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Old 29th December 2012, 06:45 PM   #61
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i use some lundahl 1620 /120ma which can be set for 11k5 which is what i use, think at that impeadence they are only rated at 14w, i was testing yesterday and saw 16v into 8.8r so 29 w at some massive clipping.
still they were 200 delivered about 15 years ago

enzo
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Old 29th December 2012, 06:47 PM   #62
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I thought they were more like 2k6?
enzo
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Old 31st December 2012, 09:24 PM   #63
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Quote:
Originally Posted by cotdt View Post
Of course distortion will get lower with higher impedance transformers, but that is not the goal for a SET amp. The GM70 has output impedance at 1.1k, so a 4.4k transformer makes more sense. I use a 5k but it sounds great even with 3.5k transformer.
True, but not the point I was trying to make. The GM-70 (in particular) is more sensitive to loading (in terms of IMD) than many tubes of its ilk (845, 572, 211, etc.) so I don't agree that "4.4K makes more sense" - (you can talk with Jack Elliano of Electra-Print if you need actual test results).
Also, I would say that lowering IMD is the goal of any amp as it's by far the most objectionable form of distortion.

That being said, much of this depends on operating bias (quiescent voltage/current) and you do get wider frequency response (less inductive roll-off) with a 5K winding than w/10K.

All designs are compromises of one sort or another.
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Old 18th June 2015, 10:40 PM   #64
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Hi

Need some help or advice
Just purchased 4PC GM70 copper plate and the four are not the same
Each two piece were produced in different year
Two has clear top and two has glossy top.
Can someone let me know if those can perform the same or close? I must worried about that or that does not effect the performance
Something similar like on the attached picture
Thank you
Also purchased 4PC graphite plate - those are 100% same

Greetings gabor
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File Type: jpg $_57.jpg (213.0 KB, 150 views)

Last edited by gaborbela; 18th June 2015 at 10:42 PM.
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Old 19th June 2015, 01:01 AM   #65
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Quote:
Originally Posted by gaborbela View Post
Hi

Need some help or advice
Just purchased 4PC GM70 copper plate and the four are not the same
Each two piece were produced in different year
Two has clear top and two has glossy top.
Can someone let me know if those can perform the same or close? I must worried about that or that does not effect the performance
Something similar like on the attached picture
Thank you
Also purchased 4PC graphite plate - those are 100% same

Greetings gabor
Hard to find a 2 tubes the same.
The pointed tube seems has some use as shown this black spot.
That spot is result of alectrons action on the copper plate.
It will increase with tube use, by this copper plates lasting less than the graphite plate version at a considerable more price.
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Old 19th June 2015, 01:41 AM   #66
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Hi
That type of tube has a ring on the top the others not.
I don't know the function or the name of that ring. Around that ring the tube the glass get like smoked, glossy.
Some tube has that ring on the top some not.
On these picture you can see that ring I talk about.

Greetings gabor
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Old 19th June 2015, 01:46 AM   #67
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These donuts are Zirconia getter.
GM70 also had the usual dust Barium getter.

OBS.: The black smoke are result of the getter action to get gasses etc
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Last edited by FullRangeMan; 19th June 2015 at 01:53 AM.
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Old 19th June 2015, 02:16 AM   #68
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Thank you!
My question two copper plate tube has that getter on the top and two does not.
All GM70 has two at the side of the bottom but not all has on the top.
Any difference how they perform, do that getter on the top make any difference sound wise.
Take a look at both picture I posted and you can see it. Some does not have that donuts at the top.
Actually the 4PC graphite plate does not have either. At least those has the same structure.
Greetings
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Old 19th June 2015, 02:26 AM   #69
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I believe the ring getter dont change the sound quality, at least directly.
If the tubes are really missing the ring getter may develop some gas with time.
So the tubes should be carefuly burn in before regular use in the amp.
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Old 19th June 2015, 06:08 AM   #70
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Quote:
Originally Posted by digitrax View Post
True, but not the point I was trying to make. The GM-70 (in particular) is more sensitive to loading (in terms of IMD) than many tubes of its ilk (845, 572, 211, etc.) so I don't agree that "4.4K makes more sense" - (you can talk with Jack Elliano of Electra-Print if you need actual test results).
Also, I would say that lowering IMD is the goal of any amp as it's by far the most objectionable form of distortion.

That being said, much of this depends on operating bias (quiescent voltage/current) and you do get wider frequency response (less inductive roll-off) with a 5K winding than w/10K.

All designs are compromises of one sort or another.
Regarding Ri of GM70, it is around 1k6...1k8 at 800-1000V plate voltages, and around 1k8...2k0 at 1200-1400V. So, for sure, Ra around 8k...10k is a must.
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