The official G9 help thread

[chrispbass]

Ok, a few questions, need to double check on a few things...

1. My AC input for my B+ supply is aprox 275 volts, is this too high?

2. If yes, as I'm using a 15V tx(recommended) powered from my 12V Tx, would just using the one 15V secondary(primary) help bring the voltage down, due to less inductance. At the moment, my two 15V secondaries(primaries) are paralled up, but I'm not sure what current the B+ supply requires. Are we talking a few mA?

3. If the three 39V zeners have blown, will the circuit still function?

Thanks for any help

Chris

 

[MeToo2]

inline

Ok, a few questions, need to double check on a few things...

1. My AC input for my B+ supply is aprox 275 volts, is this too high?

Is that peak?
The output should be around 240 -250 V

According to the datasheet, the TL783 can regulate up to a maximum voltage drop of 125V differential, and a minimum of approx 10V to get decent ripple rejection. One other important point, the TL783 needs to be supplying a minimum current of approx 10-15mA before it starts regulating effectively. If nothing is drawing current e.g. due to blown tubes or no load, the output voltage will appear to be too high as the TL783 won't kick in.

So 275 at the input should be well within spec (275 -245 = 30V drop across the TL783 which is just fine).

2. If yes, as I'm using a 15V tx(recommended) powered from my 12V Tx, would just using the one 15V secondary(primary) help bring the voltage down, due to less inductance. At the moment, my two 15V secondaries(primaries) are paralled up, but I'm not sure what current the B+ supply requires. Are we talking a few mA?

Yes, just a couple of milliamps to each plates/anodes of the first stage and around 5mA for the buffer stage if my datasheet skills are about right.
Bear in mind though that at 15V step up transformer this gets multiplied up 230/15 times of low voltage DC current.
But I see no immediate reason to switch out your transformers given the symptoms and values you have stated.

3. If the three 39V zeners have blown, will the circuit still function?

Thanks for any help

Chris

I would not rely on this. As said the TL783 has a max regulating voltage of 125V.

The 3*39V zeners switch on at 117V. This is no coincidence.

During normal operation they won't do anything (unless your input voltage peaks above 245V + 117V), but when the circuit first switches on C15 is fully discharged at 0V whilst your input rises quickly to 275V or even higher peak (C14 charging via 470R). So there's potentially a large proportion of the 275V across the TL783 at quite some charging current for C14 & C15 via the 2 470R resistors.

So the 3 * 39V zeners will trigger to provide this charging current, and thus protect the TL783 during startup.

[Just for completeness, D2 protects the TL783 during shutdown when there's potentially a large reverse voltage if C14 discharges before C15]

It could well be that your TL783 is blown, although they are pretty tough beasts.

 

[chrispbass]

MeeToo2 thank you very much for taking the time to give me great detailed answers.

I've managed to get some new zeners today, so will nail them in tomorrow, along with a new regulator. My old reg was showing a low resistance between legs, which explains why my R33 was taking a beating. Will check D2 as well.

Hopefully this wil now cure the problems. If not, I may try using a single secondary(primary) to get the ac volts down a bit.

cheers

Chris

 

[chrispbass]

Ok, tried again, not successful!! R33 still getting too hot, too much current draw through it.

In a previous post, I mentioned using a 15v tx reversed for my B supply, meant to say it is a 12v being fed from a 15v.

I tried un-paralleling the secondary(primary) to get the voltage down, didn't make any difference.

The only other tx i had was a 2 x 9v, which i put the secondary in series to get an 18v(primary), this only gave me about 180v though and wasn't enough to produce any DC voltage.

The only thing I can think of is that I should get another 15v tx to replace the 12v, which should give me an AC voltage hopefully within spec to start with.

Also, I powered up the main board with just the heater and phantom supply and thought I'd connect a voltmeter to the where the B supply connects. i got a small voltage of about 1.3V, but it was opposite polarity to the B+ connections. Is this of any significance?

Thanks for any help

Chris

 

[MeToo2]

Ok, tried again, not successful!! R33 still getting too hot, too much current draw through it.

Excessive current draw is generally *not* caused by PSU voltage. I doubt changing transformers will help. Excessive current draw is normally caused by an unexpected/short circuit path to earth or incorrect DC tube biassing. So you should be looking downstream to find out what is drawing the current.

To confirm whether the problem is in the PSU itself or on the main board, first discharge caps C14/C15 and try disconnecting the B+ from the main board completely and measuring the DC voltage drop across R33 quickly after powering up (connect up your multimeter in advance so you can do this quickly). If R33 is still getting too hot / has a large voltage drop across it without any connection to the main board, you have an issue in the PSU build itself, and likely some kind of short to ground there (because there should be zero load on the PSU in this state).

Once you confirm that the problem is not on your PSU build, remove the B+ PSU completely (after discharging caps C14/C15) and check for shorts between the B+ and ground on the main board with an ohmmeter. It should read really high resistance (megohms).

Next power up with B+ connected to the main board (again after discharging caps C14/C15), but no tubes inserted and measure R33 voltage drop. Again there's no path to ground for B+ so it should not get hot and there should be negligible current draw.

Once you have eliminated this, try turning on and off the unit quickly and measuring the DC voltages on R25/R125 and R29 /R129 with tubes inserted: that'll tell you what is drawing the current (try to connect up your multimeter in advance so you can do this quickly/safely and you don't have to stick your hands in a live box). As an alternative, you can also selectively insert individual tubes one at a time and check the voltage drop across R33 each time on power up (again don't forget to discharge your B+ between each step when you power down). When excessive current draw occurs, you'll know which stage(s) is/are the culprit, and can narrow the search.

Because this then sounds more to me like one of the SRPP buffer stages isn't biassing properly (R28/R128 => grids floating? or a duff tube) if the problem isn't within your PSU build itself or a plain short circuit somewhere.

 

[chrispbass]

Thank you again MeeToo2. I haven't tried your suggestions yet, but i was beginning to wonder if the problem was on the main board. The reason I was hoping it wasn't and isolated to the psu, was that initially, even with my incorectly wired B psu, the main board worked perfectly and scoped out fine, but with hum. The problem with R33 overheating has only been since I realised I hadn't got the 783 in circuit and corrected this! Not to say a fault hasn't developed since though and i'm positive the psu is wired correctly now!

Hmmm...confusing, but I'll keep trying. Thank you again

Chris

 

[chrispbass]

Success!!!

Inspired by your last post MeeToo2, I decided to open the lab again tonight and did some checking and found the culprit...the tab on the TL783 was shorting to the chassis!!! Why I hadn't done a continuity check there sooner, I don't know?!  :
Anyway, even though I'd got the proper isolation kit installed, for some reason the bush wasn't doing it's job, so after installing a new reg and temporarily not screwing it to the heatsink, I now have a nice healthy 250V B+ supply. Hum's gone now and a quick check with a condensor mic shows that all sounds really good!!  Looking forward to trying the DI out with my bass now.

Thanks again MeeToo2 and Jakob and will post some pics when I've got the front panel properly finished.

Chris

 

[phelar]

Need some advice about the higher gain settings. I've got some activity on the scope moving through the lover gain settings but when passing the fifth it's all dead( no more gain ), the same on both channels. Am I missing something? I have a tone generator on the instrument input and the scope on the output xlr.

 

[MeToo2]

Need some advice about the higher gain settings. I've got some activity on the scope moving through the lover gain settings but when passing the fifth it's all dead( no more gain ), the same on both channels. Am I missing something? I have a tone generator on the instrument input and the scope on the output xlr.

Have you already done the instrument DI mod?

http://www.gyraf.dk/gy_pd/g9/G9-EDIT.GIF

If you haven't, you might get HF oscillation, which will appear to kill all gain at high gain settings,

 

[phelar]

I've done this mod already.  :-\

 

[MeToo2]

I've done this mod already.  :-\

Then it's time to use an ohmeter to buzz out connections to your gain switch SW2 (without power applied)

You say you get 4 stops of good gain then nothing?

Would seem to suggest dry joint/ bad connection from C7 on the main board to R11 on the switch board [the one marked 'gain']?

 

[phelar]

Got it, r11 a 1M  :-[.  Thanks! Oh, and thanks for the earlier check list and tips
about the psu section. Switched toroids to a couple of blocks 40va and kept myself
from messing up the phasing on the 12v secondarys this time.....and used a variac

 

[stevan.siemens]

Hey there,
I just finished putting together the G9 and so far, the unit is non-functioning. I was wondering if anyone has a list of target voltages, particularly at the tube pins. I've been checking and double checking components, connections and traces and am so far at a loss to explain some things. The following is a list of voltages I've measured. I know a bunch of these are squirrelly and a list of target voltages would help out a lot. Thanks in advance.
Mic Preamp voltages:

Channel 1:
V1A:
Anode - 163
Grid - 44
Cathode - 73

V1B:
Anode - 90
Grid - 0
Cathode - 1

V2A:
Anode - 239
Grid - 89
Cathode - 95

V2B:
Anode - 89
Grid - 0
Cathode - 0

Channel 2:
V1A:
Anode - 57
Grid - 0
Cathode - 0

V1B:
Anode - 84
Grid - 0
Cathode - 1

V2A:
Anode - 238
Grid - 86
Cathode - 92

V2B:
Anode - 86
Grid - 0
Cathode - 1

 

[stevan.siemens]

Well, I've worked a few things out. The tube I was using for channel 2 V1 had a filament pin shorted to one section's cathode, resulting in 0V on the cathode and 0V on grid. With a new tube, voltages on channel 2 are more in line with channel 1. I'm going to go back and check again though because by my calculations, V1B in both channels should have around 3V on the cathode rather than the 1V that I've been reading. I did have some errors in the output circuit that I've taken care of so that now in both channels, I hear some hum, leading me to believe the output stage is working more or less properly. If anyone has any voltage readings they can point me to, I would be very much grateful. Thanks.

 

[Majestic12]

My little bunch of G9 Preamps....

 

[rmaier]

I'm guessing you like 'em?

 

[Studio Mollan]

Hi
I'm about to order parts for two stereo units and when ordering parts I came to think that maybee it's possible to use one power supply to power both of the stereo units? It would make a cleaner build and would also drop the price considerably. Maybe get 50va transformers instead of the 30va from the bom. Then comes the question if on psu could power even more units?
Thanks Jakob for the project and all of you others for sharing!
Mange tak!

 

[gemini86]

Could work, but do this: build the PSU into an external box, with basic R-C filtering, then feed the preamps through a multi-conductor cable, regulation in each preamp case to prevent cross-talk. I don't know how much power you can squeeze out of the back-to-back transformer arrangement without lots of heat or power loss.

 

[Studio Mollan]

Could work, but do this: build the PSU into an external box, with basic R-C filtering, then feed the preamps through a multi-conductor cable, regulation in each preamp case to prevent cross-talk. I don't know how much power you can squeeze out of the back-to-back transformer arrangement without lots of heat or power loss.

Ok thanks for your reply!
So to reduce cross talk there should be separate regulation on each PCB. Is there not already cross talk between the two channels on the main PCB? When you say basic R-C filtering do you mean an extra filter or to move the filtering, or some of it, from the PCB  to the external PSU box?
/
Emil

 

[MeToo2]

Ok thanks for your reply!
So to reduce cross talk there should be separate regulation on each PCB. Is there not already cross talk between the two channels on the main PCB? When you say basic R-C filtering do you mean an extra filter or to move the filtering, or some of it, from the PCB  to the external PSU box?
/
Emil

When I built an off-board PSU, I measured on-board inter-channel cross talk at ~-70dB down between 2 channels on one board (L input terminated with 200 Ohm no input, R channel 1KHz tone -33dB in +4dBU output = -66dBU 1Khz output on "quiet" L channel) So I wouldn't worry about that inter-channel cross talk if you're running one board L+R for a stereo pair of mics: you're going to get way more in-room acoustic bleed than that. Cross talk did increase at higher frequencies to ~-50dB, which points to PSU regulation. Really not sure how much money you're going to save with an out of box PSU (or an inter-connect between 2 boxes) by the time you buy some high quality high voltage connectors (like 7 pin C091 Tuchels) and multi-core cable. It's inevitable you will get some cross talk via the PSU if you have many stereo channels using one single PSU: the circuit is not balanced, especially the output buffer. Whether it'll be a significant issue for your application is hard to say.