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In Reply to: RE: Michael, a broader question posted by M3 lover on July 16, 2016 at 09:38:16
The 6CA7 and the EL34 are only identical in the sense of what they present for a load to the circuit they are put in..The EL34 is a Pentode,the 6CA7 is a tetrode.The KT77 is also a tetrode.I have in my possession,the best of EL34s ever made and I have them in NOS/NIB,and I have many very close to NOS in the ones I run out of the box.What I have found with EL34s,if they are well matched running at the higher GM ranges,they have a much better chance of maintaining their clarity all thru the spectrum.I have also found that you need the most pure.low distortion and linear driver stages to correctly run EL34s.
The 6CA7s are a bit more forgiving by nature being a tetrode IMHO."Now I see you specify the 8200 to 8700 range for the 6AC7s. But can you comment on 6AC7s which fall outside that range, are they still better than EL34s or quite similar?"
It depends how far you fall out of that range..Higher GM typically favors lower distortion numbers from my experience. Groove tubes had this rating system that was fantastic.They did their ratings on numbers 2 thru 10.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/18/16Follow Ups:
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"The piano ain't got no wrong notes." Thelonious Monk
Thanks,I'll need it..I have had them ever since I was 30 off and on and I always passed them but not this time.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Mike, wouldn't the higher GM only come into play when one turned up the amp? Lower would distort sooner right?
Mike, wouldn't the higher GM only come into play when one turned up the amp? Lower would distort sooner right?
That would be my guess but I haven't studied output tube curves that much.Maybe Tre, or Jack,Eli, or Jim McShane would be better to answer that.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I hate to rain on everyone's parade - but the Gm of a tube varies with the operating conditions under which the tube is being run. So unless you can test using the exact conditions (or quite close to them) under which the tube will operate in the amp, the transconductance readings will be significantly different. In fact the Gm will be significantly lower in the amp under typical amp conditions than it is under the much lower voltage and current tube tester conditions.
The readings that New Sensor puts on their tubes is GM and and current..The problem with this is the fact that we have no idea what plate voltage or G1 voltage they were running to assume these readings. I'm only going by what they set as a reference for testing tubes and we would have to assume that they are pretty consistent with the voltage parameters they use,no?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/18/16
I've found they do a fairly decent job on many (not all, but most) power tube Gm readings - but as you say - what test conditions were the tubes subjected to?
Hey, Gm readings on a given tube vary from one tester to another - so the Gm range that works best in Mike's amp may be significantly different from what would be shown in another tester.
They will probably give testing conditions if we ask. We can only assume they're "very good" or "less good" right now.
Jim
From what I remember reading as well as talking to Myles at Groove Tubes,Hifi applications typically like higher GM readings than say guitar tubes which a lot of those guys seem to want a higher distortion product.I've always thought of the EH 6CA7 fat bottle as decent for the money but the 2 quads I have were in the 7200 to 7500 GM range.The 3rd quad I have that tests at 8300,is far and away exceptional.These tubes are 2012 while the others were from 2008 and one of those quads came with an amp I bought.Are they using dual halo getters on the new EH 6CA7s or was that a one time thing?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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