G-PULTEC problems, please help!

[frazzman]

Fabio (Audioman) and frazzman,

We've been through this a couple of times before:

Gain through the GPultec depends on two things:

1 - The actual raw gain of the tube used - this varies some +/- 2½dB on NOS tubes, probably more on new-production ones. You could use Russian 6N23P for slightly more gain in average (if you can find those).

2 - The ratio of the values of hi-boost potentiometer (10KA) and the hi-cut potentiometer (1KA) - this is where we do the actual -20dB no-eq-attenuation (Schematic - http://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif )

For adjusting to unity gain, you could probably put a 1K Ohm trimmer potentiometer at the TOP (clockwise end) of the hi-cut pot and adjust for unity at no-hicut-setting. Only side-effect of this will be slightly less available maximum-hi-cut at pot extreme setting.

If you do this, let us know how/if it works - would be nice to have in this thread

Please also note that this circuit was made for LL5402's in and out - other brands of transformers or no-input-transformer may very well give varying results.

Jakob E.

ps: I've had quite a few JJ tubes fail me after short time. Maybe it's poor production standards, maybe they are specified slightly different than real E88CC's, and don't like us going to extremes of cathode/heater potential differences. I use Philips 6922, and like them a lot.

DO NOT run this circuit above +250V HT - the tubes don't like this. Raise series resistor value until you have some 200-250V HT under load (tube heated, ca. 30sec.). A standard 0.6W type resistor will nearly always do fine - you do the math of power dissipated at given resistor and given voltage drop....

Thanks Gyraf for your reply!

I dropped my 3k3 resistor to 22k and now have a healthy HT voltage within the acceptable range.

I still have a about a 6-7db volume drop when the unit is inserted (in bypass and with circuit in).

I always used the lundhals in the past but thought Id give an edcor xs1100 a shot as don-audio had an adaptor board that made it easy. As you said, you didn't design it for this transformer so it probably has something to do with that.  Never had an issue when I built with the lundhals in the past.

Anyone had any experience with the edcors on these builds?

I was wondering if the 10k and 2k2 resistors strapped across the input and output connectors should be omitted when not using lundhals?

Cheers

 

[Audioman]

@ Gyraf  sorry for the delay in responding you , it's been a busy period .
The gain variation with the 1k trimmer is 48 dbu
let me know
Thanks

how much gain variation do you get with a 1K trimmer?

 

[Rocinante]

Fabio (Audioman) and frazzman,

Thanks Gyraf for your reply!

I dropped my 3k3 resistor to 22k and now have a healthy HT voltage within the acceptable range.

I still have a about a 6-7db volume drop when the unit is inserted (in bypass and with circuit in).

I always used the lundhals in the past but thought Id give an edcor xs1100 a shot as don-audio had an adaptor board that made it easy. As you said, you didn't design it for this transformer so it probably has something to do with that.  Never had an issue when I built with the lundhals in the past.

Anyone had any experience with the edcors on these builds?

I was wondering if the 10k and 2k2 resistors strapped across the input and output connectors should be omitted when not using lundhals?

Cheers

Hey having some difficulties with the Edcors as well did you figure out the problem?
Thanks

 

[frazzman]

Fabio (Audioman) and frazzman,

Thanks Gyraf for your reply!

I dropped my 3k3 resistor to 22k and now have a healthy HT voltage within the acceptable range.

I still have a about a 6-7db volume drop when the unit is inserted (in bypass and with circuit in).

I always used the lundhals in the past but thought Id give an edcor xs1100 a shot as don-audio had an adaptor board that made it easy. As you said, you didn't design it for this transformer so it probably has something to do with that.  Never had an issue when I built with the lundhals in the past.

Anyone had any experience with the edcors on these builds?

I was wondering if the 10k and 2k2 resistors strapped across the input and output connectors should be omitted when not using lundhals?

Cheers

Hey having some difficulties with the Edcors as well did you figure out the problem?
Thanks

Hi there,

Nope, just ordered lundhals instead. I've built about 5 units in the past with lundhals and had no issue at all. That was the knot variable that changed in this instance.

Cheers

 

[Rocinante]

Bummer. Yeah in the other G-pultec I built I used Lundahls as well. Thanks for replying. I'll keep searching till i give in I guess.

 

[frazzman]

Bummer. Yeah in the other G-pultec I built I used Lundahls as well. Thanks for replying. I'll keep searching till i give in I guess.

Do you have a loss of volume, like I do, when the circuit is engaged?

I have this, even in bypass. Apart from that, everything works and sounds nice as it should.

I'm wondering if the 10k and 2k2 resistors strapped across the input/output should be omitted when not using Lundhals?

I haven't tried that yet, but maybe an idea.

 

[Rocinante]

I can follow the circuit with a tracer and everything is perfect up until the edcor.  There the volume drops tremendously.  Ive seen people with the edcors but I believe its been with Purusha's adapters.  I'm actually looking for his to compare the pin outs to the Don audio one's.  Cant seem to find any pics yet though.  Via a suggestion I read on here I'm gonna desolder both the transformers and the resistors and then put everything back one at a time while running a sine wave.

 

[frazzman]

I can follow the circuit with a tracer and everything is perfect up until the edcor.  There the volume drops tremendously.  Ive seen people with the edcors but I believe its been with Purusha's adapters.  I'm actually looking for his to compare the pin outs to the Don audio one's.  Cant seem to find any pics yet though.  Via a suggestion I read on here I'm gonna desolder both the transformers and the resistors and then put everything back one at a time while running a sine wave.

An update for you....

I removed the 2k2 resistor from across the output tx.... seems to have fixed the problem.

I now only have around a 3dB difference when the circuit is in.

It sounds pretty good. I'm not sure if removing that resistor is 'normal' practice when using the EDCOR XS1100 as a substitute. I'm just doing some comparisons now with another unit that has Lhundal's. So far all the boost and cuts work as expected, i'm just interested to see if the frequency response is hindered by the omission.

So far so good.

Try it and tell me what you think.

 

[Rocinante]

I can follow the circuit with a tracer and everything is perfect up until the edcor.  There the volume drops tremendously.  Ive seen people with the edcors but I believe its been with Purusha's adapters.  I'm actually looking for his to compare the pin outs to the Don audio one's.  Cant seem to find any pics yet though.  Via a suggestion I read on here I'm gonna desolder both the transformers and the resistors and then put everything back one at a time while running a sine wave.

An update for you....

I removed the 2k2 resistor from across the output tx.... seems to have fixed the problem.

I now only have around a 3dB difference when the circuit is in.

It sounds pretty good. I'm not sure if removing that resistor is 'normal' practice when using the EDCOR XS1100 as a substitute. I'm just doing some comparisons now with another unit that has Lhundal's. So far all the boost and cuts work as expected, i'm just interested to see if the frequency response is hindered by the omission.

So far so good.

Try it and tell me what you think.

This is good news. Thanks I will

 

[weiss]

i want to build a stereo unit but both channels should be affected exactly the same. how could one use a single set of controls (frequency, attenuation, boost etc.) for the two channels?
is there anything important? dual gang pots should work, right? what about the switches?

thanks for your answer

weiss

 

[Gustav]

i want to build a stereo unit but both channels should be affected exactly the same. how could one use a single set of controls (frequency, attenuation, boost etc.) for the two channels?
is there anything important? dual gang pots should work, right? what about the switches?

thanks for your answer

weiss

Same thing. Should work with double double stacked switches. (4 x6?)

Or, you could do two separate channels on the board I made.

Gustav

 

[weiss]

hey gustav, thanks. what do you mean by "do two separate channels on the board"? I will try that, but it must work, right?
It is just a matter of saving space on the front panel, so i don't need duplicated controls for each channel

 

[gyraf]

Notice that dual-gang potentiometers rarely track better than 10%, so don't expect good stereo tracking unless you are heavily selecting all components.

This is the primary reason why so few companies make stereo eq's.

Jakob E.

 

[weiss]

Notice that dual-gang potentiometers rarely track better than 10%, so don't expect good stereo tracking unless you are heavily selecting all components.

This is the primary reason why so few companies make stereo eq's.

Jakob E.

Ah okay, now i get the problem. Maybe you can tell me how these guys achieved to do it in stereo?

 

[Harpo]

... unless you are heavily selecting all components.

Ah okay, now i get the problem. Maybe you can tell me how these guys achieved to do it in stereo?

Maybe ask Steffen ?...

 

[fatheaddrummer]

Hi everybody,

I just finished my G Pultec with the PCB from Gustav, Lundahl Transformers, ECC88....
Everything works fine, but i have a very little hum. A little less when in bypass mode, a little more, when it's turned on. Compared to a normal signal it is really really quiet. I only can hear the hum when i turn on my speakers to nearly maximum - so it's actually not a big deal, but I think it is not neccesary?!

I connected the main PCB to ground.

Is there anything else i should connect to ground? What are the stars on the Filter PCBs for?

In addition i realised that the wire of my power LED is causing a lot more hum. It nearly dissapears when i movr the wire outside the case....should i use a shielded wire and connect one end of the shield to ground?

Is it possible that i mounted the main PCB too close to the Power Trafo?

Any ideas?

Thank you very much!!

Best,
Christian

 

[Gustav]

don't expect good stereo tracking unless you are heavily selecting all components.

Ah okay, now i get the problem. Maybe you can tell me how these guys achieved to do it in stereo?

Didnt he just do that!?

Gustav

 

[frazzman]

Fabio (Audioman) and frazzman,

We've been through this a couple of times before:

Gain through the GPultec depends on two things:

1 - The actual raw gain of the tube used - this varies some +/- 2½dB on NOS tubes, probably more on new-production ones. You could use Russian 6N23P for slightly more gain in average (if you can find those).

2 - The ratio of the values of hi-boost potentiometer (10KA) and the hi-cut potentiometer (1KA) - this is where we do the actual -20dB no-eq-attenuation (Schematic - http://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif )

For adjusting to unity gain, you could probably put a 1K Ohm trimmer potentiometer at the TOP (clockwise end) of the hi-cut pot and adjust for unity at no-hicut-setting. Only side-effect of this will be slightly less available maximum-hi-cut at pot extreme setting.

If you do this, let us know how/if it works - would be nice to have in this thread

Please also note that this circuit was made for LL5402's in and out - other brands of transformers or no-input-transformer may very well give varying results.

Jakob E.

ps: I've had quite a few JJ tubes fail me after short time. Maybe it's poor production standards, maybe they are specified slightly different than real E88CC's, and don't like us going to extremes of cathode/heater potential differences. I use Philips 6922, and like them a lot.

DO NOT run this circuit above +250V HT - the tubes don't like this. Raise series resistor value until you have some 200-250V HT under load (tube heated, ca. 30sec.). A standard 0.6W type resistor will nearly always do fine - you do the math of power dissipated at given resistor and given voltage drop....

Hi Jakob,

Just letting you know the 1k trimmer at the Hi cut CW position does indeed work and allow for trimming to unity gain. I now have no level difference with pultex engaged.

Good solution!

Thank you

 

[Patch]

Notice that dual-gang potentiometers rarely track better than 10%, so don't expect good stereo tracking unless you are heavily selecting all components.

This is the primary reason why so few companies make stereo eq's.

Jakob E.

When you say 'heavily selecting components' is there anything else to take into account other than using identical components bought in the same batch used across both boards, matched inductors, matched valves?

I plan on attempting a 1U version stereo G Pultec and am looking for ganged pots (haven't managed to source all yet) and double deck rotary switches (oof expensive!) and would obviously like a near perfect stereo tracking.

"rarely track better than 10%"............. oooh nooo that doesn't sounds great. Really that bad? I really can't fit 2x every pot/switch on a 1U stereo....  Would the stepped switches with resistors instead of pots be more precise? I'd rather have pots than steps but if needs must....

Any tips on on getting good tracking for dual gang would be mucho appreciated.

 

[Patch]

Also forgot to ask, I can only find a  2k ohm dual gang pot rather than a 2k2....is 2k ohm ok to use, will it affect the High Q in a bad way?

http://www.digikey.co.uk/product-search/en?pv1092=2&pv1=380&pv1849=3&FV=fff40004%2Cfff80540&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25