I need some tube related closure re: Tubes as Diodes.

[brad]

This tube related dilemma has been haunting me since the beginning of 2004:

"How do you connect a regular triode tube to work as a clipping diode?"

I've read books...asked on boards...but I've never found a clear cut answer.  Is it as simple as using the plate and cathode as if they were from a modern diode?  Is the heater needed? 

Please end my suffering 

[A.S.P.]

Is the heater needed?

yes.

http://www.elixant.com/~stompbox/smfforum/index.php?topic=2797.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=31089.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=41316.0

[brett]

Hi.
Do I need the heater?
To work as an amplifier or diode, you need the heater.  It's only the heat and electropotential difference that encourage electrons to move about.
Why not use a rectifier or diode valve instead of a 12AX7?  They're cheaper and will probably work better, but maybe not as well as a solid state diode, which has the advantages of the Vf threshold voltage.
cheers

[R.G.]

All vacuum tubes work as diodes if you ignore all the grids and connect only the cathode and plate. Power beam tubes like the 6L6 may act funny this way, maybe not. I haven't thought about it. You can't disconnect the beam forming electrodes on those. But a 12A?7 and all the rest of small tubes including pentodes will be vacuum diodes.

You just have to heat the heaters to boil electrons off into the vacuum, and then voltages can pull them to the plate in one direction, not in the other.

There are several tubes with internal diode sections, intended for AM recivers, I think.

I've seen several guys make triode-diode clipping sections. Never heard that the result was all that great. It's mentioned in Kevin O'Connor's "The Ultimate Tone".

There are actually two ways that a tube can act as a diode. There's the plate-cathode diode action, and the grid-cathode action. If you raise the grid of a tube above the cathode voltage, then the grid starts conducting. This conduction is quite sharp under some circumstances. It's not a very pleasant form of distortion.

[brad]

EUREKA!   

Thanks for the straight forward answers guys!  Everywhere I looked into this topic would either concern proper vacuum tube diodes or my search results would only turn up things about tubescreamers and diodes.

Seeing Slajeune's tube driven diode clipper resurrected this nagging question for me because it seems more elegant to use tube diodes when the clippers would otherwise be the only place semiconductors would appear in a circuit.  The funny thing about 12AJ6 tubes (datasheet) in this case is that they acually contain 2 diodes on pins 5 and 6, but because they share the same cathode, I guess 2 seperate tubes would still be needed for the two clipping diodes.

[A.S.P.]

All vacuum tubes work as diodes if you ignore all the grids

I never ignore the grids, but rather "terminate" them to a certain ( ) potential...

(grid does strange static things with electrons, when left floating...)

[brad]

Ah...that would explain why I've seen the grid tied to the plate in various tube schems.

[A.S.P.]

 

[amz-fx]

United States Patent      5,727,069
Hughes ,   et al.    March 10, 1998
Solid state musical instrument amplifier with vacuum tube signal magnitude limiting
Assignee:     Fender Musical Instruments Corporation (Scottsdale, AZ)
Filed:    January 14, 1994

But not valid in my opinion because there is a tube book in my local public library, with a copyright date in the 1950s, which shows tube clippers...

regards, Jack

[brad]

That Fender patent goes to a lot of trouble to get a tube sound from a solid state device.  More trouble than it's worth!

I went ahead and bought 5 old CBS 12AJ6 tubes for $6 (+$27 shipping), so hopefully I'll be able to try out an all tube version of Slajeune's design soon

[theblueark]

Hi okay I know the whole tubes as diodes thing may not actually sound all that good, but I do want to try using them as clippers.

I tried to understand everything that has been said, and have read multiple threads on this topic, including all the links posted by A.S.P.

However I am still a little lost. A.S.P says yes, the heater is required. And R.G says all vacuum tubes work as diodes if you ignore all the grids and connect only the cathode and plate. Refering to R.G, does that mean that we do not need the heater? Or are you saying that include the heater, then the vacuum tubes work as diodes if we ignore the grids and connect the cathode and plate?


I'm going to be using the 12AX7 and would like some visual confirmation. Here's a pic with the pin layout but with some unrelated connections.

If I compare this to a diode, which pin would be the (+) and which pin would be the (-)?

And if I need to include the heater, which pins do I need to connect what voltage to?

I guess what I'm really asking is, can anyone show me what I need to connect to these pins to make the 12AX7 work as a clipping diode? And do I need 2 of them for 2 clipping diodes?

I'm sorry if I'm asking questions with answers that can be found in articles, but the articles are really long and i'm having a lot of trouble sieving out the information I need.

[R.G.]

I was not being clear. NO ordinary vacuum tube works if you do not heat the heater. The heater is responsible for making all the electron emission that the tube works on. There is a class of vacuum tubes that emit electrons from a submicroscopic array of very sharp electrodes by electric field effects, similar to corona emission. No vacuum tube of classic construction, and no 12A?7 tubes in particular work that way. So yes, heaters are always required.

A 12A?7 tube works as two diodes if you heat the heaters and ignore the grids. In pentodes, you must open circuit all three of the grids.

In indirectly heated tubes, where there is no internal connection between the heaters and the other electrodes, you may have voltage between the heaters and the cathodes since they are not connected. They are insulated to about 300V by an oxide layer between the heater and the cathode proper. So since the heater is not connected to any electrode in a duotriode, you may use the two diodes independently.

A single 12AX7 can be a clipper pair. Connect pin 1 to pin 8, pin3 to pin 6, leave pins 2 and 7 open. Supply power to pins 4, 5 and 9. You will need 6 or 12V to power the heater. If you connect pins 4 and 5 together, you can run it on 6V at 300ma between 4&5 and 9. If you leave pin 9 open, you can run it on 12V between 4 and 5. If you want individual diodes, both plate pins (1 and 6) are anodes, both cathodes (3 and are anodes (that's the bar across the diode symbol).

I personally would run the heaters on DC for this application, because you can get hum pickup from the heaters.

[theblueark]

Ooo thanks a lot!

Hmm ok one more question. How should I go about getting the 6V from a normal 9V supply i'm recieving from a d/c? I'm planning to power the pedal with the 1-spot. Or can I just use 9V?

[R.G.]

You can't use 9V. It's too low to get good electron production on the cathode.

Short pins 4 and 5. Use a resistor of (9V-6v)/0.3A = 10 ohms, 2W in series with pins 4 and 5. Tie +9V to this. Ground pin 9.

If you can't easily get a 10 2W or more resistor, put ten 100 ohm 1/4W resistors in parallel to make a 10 ohm 2.5W resistor.

[brad]

If I compare this to a diode, which pin would be the (+) and which pin would be the (-)?

The arrow side of the diode would be the plate (pins 1 & 6), and the stripe side of the diode would be the cathode (pins 8 & 3).

I don't know if tubes necessarily sound "bad" as clippers, but at least it's another type of tone you can add to your palette.  Perhaps even mix them with conventional diodes?  It would be interesting to see what tube diode clipping looks like on a scope.

Anyhow, speaking of heaters:  Say I've got Slajeune's tube driven diode clipper set up with tube diodes for clippers (making for a total of three 12AJ6 tubes) -does it matter whether I hook up the heaters in parallel or series?  I'm trying to come up with a free-form layout where the components are soldered directly onto the socket lugs, and I'm thinking I could use the heater lugs on the first tube to connect the cathode and plate resistors to ground and 12v.  This would require parallel heaters (assuming it would work at all).

[slajeune]

Hi Brad,

you need to wire the heaters in parallel.  If you wire them in series, it will be like a voltage divider (i.e. 2 heaters in series will have 6v accross each heater).  If you have a 12A?7 tube lying around, you can use that one for a clipper (to replace the 2 12aj6).

Cheers,
Stephane.

[brad]

Thanks Stephane! 

Sorry about all the questions.  I know just enough about tubes to be dangerous.

[coxter]

So I take it that pin 4, 5 and 9 are the "heating" elements of the tube circuitry?

[brett]

That's right.
Join 4 and 5 and connect them to 6.3V.  Connect 9 to 0V.  AC or DC doesn't matter.  As RG said, you can get 6.3 volts across the heaters with a 9V battery if you place a 10 ohm 2W resistor in series with the heaters.

In summary: +9V > 10 ohms > [pins4 and 5 > pin 9] > 0V.
[] indicates connections inside the valve.

Why not get an old 12AX7 or similar and experiment with some batteries and hookup wire.  Use a multimeter and measure everything. Then download some articles about valves, and ask some more questions.

have fun

[zachary vex]

a recent schematic shown here had the grid tied to the plate for diode use.