G-PULTEC problems, please help!

[paolomunarini]

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

I ultimately solved my problems.

Filter pcb was wired in reverse. Output from the trafo was wired to the output of the filter board and the input of the gain stage to the input of the filter board.

Furthermore I had to change inductors because the DC series resistance of the ones I had was too high and the HF boost was completely attenuated by it.

I attach a few plots

 

[dario84]

glad to

I ultimately solved my problems.

Filter pcb was wired in reverse. Output from the trafo was wired to the output of the filter board and the input of the gain stage to the input of the filter board.

Furthermore I had to change inductors because the DC series resistance of the ones I had was too high and the HF boost was completely attenuated by it.

I attach a few plots

glad you made it work!

 

[SmokeyJoe]

Hey folks, just finished my G-Pultec build and getting a lot of low end roll off in bypass - looks like it’s right across the spectrum but worst from around 300hz. Round trip signal level is also ~3dB down with a 1k tone.

I've measured AC signal voltages with white noise:

XLR in hot: 0.255v
XLR in hot/cold: 0.56v
Filter in: 0.556v
Filter out: 0.062v
XLR out hot: 0.186v
XLR out hot/cold: 0.471v

I've checked that all connections between boards and connectors are correct - in goes to in and out to out, right?

Edit:
I've just tested it into a high impedance mic input and the signal loss with a 1k tone is now only 1dB. Possibly an impedance issue with my interface?

Interestingly with white noise, the return signal is actually ~5dB up after the bypassed Pultec (same with mic or line input). That’s weird, right?

 

[SmokeyJoe]

Ok I think it must be an impedance issue. If I go via my TG1 clone, which is 10k input and 600 ohm output, the bass rolloff is only below 40-50Hz. I thought my interface would be up to the task but apparently not (RME Babyface Pro FS).

Ultimately I wonder if some kind of buffer box with a high input impedance and low (or 600 ohm??) output impedance would be the way to go here - is anyone able to point me to a project on here?

 

[SmokeyJoe]

@gyraf picking up the discussion from the other thread, testing frequency response directly after the input trafo suggests that the issue is indeed with my interface's ability to drive it (first graph). I previously built a driver circuit in an attempt to overcome this and it flattened out the response nicely (second graph). However, based on some measurements I took at the time, I was concerned that the THD was a bit high, although this time I don't think the results are too bad (third graph). Any thoughts on this?

 

[BGroen]

On a side note, you can see that we can present a VERY low load impedance to the input transformer if we boost highs AND cut highs at the same time. This why input transformer must be relatively low-impedance and low-resistance: otherwise it will limit amount of boost available by introducing a virtual resistance in series with the 10K. This is why I suggest e.g. the LL5402 as input transformer.

Hi all,

I am going to build a set of MEQ5's and I'm double checking if I use the correct input transformer.
The LL5402 is an output transformer used as input suggested by Jakob for the above reason.

Is this also a suitable choice for the MEQ5, or is there a better alternative?

Hope hearing,

Kind regards,

 

[gyraf]

I don't think the MEQ5 has similar low-z loading on the input trafo - just use any 600:600 transformer you have around..

/Jakob E.

 

[staras]

Is it ok to use 10uF for output cap?

Thanks

 

[gyraf]

Yes, as long as rated voltage is at least as high as your supply

 

[tfpmusic]

Hi Jacob,

I have build my dual G-pultec 17 years ago (and enjoying it every day) but it was time to change the tubes. I bought some nice vintage NOS tubes but one is lower in gain. So I read all post again this summer and applied (tested) the trim on the input (20K trimmer) and a 1K trimmer on the High cut. Both did the same job. But I was wondering what the best place for this mod is. Input trimmer (maybe impedance is changed too much... i needed to turn the trimmer a lot) or on the High cut?

Thx for answering!

RJ

 

[k brown]

How did you determine "it was time to change the tubes"?

 

[gyraf]

Agree with Mr. Brown: Usually the tubes last a very long time in there.

To the point: I would adjust at the hi-cut for unity gain in bypass, at input to correct for tubes..

 

[tfpmusic]

Hi Jakob and Mr. Brown,
One of the tubes was spiking in 5Khz region and when checking the 17 year old build I also noticed that my HT voltage was at 265 volts all the time. So back then I did not had a good look at the voltages.

Thx for your answer and will do that!

cheers! RJ