What to do with some WE tubes and others

[JPeitzman]

I have been going though about 3 boxes of tubes that are mine/my dads, taking inventory. About 90% of them are NOS RCA, and the rest are mixed NOS tubes from 1940 - 1972 or so. Just a teaser:

4x RCA 807 ( 13 total...6 Amperex, 2 Raytheon, and one Sylvania JAN 1625)
2x RCA 5U4GB ( plus 2 GE 5R4, 2 Magnavox 5U4GB, and a RCA 5U4G)
About 4 to 6 each of RCA 12AX7, 12AU7, 12AT7, and I think some AY and BX (I haven't finished inventorying that box yet))
A single RCA "Grey-Plate" 2A3 vintage 1954...I wish I had 2
4x RCA 6SL7GT
4 RCA 6SN7GTB (plus others such as Rauland, & Tung-Sol)
4xRCA 6L6
7xRCA 6J7 still in the display holder!
And a lone RCA 56...again

Just for a start...for real...and no nothing is for sale or anything. Oh, and I have 3x RCA 868's, if anyone knows what they are I will give them a smiley...and yes I know what they are.

I have a couple questions though regarding some projects I am thinking about. I am thinking about building a pair or 807 based mono-blocks. A pair of 807's, a 6SL7 input and a 6SN7 driver/splitter...sound good? I was thinking of a 12AX7 input instead, but I don't think it would matter. I plan on using some 5R4's or 5U4's for rectification. Either that or all the same but driving a 6L6 pair instead.

I was also thinking also about a 6L6 or 807 SE... thought it might be a nice project for sensitive speakers or for bi-amping. Using either a 6J7 or 6SN7 as a driver and input.

My big question though is I have 3 Western Electric 264's, 2 B's and one C. How would they sound in a preamp? If not is there anything else I could do with them besides display them with some of my other old WE, Cunningham, and RCA stuff? Same question but with the RCA 56, what can I do with it if I can get another one?

So... Any ideas on the 264's, RCA 56, or my 807/6L6 amp ideas?

Thanks for any advice or feedback, and thanks to all for such a great community resource!!

[Miles Prower]

Originally posted by JPeitzman
I have been going though about 3 boxes of tubes that are mine/my dads, taking inventory. About 90% of them are NOS RCA, and the rest are mixed NOS tubes from 1940 - 1972 or so. Just a teaser:

Look 'em up at Frank's. That should give you a good idea as to what's what.

I can tell you that 807s and 6L6s make excellent audio finals. Of course, these are really the same, the exception being that the 807 has a plate button up top that allows for both more convenient useage as an RF amp, and allows higher operating voltages. The latter not being much of a consideration for audio since the really good audio Q-Points are all at lower plate voltages.

"A pair of 807's, a 6SL7 input and a 6SN7 driver/splitter...sound good?"

I found that this combination didn't have quite enough open loop gain, and so I added a 6J5 stage operating with an unbypassed cathode bias resistor (also the summing node) to make up the difference.

"Same question but with the RCA 56, what can I do with it if I can get another one?"

Not much, I'm afraid. It's a low gain, small signal triode. Though it looks like it has some awesome characteristics, the u-factor is 13.5, and it has a funkey 2.5V/1.0A heater. Might be difficult to power that up. (Maybe regulated DC from the 6.3V heater xfmr?) Due to its limited gain, you may have difficulties finding a use for it.

868: Hollow state equivalent of a photodiode with a gas fill to increase sensitivity.

[JPeitzman]

Wow, that link is great, I have never seen it before, so much for just using google!

After looking at that too it seems like it might be the same with the 264B as with the RCA 56. Maybe I will just do some experimenting and see what happens, looks like the Western Electrics might just end up on the shelf though.

Thanks for the info, I have more 807's than I have 6L6's so I might start there. Though both are fairly cheap if I had to by some. I have also thought about KT88's (6550 ... w/e) or EL34's, these can be acquired for a reasonable price too. I have plenty of tubes for drivers. Would the 6J5 drive a 6L6/807 by itself, ie 6J5 + 807. I will do some looking into it as well, just curious though.

RCA 868! I didn't want it to come off like a "know-it-all" Just curious to see if anyone knew what it was Good Job...lol. I have them and most of the rest of the tubes because my dad used to service theaters and worked for RCA for a while. I have exciter lamps, reels of sound test film, lamp focuser's, 868's (and the SS replacements for them), gears from projectors, all that fun stuff. I also have a whole big book of theater amp schematics! I even have a RCA "Emergency Amp"; it uses 6L6's + 6N7's + 6J7. Very nice amp, but I have been doing some work on it. The caps are old and some of the wiring needs some touch up work.

Again thanks for the input!

[JPeitzman]

Sorry for the double post, but how would it sound to make it a SE boosted triode? It should be the same about the same as the triode mode, but with a higher power output right? Just a curiosity.

[Miles Prower]

Originally posted by JPeitzman
Thanks for the info, I have more 807's than I have 6L6's so I might start there. Though both are fairly cheap if I had to by some. I have also thought about KT88's (6550 ... w/e) or EL34's, these can be acquired for a reasonable price too. I have plenty of tubes for drivers. Would the 6J5 drive a 6L6/807 by itself, ie 6J5 + 807. I will do some looking into it as well, just curious though.

6J5 + 807 might work. The 6J5 can source enough current to charge up the 807's Ci + Cmiller + Cstray if you don't push it into grid current. All by itself, though, it doesn't have enough gain margin if you add gNFB, and you would want to do that since pentodes without it have a certain amount of nastiness, especially in SE. You will also want to add gNFB to tame woofer resonance that can make the bass sound too much like monotonic thumping. Better to do something like: 6SL7 --> 6J5 --> 807, where the 6J5 is run as a cathode follower driver. That will give enough open loop gain for the addition of gNFB without losing too much sensitivity. 6SL7s won't have any problem driving a 6J5 CF stage, and the 6J5 will take care of slew rate limiting at the high end.

Sorry for the double post, but how would it sound to make it a SE boosted triode? It should be the same about the same as the triode mode, but with a higher power output right? Just a curiosity.

Dunnow 'bout that. 807s work best in pentode mode since it's one of those types with a limited screen rating. That works out quite well for efficiency and low THD, but makes things a bit awkward so far as using them in UL or trioded.

[Jim McShane]

Originally posted by Miles Prower


"A pair of 807's, a 6SL7 input and a 6SN7 driver/splitter...sound good?"

I found that this combination didn't have quite enough open loop gain,

Yow! That setup with the 6SL7 didn't have enough gain?? How did you have it configured, I'm really curious...

[tubelab.com]

Sorry for the double post, but how would it sound to make it a SE boosted triode?

If you are referring to the "Boosted triode" article in EDN magazine, I tried it with a 6L6GC which is similar to the 807 with higher ratings. What I got was a melted screen grid. The article sounds feasable, but after I reread it, it seems that the concept only exists in a spice simulator. There is no mention of anyone actually building such an amp. 6L6's and some vendors 807's have a wide variability in the grid alignment. Some may work, but mine (a Chinese Shuguang) blew up after about an hour of operation with the screen grid 100 volts more positive than the plate.

[kevinkr]

56 is an excellent choice for use in a transformer coupled line stage and that is where I would use it.

I'll second Jim's comment on the loop gain issue with 6SL7/6SN7 - I spent a great deal of time degenerating the LTP stage gains so as to avoid having too much gain in my last 300B PP amplifier design which was designed to run without global feedback. (300B PP article on my site)

[Miles Prower]

Originally posted by Jim McShane
Yow! That setup with the 6SL7 didn't have enough gain?? How did you have it configured, I'm really curious...

A 6SL7 all by itself with a gain of 52 sounds like a lot. However, connect them in an LTP and you lose half of that for a gain of 26. So you have the LTP, the cathode follower, and the 807s: (26)(0.94)(13.1)= 320.2. Still seems good enough, right? But then, add in the OPT voltage drop, and that gain becomes more like 22.4, or 27db(v). Add some 12db(v) of NFB, and you have a gain of 5.6. Sensitivity goes straight to hades. So that's why the setup there doesn't have enough open loop gain. The fix was easy enough: a 6J5 with an unbypassed cathode resistor with an additional gain of 10, and use the cathode for the gNFB summing node.

[arnoldc]

since you only have one 56 and one 2A3, i would suggest a mono amp. 56 -> 6J5 CF -> 2A3.

if you have a turntable with a mono cartridge, that would be one good mono circuit