• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Graaf GM20 6C33

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#2
From my experience with gm20, they last maybe 1000 hrs. Some more, some less. They don't slowly die like many lyver tubes, they go out with lightning so you won't be in doubt when they die. Be careful to leave them alone for a long time. I once forgot to turn of my amp before I went to work, and when I got home, one tube har died and fried the speaker.
 
#4
Graaf GM20

Hi FG,
I agree with nightpuma, about 1000Hrs or so for a set.
One thing that will lengthen life, the bias is stated as 30mV across R32L/R on the PCB, but some years ago Graaf reduced this to 20mV. I find the 6C33 very variable and often spend a lot of time getting the bias and offset right with new pairs. I usually start off with about 16 and select 2 or 4 accordingly.
The small signal valves seldom cause trouble, but the 6922 can and do cause DC drift on the output (along with the 6C33) and often replace them with E88CC (if I can get them) as they are better matched.
I replace both the bias (50K) and offset (200R) potentiometers with 20 turn upright varieties glued in place with epoxy and tinned copper wire connections to the PCB, which help make settings easier.
The other items that have caused me grief are the "block" transformers going shorted turns,a real swine to replace, and the small decoupling capacitors C8L/R & C10L/R 22µF/450V

John
 
#5
John Caswell said:
Hi FG,
I agree with nightpuma, about 1000Hrs or so for a set.
One thing that will lengthen life, the bias is stated as 30mV across R32L/R on the PCB, but some years ago Graaf reduced this to 20mV. I find the 6C33 very variable and often spend a lot of time getting the bias and offset right with new pairs. I usually start off with about 16 and select 2 or 4 accordingly.
The small signal valves seldom cause trouble, but the 6922 can and do cause DC drift on the output (along with the 6C33) and often replace them with E88CC (if I can get them) as they are better matched.
I replace both the bias (50K) and offset (200R) potentiometers with 20 turn upright varieties glued in place with epoxy and tinned copper wire connections to the PCB, which help make settings easier.
The other items that have caused me grief are the "block" transformers going shorted turns,a real swine to replace, and the small decoupling capacitors C8L/R & C10L/R 22µF/450

John
Click to expand...

When doing bias regulation, better to make a first one, leave the amp switched on for half an hour so it gets hot and check it again, At that time 18 or even 16 mV can be even better to have a longer output tube life.
 
#6
Hi John and Clack,
Thanks for this input. The former owner of my GM20 replaced the tubes for a matched quartet 6c33 and specially matched and selected signal valves and rectifiers in 2014. It has about 300 hours on the clock now. He also chose a 20mV bias setting and tought me to adjust it myself as well as the offset. I have used it at 20mV ever since i bought it. I'll try 18mV next time.
Regards,
FG
 
#7
I always ran my gm-20 at about 24-25 mV. You could adjust for best sound, the tube life will not be much shorter, and tubes are not so expensive. You could also try PCC88 tubes instead of E88CC. It is the same tube with 7V heater. It will run a little bit starved, but that will be fine. A friend of mine find they outperform E88CC in the GM-20.
 
#8
FG1967 said:
Hi John and Clack,
Thanks for this input. The former owner of my GM20 replaced the tubes for a matched quartet 6c33 and specially matched and selected signal valves and rectifiers in 2014. It has about 300 hours on the clock now. He also chose a 20mV bias setting and tought me to adjust it myself as well as the offset. I have used it at 20mV ever since i bought it. I'll try 18mV next time.
Regards,
FG
Click to expand...

Well, there is not much difference soundwise between 20 and 18/16 mV, but the lower value will help to have a longer output tubes lifespan.
Remember to check bias when the amp is at working temperature as well, at least half an hour since it has been switched on.
For this purpose you can use a fan to avoid problems while the amp is working such a long time in vertical position, that could affect the output tubes correct heat dissipation.
Bias and offset need to be regularly checked, so don't let pass years forgetting to do that.
These advices were given to me directly by the designer, Mr. Giovanni Mariani.

GM 20 has solid state rectifiers. The EF 184 tubes are pentodes used in the input/driver stage. Together with the E88CC's they are very important for the offset regulation as well, that in turn greatly influences the correct working of the output stages, not only for the output tubes lifespan, but for the whole amplifier.
So, be sure to use only well balanced tubes even at signal level.
 
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#12
Graaf GM20

Hi all,
The EF184 maybe pentodes but they are wired as triodes for the driver stage.
I have tried several types here and they don't seem to make any difference to the bias/offset settings.
When repairing a GM20 I always check output levels and usually find I have to adjust the bias on one channel to match gains, generally can get them to within 0.25dB or less. This often means one of the channels biased at between 16- 20mV.
FG wrote "specially matched and selected signal valves and rectifiers in 2014" I can understand the signal valve selection, but not the rectifier bit. How you match SS rectifiers I do not know - a bit of a waste I think.

John
 
#13
Thank you all for your input.
I am just a music lover and not technically educated, so i can use any help i can get to get the most out of my equipment.
Many many years ago i heard the GM20 on the Diapason Adamantes. I fell in love. About 18 months ago i found an affordable pair Adamants III, and started my search for a GM20. About a year ago i bought one in almost new condition with almost new tubes. Very recently i obtained the WFB-one from the same reliable source with very few hours of use since the tubes were replaced.
A 13.5 would perform even better, but -besides the price and availability- the phono section in the WFB one makes it very attractive to me. Every day since i replaced my solid state prepre and preamp, i hear new elements in my record collection. At this price level it is one of the best setups i can imagine 🙂
Regards,
FG
 
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