6U8A... I have a bunch... any ideas?!?

[tarzanalog]

Anyone aware of any simple amp circuits designed around these 9-pin triode/pentode tubes?

I also have a lot of loctal pentodes and double triodes, FWIW.

EDIT: The only thing I could find was the Vox AC-10 which used one... looks like as part of the Tremolo circuit?

http://www.freeinfosociety.com/electron ... oxac10.pdf

[tunghaichuan]

Anyone aware of any simple amp circuits designed around these 9-pin triode/pentode tubes?

I also have a lot of loctal pentodes and double triodes, FWIW.

EDIT: The only thing I could find was the Vox AC-10 which used one... looks like as part of the Tremolo circuit?

http://www.freeinfosociety.com/electron ... oxac10.pdf

Since no one posted, why not try using the pentode section as a preamp?. Use the triode section to drive a TMB tone stack. Phil pointed out that a similar tube (6AN8) has a 5K anode resistance for the triode section. The 6U8 says 5K as well. You could drive the tone stack with the anode of the triode section.

Add two 6BM8s or 6GW8s and you have a 10-13w push pull amp with a full tone stack using only three tubes.

I have a bunch of 6GH8, 6U8, and 6BL8 tubes I'm going to try this with.

tung

[briane]

the sunn 100s (nov 1967, no jfet version), on schematic heaven uses 6an8 for a gain stage + PI. I have built this and it works well, should be similar to a 6au8 in design, probably need a couple of tweaks.

I have a design for a 6an8 pre (pentode) + cathode follower(triode), but I have not tested it yet. I'll probably get it drawn up first and post it here so as to catch any issues. maybe next week or so.

like tung said, might be better to drive the tone stack from the triode. not decided as yet myself.

[tarzanalog]

Add two 6BM8s or 6GW8s and you have a 10-13w push pull amp with a full tone stack using only three tubes.

I have 4 of the 6GW8s on hand so that sounds like a great idea... thanks!

I'll probably get it drawn up first and post it here so as to catch any issues. maybe next week or so.

Please do. Thanks!

Q: So using a 6U8A and two 6GW8s what specs should I be looking at for the iron? BTW, I'm also thinking SS rect. at this point to keep the build size as small as possible. Probably try to use a '67 10" Vox Bulldog speaker (8 ohm) that I have lying around. Just wish I knew the wattage... don't want to blow it.

[zaphod_phil]

I just bought myself a couple of NOS 6U8s for $5 each.

All these triode-pentodes are great tubes - 6U8s, 6GH8s, 6BR8s, 6AN8s, ECF82s. You could also consider the 7199, although it's triode has less gain and a higher anode impedance (Ra).

The easiest thing is to use them anywhere you might want to use an EF86. Treat the pentode half exactly like an EF86 and use the triode side as a built-in cathode follower, with the triode's grid directly connected to the pentode's anode. This makes it much easier for the pentode to work well with the other circuitry that comes after it, and in many cases should help improve tone and definition.

Or take a simple two stage preamp, like in the AX84 P1, the TMB SuperLite or VJr. You want to give it some pentode tone? So use one of these tubes, with the pentode as the the first stage, connected like it's an EF86. Then the triode side is the second stage, exactly like before, and the triode's low impedance will drive any tone stack a lot better than a 12AX7.

Did I say, these are real useful tubes!

Q: So using a 6U8A and two 6GW8s what specs should I be looking at for the iron? BTW, I'm also thinking SS rect. at this point to keep the build size as small as possible.

You use regular 18W iron, although for SS you should probably use a lower voltage PT. It's probably best not to put more than 320V on them - then build the PI power amp exactly like a Lite IIb, with a 220 ohm cathode resistor.
ECL86/6GW8s are basically the same as a single 12AX7 triode and a mini EL84 in the one bottle. The power tube can be treated and sounds exactly like an EL84, just you need to reduce its power dissipation inline with its 9W max dissipation rating. I've built a Lite IIb with these tubes and it rocks.

[bigsbybender]

Probably try to use a '67 10" Vox Bulldog speaker (8 ohm) that I have lying around. Just wish I knew the wattage... don't want to blow it.

Likely 10-15wRMS

[tarzanalog]

Z_P, VERY helpful... as always!

Bigsby, These are the Oxford AlNiCo 10-inchers that went into the late 60's stuff after Thomas Organ took over. (Tube and/or SS combos mostly.) I'm having a REALLY hard time finding out anything about them. My guess is 15W, too... but better safe than sorry.

[zaphod_phil]

Oh, yes, one more thing - the Vox AC10 uses these tubes as well. ECF82s and 6U8s are exactly the same tube.

[bigsbybender]

These are the Oxford AlNiCo 10-inchers that went into the late 60's stuff after Thomas Organ took over. (Tube and/or SS combos mostly.) I'm having a REALLY hard time finding out anything about them. My guess is 15W, too... but better safe than sorry.

Got speaker codes?


I got an 8" AlNiCo Bulldog speaker from that same era in a Thomas Organ Vox Tube Pathfinder...a decent sounding speaker for it's size.....but not one I'd trust plugged into a super reverb.

[tarzanalog]

Mine are recones that Guitar Oasis out west did in the 80's. I spoke with MY recone guy and he said that they could probably handle 25W/ea. He said they used these speakers alone in several 17W tube combos and also in the 32W SS Berkley II 2-10. I have 3 of 4 speakers that came out of a SS Scorpion 4-10 that was rated at 60W(120W Peak).