Power Tube Options?

[Wmacky]

What are the chances of using non el-34 power tubes in a clone with Shannons board, SDS PS, and Triodes better Power iron? How about the KT66, KT88, and 350B. I've seen all these in modded 70's around the net. I have all the sudden grown fond of those fat bottles! Thoughts?

[dhuebert]

(bragging) MY tube guru has been repairing, designing and building tube equipment since 1939, and it is the considered opinion around the watercooler that 6L6 is an excellent upgrade for ST70. Generally, it is agreed that any beam power is better sounding for HiFi that any pentode. There, I said it! (run for cover)

Don

[EWBrown]

The other good replacement bottle is the new JJ KT77 (or original ones if you can even find them, much less afford them). 6L6GCs sound like a good thing to try. They're about 15 bux each, and well worth it.

Or go "retro" and add plate caps, and 6BG6GTs, they're basically 807s with octal sockets, or ballzy 6L6Gs with plate caps. Some socket rewiring will be required. This has been discussed earlier.

Or go for 807s, change the octal sox to 5 pinners, and add the plate caps.

KT66s like a higher OPT primary, around 6600 CT to be truly happy in their little tube nirvana Yellow_Light_Colorz_PDT_01

HTH

/ed B in NH

[erichayes]

Hi All,

As one who uses both power pentodes and beam power tubes in my designs, I'd like to think that I'm fairly unbiased on the subject. At one point, I was a huge proponent of the EL34, and still think it's a pretty good tube. But when I took the same amplifier and loaded it with (specifically, JJ) KT88s . . . well, I now have 12 NIB JJ EL34s that will probably find a home in a Marshall somewhere, sometime.

On the other hand, I've never heard a bad word spoken over 6BQ5s in a generic sense--some brands are better thon others for specific purposes, but none has ever driven me from a room (except pre-broken-in JJs. They are Godawful). In the early days of the 1773's design, I experimented with 6973s, which are beam power tubes with roughly the same specs as the 6BQ5. The results weren't even close. The 6BQ5s had an ethereal quality that made the speakers seem to disappear; the 6973s made it very clear that there were speakers in the room.

Diff'rent strokes . . .

[Wmacky]

Would anyone care to comment on the 350b in paticular. Is this a drop in? Would any componets "have" to be changed? Is the iron up to it? I know of one person running them with the VTA board. I'll be using the DIYTUBE driver.

And i'm still unclear if the KT88 will work without mods?

[erichayes]

Hi All,

The 350B falls somewhere between a 6V6 and a 6L6 performance-wise. Physically, it's a drop-in, but you'd want to twiddle with the bias; the 350B is a wee bit more sensitive. WE says it wants to look at a 5000Ω P-P load in push-pull, but that's always nominal. In short, it's close enough at least to do one channel and do some A-B comparisons.

The main considerations that need to be addressed with the KT88 or 6550 (and they are not the same tube, regardless of spec's) is increased B+ current capability and higher negative voltage in the bias supply. I like to have at least -75 volts available--doesn't mean I'll use it all--so I can bias the tubes to cut-off while I'm 'sperimentin' around. Much cheaper than a new pair of tubes.

As far as high voltage goes, the stock power tranny is definitely out. If I were going to use a new stock OTC power transformer, I'd probably go with the Hammond 278CX and a 166L6 or M6 to handle the excess heater requirements. Finally, a little 25.2 volt (at least) transformer such as Mouser's 41LK010 hooked to a voltage doubler to take care of the bias supply. Now ya gotta worry about the output iron . . .