6k11 tubes / Wurlitzer

[jhaible]

Hi

Maybe I'll replace the transistor amp in my Wurlitzer 200A epiano with a tube amp. I found that the early tube Wurlies used a 6k11 triple triode.
It seems to be quite expensive, but I might be able to buy one.

Is there any reason to go for a triple triode? I saw the bulb is rather stout, compared to normal preamp tubes. Maybe this is an advantage in an invironment of extreme vibrations, like the inside of an epiano?
Any other reasons? Any suggestions what else I could use (except the typical standard stuff)?

JH.

 

[PRR]

> early tube Wurlies used a 6k11 triple triode. Is there any reason to go for a triple triode? I saw the bulb is rather stout, compared to normal preamp tubes. Maybe this is an advantage in an environment of extreme vibrations, like the inside of an epiano?

The -only- advantage of the triple-triode bottles is to reduce tube count in mass-produced TVs and other complex systems.

The bottle is fat just to get enough pins on the bottom. That means an unusual socket: good luck finding one.

The works inside are plain 12AU7 12AT7 12AX7 etc guts, just three instead of two.

Most such tubes were made for the TV market, and may be of marginal quality.

I think you should use plain old dual-triodes. An extra 9-pin socket is much easier than finding those funny 11- and 12-pin sockets, the tubes are available everywhere, in mil-quality if you like.

 

[jhaible]

[quote author="PRR"]> early tube Wurlies used a 6k11 triple triode. Is there any reason to go for a triple triode? I saw the bulb is rather stout, compared to normal preamp tubes. Maybe this is an advantage in an environment of extreme vibrations, like the inside of an epiano?

The -only- advantage of the triple-triode bottles is to reduce tube count in mass-produced TVs and other complex systems.

The bottle is fat just to get enough pins on the bottom. That means an unusual socket: good luck finding one.

The works inside are plain 12AU7 12AT7 12AX7 etc guts, just three instead of two.

Most such tubes were made for the TV market, and may be of marginal quality.

I think you should use plain old dual-triodes. An extra 9-pin socket is much easier than finding those funny 11- and 12-pin sockets, the tubes are available everywhere, in mil-quality if you like.[/quote]


Hi PRR,

thanks for your reply!

Nothing special inside, expensive tube, and probably even more expensive socket.

So - but would there be any advantage, using this tube?
My thought was that a socket that is rather wide, but also rather flat, will help to keep the forces from vibration low. But I have no idea if this is an issue here or not. I'm glad if it's not - jut want to be sure.

JH.

 

[BradAvenson]

I actually have done the mod that you are talking about before. I haven't found that the tube is bothered any by the vibration/shock of moving the wurly around. (I mean have you seen how the guitar player treats his tube amp! :wink: )

You also don't need much gain. 2 triodes are more than enough. You really only need about 10-15 db to get to line level from the reed bar.

The most important thing is to get a really clean HV supply for the reedbar. I actually increased my supply to about 250V.

 

[jhaible]

[quote author="BradAvenson"]I actually have done the mod that you are talking about before. I haven't found that the tube is bothered any by the vibration/shock of moving the wurly around. [/quote]

Great!

(I mean have you seen how the guitar player treats his tube amp! :wink: )

(;->) But have you seen how I hammer on my Wurlie!
Seriously: I'm glad you gave me some first hand experience. So it will be a standard tube when I do it.

You also don't need much gain. 2 triodes are more than enough. You really only need about 10-15 db to get to line level from the reed bar.

I know. I want to do the mod because my transistor preamp is clipping!
The only thing that really surprised me was that passive output on the 140 (tube) model. No gain at all there.

But then again, when I built a tube circuit in there, I also want to have that variable Mu tremolo. I think about making a 600R output, but with a push pull configuration just like the speaker amp was. Then I can simply modulate the common mode grid voltage of that amp.

The most important thing is to get a really clean HV supply for the reedbar. I actually increased my supply to about 250V.

That's approximately what the tube models had.
I wonder if there is an optimum for this voltage, as low voltage will give lower amplitude, but high voltage will more likely attract dust in the pickup.

JH.

 

[BradAvenson]

As the 200 turned into a 200a, the reedbar voltage was reduced to about 130V from 170V. I used a doubler to get the voltage up to 250-275V and I thought it sounded better.

I thought about including another tube or 2 and using a vibro-champ type circuit, but I kept it simple so that I could use the original transformer. I didn't need a speaker out, so all I have is a line level transformer balanced output. I just made a PCB to fit the space that is already there, but watch out the holes aren't all that evenly spaced.

 

[jhaible]

[quote author="BradAvenson"]As the 200 turned into a 200a, the reedbar voltage was reduced to about 130V from 170V. I used a doubler to get the voltage up to 250-275V and I thought it sounded better.

I thought about including another tube or 2 and using a vibro-champ type circuit, but I kept it simple so that I could use the original transformer. I didn't need a speaker out, so all I have is a line level transformer balanced output. I just made a PCB to fit the space that is already there, but watch out the holes aren't all that evenly spaced.[/quote]


I have the power amp components removed on the main board of my 200A. The board is still there because of the tremolo circuit.
I don't have the original transformer anymore. Replaced it with a toriodal transformer, plus a little print transformer connected backward to get the high voltage. I found that after replacing the PSU stuff, running the 230V cable thru that aluminium pipe (as before) is very important. Magnetic field from the transformer is no problem, but electric field from the PSU cable is a big problem! (No wonder, as the pickup is electrostatic.)

I'm not sure if I will put the electronics at the original place, in the middle of the metal noard that runs across the keys, anymore. Someone told me tube wurlies had the electronics all at the left side. I haven't seen this myself, but it makes some sense not to build a mechanical resonator, and drag the 230V AC across 2/3 the piano's width.

Don Tillman once told me that the cable from the pickup to the electronics will increase noise. He says that's why they moved the preamp right onto the pickup in the 200A. If it wasn't for this, I would probably just place the whole amp outside. But I could still build it like a phantom powered microphone: Place a single tube inside, and feed the supply voltage from the outside. This should be more than 48V then, of course.

Just thinking out loud and playing with some ideas ...

JH.