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Any thoughts on using 6GK6s (or the cheap as dirt 10GK6s or 16GK6s)
in place of EL84s? Other than a slightly different pinout, the tube characteristics are fairly close to 6BQ5s.

G2 connects to pin 8, and K and G3 are separately connected to pins 3 and 1/9 respectively.

6CW5/EL86s and their 10V and 15V (PL84) filament brethren have the same pinout as EL84s, but their ratings are somewhat less robust, they might not be too happy with 360VDC on the plate and G2.

/ed B in NH (always looking for a cheaper way out...)

YES! I tried 6GK6's in a modified Heathkit AA-151 a few years back, and they worked great. Would be nice for a SCA-35 on a budget.

Ed,

I never got around to testing these triode strapped, but they would be perfect for that. FWIW, pins 1, 6 & 8 aren't tied to anything on the ST35 board.

Shannon

I have a Heath AA-151 that was oiriginally destined to be an iron-donor, but it was in nice enough shape to save it from the surgeon's "knife". Should be relatively easy to try some 6GK6s in it. THe original 6BQ5s were a tad weak, and I installed a home-matched 6P14P-EV quad into it recently.

Also have some original Dynaco ST-35 and SCA-35 restoration projects in the pipeline. New boards, and upgraded power supplies for all.

/ed

I haven't looked this over for logic yet - hey, I gotta drink a couple cups (pots?) of coffee first. But Donald from up North sent this - a possible no switch mod for 6GK6-6BQ5 operation:



Shannon

I've glommed onto 6GK6s, 10 GK6s and 16GK6s for dirt cheap at local flea markets and hamfests, often as low as 25 cents each.

6CW5s (EL86s) and 15CW5s (PL84s) are also available cheap.

/ed B in NH

Speaking of 16GK6's, since they have a 300ma current rating, I thought of trying 4 of them in a SCA-35, and add 2 - 25CT3 damper tubes for the rectifiers. If you connect all the above in series it equals to 114v. This way, you could stuff your amp with cheap tubes!

Is there any disadvantage to connect the heaters across the AC line?

TerrySmith wrote:Is there any disadvantage to connect the heaters across the AC line?

Hi Terry,

I think the DIY nature and/or metal chassis automatically eliminates using non-isolated AC mains. This is unfortunate for many reasons: cost, weight and less efficiency (heat). Sigh.

FWIW, I had a pair of 6922 tubes in a phono amp using 12VDC on the filament line - easy to do as it's a standard switcher voltage. One of the tubes went south and was only dropping 5V across the filament while the other dropped 7V (folks, just think of the the two filaments as two 21 ohm resistors in series). Plus, since some tubes have slower warm ups than others, I'd hate to damage a tube this way.

All that said, I'm sure it's been done a million times. Yellow_Light_Colorz_PDT_15

Shannon

This guy (Fred Nachbaur) uses this approach in an interesting little PPP 8 watt guitar amp, using fixed bias 35C5s, with heaters fed by the AC line.

http://www.dogstar.dantimax.dk/tubestuf/dv8index.htm

An interesting design approach, but it seems a tad "scarey" to me...

It "can" be done, but I wouldn't recommend it for the DIY35 design, for safety reasone, for hummmmm elimination, and for the fact that it would require quite a bit of "surgery" on the PC board filament power traces.

Another consideration with using "damper diode" type rectifiers is the cathode-to-filament voltage rating, this can be as low as 100V in some examples, like 6AU4. These should have their own filament transformer, and it should be "floated" at or near the B+ voltage present on the cathode(s).

/ed B in NH

I was'nt going to use series connected ?gk6's on Shannon's board, but try them on stock ST or SCA boards, or maybe a Heathkit AA-151.

I found in my plinker stash 5 nos 29GK6's, 4 adds up to 116v. And 29GK6's are CHEAP for NOS!

I would never try a direct line PS. I was going to use a PA774, and just have the filaments run off the A-C line, and maybe use a winding for a tube rectifier.

Another way around the line operated filament situation, is to get an isolation transformer, these can be found for cheap at hamfests, swapmeets, etc. This at least gets the "hot" line separated from the circuitry. If one can live with lower b+ voltages, then using a voltage doubler circuit will work fine, delivering around 300VDC to the tubes.
I've not run across 29GK6s, will have to keep a lookout for them at the upcoming flea market season.

I've found 50 and 100VA isolation and stepup/down 230/115 (3 separate 115/120V windings) trannies for a buck or two. A lot cheaper than new iron...

/ed B in NH

Speaking of EL86s, 6CW5s and PL84s, there is another "weirdo" in this family, the UL84 which would be a 45CW5, if there were such a thing.

45V, 100 mA filaments. No "standard" US equivalent. Yellow_Light_Colorz_PDT_11

The higher and "oddball" voltage filaments ar generally intended for series-string operation, where 6.3V and 12.6V tubes are generally used in "parallel" filament operation.

/ed B

I think I might have a couple of UL84's deep down in my plinker box. Let's see, two UL84, one 20EZ7 for VA-PI- looks like the ultimate plinkertron to me!

How about those Luxman amps that use 50CA10 compactrons, the filaments are connected in series across the AC mains. Works out great in Japan as they have 100v mains. Yellow_Light_Colorz_PDT_07