• 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.

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 don’t 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 .
 
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!
 

Attachments

  • GM70 SE 40W 3.pdf
    15.5 KB · Views: 1,718
  • OT SE GM70 3.pdf
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Pyre

Member
2006-05-29 10:00 pm
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 don’t 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.
 
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!
 

45

Member
2008-12-18 2:29 am
UK
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..!!!:D
 
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45

Member
2008-12-18 2:29 am
UK
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.
 
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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.
 
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!
 
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45

Member
2008-12-18 2:29 am
UK
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"!!

No LAZAROIU, the grid of U5 will be negative but its cathode will be positive because the 30H choke is connected to ground in your schematic. The DC resistance of the choke will make the bias for U5 even more negative BUT for the GM70 it can only be a positive voltage.
In your design if, for example, you want the anode current for U5 to be 40 mA the cathode will be at 120 ohm x 40 mA = +4.8 V and this is the bias for the GM70. The 4.7 ohm cathode resistor of the GM70 will be not sufficient to limit its anode current.......
 

45

Member
2008-12-18 2:29 am
UK
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.

This is not correct as well because R9 + R10 doesn't work as voltage divider. Basically the current will flow only in the 6K1 resistor and thus the bias will be -270V if U5 doesn't have any grid current of itself!

If you tell me the bias that you want for the GM70 and the anode current for U5 I can give you some help. I don't have a software for drawing circuits.

45
 
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.

Actually, if you look at the triode curves for 6E5P; put a constant current source on the anode and use an operating point of 15mA,160V+ you can easily swing a few hundred volts peak to peak which should be plenty to drive the GM-70 to the required power; after all, it has slightly higher mu than 845.
 
Hi!
Something like this would be the power supply schematic for GU70.
Note that the source of + / - 270V is absolutely floating point (0), GND is the connection R9, R10.
Consequently, the grid is more negative than the U5's cathode, grid coupling of GM70 as galvanic GM70's grid will be more negative than the cathode.
Maybe I do not understand, but you should also post a schematic with your corrections.
Thank you for your interest and advice!


We see the evening!
 

Attachments

  • SURSA GM70 3.pdf
    17.2 KB · Views: 541
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Hi Lazaroiu,

Check connections of d3, d4, d5, d6; your high voltage supply will not work.

Pieter



Thanks for the warning, thousands of excuses, I drew the wrong di big hurry, but he saw and realized!
Once again I apologize for the wrong drawing.
I put together schematically corrected.
 

Attachments

  • Corectie.pdf
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This is not correct as well because R9 + R10 doesn't work as voltage divider. Basically the current will flow only in the 6K1 resistor and thus the bias will be -270V if U5 doesn't have any grid current of itself!

If you tell me the bias that you want for the GM70 and the anode current for U5 I can give you some help. I don't have a software for drawing circuits.

45


Hi!
What value do you think is the grid voltage polarity of the GM70 and GM70's katod from??
 

Attachments

  • polaritate.pdf
    7.1 KB · Views: 233

45

Member
2008-12-18 2:29 am
UK
Hi!
What value do you think is the grid voltage polarity of the GM70 and GM70's katod from??

Your schematic will not work.

First, the bias supply and bias regulation are wrong.
In your schematic the dual supply has a reference to ground anyway..... can you see it?


If we suppose that you have a correct dual supply for U5 and the negative voltage is -270V then your bias for U5 will be very close to -270V because between -270V and the grid of U5 there is only R10. Can you tell how do you calculate -50V?
The current in R10 can only be its grid current which is very small (tens of microampere in worst case). The only case where you have a substantial current draw is when the grid voltage is positive respect to the cathode!

In your schematic the cathode of U5 can only be positive because its plate is at positive voltage and the choke is connected to ground.
Then apply the ohm's law: choke resistance x U5 current = (positive) cathode voltage. This is the answer to your question in the schematic.

Why do you think it is negative?
What happens if you remove the ground from R9-choke node and put the 0V terminal of the dual supply to ground?

In your original schematic you can bias the GM70 increasing a lot the value of R3 (few hundreds ohm) but this will be a self-bias! However if R3 is 4.7 ohm like in your original schematic the bias will be close to zero volts. However the driver is not working yet...

Please answer the questions otherwise I cannot help you.

I am afraid I cannot draw a schematic because I don't have any software.....



45
 
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