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6X8 ?
Posted:
Sun Jul 29, 2007 1:03 pm
by Vince
I have inherited a bunch of these tubes from an organ. Do they have any use
in amp or preamp building? Thanks, Vince
Posted:
Mon Jul 30, 2007 7:41 am
by EWBrown
This is a triode-pentode combination, with a shared cathode.
It was intended as an RF local oscillator and pentode mixer.
THe shared cathode connection is the real "zinger" here...
http://frank.pocnet.net/sheets/093/6/6X8.pdf
It may have some very limited usefulness, using just the triode section.
It basically rates as a "plinker"...
AS Gary K says, no tube is totally useless, the filament can always be used as a low current fuse Yellow_Light_Colorz_PDT_01
/ed B in NH
Posted:
Mon Jul 30, 2007 8:51 am
by Tom Bavis
Only Fred Nachbaur would see the use of these...
http://www.dogstar.dantimax.dk/tubestuf/paraph.htm
Clever!
Posted:
Mon Jul 30, 2007 10:54 am
by Vince
Thanks Tom.
He's a man after my own heart!!! Except he knows what he is doing!!
Vince
Posted:
Mon Jul 30, 2007 12:46 pm
by EWBrown
I also see that he has used a voltage quadrupler for the B+ voltage.
(I'll post THAT over in the "voltage multipliers" topic).
The man was a genius, in the use of "plinkers" in decent amp designs.
/ed B in NH
6x8 as preamp tube
Posted:
Sat Apr 03, 2010 11:36 pm
by quetz
I say this with some trepidation, but I beg to differ. I concocted a 6x8 driver for triode strapped 6as5s, and I am very please with the sound.
Posted:
Sun Apr 04, 2010 11:21 am
by Vince
Wonderful!! Could you please post the schematic? I certainly would appreciate it!!! Vince
Posted:
Tue Apr 06, 2010 12:54 am
by Impmon
6X8s are useful for a couple of reasons: if you make a pseudotriode of the pentode section, the characteristics are quite similar to the triode section. This might prove useful as a differential amp. I also find them very useful as voltage regulator error amps. The common cathode connection and a high gain pentode are just what I want.
Posted:
Sun Apr 11, 2010 8:26 am
by soundbrigade
I have built Nachbaur's PP-amp with 6AQ5 and 6X8. One of my best amps, actually.
Would an (almost) direct swap to 6BQ5 work? (Increasing B+).
Posted:
Sun Apr 11, 2010 7:12 pm
by Impmon
^^^^
It looks like the 6BQ5 is close enough in characteristics to the 6AQ5 for this to work. The peak grid voltage is quite close, though the 6BQ5 likes a somewhat heavier plate load (8K P-2-P v. the 10K P-2-P of the 6AQ5). A lighter load would give less power and less THD.
Posted:
Mon Apr 12, 2010 1:22 am
by soundbrigade
Just before uploading som pics on the workshop theme, couldn't a tube like 6X8 be used in various configs, like mu-followers an such?
Also I have a faint memory that I have seen this tube in the Super Triode Connection amps, where the pentode serves as a CCS and the triode as the feedback element for the OP tubes.
Posted:
Mon Apr 12, 2010 10:08 am
by EWBrown
I forgot about Fred's unique application for the 6X8 here... Sometimes I believe that he could have created a great-sounding amp from a roll of duct tape, seven fish hooks and a string of Christmas tree lights. He was the True McGyver of the Tube Amplifier World.
The 6BQ5s should work well with Fred's 6X8 driver, just increase the B+ to about 325VDC, in order to get the full "benefits" of the upgraded power tubes. (more power output is always a good thing)
I wouldn't use the ST-35's "standatrd" B+ voltages in this application (360 to 380VDC), the 325V B+, along with cahhode biasing (use a shared 150 ohm , 5W resistor, ot separate 300-330 ohm 2W cathode resistors, bypassed with 100-220 uF / 35V) should deliver around 14-WPC, with a 6.6K to 8K A-A OPT. Or use 6GK6s, ( or 10GK6, 16GK6, 29GK6, etc) for a true "Plinkertron" design.
With fixed biasing, set the current for each tube to no more than 40 mA, else the PD will be close to the upper safe dissipation limits .
/ed B
Posted:
Mon Apr 12, 2010 10:10 am
by soundbrigade
True McGyver
.... we need more of that kind.