Hello from a noob

[hembrook]

Hi all

I am not new to tubes, but I am new to the Get*SET*Go amp.

I have some NOS Sylvania 6A5G tubes that I would like to use in a GSG. Sort of a 2A3/WE300 assembly on a octal 6.3V base. Any advice you have would be welcome.

I also am in need of a good 220V or better a 110/220V power trafo. I have already asked the Dynaclone guys if they can spin a dual winding on one of theirs, I'll see what they say. Else I may have a custom toroid wound.

Anyway, looking forward to the conversations.

Robert
Mannheim

[EWBrown]

Hi Robert

The 6A5G is essentially a 6B4G, with an indirectly heated cathode, which is also connected to the center of the filament, and then brought out on pin 8.

Rp = 800 ohms, gm = 5.25 mA/V, mu = 4.2

http://www.mif.pg.gda.pl/homepages/fran ... 6/6A5G.pdf

(It's in French language, but is easily translatable)

It appears that the best application would be to apply the 6.3VDC to the filament, and don't use the 47 and 51 ohm "hum balancing" resistors, and then connect the cathode resistor directly to the cathode at pin 8.

With a little creative liberty (and keeping the 47 and 51 ohm resistors) the GSG could be made to be 6A5G or 6B4G "plug-and-play" convertable.

Trafomatic, in Serbia, produces an ST-35 compatible ST-35 potted and shielded toriod power transformer, which has dual primaries, in order to accomodate 110-120 or 220-240VAC, 50-60 Hz operation. These occasionally show up on e-bay, or they could be ordered directly from the factory.

HTH

FWIW, the 6A5G looks like a good candidate for use with the "Ike" board, similar to the 6B4G version. Pinout appears to be fully compatible, so no etch-chopping should be necessary.

/ed B in NH

[hembrook]

Thanks for the details.

I have both English and French data sheets on the tube, and I agree that it is really similar to a 6B4G with an indirectly heated cathode. Seems like a cinch for a Get*SET*Go. I really think it is the same internal structure as a *A3, just set up for 6.3v humbucking indirect heated filament. Many tubes seem to all be variations on a basic theme to meet a need. Apparently they used 6A5Gs for voltage regulation as well.

I have an octet of military (1943 vintage) Sylvanias, NOS, out of a packing crate that never got used. All matching from the same lot. I hope after 64 years they are not TOO gassy... ;-)

So, the Ike sounds like fun. What voltage would I be seeing on them and what power output p-p would I see? Might be nice for the office. I currently use my ST70 (modded by Pat Morford of Canada) for the office and it is WAY too much power, even with my relatively inefficient (but gorgeous sounding) Realistic LX-5s

Good tip about Trafo. I will ask them about OTs as well as PTs. 2500ohm primary for a 8 ohm load sound right for SET?

[EWBrown]

The B+ is around 330VDC, and actual plate voltage will be determined by the OPT's primary DCR and plate current.

With the James 6113 / 6115 OPTs and an estimated 56-57 mA plate current (51V across 900 ohms) the plate voltage should be around 305-310VDC, as the drop across the OPT primary will be around 20VDC.

The cathode / filament normally sits around 51VDC above ground reference, so the total plate to cathode voltage is approximately 250-260VDC, which is running the tube fairly conservatively, about 85-90% of its plate dissipation rating of 15Watts - which is conservative in its own right....

With these vintage tubes, it seems apparent that solid-state rectification is not a good idea, unless the power transformer has lower secondary voltage and / or the cathode resistance is increased to 1100 -1200 ohms.

With everything according to the GSG manual, and using the 6AX5G rectifier tube, then 900 ohms cathode resistance gives better performance, and should provide fully safe operating conditions for your 6A5Gs.

It should be possible to run the 6A5G filaments from 6.3VAC, and save on eight SS rectifiers, four caps and two WW resistors, as the cathode should reduce or eliminate the usual DHT AC filament hum problems. Shannon ?

They should still be good adter all these years, if they were gently stored and handled... A little blue glow inside the glass is perfectly normal for these kind of power triodes, either 60 years old vintage or new production Sovtek, J/J, etc.

The "Ike" would run a PP pair around 10-15 watts (check the 6A5G spec sheet). I think the Ike boards are no longer available, but the circuit could be duplicated using point to point wiring. The real name is "eiclone" and the basic schematic is here:

http://www.diytube.com/6B4G_Ike.pdf

The LX-5s are great little speakers, I did the "Black Dahlia" mod to my smaller LX-4s, (bypass the crossover caps to Solen "Fastcaps" and add an indctor for the woofer) and they sound even better.

LX-4 mods:

http://www.blackdahlia.com/tipindex/Tip__3/tip__3.html

Excerpted from the site:

In the end I incorporated three inexpensive mods that elevate the LX-4 sonic report card from a C to a B. I'll describe these for those of you out there who aren't intimidated by a soldering iron.

(1) 0.47 mH air coil in series with the woofer. Pop out the woofer and terminal cup. Pull off the woofer's clip-on connectors and set it aside. De-solder the woofer's yellow lead wire at the terminal cup. Solder the coil leads between the lead wire and terminal cup. Locate the coil under the padding material, which is inside the box.

(2) Bypass the stock cap with a 0.1 microfarad high-grade cap (eg, Kimber or MIT). Solder the bypass cap in parallel with the existing cap.

(3) Insert a small square of acoustic-grade foam into the enclosure directly under the woofer. Reconnect the woofer's lead wires and re-screw the woofer and terminal cup

Some of the ceramic based tubes of the era did tend to get gassy, I have a NOS pair of WE 276As, which start flashing over around 450VDC, so they are not good for much beyond serving as ornaments, lamp bases or possibly the foundation for home made plasma globes Yellow_Light_Colorz_PDT_04

I tried all the tried and true techniques to "burn out" the gas to no avail.

/ed B in NH

[hembrook]

Ed:

Thanks again for the input. Looks like I will get a pair of boards and use the tubes as a pair of SETs for now. The idea of the EICLONE makes sense, but I have scads of USSR vintage 6L6 and others available for push pull, so maybe I will save the Syvanias as spares. It will be cool to light them off on their 100th birthday in 2043 (if I live so long)

OT, I do have 10 6Y6GT tubes (at a couple bucks a pop 1983 vintage GE JAN) that I am considering making a 5-channel amp (using 6SN7 drivers, similar to Bob Danielak's car amp (RIP, Bob, you were a good dude) but 5 channels which would be run thru a set of 2 Voigt pipe single driver HT speakers (R, L) and 3 matching single driver box speakers (RS, LS, CC) for my home theater setup (along with a SS 200 watt Sub, based upon Parts Express's model). Should be good for 15-20 watts P-P which with 97dB speakers would meet THX standards of 120 watts into 85dB speakers. The only rub with 6Y6 is the 1.5A 6.3V heater current times 10, which along with the 5x 6SN7 mans over 16 amps for the 6.3 V winding on the trafo. Woof.

[TomMcNally]

??? Did something happen to Bob Danielak ?
Are you confusing him with Fred Nachbaur who
passed away a few years ago ?

[hembrook]

??? Did something happen to Bob Danielak ?
Are you confusing him with Fred Nachbaur who
passed away a few years ago ?

Perhaps I m wrong, but I thought he passed away frm cancer a couple years ago. His website http://www.geocities.com/TimesSquare/1965/links.html has not been updated in a long time. I hope I am wrong.

[TomMcNally]

I believe you mean Fred Nachbaur of 6EM7 MiniBlok and
other amp fame. One of the Darling builders just emailed
with Bob Danielak a few months ago ... I think he just got
bored of it all. Look at my website ... most of it is from
1999 and I'm not dead yet !

[hembrook]

Not heard of the other gentleman that you mentioned. Anyway, back to our regularly scheduled program on the 6A5G!

Thanks to all for sharing your knowledge.

Robert

[Shannon Parks]

Man, Hallowe'en waylaid me this week!

Robert, Ed is on the money. I would leave off R11-R14 and attach the cathode CT pins direct to TP1 and TP2. Should would like a charm. I'll guess that maybe there might be a slight greater amount of hum, though - maybe 200uV? The offset resistors in my method (ie the ones at R11-R14) additionally null some residual B+ hum I think. Just swagging.

Too bad there aren't stockpiles of these tubes for cheap.

[EWBrown]

I noticed that in my GSG, there was a little minor bit of hum in one channel, not-so-coincidentally the same channel where the choke is mounted under the chassis and fairly close to the bottom of its James OPT. The hum is minor enough to ignore, so I doubt that I'll take any preventative measures at this point.

I'm not sure of the orientation of the James' cores, but I SWAG that the choke is magnetically coupling directly into the OPT.

/ed B in NH

Part II, The Sequel: Saturday the 14th...

Now for my crazy idea #469 Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_02

Given that the 6B4G (or 6A5G) is essentially pin-compatible with the standard 6L6 / EL34 pinout, and that the James OPTs already have a UL screen grid tap, the following little bit of evil hallowe'en thought occured to me:

Wire the SG connection from pin 4 of the tube socket to the OPT's UL/SG tap, and add a 1.2K WW resistor, for each channel, which could be switched in parallel across the existing 1100 / 900 ohm cathode resistors, in order to accomodate the 6CA7s. Use the plug-in SS rectifier to replace the 6AX5, and presto! you have a 5 to 6 watt UL SEP amp! This way, it is simply a matter of swapping the rectifiers and flipping a switch, to use either triode or pentode modes!

If this seems a tad complex, just use a 6V6GT instead of EL34, stay with the 6AX5 rectifier, and adjust the switched-in extra parallel cathode resistance as required to bring it down to around 330 to 360 ohms (12-13V @ 35-40 mA)..

The EL34 is happier with a 3.5K plate load, and the 6V6 with 5K, YMMV, etc...

Several years ago, I built up an SEP / UL EL34 amp, using James 6115 OPTs and a 12AX7 driver (close enough to 6SL7) and it sounds pretty darn good, and with no NFB loop needed! The power trannie was the "standard" Hammond 272FX, 300-0-300 @ 150 mA, and using SS rectification. B+ in that amp is around 365VDC. The EL34s are cathode biased for around 50 mA, the cathode resistors were either 500 or 560 ohms (I'll have to check under the hood to verify which).

/ed B in NH

[TomMcNally]

Jeez Ed - I feel like a dork leaving a one line reply,
but I wonder if rotating the round James OPT would
help ...

... tom

[hembrook]

This is a hobby, so parts count for me is not very relevant, so I don't mind doing the DC on the heaters. DC + humbucker filament = really really small hum! Whee!

Good suggestions tho.

How small a format can this be squeezed into? I have a nice tin from Reims that held cookies I am eyeing. It might be kinda small to get all the iron in or on. About 6x10 inches, 2 1/2" thick. I can strengthen the chassis with strategic inserts of additional steel to stiffen up the lid. Ideally I would fly the board and sockets from standoffs from below, so I can just pop out the tubes and open the lid. We shall see. I could always get a REALLY flat toroid for the power tranny and toss it into the can or hang it under with tall legs to make up for things. Its a thought. Naturally I will vent it a bit to help with the heat load from the resistors inside.

I am currently putting my K501 kit into a chocolate tin I got at the 2006 Torino Olympics. I see a theme here... Travel, Sweets, Tubes.. Yummy!

Loads of fun ideas!

[EWBrown]

Hi Tom, great idea!

I'll try that (why don't I ever think of the easy cures first) Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_16 .

Currently, they are oriented so that the label reads "correctly" from the front, with the primary in front, and secondary to the rear. I think I have enough lead "slack" I so can rotate them +/- 90 degrees. The choke core is oriented vertically, and still perpendicular to the power trannie (which could possibly couple into rotated OPTs? )

-------------------

FWIW, the PC board is (IIRC) 6.5 inches X 5 inches, figure in enough room for the OPTs, and 6A5Gs, the choke and power trannie can go under the chassis, hang off the back, etc... Definitely some additional stiffening brackets will be neded with a cookie tin.

----------------

I'm still running SET CAD scenarios for using 6BL7GTAs in this design, with a 5K primary OPT. . As y'all can see, I never lleave anything as originally designed Yellow_Light_Colorz_PDT_07

Getting the max PD down, and still keeping a reasonable output level and distortion %age.

/ed