G
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You know the maximum ratin g of the 6X4 rectifier the old circuit had but that dosen't mean the transformer could deliver that. You still need to know the transformer specs. It could be more, or less than 70mA on primary.
G9 problems continued...
- Thread starter Bjorn Zetterlund
- Start date
Help Support GroupDIY Audio Forum:
Rob Flinn
Well-known member
Which were the cables that have to be screened to eleiminate the high gain oscillation problem ?
I have screened input & output cables, but have oscillation on only the highest gain setting on the right hand channel which is nearest to the PSU.
I`m not quite sure whether this is a screening problem or not. I originally had a gain switch that I got quite cheaply made by a company called alpha from rapid electronics. I was getting the oscillation on pos 11 & really low gain on pos 12. After checking things out I discovered that this switch is in fact an 11 postions switch, although it was listed as 12 positions (be warned). Now I have replaced it with a 12 position switch the oscillation only happens on position 12 ??
I have screened input & output cables, but have oscillation on only the highest gain setting on the right hand channel which is nearest to the PSU.
I`m not quite sure whether this is a screening problem or not. I originally had a gain switch that I got quite cheaply made by a company called alpha from rapid electronics. I was getting the oscillation on pos 11 & really low gain on pos 12. After checking things out I discovered that this switch is in fact an 11 postions switch, although it was listed as 12 positions (be warned). Now I have replaced it with a 12 position switch the oscillation only happens on position 12 ??
12 positions - is this right, Rob?
http://www.gyraf.dk/gy_pd/g9/g9_sch.gif
I just looked at the Gyraf page and it states only 11 positions for this: 1-4 = attenuation & lowest gain, 5-11 = gain, no attenuation.
:?:
I've built two G9s now. The second one (the 'BBC' variant) has some noise on channel two at high gain, but I'm beginning to suspect pickup from one of the film caps.
Stewart
http://www.gyraf.dk/gy_pd/g9/g9_sch.gif
I just looked at the Gyraf page and it states only 11 positions for this: 1-4 = attenuation & lowest gain, 5-11 = gain, no attenuation.
:?:
I've built two G9s now. The second one (the 'BBC' variant) has some noise on channel two at high gain, but I'm beginning to suspect pickup from one of the film caps.
Stewart
Rob Flinn
Well-known member
Ah Ha !!!
But, thinking about it does this explain what happened with the 1st switch I was using ?
Yes, the gain switch is only 11 positions. The schematic is probably a bit flawed around that area.
The PCB's are know working, and can be used for reference if you're in doubt.
Jakob E.
The PCB's are know working, and can be used for reference if you're in doubt.
Jakob E.
Rob Flinn
Well-known member
Right I`ll notch the switch back 1 place
Rob Flinn
Well-known member
No, this has not solved the problem. It appears that all these switches are really 11 position & not 12. If I notch the switch back a peg then I don`t have oscillation on the highest setting, but I only have 10 postions & less gain than the other channel is giving.
That's the classic oscillation thingy alright. You'll need to experiment a bit with the interconnection wires to get rid of it - please report back on your findings..
Jakob E.
Jakob E.
Rob Flinn
Well-known member
Jakob
Do you know whether thsis is usually just the wires to & from the XLR`s or whether it can also be caused be the wries that interconnect the control pcb`s ?
Do you know whether thsis is usually just the wires to & from the XLR`s or whether it can also be caused be the wries that interconnect the control pcb`s ?
Some observations:
I managed to improve this on one of mine by shielding around the HPF capacitors with some copper mesh, and even bending one of the caps! The other unit seems OK. Having read some posts here on the asymmetry of film caps, I'm going to try swapping the cap around to see if that helps things.
Stewart
I managed to improve this on one of mine by shielding around the HPF capacitors with some copper mesh, and even bending one of the caps! The other unit seems OK. Having read some posts here on the asymmetry of film caps, I'm going to try swapping the cap around to see if that helps things.
Stewart
Zebra - that sounds very sensible indeed!
Rob - it's probably not the XLR wires, but the PCB interconnect wires.
Rob - it's probably not the XLR wires, but the PCB interconnect wires.
Rob Flinn
Well-known member
I tried to keep the pcb interconnect wires as short as possible. Maybe putting a steel screen round the mains might do it .
Rob Flinn
Well-known member
I have sorted out this problem after a bit of playing around.
I tried a sheild for the PSU, which made no difference. I then agitated the pcb interconnect wires. The row with 10 odd wires made no difference.
However, prodding the row of 5 wires altered the image on my scope, so I decided to concentrate on this area. I disconnected these 5 wires & twisted them, then reconnected, & the problem was solved.
I tried a sheild for the PSU, which made no difference. I then agitated the pcb interconnect wires. The row with 10 odd wires made no difference.
However, prodding the row of 5 wires altered the image on my scope, so I decided to concentrate on this area. I disconnected these 5 wires & twisted them, then reconnected, & the problem was solved.
Val_r
Well-known member
[quote author="Bjorn Zetterlund"]Right...some further experiments!
V1A
Plate: 160,158,157
Grid: 47,48,50
Cathode: 77,80,81
Node: 74,76,78
HT: 234,234,234
V1B
Plate: 83,78,75
Grid: 0,0,0
Cathode: 2.2, 2.3, 2.3
Having established that the results seemed to have a reasonable spread between different valves, I thought I'd try lowering the HT until the V1A cathode voltage dropped to 60, which would be acceptable for C7. I found that I had to drop the HT to 180 for this. According to the G9 schem, that would mean (245-180=) 65 volts dropped by R31, which is 1k...hence it must pass 65mA. But I just read in one of Jakobs posts to another G9 question that each G9 channel needs about 8mA, so something must be wrong here! I do use 6.3V heating, so it will be slightly hotter that the standard G9, but surely it can't be due to that? And should the 'node' voltage and the grid of V1A differ by about 28V? That would suggest a grid current of 27uA?
So just where am I going wrong with this :roll:
Bjorn
...and these DC readings do correspond to the values of one of my G9 channels posted over at RO.[/quote]
Hi,
I can read virtually the same voltages on my G9. Did anyone measure the correct values to read?
Thanks,
Val
:roll:
V1A
Plate: 160,158,157
Grid: 47,48,50
Cathode: 77,80,81
Node: 74,76,78
HT: 234,234,234
V1B
Plate: 83,78,75
Grid: 0,0,0
Cathode: 2.2, 2.3, 2.3
Having established that the results seemed to have a reasonable spread between different valves, I thought I'd try lowering the HT until the V1A cathode voltage dropped to 60, which would be acceptable for C7. I found that I had to drop the HT to 180 for this. According to the G9 schem, that would mean (245-180=) 65 volts dropped by R31, which is 1k...hence it must pass 65mA. But I just read in one of Jakobs posts to another G9 question that each G9 channel needs about 8mA, so something must be wrong here! I do use 6.3V heating, so it will be slightly hotter that the standard G9, but surely it can't be due to that? And should the 'node' voltage and the grid of V1A differ by about 28V? That would suggest a grid current of 27uA?
So just where am I going wrong with this :roll:
Bjorn
...and these DC readings do correspond to the values of one of my G9 channels posted over at RO.[/quote]
Hi,
I can read virtually the same voltages on my G9. Did anyone measure the correct values to read?
Thanks,
Val
:roll:
Rob Flinn
Well-known member
Quick question that`s probably been asked before:
On the diagram for the G9 R9 is marked as 220K, whereas on the board overlay it`s marked as 47K. Can anybosy tell me what the correct value is ???
On the diagram for the G9 R9 is marked as 220K, whereas on the board overlay it`s marked as 47K. Can anybosy tell me what the correct value is ???
??
Both the schematic
http://www.gyraf.dk/gy_pd/g9/g9_sch.gif
and the overlay
http://www.gyraf.dk/gy_pd/g9/g9_pcbs.pdf
says 47K for R9...
??
Jakob E.
Both the schematic
http://www.gyraf.dk/gy_pd/g9/g9_sch.gif
and the overlay
http://www.gyraf.dk/gy_pd/g9/g9_pcbs.pdf
says 47K for R9...
??
Jakob E.
Rob Flinn
Well-known member
Ah !!
The diagram I`ve been looking at is dated 10-2002 !!!
The diagram I`ve been looking at is dated 10-2002 !!!
Rob Flinn
Well-known member
The second G9 I`ve built is experiencing the same problem as the first. However the solution to sort out the oscillation on the first one does not cure the problem on the second G9. This was twisting the group of 5 wires to the switch PCB.
Messing around with my scope I notciced that the oscillation dissapears if I put a probel on the grid of V1a. Looking at the circuit diagram I notice that there are no grid stopper resistors. Do you think maybe this would be a cure ? I`m thinking that the whole amp might be more stable with them. Would 1K be a good value to try ?
Messing around with my scope I notciced that the oscillation dissapears if I put a probel on the grid of V1a. Looking at the circuit diagram I notice that there are no grid stopper resistors. Do you think maybe this would be a cure ? I`m thinking that the whole amp might be more stable with them. Would 1K be a good value to try ?
Brizco
Well-known member
heloo
i have a prblem: i dont get the 12 volts!
i already changed the 78s12 and checked pcb, caps, diodes and resistors but no 12v!
for the 245v i get 276 volt
the 48 volt phantom is exact 48v
has anybody some values i can measure or hints where the problem could be?
thank you very much!
i have a prblem: i dont get the 12 volts!
i already changed the 78s12 and checked pcb, caps, diodes and resistors but no 12v!
for the 245v i get 276 volt
the 48 volt phantom is exact 48v
has anybody some values i can measure or hints where the problem could be?
thank you very much!
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