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Old 8th October 2010, 04:55 PM   #1
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Default GM70 SE Amplifier

Hi
A few months ago I finished my first tube project a 6C33C SE amplifier (I love it) and now I decide to go to another level so I ordered a pair of GM70.
I would like to build an amplifier with at least 35 40W. I see that most of the diagram use inter stage transformers or mosfet driver like in Lamm 3.but I donít like this approach.
Any idea about a tube for driver to excite the GM70 and produce at least 35w or more.
Where I can find irons for the OPT, I would like double C core.
The project I want to made will have the power supply separately.
Best regards .
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Old 8th October 2010, 05:13 PM   #2
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Hi!
Two months ago, I listened to your amplifier 6C33C, although the distance between us is over 300km luck you can do to listen.
Impressions are above expectations, I heard a great sound and incredible achievement is beyond what is a DIY so I'm glad that you approach a more complex schematic of a high level even though you started skipping directly 6C33C stages: EL84, 6P3S ........ and other !!!!... I can only congratulate you!
Now try to make this a purely theoretical schematic, with minor adjustments to certain inherent in the practice will work well sure it will depend very much on the quality of the parts but the first OT.
Propose the scheme, but I OT here and database schema.
Thank you and your opinions!
Attached Files
File Type: pdf GM70 SE 40W 3.pdf (15.5 KB, 1017 views)
File Type: pdf OT SE GM70 3.pdf (14.6 KB, 519 views)

Last edited by LAZAROIU; 8th October 2010 at 05:19 PM.
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Old 8th October 2010, 05:19 PM   #3
Pyre is offline Pyre  Canada
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Quote:
Originally Posted by gabrielbecheanu View Post
Hi
A few months ago I finished my first tube project a 6C33C SE amplifier (I love it) and now I decide to go to another level so I ordered a pair of GM70.
I would like to build an amplifier with at least 35 40W. I see that most of the diagram use inter stage transformers or mosfet driver like in Lamm 3.but I donít like this approach.
Any idea about a tube for driver to excite the GM70 and produce at least 35w or more.
Where I can find irons for the OPT, I would like double C core.
The project I want to made will have the power supply separately.
Best regards .
First open your wallet and get a lighter ready. 35-40W SE out of a GM-70 requires 1kv+ on the plates and parts get real expensive. I kept my GM-70 amp at around 900V so I could buy 150VA output transformers and not break the bank on caps. GM-70 tubes are cheap compared to the parts required to make them sing.

If you do a quick search of the tube section for GM-70 you will come up with a few threads. Check out:


Hi-end GM70 amplifier with only 2 stages


GM-70 Build Issues

There was one on transformers but I can not seem to find it. If you need any schematics PM me and I can zip the load that I have.
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Old 8th October 2010, 07:06 PM   #4
piano3 is online now piano3  United Kingdom
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Why not stay Soviet and use 6E5P as a driver. More than 35W SE seems a rather tall order though. Rather you than me with these instantly lethal voltages!
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Old 8th October 2010, 07:41 PM   #5
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Quote:
Originally Posted by piano3 View Post
Why not stay Soviet and use 6E5P as a driver. More than 35W SE seems a rather tall order though. Rather you than me with these instantly lethal voltages!
Hi!

Sure lethal voltages are all greater than 100V voltages are dangerous, but we can not achieve performance GM70 at low voltage and a driver with 6E5P is too little to attack GM70 grid. With an anode voltage 300-600V for GM70 and a driver will not get more classic than 12-15W sinus unlimited power.
This scheme is designed for people with great experience in very large assemblies to Ua, and so must use all safety rules.
In my answer I tried to recommend a schematic of <gabrielbecheanu>.
With respect!
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Old 8th October 2010, 08:53 PM   #6
45 is offline 45  Italy
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Quote:
Originally Posted by LAZAROIU View Post
Hi!
Two months ago, I listened to your amplifier 6C33C, although the distance between us is over 300km luck you can do to listen.
Impressions are above expectations, I heard a great sound and incredible achievement is beyond what is a DIY so I'm glad that you approach a more complex schematic of a high level even though you started skipping directly 6C33C stages: EL84, 6P3S ........ and other !!!!... I can only congratulate you!
Now try to make this a purely theoretical schematic, with minor adjustments to certain inherent in the practice will work well sure it will depend very much on the quality of the parts but the first OT.
Propose the scheme, but I OT here and database schema.
Thank you and your opinions!
It looks like there are a couple of mistakes in your schematic:

1) there is a missing resistor (in series with the cathode inductor).
There is a number (6K1 5W) but not the resistor.

2) the negative voltage supply for the driver is wrongly connected. The 6k1 resistor should be connected to -270V. If you connect it to the ground you will probably kill the GM70..!!!

Last edited by 45; 8th October 2010 at 08:57 PM.
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Old 8th October 2010, 09:03 PM   #7
djn is offline djn  United States
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What speakers do you have that require that much power? I am building a GM70 and running 850v B+. That is plenty to make that tube sing but not enought to really worry about Crona effect. You could do push pull and make your target.
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Old 8th October 2010, 09:07 PM   #8
45 is offline 45  Italy
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P.S.
If 6K1 5W is the value for R10 then you must add a resistor in series with the inductor and connect it to -270V otherwise your GM70 will get a positive bias or something close to zero volts which at 950 V anode voltage will result again in huge anode current!
It would be better if you put 1 or more resistors + trimmer in place of R10 in order to regulate the correct bias.

Last edited by 45; 8th October 2010 at 09:11 PM.
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Old 8th October 2010, 09:13 PM   #9
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Hi djn
I don't need that power but I don't want to build a GM70 with the same power like my 6C33C. I want it to be my best amplifier so i want all the power i can take it.
I like SE
Now i work on a PP G807 amplifier around 50W
With this GM70 i would like to drive a Jamo Oriel an 87db speakers.
My main speakers are a pair of Grundig Super hifi box 450 drivers made in 1978 in a DIY cabinet.
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Old 8th October 2010, 09:18 PM   #10
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Hi!
Source of + /-270V is connected to minus divider R9, R10 ( R9 = 330K and R10=6.1 K), so the point we had about minus50V R9 R10, so the grid will have a voltage of tube U5 negative.
So in his katod U5 tablecloth we have the correct voltage scale attack by GM70.
I hope so, but if there is something wrong, please correct you just proposed schematic theory!
Thanks for the observations!

I apologize for the "English" my, but I translate with "google translate"!!

P.S. source +/ _ 270V galvanic table is separated, one amplifier is connected to the mass through resistance R10 to 6.1 K.
To make me understand I have to put the power supply here.
Yes, you're right, it made a perfect divider to adjust the variable current through the GM70.
Thanks!

Last edited by LAZAROIU; 8th October 2010 at 09:30 PM.
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