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In the Forum: Audio News
In the Thread: Lamm Industries: a special interview with a special company
Post Subject: I do not think so but I migh be wrong.Posted by Romy the Cat on: 10/14/2013
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 haralanov wrote:
According to a friend of mine, who has a lot of experience with GM70, this tube starts to sing only with (very high quality) interstage transformer coupling. He told me that when it is used with coupling capacitor (as is the case with ML3), no matter how good the capacitor is  - the sound loses energy and tone density from the midrange down. Later, when I heard his GM70-based amplifiers - I can 100% confirm his words.
Haralanov,
 
I can’t argue this point as I do not have my own experience with GM70 and my hearing the already made by somebody GM70 amps would not qualify me to be an expert as I have no idea how those amps were designed and made. Still, what your friend says does not make any since to me. The presence of absence of coupling capacitor in case of amp enter grid current has absolutely nothing to do with the tube. Any tube that might work deep in A2 would from point of view the coupling capacitor to behave identically and I do not feel that GM70 would demand anything different then let say 211.
 
The interstage transformer is a fine solution and most of the GM70 amps use it. It is VERY difficult to make/buy a good interstage transformer but possible. Do not forget that the interstage transformers are not a panacea and also has theoretical problems not different then the capacitors. The caps when the grid bias goes mover zero begin to re-chard the dialectic and this is bad we all agree with this. The transformer it looks like free from this problem but it is only on surface. In reality transformer have inductance and as the polarity is changed the transformer inductance acts like a choke filter, trying to stabilize current and eating the minute current transients. There are ways to deal with it in case of cap and in care of transformer but this is a whole another subject.
 
Still, I would like to stress that the preference of interstage transformer vs. interstage cap I do not think that would be something specific to GM70.
 
My personal recipe is use higher sensitively acrostic, lower power and do not run output stages into grid currents.  From a different perspective the guys who use 90dB sensitive speakers do not have drivers capable to react to low driving currents to begin with, so they might not be feeling any interstage coupling difference to begin with.
 
Again, that all theory. All that I personally trying introduction a cap to Milq MF channels filter.  No matter which cap I tried I was not able to find anything transparent and went for RL subtraction filter. In my case it was filter constantly polarized. I am sure that if it was a cap that change polarity constantly then it would be 100 times worse.
 

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