6X8 ?

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6X8 ?

Postby Vince » Sun Jul 29, 2007 1:03 pm

I have inherited a bunch of these tubes from an organ. Do they have any use
in amp or preamp building? Thanks, Vince
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Postby EWBrown » Mon Jul 30, 2007 7:41 am

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
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Postby Tom Bavis » Mon Jul 30, 2007 8:51 am

Only Fred Nachbaur would see the use of these... http://www.dogstar.dantimax.dk/tubestuf/paraph.htm

Clever!
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Postby Vince » Mon Jul 30, 2007 10:54 am

Thanks Tom.
He's a man after my own heart!!! Except he knows what he is doing!!
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Postby EWBrown » Mon Jul 30, 2007 12:46 pm

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.

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6x8 as preamp tube

Postby quetz » Sat Apr 03, 2010 11:36 pm

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.
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Postby Vince » Sun Apr 04, 2010 11:21 am

Wonderful!! Could you please post the schematic? I certainly would appreciate it!!! Vince
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Postby Impmon » Tue Apr 06, 2010 12:54 am

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.
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Postby soundbrigade » Sun Apr 11, 2010 8:26 am

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+).
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Postby Impmon » Sun Apr 11, 2010 7:12 pm

^^^^

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.
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Postby soundbrigade » Mon Apr 12, 2010 1:22 am

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.
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Postby EWBrown » Mon Apr 12, 2010 10:08 am

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 .

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Postby soundbrigade » Mon Apr 12, 2010 10:10 am

True McGyver
.... we need more of that kind.
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