Best GM70 version suited for audio??

[FullRangeMan]

Hi guys,
Please, what is the best GM70 tube version suited for a SET parallel amp??
I know three versions of this tube:
Copper plate(Ulyanovsk),
Graphite plate(Ulyanovsk)
Metal Plate(Photon Uzbekistan plant).

Anyone know what is the output plate impedance of this tube??
The PDF specs files I have see do not show this detail.
Thanks for any info, Gustavo

[kavermei]

Many people will tell you the copper plate version is best (probably because it is the most scarce )

Technically the graphite plate should be superior, it can radiate heat more efficiently and can stand higher temperatures.

My suggestion would be (and some people will disagree with this), to use the cheaper graphite types and spend the saved money on better output transformers.

You can calculate rp as rp=mu/transconductance. However, for the GM70, the parameters vary a lot with B+. Most schematics I have seen use quite high load impedances, such as 10 kohms, for high B+ like 1000--1200V.

Kenneth

[radiostar]

To my point of view copper plate is better sounding, but graphite plate version is more powerfull.

[djn]

I am building a 12HL7/GM70 and have the graphite tubes for now. I have friends that have the copper plate version. Once the amp is done and broke in nice, I will roll the coppers in it and see how they sound. Do you have a scheem yet? What B+ are you running? two stage or three stage?

[kevinkr]

I'm also in the process of building a GM70 amplifier and for the time being will use graphite plate GM70, but I am tentatively thinking of getting some copper plate ones later. I'm running this design pretty hot so for copper I would probably have to reduce the plate current significantly.

Others more familiar with the GM70 than I currently am have indicated that both types have their virtues, but that possibly the copper version sounds slightly better. So far I have heard only the graphite type and it sounded just fine to me..

I'm going to be running 1KV/125mA with graphite, driven by D3A via IT. Both stages will be fixed biased, and supplies fully regulated. (On separate chassis)

Project is moving rather slowly, I have posted details on another long dormant thread. (When I was planning to use choke loading.) Once I actually start putting the thing together I will post to that thread.

[h00hbt]

Hi FRM

I find the graphite more detailed and sharp or quick and the copper maybe has a tad deeper bass but not as much power. I have a pair of the extreamly rare plain metal plate with brass base but haven´t got round to hear them out yet, one of these days.......

[djn]

Hey Kevin, I am running 840v B+ at somewhere around 80 to 100 ma. My horns are 115db efficient so I don't need more that the 7 to 10 wpc I will get out of it. I am also using the OA2 controll tube on the 12HL7.

[FullRangeMan]

Hi Boys,
Thanks for your valuable and detailed info. Iam surprised how many GM70 amps are being manufactured.
This GM70 looks a good tube for the price, in view of a 500dollars 845 single.
Go ahead it worth the work.

[FullRangeMan]

Originally Posted by kavermei

Many people will tell you the copper plate version is best (probably because it is the most scarce )

Technically the graphite plate should be superior, it can radiate heat more efficiently and can stand higher temperatures.

My suggestion would be (and some people will disagree with this), to use the cheaper graphite types and spend the saved money on better output transformers.

You can calculate rp as rp=mu/transconductance. However, for the GM70, the parameters vary a lot with B+. Most schematics I have seen use quite high load impedances, such as 10 kohms, for high B+ like 1000--1200V.

Kenneth

Mr.Kenneth: Thanks for this formula. So, as you said, looks this GM70 has a plate impedance of 9k ohms!! Are this too high? This could be a problem?
As a 845 plate impedance is around 3k ohms. Looks the output transformer will be big...
Regards, Gustavo

[FullRangeMan]

Originally Posted by h00hbt

Hi FRM
I find the graphite more detailed and sharp or quick and the copper maybe has a tad deeper bass but not as much power. I have a pair of the extreamly rare plain metal plate with brass base but haven´t got round to hear them out yet, one of these days.......

Dear H00HBT,
By this good description you and others said on GM70 sound quality, I will choose the inexpensive Graphite Plate.
Iam in a very begining stage of choose the parts, buy it and store for later building. I will no build myself, as I know only made solder, I will use a tube builder in my city, or any other in Brazil that accept the task.
Many thanks for your support guys, Gustavo