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compared to a 6CG7?
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My experience with the 6FQ7 and 6CG7 aligns with Len_'s. I use these tubes in the driver/splitter positions of Conrad Johnson amplifiers, modified MV-75s and stock Premier 12s. Swapping this tube makes a big difference in Premier 12s, much less difference in the MV-75s.Rolling in these amps nearly every variant of different 6FQ7 and 6CG7 I can find, some with/without plate shields, with/without filament shields, I found a large sonic variation among the different tubes, but I don't feel there's a difference that's resovable to shield/no shield. In the system I've most recently used the Premier 12s in, my favorite tube is a 6FQ7, the late production GE with the bridge wire and round plate cut outs, which is what CJ used as OE in these amps.
Now, CJ has always spec'ed a 6FQ7 in all of its products that used this tube. Since CJ specs this tube, I'm guessing that there isn't a connection for the 6CG7 plate shield, but I should check.
- SJ
I've always assumed the purpose of the shield was to lower crosstalk coupling between sections. The tube was originally designed for HF service as a vertical oscillator in TV receivers.Comments?
jarthel,While more or less "finishing" a tube collection recently, I relaised that I had only 4 NOS RCA "cleartop" 6CG7/6FQ7s and as the 4 of these in the ol' Audio Research D115 are over 16 years old, I thought I'd better make sure I have a spare set of whatever the "best" of these might be.
AS it would be some time before I could try these, I had to rely on means other than listening to choose. Looking into the archives here and dealers, it seemed the 6CG7 with the "center shield" was the one to get, and the most consistently praised were the "cleartop" and the early black plate RCAs.
Along the way, I also became interested in what the shield was doing,and, if this was necessary in a larger coulbe triode, why not in 12AX7's. Actually, I never got a consistent answer as to it's function, but everyone agreed that tube makers must have found out it wasn't necesssary and they could save costs by eliminating it- and voila!- the 6FQ7 was born. The thing is, the centre shield has no function is the tube socket is not wired to ground the shield and who knows how many designers used that possibility. Cross- electron splatter from triode plate to plate must nothave been a problem as it seems the shield was dropped after the 6CG7 was made for something like 10-15 years, until the mid -60's (?) the shieldess version came out, still called "6CG7"- for awhile.
Who knows what forces were at work, but it seems that soon the makers called the new, shieldless design the "6CG7/6FQ7" to demonstrate this is the same tube electrically, but not a straight 6CG7. Perhaps, there is some bureau of standards that required a separate designation when a tube is different enough. Possibly it's so the makers could keep it organised in their catalogues.
Apparently, the 6CG7 is a 6SN7 in a smaller bottle and socket. When they shrank the volume the plates occupy to the sort of 12BH7 size, they must have seen those big plates- with always higher voltages- moving closer together and added the shield as a safety measure. And look at the scale of the plates in a 6SN7 compared to a 9 pin like a 12AX7- that's a lot of surface area- 6SN7s seem to cram as much plate area as possible. I don't use teh 6SN7, but I have an old RCA and the plates are so absolutely pushing up to the top of the tube, they had to put the getter between the leads to the pins- under the lower mica- it's the only "bottom" getter tube I have. Similarly, the 6CG7 and 6FQ7 were made with sidegetters- the "cleartop" which allows the plates to be taller. It's possible the more compact 6CG7 was intended to have a lot of communications and/or communications use and the shield is a ruggedising/military feature.
An interesting type and I decided to go after what I think is the higher quality construction of the centre shield 6CG7 and I bought 12 "Cleartop" RCA, three of the black plate RCA, some black plate TungSol and surprisingly- 10 Hitachi black plate, square getter from 1961 clearly patterned after the RCA. Later, I intend to cull this down to three good replacement sets for the D115 (uses 4) and as these tubes appear to last in that amplifier for 15-20 years, three sets will see me through!
Cheers,
has the center shield ( 1978 date code ) , the other ( 1985 code ) doesn't. On these, one leg of the shield is most definitely connected to a pin; there is a discernable drop in the noise floor between the shielded, and unshielded tubes, in my Amp, all other factors remaining equal
grinagog,Yes, the 6CG7's shield is always wired to- I forget, perhaps it's PIN 9(?), and the question is whether the corresponding socket is wired to take that pin to ground. Since the shielded ones are quieter in your amplifier than the unshielded, the shield demonstrates a discernible sonic affect when accomodated- wired properly.
-Very good to know, as this is a key feature in choosing between the various 6CG7 and 6FQ7s. And it means shielding between plates that catches "overruns" off the plates does make the tube quieter.
Other shield tubes: I'm curious now how many other tubes had similar isolation between the pieces, as I came across a couple of tubes with shields when buying some NOS for a McIntosh MR67 tuner. I'm looking at a black plate, halo getter RCA 6GH8A and there is definitely a shield between the pentode and triode sections. The shield is unplated or nickel in contrast to the plates. An Amperex 6GH8A (made by LRT, France) is interesting as the shield pierces the upper mica and two triangular ears project up and support the getter. This shield is shiny also and is a very clever, very strong looking construction. 6AV6s have this kind of gothic-looking architecture as well. I like the way it locks thwe whole tube structure together. It must be quite rigid.
Well, I'm learning something this evening- those shields do work for a living. I wonder why the trade off in noise was thought worth the cost savings when going to the 6FQ7- was the 6FQ7 by then a mainly a TV or tabel radio tube with wider tolerances for noise?
Cheers,
Nope, I've heard no distortion or crosstalk in any of the 6FQ7 I've used. I can't imagine this being a real problem. The vast majority of twin triodes have no shields between them plates.
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