it? My HT voltage is 236V and heater is 6.1V.

Correct that I have to LOWER the 1k resistor as JW did? I would expect increasing it to get a higher voltage drop.

And then should I also adjust 470R? Reading a little about it on The Tube CAD Journal,SRPP Decoded I understand that the stage is optimized for a specific impedence loading, and I am afraid changing only Rak ruins the optimization.

 

[Lakewood]

Hey!

I just finished another pultec-build, voltages seemed fine so i inserted some noise (-12dBFS)..
The output signal of the unit in bypass is much lower than the Input signal (-30dBFS).. So it's about 18dB is signal loss.
I double checked everything but i can't find the mistake. Maybe someone can give me some hints what this can be?



Kind regards, Leo

 

[petermontg]

Check the phase wiring of I/O. You could have a solder bridge/connection in there.

 

[gyraf]

check it partially:
Input signal transformer? signal should be ca. 0dB after this.
Filter stage? signal should be down to ca. -20dB after this.
Amplifier stage? signal should now be back to ca. 0dB after this

 

[Lakewood]

check it partially:
Input signal transformer? signal should be ca. 0dB after this.
Filter stage? signal should be down to ca. -20dB after

Thank you for the fast replies! What is the best way do I measure these partial stages in the unit?

 

[gyraf]

Yes, simply trace the signal through the unit: Your AC voltage at the input to the filter? At the output of the filter?


(ps: you are aware that you must ground xlrs pin3s for connecting to unbalanced inputs/outputs?)

 

[Lakewood]

Yes, simply trace the signal through the unit: Your AC voltage at the input to the filter? At the output of the filter?


(ps: you are aware that you must ground xlrs pin3s for connecting to unbalanced inputs/outputs?)

Hey, I just checked the AC Voltages of the unit! I fed it with balanced signal (white noise @0dbfs). Ground Connection between XLR1 Pins and chassis is intact! Here is what i measured:

• BAL. IN Hot Wire -> 0.82V (thats the white noise signal)
• Filter IN -> 0.23V (should equal -11dB)
• Filter OUT -> 0.004V (should equal -41dB)

I also found some discrepancies in the schematic and the PCB layout on the gyraf website.. I have designed a new PCB according to the existing layout, so maybe this could make some troubles:

In the PCB Layout:​

LL5405 Input PIN7 -> GND​

LL5402 Input PIN10 -> FILTER IN​

​

​

In the Schematic:
LL5405 Input PIN7 -> FILTER IN
LL5402 Input PIN10 -> GND

 

[dario84]

Hi everyone, I need a quick advice.

I am finishing my build. Everything works but the two following:

• Low atten kicks in all together in the first 5-10% of the pot. I swapped for a 100K log and the response is better. But it still appears to be attenuating too much at the beginning of the pot.
• High boost does not work at all. No change in bandwidth and high frequency gain at any position of the two pots.

I started analyzing the DC resistance of the high boost signal path. I suppose that the inductors that I bought have a too high DC resistance. The 4 inductors in series sum up to almost 1K. Could this be the reason why the high boost is totally inactive?

Edit: build is based on creative clowns PCBs. Bandwidth pot is 2K5 log, high boost is 10K log, HF capacitor network is styroflex. Inductors are CTCs found on banzai

Edit 2: Low boost is also working strange. It boosts only when Low atten is active.

strange behaviour indeed. never had that problem. photos of the wiring and build would maybe help us figure out the problem.