The official G9 help thread

[wolan42]

The last picture.

 

[baol]

what measures to do before tubes mounting?
thank you
Andrea

 

[wolan42]

I measure the HT voltages.
Something between 280 to 290 V without tubes.
Calibrate the 48 V and check the 12 V rail.

 

[baol]

Hi Wolan42, where do you misure them?
thank you

 

[wolan42]

Were they are marked.
Between the both resistors indicate with 245 V.
The wire were 12 V indicated.
But this all explained in the threat.
If you want to build a G9, read it. There are almost all answers and hints.

 

[baol]

you're true wolan42, but with my english knowledge it takes me weeks to read the 81 pages, I read the more I can but it happens that I miss informations

 

[knight84]

Hi again, I did some measurement, but still have 50dB max gain. I know it is enough for most applications but I want to get use of more of the transparent side of G9 with a few lower input settings. Here is some of my measurements.  HT is 233VDC, heater is 11.8V and no problem with 48V also.  I measured the input transformer stage (Lundahl). When I give 1.2V AC input signal I got 7.2V at the end of the input transformer(greater than 1:5 but should be better for gain) I got 2:1 decrease at the output transformer as well as described. I measured the gain switch resistors and they are right. I checked the resistor values on the gain stage as well. I used telefunken ECC82 tubes. I used wima,ERO, DME, Vishay poly. caps. with at most %10 tolerance. Any idea can show me a way to look at the problem maybe.
Barbaros

 

[bruce0]

Suggest being careful with test signals to mic input trafo.  My understanding is it is best to keep them "mic level" (-20dB or so) unless you switch in the pad.

No damage from 1.2VRMS but remember what levels it is designed for.

The gain design is clever, see Gyraf notes. It is via attenuation for the first 4 steps and then via feedback for the rest of the steps.  Does the preamp behave normally (keep increasing) at the step 4 to 5 boundary?

The other part of the output level is the way the output pot is wired (Am I correct in assuming that pot is connected right to the board, and using a 40k-50K ish pot?)

I have never measured the output of mine.  Can anyone confirm that 55-57dB is typical?

Gyraf schematic calls for 1:6.45 trafo, you are at 1:6 but that only explains 1dB decrease.

 

[knight84]

Maxgain with Lundahl's is some 55-57dB. 60-62 with OEP input.

Thank you very much for your help bruce0. That was what I saw. I have 23.4 dB at 4th position and 27 dB at 5th position. The 1st position starts at 11dB and with 3.5 and 4 dB steps goes up to 50dB. By the way, my local main voltages are changing from day to night (230V to 200V). But I always get the same 50 dB at max gain setting independent from that. Maybe I am thinking about my tubes.
Another maybe different thing done; my friend connected the middle legs of all tube sockets to ground.
He said that helps a little on the noise level.

 

[knight84]

By the way, the phase problem is going on. When I am measuring with signal generator from DAW at each freq, the phase switch works fine. But when I plug the mic or instrument there is a change in especially transient response. It has an effect like a tone button or an impedance button. When it is flipped it losses some top end transient information. Because of that I get sometimes less peak gain when flipped.
Also wonder this:When I send sin. wave max gain is 50dB, when I send white noise max gain is 58 dB. Which one is right?

 

[bruce0]

Ahhh... that is a clue.

Maybe the frequency response problem and the phase problem are related.

When you send sine are you sending 1K Hz?  Try 10kHz and 100Hz.... I bet you get better gain at one of those.

As far as "transient" response, not sure what you are measuring there.  Lets just look at frequency for now. 

You said it sounded thinner, losing low end.
White noise has a lot of top end (actually more energy up there than at the low end.  Pink noise corrects this).

So if you get more gain with white noise, and gain that is appropriate, then I would say you have a high pass filter in there somewhere.

Additionally you seem to have a high pass filter that varies based upon phase.  This is really hard to do with Jacobs board because the phase is relay switched.

I am wondering if you have the front panel to main board wiring screwed up, and when you switch the phase you are also somehow activating the high pass, or visa versa, or something wierd.  I did this once (crossed a couple of wires) and the result was strange behavior.  The connectors go one for one down the pins same same front panel and main board.  check each one.


Another possibility is that you put some rf filter caps on the XLR's?  Send us a pic... .  Wrong caps there would make a low pass filter not a high pass filter, and shouldn't vary by phase..

Does the gain vary by phase?

This behavior is symetrical channel to channel right?

 

[knight84]

I did already tests with 10k and 100Hz but didn't change any gain value with phase switch. But I recorded a DI guitar with two settings. I stopped in the middle to flip the phase. Then it gets more transient. I mean with transient, the first attack of the sound which is a very irregular signal. Here, the attack of finger touch is my transients. When I flip the impedance button, I have very similar effect on another preamp. Here the difference is very subtle but it is there. I am curious about problem solving and learning so I am getting on details like that.

https://soundcloud.com/barbaros-bensoy/phase-switch-position-test-of-g9-gyraf-pre

I already checked If I have a signal like a high pass filter but it is not the same sound. Here are some photos from the preamp;

 

[bruce0]

Did you etch your own boards?  Have you checked trace continuity on the back side?

Also, did you check the connection between front panel and main board.

I have to tell you sounds like a high pass to me.  Just listening in mono.  First half of the track has more low end?
Is that what you hear (I am doing this on a PC headphone jack).

 

[knight84]

front panel pcb

 

[knight84]

Yes on the second one I hear a little bit of low end loss and also more high end increase (just the first bright part of finger touches) . So thats why I thought this is similar to an impedance button like a tone button on Neve ams pre.
The strange thing is when I give white noise or sign wave it doesnt change with phase button but with instrument or microphone plugged, the signal changes. 
Yes I etched my own board. I checked for shorts on suspicious places. I have seen one undesired short and solved a problem, not 100% sure for all of the ways but I checked as much as I can. Checked for ground in various parts of pcb also. Checked for shorts at filter stage and switch positions there. The filter drops 5-6dB at second setting from my sin. wave.Upgraded the filter caps with higher quality ones.

By the way we grounded pcb from c14-15 to mains. Is this right to do? We used EMI fiter power inlet. We grounded all input and output xlrs to main. I am writing anything comes to my mind, just in case we made something wrong. Also does the input-output of log pot changes due to brand? I did as mostly I see from pictures as 3rd pin input and 2nd pin output and it worked. I used not 470nF but 220nF as interstage caps as in old version.

 

[bruce0]

Do you mean C5 is 220nf?

Phase doesn;t change the di impedance (unless something is miswired).  DI impedance is fixed.

 

[knight84]

Yes, C5 is 220. I will check the connections tomorrow. Thank you

 

[bruce0]

I am not sure about this but I Think C5 behaves as if it is in series with C4 and driving r26 in series with P1|| (parallel) R27.

I think it acts like a 191nf cap into 91K ohms.

The corner frequency would be around 9 hz which means it will have an effect (subtle) effect in the audio range in the audible range. 

It am wondering how it changes the load on V1B. May change (lighten) the current load and thus provide more negative feedback to V1A (via R10) and that could reduce gain maybe?  I am a bit out of my depth here.

This could explain gain issues
It could explain a subtle audible low cut (but the cut would be small and down around 20hz and I know I couldnt' hear it
And any low cut would not be phase dependent

Here is what I think.

Change the cap to .47 (just parallel something bigger temporarily... even another .22)
See if gain gets better

As far as phase switch is concerned test continuity on both pin 2 and pin 3 back to the transformer, and test the resistance between pin one and pin 2 in both phase positions.  Maybe you have an OPEN (opens are common on self etched boards) on the PCB or maybe on one of those two load resistors that only effects one phase position.  Nothing should be open and it should be the same resistance with both phases (check that first).  This is unlikely because you say it happens on both channels right?

 

[knight84]

I took the board off to replace the 220 with 470 caps. I have a question: when it is off is it normal to have an open between 12V and ground of the pcb. I tracked the traces but couldnt see anything. I was measuring 11.8 V DC at the heater leg of tube and I have the light on when turn the unit on. But maybe there is something wrong. I touched the 78S12 and it is just a little warm, 783 is hoter than it.

 

[bruce0]

The only thing connecting 12V to ground are:
  The heaters should always be connected.
  The front panel switches the relay coils on for mic and 48 and for phase
  The schematic shows a 12V light, is that installed, it would show some continuity.
  The 78012 regulator I think would show open (not sure on that)


If you removed the tubes and had the relays switched off (phase off and mic/line set to line) and don't have a front panel light then I think you could see an open to ground on 12V.  Might look not open as the caps charged up momentarily.

Just from looking at the schematic which is nicely drawn and an easy schematic to read .


http://www.gyraf.dk/gy_pd/g9/g9_sch.gif


A suggested test for the phase thing....

When you put it back together, turn off phantom power in case you touch the input, turn the amp on on and measure the resistance between pin 2 and 3 of the output xlr1.  Then switch the phase and do the same measurements.  Those should be pretty darn identical measurements. All you are doing there is measuring continuity from the output to the transformer, including the load resistors and the transformers secondary. 

If they are not the same, that is not good.

Make the same measurement on output XLR2.

Why?  I was thinking that you might have one side open in one phase.  This can test normal depending upon the cabling and input, but might switch out the load resistors in one position.  I have not looked at the PCB to see if this could happen, but it is a simple test.