diy 6n2p to 12ax7 adaptor for 6.3v heater taps only

[pinkjimiphoton]

so i been messing with russian analogs of more common tubes, and my new fav is the 6n2p. they're cheap, robust, plentiful, and have a really great presence in the midrange going on.

they're only for fender style 6.3volt heaters.... some 12ax7's sockets are wired up for 12 volts, the 12ax can do 6 or 12,  but the 6n2p can only do 6. but 6.3 is fine.

to start, go to tubedepot or whatever and get some socket savers, the kind that have a screw running thru them. check the pics.








unscrew the screw, and set the washer and screw and nut aside. i stand it up to unscrew it so the little nut just falls out. this is what the guts look like




here ya can see where i bent pin 5 across <with a slight arch in it to accomdate the screw>
and connected it to pin 9









here ya can see where i used my fiskars to nip away pin 9 from the socket where the tube will be installed. first i cut up by the phenolic as close as i can, then a little bit below where i soldered pin 5 to pin 9. the pin is stiff enough to support the soldered on post, make sure ya get a good joint before ya snip it.









pins stuck back thru the phenolic... note no pin 5. be careful with tube insertion, as effectively there's now two possible keyways on the tube. you could take a piece of medium paper clip and solder it to pin 4 if ya really need a pin 5. i just said the hell with it.




some peeps suggest leaving pin 9 connected to pin 8, the cathode, but then you're floating the shield above ground potential cuzza the cathode rc networks in most guitar amps. since its just a shield no need to even bother with it. when i did connect to the cathode i got more of a hum. you also don't want it connected to the heater supply, as it would be like an antennae i'd imagine.

anyways.. easy breezy, way easier than i thought it would be. here's the assembled socket sporting one of them beautiful tubes





and here's a crummy shot in action inside my pro reverb, first tube <dry channel>

so .... if ya been wondering, this is how ya do it. its easy, and the tubes are cheap and sound freekin great!

[pinkjimiphoton]

a bit more experimentation

you can indeed tie pin 9's stub to pin 8, and take advantage of the internal shield. just run a jumper and solder it in.

these tubes sound KILLER in the first stage of my fenders. devastating difference in midrange. that said...

they ONLY work well as 12ax7 analogs. for reverb or driver tubes, they will work, but not as well as say, a 12at7.

THAT said, it may be because i found these tubes to be exceptionally noisy....
UNTIL i played thru them for a couple minutes. then they became dead quiet. at idle they were making all kinds of horrible frying bacon noises, oscillation, what sounded like a flanger sweeping in the background even. all of this stopped after a couple bursts of power chords.

i postulate the noise is from a tiny bit of atmosphere still in the tube. slamming some audio into it seems to burn it off, then the tubes work and sound great.

all THAT said,
i could NOT get them to work well in cathode follower stages, and they did nothing but hum in my silkyn.

but in my princeton reverb and especially my pro reverb they are pretty freekin devastating.

and they work REAL well in a valvecaster circuit, tho they probably won't last super long in that application as the  6n2p is a 6 volt heater ONLY, but the valvecaster runs the heater at 12v.

i got one in mine cuz i am an insane, sick, bastard. it sounds great. i postulate the increased power to the heaters is supplying more power from the cathode to the plate... but expect it to halve the rated tube life <data sheet gives these tubes 10,000 hours or something!!! sick!!!>

it runs a LOT hotter. the tube actually glows in this application, and the enclosure hits around 120 degrees farenheit. but the increase in headroom is marked, and the distortion tone is a lot stronger.

do this at your own risk tho. i'm only responsible for ME blowing shit up.

[thermionix]

getter flash

[davent]

If you're interested in building an amp with them, people at Wattkins have done a lot of experimenting with them, built a number of different amps using them.

Can be a stress test to get past the gatekeeper...

http://www.wattkins.com/search/node/6n2p

dave

[Phoenix]

all THAT said,
i could NOT get them to work well in cathode follower stages, and they did nothing but hum in my silkyn.

That's because they have a max Vhk of +/-100V compared to the 12AX7's +/-180V.

[pinkjimiphoton]

getter flash

i figured that was what it was. at first i thought the tubes were bunk, but once ya play thru 'em a minute, they sound killer!

[pinkjimiphoton]

If you're interested in building an amp with them, people at Wattkins have done a lot of experimenting with them, built a number of different amps using them.

Can be a stress test to get past the gatekeeper...

http://www.wattkins.com/search/node/6n2p

dave

thanks dave, you weren't kidding!! more checks n balances than... i dunno what... but i got in, thanks for the link!!
looks to me to be a kewl place where i have no doubt i'll learn something, if only by osmosis

[pinkjimiphoton]

all THAT said,
i could NOT get them to work well in cathode follower stages, and they did nothing but hum in my silkyn.

That's because they have a max Vhk of +/-100V compared to the 12AX7's +/-180V.

greg, forgive my ignorance, but what the heck is vhk? all i find with google is some rare autoimune disease

[Phoenix]

greg, forgive my ignorance, but what the heck is vhk? all i find with google is some rare autoimune disease

Heater to cathode voltage - basically the insulation rating. If you exceed it, you get heater to cathode breakdown and the AC heater signal superimposed on the cathode. You can exceed this anywhere the cathode is at a high potential, so most commonly cathode followers, but also phase inverters. You can often however elevate the heaters at some DC voltage (to the midpoint between the highest and lowest potential cathode in the heater string, so long as that remains within the ratings of all tube types used), so rather than have them tied to ground, tie them to a voltage divider off the B+ string, or in a cathode biased amp, it's often convenient to tie them to the cathode of the power tube to get the heaters just a few 10's of volts above ground. There are additional benefits to elevation beyond just avoiding heater to cathode breakdown, if you elevate by at least more than the peak of the heater voltage (so ~8.9VDC for 6.3VAC heaters), you also swamp the normal heater to cathode leakage current, and reduce noise compared to un-elevated heaters.

I elevated the heaters in my recent amp build, and despite being quite high gain it's dead silent. Here's the thread.

[pinkjimiphoton]

thanks for the explanation greg. i saw the thread.... man, for a "wing it", that thing is gorgeous in all respects!!

a question, if i may...

what happens if ya run one of these tubes at 12v instead of 6v on the heaters? would that cause the problem you describe with heater to cathode shorts?
just curious... i'd tried one of these "adapted" 6n2p's in my valvecaster, too dumb to remember it runs on 12vac... and it didn't work. so i plugged it in direct, and it not only works, but sounds great.
i expect the tube life will be substantially shorter, and am surprised it even works at all.

its been years really since i did tube amps... i messed with them before stompboxes. but same story now as then, far from an EE, just a monkey with opposable thumbs n a breadboard.

the valvecaster circuit sounds killer, and i've left it on a few days to see what would happen. seems to be doing fine. i expect a shorter life on the tube, but have you ever experienced anything like that?

i'm wondering if the increased heater voltage is making the cathode release a bigger cloud of electrons to the plate? it seems to have garnered a substantial increase in headroom and bite.

thanks for the patience man. its wicked appreciated.

[Phoenix]

Thanks for the compliment!

Well yes, you're going to get sustantially shorter tube life vs. running them in spec, but it's impossible to put a firm number on it. I'd expect to only get about 10% of normal life out of it at best running double voltage, but different individual tubes might fail instantly on power up, while others you may get lucky and may last almost as long as normal - but those would definitely be outliers. You'll definitely reduce life by more than 50%, as you're putting 4x the power into the heaters by doubling the voltage.

The cathode will be emitting a slightly greater space charge, but not significantly more - the heater is designed to get the cathode to the temperature just below the cathode coatings saturation point, so there's a tiny little bit of extra headroom, but nothing to get excited about. It'll also wear out the cathode and potentially build up interferance resistance faster than normal too. And anyway, the valvecaster runs the tube NOWHERE near saturation, so more cathode emission just isn't going to get taken advantage of.

As to the difference in sound, I'd say that's probably just down to it being a different tube type and your prefering this one, I doubt it's because you're running the heaters out of spec, although some strange behaviours do happen with these starved plate designs, so I could be wrong. Only way would be to try it with 6V on the heaters - chuck a 7806 or 7805 with a couple of 1N914/1N4148's in the ground leg in the circuit and see how it sounds.

[pinkjimiphoton]

most welcome!
i like beautiful shiny green things... i been using some old retired audio snakes for hook up wire for the last several years. lots of color codes, good quality wire, and it was FREE

here's the thing with this tube ... with a 12axwhatever in there, it doesn't glow at all, or even get warm.. there's an led in there to fake it
its ok sounding, but distorts almost unuseably. the 6n's do, too all the way up... but there's way more... the best i can describe it is "glass" or "crystaline" about it and its perceptibly higher output before it turns to total noise.

i gotta gig in a couple hours, i'll try and get some vid tomorrow of it, and swap the two valves around to compare.

you're right, it quite probably is all in my head. full moon madness seems abundant this time out

but with this thing,

[Phoenix]

most welcome!
i like beautiful shiny green things... i been using some old retired audio snakes for hook up wire for the last several years. lots of color codes, good quality wire, and it was FREE

here's the thing with this tube ... with a 12axwhatever in there, it doesn't glow at all, or even get warm.. there's an led in there to fake it
its ok sounding, but distorts almost unuseably. the 6n's do, too all the way up... but there's way more... the best i can describe it is "glass" or "crystaline" about it and its perceptibly higher output before it turns to total noise.

i gotta gig in a couple hours, i'll try and get some vid tomorrow of it, and swap the two valves around to compare.

you're right, it quite probably is all in my head. full moon madness seems abundant this time out

but with this thing,

You know valvecasters are typically stuffed with a 12AU7, not a 12AX7 right?  That'd be why you're finding it unusable. To be honest, I'm not really a fan even with a 12AU7, but lotso people love em.