Evening all - I'm afraid I'm a newbie...
I have a pair of Graafs GM20's (schematic via link) but the constant tweeking of bias and offset is driving me mad!
https://cid-a332bc3aefc89f6b.skydrive.live.com/self.aspx/Public/GM20 schematic.pdf
Is there a way of adding an auto bias circuit?
Also, can anyone suggest any tweeks / improvements? I've scoured the net but nothing forthcoming.
Ahh, can I reduce the gain / adapt the present circuit to suit a higher input voltage without using a pot and thus destroying the dynamics? I'm using a Weiss Medea DAC with +27dBu via XLR's....
Thanks guys
I have a pair of Graafs GM20's (schematic via link) but the constant tweeking of bias and offset is driving me mad!
https://cid-a332bc3aefc89f6b.skydrive.live.com/self.aspx/Public/GM20 schematic.pdf
Is there a way of adding an auto bias circuit?
Also, can anyone suggest any tweeks / improvements? I've scoured the net but nothing forthcoming.
Ahh, can I reduce the gain / adapt the present circuit to suit a higher input voltage without using a pot and thus destroying the dynamics? I'm using a Weiss Medea DAC with +27dBu via XLR's....
Thanks guys
Luke said:
Well spotted - there's a little room and I can always extend the base downwards. (I'm also happy to construct a third box if necessary, or maybe two full width units, one for the power supplies, the other for the tubes etc)
I'm toying with the idea of a passive line level crossover using one amp for each channel but until I can get the amps running in a consistent and stable manner it's pointless...
BTW Thanks for the interest, TENT module noted, email sent.... Cheers!
Koifarm said:
From Graaf....
Usually the gain of the preamplifiers is around 15 db, and yours is even 27 db, so it’s not easy. If you want to decrease the gain of GM 20, you can utilize an attenuator in parallel by input of GM 20, working on resistances R2, R3 and R4, R5. Doing so the dynamics will be very low, or disappears at all, this is important for you to know. You have to bring the R2, R3 to 32 KOhm instead of 24 KOhm, and R4,R5 to 16 KOhm instead of 24 KOhm.
This unfortunately kills the dynamics, I'm looking for a more subtle way, reducing further gain rather than "wasting" my pre-amp gain...
Oke, use a 6n6p tube in the first stage. It is fully compatible with the 6922 pin layout only the heater current is two times higher. But the Graaf amp has lots in reserve. Because of his lower amplication factor ( 22 i.s.o. 33 ) the input can be higher.
Another possibility is to use a Ecc82 ( 20 x ) tube but then you have to change heater pin layout. Or use noval adaptor.
Ronny
Another possibility is to use a Ecc82 ( 20 x ) tube but then you have to change heater pin layout. Or use noval adaptor.
Ronny
Top idea, just the knowledgeable advice I need - many thanks Is there any sonic performance to chose between the two suggestions? Microphonics / a golden reputation? I've just searched google...
http://theothor.googlepages.com/6n6ptweakforgm-20
"After using the russian tube for a long time, the midbass hump became to annoying, so I couldn't live with it. After talking to a DIY buddy, I also concluded that using the 6n6p tube, the output impedance of the amplifier changed, and that's not so good."
Maybe the 6n6p is not a favourable option - do you agree with the impedance comment?
Seeing as you obviously understand a tube or two.... in your opinion will the Tent lab bias module work within the Graaf schematic?
Much obliged
Is there any sonic performance to chose between the two suggestions? Microphonics / a golden reputation? I've just searched google...http://theothor.googlepages.com/6n6ptweakforgm-20
"After using the russian tube for a long time, the midbass hump became to annoying, so I couldn't live with it. After talking to a DIY buddy, I also concluded that using the 6n6p tube, the output impedance of the amplifier changed, and that's not so good."
Maybe the 6n6p is not a favourable option - do you agree with the impedance comment?
Seeing as you obviously understand a tube or two.... in your opinion will the Tent lab bias module work within the Graaf schematic?
Much obliged
I like the 6n6p tube a lot and use it in al my stuff. There is no big hump around 400Hz. It just has more dynamics in the kickbass region. And it is cheap. Just try it.
The tentlabs auto bias just need a minor adjustment because your bias current is about 250mA and the design is for 175mA max.
Probaly the resistor who measures the current needs to be replaced for a lower value. You can ask the people at tentlabs.
Ronny
The tentlabs auto bias just need a minor adjustment because your bias current is about 250mA and the design is for 175mA max.
Probaly the resistor who measures the current needs to be replaced for a lower value. You can ask the people at tentlabs.
Ronny
I have been playing with the GM20 topology on a breadboard and liked its performance a lot, but disliked the electricity bill it gave me
Changing the input tube can be done but watch the feedback loop from the output tubes right to the anode supply of the input tubes!
The Av input to Av driver+output relationship makes the amount of feedback in the circuit. Which changes also with speakerload.....
Also the inputtube Va is something like 50V, which is probably too low for a 6n6p.
An easy way out could be to use an input stepdown transformer, but no more a true OTL then...........
Edit
6c33 is sensitive to changes in heatervoltage wich will trow them off bias.
This can come from changes of your wallvoltage or very likely dirty tubesockets ! Try cleaning your powertube sockets first!
Changing the input tube can be done but watch the feedback loop from the output tubes right to the anode supply of the input tubes!
The Av input to Av driver+output relationship makes the amount of feedback in the circuit. Which changes also with speakerload.....
Also the inputtube Va is something like 50V, which is probably too low for a 6n6p.
An easy way out could be to use an input stepdown transformer, but no more a true OTL then...........
Edit
6c33 is sensitive to changes in heatervoltage wich will trow them off bias.
This can come from changes of your wallvoltage or very likely dirty tubesockets ! Try cleaning your powertube sockets first!
Bad news from Menno re the autobias Tent module....
Unfortunately it is NOT possible to implement the auto bias in this Graaf Circlotron circuit. The reason why is because the cathodes of the power triodes are NOT refered to ground, but floating. The condition for proper functioning of the auto bias unit is that the cathodes continuesly are on ground potential en in this circuit they are on differencial signal potential, they are floating. Then the auto bias unit has no reference where to compare with, and consequently can't do its work properly.
I am sorry, but in this case it is not possible.
..... ho hum...
Unfortunately it is NOT possible to implement the auto bias in this Graaf Circlotron circuit. The reason why is because the cathodes of the power triodes are NOT refered to ground, but floating. The condition for proper functioning of the auto bias unit is that the cathodes continuesly are on ground potential en in this circuit they are on differencial signal potential, they are floating. Then the auto bias unit has no reference where to compare with, and consequently can't do its work properly.
I am sorry, but in this case it is not possible.
..... ho hum...
IF the GM20 is working correctly, there is no reason for it to change bias all by itself..so something must be off spec in your wallvoltage or in your amp.
I would advice you to get help from a competent technician...
On the topic of sensitivity:
I had a similar problem when using a Croft preamp on my GM20 breadboard. Nine o'clock on the volume pots and the GM20 was already maxed out. Also I had a bit of noise coming trough from the preamp.
The solution was simple: I changed the series resistor R2 (cinch input) on the GM20 breadboard to 100k. Now the preamp vol pots could go to 12 o'clock. And as a bonus the preamp noise was reduced as well.
No ill effects at all. Dynamics were fine.
You can try the same, but since you are using balanced inputs, be sure to change R2 and R3 by the same amount.
I would advice you to get help from a competent technician...
On the topic of sensitivity:
I had a similar problem when using a Croft preamp on my GM20 breadboard. Nine o'clock on the volume pots and the GM20 was already maxed out. Also I had a bit of noise coming trough from the preamp.
The solution was simple: I changed the series resistor R2 (cinch input) on the GM20 breadboard to 100k. Now the preamp vol pots could go to 12 o'clock. And as a bonus the preamp noise was reduced as well.
No ill effects at all. Dynamics were fine.
You can try the same, but since you are using balanced inputs, be sure to change R2 and R3 by the same amount.
Hi.
I am the guy with the gm-20 page. I agree with you that the bias/dc adjustment on the GM-20 could have been made easier. If you have to adjust the bias all the time, I think you either adjust it to often because you can't leave it alone, or the output tubes is not matched good enough. How fast is it drifting?
In Norway, there are a lot of GM-20 owners who has had the bias/offset adjustment moved out to the side of the amp. Just drilled three four small holes on each side. Two for the multimeter pins measuring the bias, one for adjusting the bias, and one for offset. This mod is easy to make and works very well. You can then also use pots with several turns which makes accurate adjustment a breeze.
If you look at the picture I have attach you will see a totally rebuild GM-20. The inside of the amp was smashed becacuse a speaker landed on top of it. Somehow the exterior was without a scratch. Anyway, here you can see the bias arrangement on the side. If you find this interesting, I can make contact with the norwegian G-20 expert who rebuild this amp and has made the bias arrangement on several amps.
Regarding the 6n6p tube, the operating points are not best suited for it. But it will be no harm trying it out. You should buy at least 8 tubes, cause with some of them it is impossible to get the offset to zero.
IMO a nice pair of Telefunken E88CC is tops, but the ususal suspects like Philips and Siemens is also very nice.
Regarding gain, there must be another way around this than messing with the guts of the gm-20.
Best
Theo
I am the guy with the gm-20 page. I agree with you that the bias/dc adjustment on the GM-20 could have been made easier. If you have to adjust the bias all the time, I think you either adjust it to often because you can't leave it alone, or the output tubes is not matched good enough. How fast is it drifting?
In Norway, there are a lot of GM-20 owners who has had the bias/offset adjustment moved out to the side of the amp. Just drilled three four small holes on each side. Two for the multimeter pins measuring the bias, one for adjusting the bias, and one for offset. This mod is easy to make and works very well. You can then also use pots with several turns which makes accurate adjustment a breeze.
If you look at the picture I have attach you will see a totally rebuild GM-20. The inside of the amp was smashed becacuse a speaker landed on top of it. Somehow the exterior was without a scratch. Anyway, here you can see the bias arrangement on the side. If you find this interesting, I can make contact with the norwegian G-20 expert who rebuild this amp and has made the bias arrangement on several amps.
Regarding the 6n6p tube, the operating points are not best suited for it. But it will be no harm trying it out. You should buy at least 8 tubes, cause with some of them it is impossible to get the offset to zero.
IMO a nice pair of Telefunken E88CC is tops, but the ususal suspects like Philips and Siemens is also very nice.
Regarding gain, there must be another way around this than messing with the guts of the gm-20.
Best
Theo
Good afternoon Theo - you've saved me a mail, Johan has been in touch )
The bias seems OK, holding +/-2mv, but the offset can drift 10mv/day upto 200mV. I need consistency so I can use an active xover before it and rely on the power amps holding thier relative output.
I've just received some 10 turn 50K pots from the US and the drill is poised... gulp! Maybe replace the on/off green button with a four pin socket to connect a dedicated multimeter to....
It would be nice to build an auto sensing soft start as I often fall asleep to the music and it's a waste of tube life (and heating) waiting until I wake up. Not sure which part of the power supply sould be switched for the standby, I suppose the heaters?
My second amp does need some TLC with a large DC offset regardless of triode and a buzzing in the left channel. Have tube swapped but to no avail... could you forward some address details of your Graaf guru? dpaws at hotmail dot com
As for the gain I am sure there will be a way, albeit with a front end redesign, a project for another day.... I'm sure a passive line level second order xover will take care of some of the gain and maybe will only need to trim after this with bias... Need to chat with VacuumState, he's a clever guy!
Much obliged, Steve
)The bias seems OK, holding +/-2mv, but the offset can drift 10mv/day upto 200mV. I need consistency so I can use an active xover before it and rely on the power amps holding thier relative output.
I've just received some 10 turn 50K pots from the US and the drill is poised... gulp! Maybe replace the on/off green button with a four pin socket to connect a dedicated multimeter to....
It would be nice to build an auto sensing soft start as I often fall asleep to the music and it's a waste of tube life (and heating) waiting until I wake up. Not sure which part of the power supply sould be switched for the standby, I suppose the heaters?
My second amp does need some TLC with a large DC offset regardless of triode and a buzzing in the left channel. Have tube swapped but to no avail... could you forward some address details of your Graaf guru? dpaws at hotmail dot com
As for the gain I am sure there will be a way, albeit with a front end redesign, a project for another day.... I'm sure a passive line level second order xover will take care of some of the gain and maybe will only need to trim after this with bias... Need to chat with VacuumState, he's a clever guy!
Much obliged, Steve
I've rebuilt some GM20's. The one pictured allmost from scratch. There's nothing to improve in these amplifieres except getting the best 6922 and EF184 tubes. Input sensitivity is normal and should match any normal preamplifier. Sensitivity can be changed as described above. It wil NOT affect any dynamic performance. Beleive me.
How is your amp lokking from the underside?
Solid bars to the heaters of 6C33 or wires?
The only week point of GM20 is the "sockets" for 6C33. I've replaced "a few" with ceramic transmitting tube type sockets.
Mounting 20t potmeters and testpoints on the sides make testing and adjustments very easy. When a GM20 is in good shape, maintenance is not a matter.
How is your amp lokking from the underside?
Solid bars to the heaters of 6C33 or wires?
The only week point of GM20 is the "sockets" for 6C33. I've replaced "a few" with ceramic transmitting tube type sockets.
Mounting 20t potmeters and testpoints on the sides make testing and adjustments very easy. When a GM20 is in good shape, maintenance is not a matter.