6CY7 Variants

[wrenchone]

I'm adopting the 6CY7 dual dissimilar triode for preamp/driver service in a few of my projects, and I've been busy collecting NOS tubes from various manufacturers. I've noticed that the hi-mu low power tube section looks pretty much the same for all the tubes I've collected so far (though some use bright vs. blackened metal for the plate). However, there are two distinct types of construction for the high power, low-mu section. One type uses a fairly tall closed plate structure with a pair of radiating fins on the sides, while the other style uses a pair of plates in open construction with the cathode and grid clearly visible in between, sort of like a mini version of the 6080 or 7233 triodes. I would think that the first type of construction might have more dissipation capability than the open construction, but I'm not sure, as I haven't had a chance to fire up any of these tubes just yet. Has anyone used the 6CY7 and noticed a difference in performance between the two types of construction?

BTW, there are also two variants I've seen for the power section of the 6GF7 dual triode - one is a closed boxy construction with a pair of radiating fins, while another variant uses a very thin, flat wide plate construction. I have pretty much the same questions regarding this tube.

[tubelab.com]

I have not encountered the 6CY7 yet. I have not seen the open (6080 like) structure yet in any vertical output tube but I am sure that I have some somewhere, I will look out for them.

I have seen the two other structures that you mention in 6EW7's and 6EM7/6EA7's. When operated in excess of the ratings (who would do that) the flat variety gets a bright orange spot in the center of the plate. The finned variety has a dim glow that is only visible in a dark room. Experiments were caried out on 6EM7's operating as a 2 watt SE amp. I don't remember the exact power level, but I went through a lot of tubes (20 to 25) and the trend was pretty consistent.

[wrenchone]

Nice to know the difference in performance between the slim vs. chunky plate 6EM7s. I've seen the same variation in plate structure for these as with the 6GF7 - no mystery, really. Do you recall what sort of excess power dissipation you applied to these tubes (approximately) to get them to glow? The spec sheets say the big triodes are rated for 10W - I find this difficult to believe. However I have a bunch of 6AQ5/6005 tubes that supposedly can dissipate 12W, and their plates look pretty small, too.

[Miles Prower]

Originally posted by wrenchone
The spec sheets say the big triodes are rated for 10W - I find this difficult to believe. However I have a bunch of 6AQ5/6005 tubes that supposedly can dissipate 12W, and their plates look pretty small, too.

Since this was intended for use as a TV vertical deflection PA, the rating is probably on the conservative side since this was a rather demanding application. You can probably push that spec a bit in audio useage.

[tubelab.com]

I don't remember the exact test conditions of the 6EM7's (and I wasn't smart enough to write it down. I can guess that it was in the 12 to 15 watt range.

However I have a bunch of 6AQ5/6005 tubes that supposedly can dissipate 12W, and their plates look pretty small, too.

Now this is an experiment that I can remember. I have several hundred of these including some with severely corroded pins that would not be reliable in an amp. What do you do with them, you "test" them of course. I have cranked a bunch of 6AQ5 tubes of various makes and none of them glowed at 12 watts. Some would glow dimly at 14 and all were visible at 16 watts. I cranked one poor tube up to 30 watts, where it was glowing brightly. It was running triode SE where it made over 4 watts at 5% distortion, for a while. Within minutes the power started to drop, and the distortion rose. After 10 minutes the glass would melt solder. Just over an hour the glass started to deform and then one side caved in and the tube sucked air. I made no adjustments during the "test" and at 1 hour I was still getting 3 watts at 12% distortion. These are tough little tubes. The GE 6005's took the most abuse. Again I don't remember the exact voltage and currents used. I have pictures somewhere, I think they are on my old computer.

I already have a textbook P-P amp using 4 6AQ5's and $17 Edcor transformers. It makes an easy 10 watts and doesn't sound bad for a super cheap amp. When I get time I am going to triode wire it and see how it sounds.

[ilimzn]

Something I read recently reminded me that the size of the plate is not the only factor determining it's maximum power dissipation.
It is prudent to remember that ina tube, the only heat transfer mechanism is radiation to adjecent structures, which conduct the heat to other parts of the same structure, then radiate it further, in order to cool the tube.
Discounting extra cooling fins, and ehat conducted into tube pins, it is important to remember that the plate encloses all other dissipative elements, and most of the heat radiated from them, ends up in it. For most tubes, the next largest source of radiated heat is the cathode, and it is virtually completely enclosed by the plate.
hence, the plate dissipates the power generated on the plate, plus heater power, which can be quite a considerable portion of the total heat generated.
Apparently unproportionally enlarged plates are most often seen in sweep tubes, which usually require low saturation voltages and consequently large cathodes. Even taking into account conservative specs, these tubes often have seemingly overdimensioned plate structures for the declared maximum plate power dissipation.
It follows that plate size can be compared on more equal grounds if you actually look for proportionality of heater power plus declared plate dissipation, with plate size - all other things being equal.

[coresta]

I made the same test on an EL84 , with and without IERC shield / dissipator . In fact , the black "paint" faded and the IERC went grey dunno why

[wrenchone]

Coresta - Black anodizing like that used on the IERC heat sinks is simply a black dye used to color the porous aluminum oxide formed by the anodizing process. This dye will change color when heated up for a while. I doubt it has that much effect on the emissivity of the aluminum surface, though it certaily doesn't look as slick as the original black color.

The point raised by Ilimzn is interesting. This may mean that the open construction triodes for the 6CY7 may be as good or superior to the closed plate structure in dissipating power due to actual electron bombardment, because the open structure provides a better escape path for the cathode heat load. I can think of one way to find out.

BTW - Tubelab, I dub thee the tube Torquemada...