The official G9 help thread

[MeToo2]

Hey all. I'm designing my front panel and ran into a snag. I have the 12 position Lorin rotary switches  for the gain swith. I know the G9 uses 11 steps so I'll have to adjust the switch.

Assuming the normal travel is 300 degrees, should the gain index now be 275 degrees? (12positions/300degee rotation= 25degrees per position). The only reason I'm not really sure is because on all the G9 pics I've seen it appears as though they spaced 11 positions out over 300 degrees.

Thanks

Adjusting the Lorlin switches is a piece of cake although a little bit fiddly the first time you do it. Just remove the nut and lock washer and you'll see a 'stop' washer with a tag on it.(assuming you ordered the version with "stop included".) Simply move this little stopper washer tag around to the location marked with the number of positions you want to be able to select. The standard switch version has THIRTY (30) degree indexing. Normal travel for 12 positions is 330 degrees (11 spaces between positions). That means for 11 positions you have 10 spaces * 30 degrees = 300 degrees. I think you were probably counting the positions rather than the spaces between positions.

 

[sonolink]

Please I need to be sure about this:
If I'd like to change the original 12V/50mA pilot light for a 4,4V/50mA would I be ok by putting an 88 Ohms resistor on the positive side of the light? (I'm guessing this using Ohm's Law V=IxR, of course but am unsure if it applies like that)
Thanks

 

[Graph]

I want to make some questions, because i am having some troubles with the 245v PSU of the G9.

I am using a 220:15+15 and 220:12+12 like i said before.

This time i didnt burned R34, but i have made some calculations, and i think there is something wrong with the Voltage.

The output of T2, is 275Vac, after D17-20 regultates 360v. At the Tl783 input i have 360v(), and after the regulation i have 350v aprox(this was measured in the capacitors C14-15). i have Read the TL783 datasheet, and we came with the following formula:

Vo=1.25(1+R2/R1)
Vo=1.25(1+100k/517)= 240v



In the datasheet warns to dont use differential in/out more than 125v.

my question is,
1) Why the TL783 is not regulating fine?
2) If it is really regulating, why if i have 360Vdc in the input i should have 240Vdc?

One another question. I have made some calculations, and its ok with that input voltage to generate 360Vdc, isnt this a really high voltage?

Sorry if this was asked before, i couldnt find an answer with the search function.

PS: Sorry for my bad english, i am not a native english person.

btw:  Look my nice project

 

[audiophreak]

Hi All,
      I have read in different threads including this one that channel #2 on the G9 seams to have a little more noise that channel one. I have three G9s now and absolutely love how they sound, but have noticed that at least on my units this is true, channel 2 does in fact have more noise than channel one.
     I am curious to know if this is due to the approximate position of the PTs, its seams that most people, me included, placed the main PCB in the front left corner of the case  and the PTs in rear right corner, OR is this noise do to proximity of the PSU ciruit on the main PCB in relation to channel 2 on the PCB ?  

   I ask because I was thinking of mounting the PTs on the outside back panel like I've seen on Pultec EQs and other vintage gear but don't want to go through all that trouble if the noise is from the PSU circuit on the PCB.

 any and all advice welcome,
                                      Thanks,
                                                 Chip

  BTW ... Graph ..  I'm in no way any kind of expert, but I think the circuit needs to be loaded with the tubes in place to take accurate measurements ,  anyone please correct me if I'm wrong.

 

[Graph]

  BTW ... Graph ..  I'm in no way any kind of expert, but I think the circuit needs to be loaded with the tubes in place to take accurate measurements ,  anyone please correct me if I'm wrong.

Are you sure? the voltage shouldnt drop to 240v if i connect the tubes.. the voltage is regulated anyways.. thats as far as i know, maybe the TL783 works in other way. I am testing the power supplys, if this isnt working, i will have then 350v running through the 250v caps, i dont think its ok.

I will try with a 220:15+15v using it as T2, i will drop the voltage to a lower value, maybe the TL783 if its not fried up, will regulate better.

Thanks! I hope somebody can give me a confirmation of this or a better explanation
I really appreciate the info here.

 

[audiophreak]

  Graph .. take a look at the bottom of page 28 of this thread, don't know if this is the problem your facing, but that's were I got the " Tubes in " reference. as well as I've done some work with building and modding tube microphones and with tube mics, voltages need to be checked with the mic plugged in so tube loads the PSU circuit to get accurate voltage reading.

  Again, if I am wrong or misleading, someone please  correct me.

 

[sonolink]

Please can someone help me with this?
If I'd like to change the original 12V/50mA pilot light for a 4,4V/50mA would I be ok by putting an 88 Ohms resistor on the positive side of the light?  :-\
Thanks

 

[Graph]

  Graph .. take a look at the bottom of page 28 of this thread, don't know if this is the problem your facing, but that's were I got the " Tubes in " reference. as well as I've done some work with building and modding tube microphones and with tube mics, voltages need to be checked with the mic plugged in so tube loads the PSU circuit to get accurate voltage reading.

   Again, if I am wrong or misleading, someone please  correct me.

I have read again the Specs of the  TL783

It needs 14mA of current to regulate.. so maybe... it isnt regulating either... that may be the reason and its probably working fine. With the Tubes Load in, i think it will consume at least 14mA.

But anyways... 360v isnt too much? obiusly the zener will drop itif it goes more than 10v up, but making this happen you are not using the zener as protection, u are using it as a necessary element in the design. With T2 at 220:15+15 it should have 308Vdc aprox, a differential input of 63v instead of 115v(anyways it should work), arent we throwing in so much voltage to the regulator?

Thanks for your answer i am saying this because i want my G9 to work propperly and dont have burned capacitors in a few days or anything, its my first time using a regulator in a Tube design. If this is really ok, i will continue the G9

Thanks again audiophreak, u werent wrong about that.

 

[MeToo2]

I have read again the Specs of the  TL783

It needs 14mA of current to regulate.. so maybe... it isnt regulating either... that may be the reason and its probably working fine. With the Tubes Load in, i think it will consume at least 14mA.

But anyways... 360v isnt too much? obiusly the zener will drop itif it goes more than 10v up, but making this happen you are not using the zener as protection, u are using it as a necessary element in the design. With T2 at 220:15+15 it should have 308Vdc aprox, a differential input of 63v instead of 115v(anyways it should work), arent we throwing in so much voltage to the regulator?

Thanks for your answer i am saying this because i want my G9 to work propperly and dont have burned capacitors in a few days or anything, its my first time using a regulator in a Tube design. If this is really ok, i will continue the G9

Thanks again audiophreak, u werent wrong about that.

The regulator really does seem to need a very significant load before it starts to regulate. Otherwise you will basically measure the smoothed output of the rectified step up transfomer (which is in itself high because the transformer has no load either)

I had the same concern as you when I first tested.

So before it starts properly regulating, you can measure anywhere North of 220V * 15 / 12 (transformer ratios) * 1.4 (full wave rectifying) = 388V under zero load. So 360V does not sound unusual in a zero load condition.

You can always first test your power supply with a bunch of 5 or 6 off 100K 1W resistors in parallel if you don't want to try live with your tubes the first time. That'll give a load of around 15 - 18 mA and it should start regulating properly then.

 

[MeToo2]

I am curious to know if this is due to the approximate position of the PTs, its seams that most people, me included, placed the main PCB in the front left corner of the case  and the PTs in rear right corner, OR is this noise do to proximity of the PSU ciruit on the main PCB in relation to channel 2 on the PCB ? 

I would really value hearing the results of your tests. I looked for quite a while at residual noise sources and eliminated quite a few of them in my build: they were nothing to do with design but certainly to do with specific build and layout. I gave up on further testing once I reached a certain level that I was happy with. I *suspect* (not proven!) that the physical layout that everyone uses combined with the proximity of the transformers & rectifier diodes + first line smoothing to both the switching board (high impedance controls) AND the second channel output transformer is the issue, but I have no proof. It doesn't seem to be the 48V voltage multiplier chain or phantom power regulation circuit which I initially suspected: completely disconnecting the 15V supply to that makes no difference at all to the noise floor. I also tried smoothing my mains before it even entered the box, and also a dummy "loopback" cable on my test gear and that made no difference at all either, so any effects are within the box. Another simple test to prove it wasn't the 12V heater supply (which was a suspect because it draws significant current @100Hz and its harmonics) was to unplug the 15V AC supply from the input to the rectifier of the 12V heater circuit and plug in a 15V DC 1.5A lab power supply instead. Required no soldering or difficult testing. But the noise floor didn't change significantly so that wasn't the direct source either. I personally doubt that much significant gets past the regulators (either the 7812 or TL783) to the rest of the amp that way because that would probably show up on both channels. You could eliminate HT noise getting in via the regulator as a cause by placing a 10H choke in place of R33. Haven't tried that yet personally but might do now as I have one lying around spare. Which would just then leave the main suspects as everything upstream of the 7812 & TL783 up to the mains socket (2*full wave rectifiers for heater & HT D13-D20, first stage capacitive smoothing C14 + C16-C18, transformers & wiring). You could always just move them around without actually bolting them on or drilling holes, as an initial test. It would be a very valuable test for a lot of people because I believe many have toyed with external power supplies (in a completely separate box), or shielding, or potted transformers, or enclosing the transformers & rectification in a separate shielding can in the same box, but I haven't seen any definitive results. Damn you've got me all interested again but I've no real time at the moment......

 

[sonolink]

Thanks for your help and time...

 

[Graph]

I have read again the Specs of the  TL783

It needs 14mA of current to regulate.. so maybe... it isnt regulating either... that may be the reason and its probably working fine. With the Tubes Load in, i think it will consume at least 14mA.

But anyways... 360v isnt too much? obiusly the zener will drop itif it goes more than 10v up, but making this happen you are not using the zener as protection, u are using it as a necessary element in the design. With T2 at 220:15+15 it should have 308Vdc aprox, a differential input of 63v instead of 115v(anyways it should work), arent we throwing in so much voltage to the regulator?

Thanks for your answer i am saying this because i want my G9 to work propperly and dont have burned capacitors in a few days or anything, its my first time using a regulator in a Tube design. If this is really ok, i will continue the G9

Thanks again audiophreak, u werent wrong about that.

The regulator really does seem to need a very significant load before it starts to regulate. Otherwise you will basically measure the smoothed output of the rectified step up transfomer (which is in itself high because the transformer has no load either)

I had the same concern as you when I first tested.

So before it starts properly regulating, you can measure anywhere North of 220V * 15 / 12 (transformer ratios) * 1.4 (full wave rectifying) = 388V under zero load. So 360V does not sound unusual in a zero load condition.

You can always first test your power supply with a bunch of 5 or 6 off 100K 1W resistors in parallel if you don't want to try live with your tubes the first time. That'll give a load of around 15 - 18 mA and it should start regulating properly then.

Yeah, its regulating fine now i hope this days i will have some time to finish it.

 

[audiophreak]

Glad to hear that Graph    keep us informed as to your progress. 

 

[JimboJohnson]

Hi all - new here, and may i offer my congrats on a great piece of gear. I'm on here as a mate bought a half finished G9, and had me rewire it properly. It was very ugly, with mistakes everywhere, but i think i've got it working pretty well. There's some oscillation, but I've seen some info previously to fix that....

The main thing that's stumped me, is the phantom out. We had 70 volts coming out....

On further investigation - 56 volts across the zener diode, 70 odd volts out of tip121, 70v out of D12.  I've managed to bodge out 48 volts out by replacing D12 with a link, and r41 with 1k5.  It works, but obviously, would be nice to know why 70v in the first place - seems like there's one too many diodes in the voltage quadrupler, but i'm just a hack....

Many thanks,
Jim

 

[gyraf]

would be nice to know why 70v in the first place

Most probably a blown P48 regulating transistor, T1. This happens sometimes when shorting the 48V-line.

Or maybe a broken connection from ZD56 over trimmer to base of T1.

Measure base voltage on T1, should be adjustable from ca 40-55V by the trimmer. If not, check circuit vs schematic. If base regulates right, but output stays up at +70, then transistor has a C-E short, replace.

Jakob E.

 

[PeteSanders]

Hello there!

Can I get some suggestions to fix my new G9 please?

Power - p48, 12v and 245v seem to be OK (I get 242v, have 110 mains or something).

It passes audio, but only when phase switch is in the - position.  The gain and output pot aren't kicking in.

If I press on the board around the input tube area, it suddenly sounds fuller and a littler louder.  Checking audio connections - might be my cheesy crimp-pin job on the xlr's.

I'm not sure what to look at.   I hard-wired the panel to the mainboard.  Using a p48 condenser mic for the testing.

 

[PeteSanders]

I'm looking over the r32 and r44 areas and schematic around the output transformer.

 

[PeteSanders]

OK,gain and ouput now work; bad etching job.  Phase prob still there, and sounds etter when I press on the main board - something kicks in.

 

[gyraf]

use ohmmeter to check signal path through phase switch. refer to schematic.

 

[PeteSanders]

Thanks!
Have been doing that; I got the 6-position 2-pole Lorlins - bends my brain.  Have that one set to two-pos.

I had miswired two legs of the relay together; now the bend-board issue has stopped. 

Sounds good, except it's low-volume, maybe half what it should be.  My TL783 gets hot; I have the thin u-shaped heatsink plus 3 large washers on it.  Getting 12.2 volts on the jumper wire.

Not noisy, and the DI works.  I don't think the hi-low filters are working.