The official G9 help thread

[innercityman]

I sent an email yesterday. My experience with don-audio e-mail support has been pretty bad in the past though. Maybe I’ll hear something in a few weeks.

Same for me. If by any chance you get an answer, please keep us informed.

 

[Clbraddock]

Response from don-audio:

“i can not recall this unfortunately but if the toroids have these values, we must change the description online. It is a customer project and the values on the toroids are for the G9 project. I know we had several prototypes and this is the final version that works great with the G9 but if you figure out it does not work for you, you can return it. I am not so deep in the G9 project. The toroid was winded by the given specs for this project and the last change to 12v 12v 275v should have been made to better match to the circuit.”

Doesn’t sound right if Jakob is saying this transformer will blow the caps. Should I swap the C14 and C15 for higher voltage or scrap the transformer and use two correctly specced together like in the schematic?

If I use this transformer do I need to change the bias resistors due to higher plate voltage?

Lastly, one other idea: on tube guitar amps with a center tap I have used a zener to ground from the center tap to reduce the voltage without causing excess sag. This has worked great when there is a center tap. This transformer does not have a center tap. Is there something similar I could do to reduce the voltage without causing sag?

 

[BGroen]

Hi all,
I read somewhere (but an't find it again) that twisting wires to a voltage regulator can cause osscilation of the regulator.
I want to attach the 78S12 to the chassis. Can I tidy up the wires by twisting them? I've seen some do it and other don't.
Hope hearing,
Beinte

 

[innercityman]

Hi all,
I read somewhere (but an't find it again) that twisting wires to a voltage regulator can cause osscilation of the regulator.
I want to attach the 78S12 to the chassis. Can I tidy up the wires by twisting them? I've seen some do it and other don't.
Hope hearing,
Beinte

I put my regulator to the chassis and twisted wires, no oscillation.

 

[Clbraddock]

The don-audio transformer is a no go. Pretty sure I fried my board. It reads 370 VCD at the test point that should be 245!! Never purchasing anything from don-audio ever again. Think Im going to bin this project and move on to something else. What a waste.

 

[gyraf]

Why not just go with the two-transformers-setup like it was originally intended?

Just replace your TL783 [and maybe (just maybe) your two reservoir electrolytics]

/Jakob E.

 

[Clbraddock]

The TL783 is definitely fried, but there are a bunch of 250 volt coupling caps in the path of the 370 volts also. Depending on if they failed short or open they are probably all toast too: C25, C3, C4, C5, C8, C9, C10. I'd just assume not dump a bunch of DC onto my V2 grids. You are way more knowledgeable than me though Jakob. Do you not think I need to replace those (or at least check if they failed)? For the HT PSU caps I used 450VCD, so those are probably fine.

It just pisses me off that don-audio is allowed to commit fraud. Might as well buy relabeled parts from AliExpress, probably a better (though minimal) chance of the parts being what they claim to be.

Edit: C3 and C4 still test okay with an LCR so maybe the downstream caps are fine. Ordered 2 MULTICOMP PRO MCTA030/15 transformers. Half the price of don-audio’s transformer for both and less likely to start a fire. https://www.newark.com/multicomp-pr...ormer/dp/38K4876?CMP=e-email-sys-orderack-GLB

 

[thelivingroom]

The TL783 is definitely fried, but there are a bunch of 250 volt coupling caps in the path of the 370 volts also. Depending on if they failed short or open they are probably all toast too: C25, C3, C4, C5, C8, C9, C10. I'd just assume not dump a bunch of DC onto my V2 grids. You are way more knowledgeable than me though Jakob. Do you not think I need to replace those (or at least check if they failed)? For the HT PSU caps I used 450VCD, so those are probably fine.

It just pisses me off that don-audio is allowed to commit fraud. Might as well buy relabeled parts from AliExpress, probably a better (though minimal) chance of the parts being what they claim to be.

Edit: C3 and C4 still test okay with an LCR so maybe the downstream caps are fine. Ordered 2 MULTICOMP PRO MCTA030/15 transformers. Half the price of don-audio’s transformer for both and less likely to start a fire. https://www.newark.com/multicomp-pr...ormer/dp/38K4876?CMP=e-email-sys-orderack-GLB

I used one 15v secondary and one 12v secondary power transformer. The updated (long ago) Gyraf BOM calls for that, and it was also discussed in this thread somewhere. I highly recommend reading everything, both on the Gyraf site and in here, and making notes of the special/important/unfamiliar stuff. Because I knew I’d be doing mine over a period of time, I made a little manual with all that so I wouldn’t forget the important stuff I came across. When I built mine I was very inexperienced, but I did my homework up front and it has worked perfectly from first fire up.

 

[Clbraddock]

I used one 15v secondary and one 12v secondary power transformer. The updated (long ago) Gyraf BOM calls for that, and it was also discussed in this thread somewhere. I highly recommend reading everything, both on the Gyraf site and in here, and making notes of the special/important/unfamiliar stuff. Because I knew I’d be doing mine over a period of time, I made a little manual with all that so I wouldn’t forget the important stuff I came across. When I built mine I was very inexperienced, but I did my homework up front and it has worked perfectly from first fire up.

View attachment 109043

Any chance you still have the manual you made?

Guess Ill order a 12+12 transformer to go with the 15+15 and just save the second 15+15 for possible future use. They werent super expensive, so no biggie. I should have looked more carefully at the BOM instead of just looking at the schematic. Thanks for catching that and letting me know!

I did read through the thread a couple times, but its been awhile. I did some of the common changes like cutting traces for the instrument inputs, but may have missed some others.

 

[Clbraddock]

So I got the new transformers wired up. Voltage readings are better, but I have a different problem.

The first transformer (connected to the iec) runs very hot and hums loudly when switched on. It acts possibly like a shorted turn. I quickly turned it off, but I am not sure why it is doing this. It is mounted to the side wall and the bolt definitely is not touching the case on both sides - it only touches the base where it is mounted. I’ve checked the wiring a couple times and everything looks correct. Only difference from the schematic is that I have the primary of the first transformer in parallel instead of series since my voltage from the wall is 120.

If I disconnect the HT the transformer stops humming and does not get hot. I would think with a true shorted turn it would be melting down even with no load, but I don’t really know. I’ve never experienced a shorted turn before so I’m just guessing.

What could be causing this?

 

[gyraf]

too much load from the HT?

 

[Clbraddock]

Is the 30VA in the spec insufficient? Seems like that shouldn't be able to deliver enough current for the circuit. 30VA/15V = 0.5 amps. 12AU7 heaters in series are going to draw 0.15 amps each, so for 4 of them thats already 0.6 amps. So, thinking about it more, I don't think its a shorted turn, I think the transformer just can't handle the current draw for the full circuit and is burning up.

Edit: What is the actual voltage that should be coming from the second transformer? Using the parts in the schematic (using ideal values) would not match the voltage stated in the schematic.

15 volts into a 12:110 transformer should be 137.50 volts. Both sides in series would by 275. Thats the value of the don-audio transformer that everyone said was too high. The schematic says the voltage before the HT rectifier should only be 220.

15 volts into 15:110 transformer should be 110 volts. Both sides in series would be 220. Thats the desired value stated in the schematic.

So why the change from 15:110 to 12:110? What is the desired voltage coming of the second transformer going to the HT rectifier????

Edit 2: From Jakob in an old post:
"You can probably use the two 15V transformers - the step to the 12V-version was just because a few people had too low a HT supply voltage.

So if you build it and it works (mine did), don't worry about it.

Jakob E."

Im getting too high HT so will try going back to the original 15vac transformers. Going to also get a transformer with a higher VA rating for the first transformer.

 

[gyraf]

30va/15v is 2A

 

[Clbraddock]

30va/15v is 2A

Oops, some idiot math on my part.

 

[innercityman]

Is the 30VA in the spec insufficient? Seems like that shouldn't be able to deliver enough current for the circuit. 30VA/15V = 0.5 amps. 12AU7 heaters in series are going to draw 0.15 amps each, so for 4 of them thats already 0.6 amps. So, thinking about it more, I don't think its a shorted turn, I think the transformer just can't handle the current draw for the full circuit and is burning up.

Edit: What is the actual voltage that should be coming from the second transformer? Using the parts in the schematic (using ideal values) would not match the voltage stated in the schematic.

15 volts into a 12:110 transformer should be 137.50 volts. Both sides in series would by 275. Thats the value of the don-audio transformer that everyone said was too high. The schematic says the voltage before the HT rectifier should only be 220.

15 volts into 15:110 transformer should be 110 volts. Both sides in series would be 220. Thats the desired value stated in the schematic.

So why the change from 15:110 to 12:110? What is the desired voltage coming of the second transformer going to the HT rectifier????

Edit 2: From Jakob in an old post:
"You can probably use the two 15V transformers - the step to the 12V-version was just because a few people had too low a HT supply voltage.

So if you build it and it works (mine did), don't worry about it.

Jakob E."

Im getting too high HT so will try going back to the original 15vac transformers. Going to also get a transformer with a higher VA rating for the first transformer.

I've replaced shitty transformer from don-audio by two pieces of this one... vigortronix vtx. This is the one in the BOM.
My G9 works great.

 

[Nyquist]

So I got the new transformers wired up. Voltage readings are better, but I have a different problem.

The first transformer (connected to the iec) runs very hot and hums loudly when switched on. It acts possibly like a shorted turn. I quickly turned it off, but I am not sure why it is doing this. It is mounted to the side wall and the bolt definitely is not touching the case on both sides - it only touches the base where it is mounted. I’ve checked the wiring a couple times and everything looks correct. Only difference from the schematic is that I have the primary of the first transformer in parallel instead of series since my voltage from the wall is 120.

If I disconnect the HT the transformer stops humming and does not get hot. I would think with a true shorted turn it would be melting down even with no load, but I don’t really know. I’ve never experienced a shorted turn before so I’m just guessing.

What could be causing this?

You have a short in the circuit somewhere. Check all PSU components for shorts and or shorted tracks

 

[Clbraddock]

I think Nyquist is correct. Something is shorting in the heater rectifier. I double checked all the diodes and the are the correct types in the correct orientation. I’ll have to take the board out to check the solder joints and traces.

I think I’m going to take a break from this for awhile and come back to it with fresh eyes in the future.

 

[studiosus]

Hi Guys,
Just wanted to confirm that in my build the donaudio g9 transformer works without any issues.

But something else: I have quite a bit of crosstalk in my two channels. Do you guys also experience this? I suspect I got mostlikely some microphonic tubes? There was a bass di with quite some gain and a rather low gain trumpet mic. There was a fair amount of trumpet on the bass track.

 

[dodgy valve]

New to this site so sorry if this is not the right place to post this. Had the problem of low gain but after many hours of reading finally solved the problem, a ground issue. Just also thought someone maybe interested in a single channel G9

 

[DKaye]

Yes, that was a scare we had some 17 years ago - turned out to be the design not entirely predictable enough for medical electronics, but was "re-issued" as a not-for-critical category device.

I have not heard about trouble since then?

I just got an email from Mouser saying the TL783 is end of life.