6sj7 Output at g2?

[NorCalTuna]

A friend of mine brought me a Rickenbacker p-p 6v6 that is from before the dawn of time- I think the 40s. I can't date it, and it's been a little hacked. He brought it to me not making any sound- it supposedly rocked- at all and I've been poking at it in my spare time. The OT had been smoking at some time- it's in the chassis nice and hot, so I popped in a Hammond 125d I had around. The amp fired up, made good bias, and made a reassuring hum and a little TINY amount of output.

I popped it back out and starting tracing voltages. When I reached the preamp tube (6sj7) g2, I had no voltage (300 on the plate) and didn't on the cathode either. Swap tube to a believed good, no difference. Turn off, discharge amp.

The resistor at g2 reads open, so it's likely bad, while the cathode resistor reads 1k (it's 820r) but here's my conundrum- The signal is coming from the g2 while the plate's capacitor is tied to ground. I don't know- and there is no schematic available (this thing doesn't have a model no! The closest I can find is an m11 which uses two twin triodes. "6sj7" is silkscreened on the amp though so I don't think it's a mod)- if the last tech who was in there (it's full of new caps n resistors) re-arranged the parts accidentally. Moving everything back to match, say, a 5c1 or ga-20 wouldn't be hard, but I don't wanna trip this thing's mojo if I'm just seeing things that came before my time. It seems all wrong and when I jumper another pre into the phase inverter, the thing roars to life.

In summary: Can the signal from a 6sj7 be pulled from it's g2?

I drew the local schematic and took the best pic I could; i apologize in advance for the quality. The red alligator is on pin 1 for reference, the yellow .1 is from the plate to gnd. Thanks!!!

[NorCalTuna]

Just an update-

I replaced the open resistor and it's functional, and seems to make full power- it's dirty! I'll take some measurements off of it tomorrow. Looking at the schematic again it's very close- all the parts are there- to a conventional circuit. I still can't shake the feeling it got hooked up funny but don't want to second guess somebody.

[mooreamps]

In summary: Can the signal from a 6sj7 be pulled from it's g2?


[ATTACH=CONFIG]13091[/ATTACH][ATTACH=CONFIG]13092[/ATTACH]

I'm going to say the answer is ; it depends.... and here is why...

The wires of the screen grid are supposed to shadow the wires of the control grid.. Thus, inside of a "perfect" tube, you would see very little screen current, and thus very little signal... Now, if you took a hot tube , and knocked it around some to get the screen wires more out-of-alignment, maybe you would see more screen current and more screen signal...

But generally speaking, the screen on this tube is rated for abouts 1/2 a watt, so it could be enough to drive another gain stage before hitting the P/I... As far as how this set-up would contribute to the "mojo", someone would have to cobble something up, and try it... Using as is, yes it would be a very low power set-up, assuming you could get any signal out of it, yes "depending" on the tube you're using.....

-g

[Stadler Guitars]

I know this is a little out of date, but I'm reading everything on this forum concerning 6SJ7s. I'm putting something together from scratch and it is requiring some study. This schematic would make a lot more sense if the .1 cap and .022 were switched. The .022 would go from the plate to the vol. leg to serve as the coupling cap. The .1 would go from the screen resistor to ground. It serves as a filter/supply cap in that position. I changed some caps on a Rickenbacker with push/pull triode output tubes years ago. It was the warmest, most clear sounding amp I've ever heard.

[Jazz P Bass]

Makes sense to me.
Thanks.

[Gingertube]

Just a note of caution.
Using G2 as the output of a pentode is something the HiFi boffins do reasonably often - usually with a 6AU6 but should work as well with a 6SJ7. It basically puts the tube into a low power triode mode (max power dissipation is then set by G2 dissipation limit rather than the anode dissipation limit). It is reputed to be extremely linear which may suggest that it is less likely to be used in a Git Amp. THis connection usually includes grounding the anode.
The 6AU6 Triode mode with output taken from G2 curves are published on the web - here:
http://www.audiophool.cjb.net/Misc/6...ed%20plate.pdf

Looking at your little schematic trace, it is far more likely that its a wiring fault - someone counted the pin numbers wrongly - post #4 above has it right.
Cheers,
Ian

[NorCalTuna]

Great thread dig up. FWIW, I think it's a mistake as well but the last guy who was in it was a great tech, I know him personally, so I don't think he f'ed it up, the wiring around it all looks "right" length wise and fit wise. Also, the amp sounds smashing, and is definitely from the time when gain and fuzz were enemies, so they may have been trying to build a tame amp? When you push it hard, it breaks up pretty well but it's clean tones are actually what stood out to me, but i do like unique tones soooo. Great info ginger thanks for letting me in on it!

[Gingertube]

No worries mate - as we say in Oz.
I'm new to the forum so am still browsing and "cherry picking" threads of interest. I'm not sure how I missed this great forum. I've been an active member at AudioAsylum,, DIYAudio, Powerscaling and DIYTube for years. Probably coz I've been concentrating on Tube HiFi design (Brag - My "Baby Huey" HiFi Amp design at DIYAudio has been built by more than 50 folk around the world).
Not sure if I got the edit in in time but I posted a link to the G2 Output Triode connection stuff.

Cheers,
Ian

[Stadler Guitars]

The grounded plate and screen as plate configuration is new to me. It seems like a waste of a pentode, with the glass enclosed dual triodes offering higher output possibilities with lower distortion. This schematic does not appear to be a mis-wired grounded plate, but who knows what the original schematic looked like? Also, the 6sj7 displays very linear response with a high supply voltage and large resistor in the screen circuit: http://www.triodeel.com/6sj7p3.gif

I have not experienced the audio difference between a pentode with a grounded plate and a true triode run with similar output levels. Gingertube, do you have a qualitative description of the difference? I would expect them to have unique characteristics.

Tom

[Gingertube]

Tom,
I haven't tried it myself. After re-reading the article it seems that it is intended to reduce hum and noise in low level audio signals - so probably only appropriate for the Input stage or a reverb recovery stage in a guitar amp.

Part of the noise reduction seemed to be from elimination of partition noise (the bugbear of all pentodes) BUT some was also attributed to the fact that for a 6AU6 at least, you end up with a grounded pin either side of the heater pins which reduces hum. That part might not apply for a 6SJ7.

Cheers,
Ian