DHT Christmas: My First SET

[EWBrown]

Thanks,

I'm just trying to figure out better means of applying DC filament power, than I have been normally using, including using switcher bricks. I kinda figured that combining tubes and SMPS bricks would be some form of heresy, and that the thermionic spirits would be displeased, but I found that even some of the "purists" who turn up their noses at any tube that isn't an original Western Electric, happily use switchers to feed their filaments.

Going one step further, I did run across a 300B SET design that got around the AC hum problem entirely, by using rechargeable batteries for filaments and B+ voltages. An elegant and expensive solution, to be sure :o

I probably have enough Li-Ion cells (recovered from discarded laptop battery packs) around to make one heluva B+ source, but being able to reccharge and maintain them properly would be a major problem, as they can't just be strung in series like NiCDs and charged up at high voltages. Each cell needs to be individually pampered, nurtured and fed Yellow_Light_Colorz_PDT_02


/ed B in NH

[EWBrown]

Last night (nothing better to do) I triaged, tested, "curved" and matched up about 20 6AV5-GAs that I had in the tube stash, including five NOS Tung-Sols in their somewhat moldy boxes..

I also tried this with them "triode-strapped" and found that the characteristics and behavior came VERY close to that of 6B4G and 2A3. So, these should work quite well in a budget pesudo-SET design. Yellow_Light_Colorz_PDT_11

I did a quick web seartch for "6AV5-GA" but didn't find any amp designs or schematics. They're out there, just well hidden, I guess...

The other tubes (only two) that I also "processed" were 6CK4s, one NOS Tung-Sol, one NOS Sylvania, they measured out very close to one another.

They're quite different from triode 6AV5-GAs, come somewhere near the behavior of the power section of a 6EM7. I'll post short details later.

Next time around: 6S4As, These also look to be promising little SET or PP amp fodder... I've seen and heard Gary K's 6S4 SET design, it sounds very good, but I wasnt something with a little more oomph, so parallel or PP will be the way to go with these.

/ed B in NH

[EWBrown]

IIRC, the Bottlehead Paramount 2A3 SETs run around 16dB overall gain, so yours are pretty durn close. The BH's use half a 12AT7 as the VA tube, and "parallel feed" output, a 30H plate choke for the DC load and AC coupled 4K-to-8 ohm OPT which doesn't have to deal with any DC at all, so an air-gap is not necessary, and HV insulatiing properties are less of an issue..

2A3 and 6B4G have nearly identical active characteristics, other than basing and filament voltage.

FWIW, a triode-strapped 6AV5G (1K, 1W between G2 and plate) acts very similarly to a 6B4G, with the addition of an iindirectly heated cathode.


/ed B in NH

[Shannon Parks]

FWIW, a triode-strapped 6AV5G (1K, 1W between G2 and plate) acts very similarly to a 6B4G, with the addition of an iindirectly heated cathode.

I definitely need to get me a pair of 6AV5s to test in the Getsetgo. Seems pretty fun if we can get clone triode performance out of this junk box tube!

[EWBrown]

I can easily send you some NOS NIMB (New In Moldy Boxes) 6AV5Gs in the next "care package'... got LOTS of 'em... Yellow_Light_Colorz_PDT_03

/ed

[Shannon Parks]

I can easily send you some NOS NIMB (New In Moldy Boxes) 6AV5Gs in the next "care package'... got LOTS of 'em... Yellow_Light_Colorz_PDT_03

/ed

Cool - I'll trade you a new Getsetgo board for some once they get in!

[erichayes]

Hi All,

I'll be damned if I can find it now, but I read a long time ago (college text book?) about operating DH tubes designed for AC on DC and vice versa. DC filaments are beefed up physically on the negative side to keep them from wearing out (opening up) prematurely from depletion. They have practically no thorium in them, so they heat up quicker and are more suitable to a portable environment that would snap a thoriated filament. They also hum like Hell when run on AC because the tungsten can follow the AC wave better than a thoriated filament can.

DH tubes designed for AC use, however, are thoriated, which explains (A) why the filament's not polarized, and (B) why they take a little longer to warm up than does a DC filament. The thorium acts as a thermal shock absorber to damp out the heat/cool cycle caused by the AC. It also means there's no need to reenforce one side of the filament, as both sides are seeing equal duty cycles.

On DC, though, the end connected to the negative side of the supply is going to deteriorate more rapidly than the positive end, and premature tube failure will result.

I inagine that, whenever this was written (Terman?), tube life was more important than a little hum . . . although that PP 45 guitar amp I restored was awfully quiet. I would suggest possibly a cross-wired DPDT switch going from the filament supply to the DH tubes that could be flipped every 20 or 100 hours of operation to even out the wear might be in order.

[Shannon Parks]

I would suggest possibly a cross-wired DPDT switch going from the filament supply to the DH tubes that could be flipped every 20 or 100 hours of operation to even out the wear might be in order.

How about this, Eric. Currently I have one channel connecting the + DC at pin 2 and the other channel connecting - DC at pin 2. The power tubes between channels could be swapped to alleviate this issue; a nice free, low tech solution.

[TomMcNally]

I think a more high tech approach would be suitable ...
I came up with this ...

[erichayes]

Hi All,

My kind of solution, Shannon, in spades. Don't throw a horse trough into the circuit if one's not necessary.

[Shannon Parks]

BTW, Eric, in related news I think Tom and I have found a decent ceramic PCB octal socket. It has U-shaped pin grabbers (like a serpents forked tongue) instead of the hit-n-miss circular style that has been around. Haven't tested mine yet, but they look to be a leap forward.

[EWBrown]

The ultimate solution, is to use the automatic DC filament polarity reverser, but then add rectification and filtering on the secondary side of the transformer, and then wire in a cross-wired DPDT latching relay which then automatically reverses the polarity each time the unit is powered up.
Of course, each tube should have its own automatic polarity reversing circuit...

Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_04 Yellow_Light_Colorz_PDT_05 Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_16 Yellow_Light_Colorz_PDT_08

/ed B in NH

PS:

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EWBrown wrote:
I can easily send you some NOS NIMB (New In Moldy Boxes) 6AV5Gs in the next "care package'... got LOTS of 'em...

/ed


Cool - I'll trade you a new Getsetgo board for some once they get in!

You have a quad of NOS NIMB Tung Sol 6AV5GAs coming your way