General 12v tube question.

Jeffj85 · September 15, 2009, 10:45:11 AM

Hi all,

My name is Jeff and I'm new to the forums. I hope to have a long productive stay.
My question has to do with some tubes I've purchased from a local shop. The tubes in question are the 12ax7,
12au7, and the 12k5.

I've read and followed a few tutorials and bits of articals around the interweb all that state the tubes heaters will glow at approx 6-14 vdc. My problem is I can't get any of them to glow. In a project or not. Now I have a tube tester from ages ago( an emc 211) and I can test the tubes, all come back a-okay. When checking the voltage on the tester across the heater pins it gives me 36vac and the heaters glow.

Are these bad tubes or am I doing something wrong. The projects I'm workig on are the hybrid- ruby tuby, the valve castor, and the tube cricket. Any help is appriciated.

-Jeff

isildur100 · September 15, 2009, 11:00:17 AM

Hi,

I am using 12AU7 tubes for a pedal and some glow more than others. The Electro Harmonix are the ones I have that glow the less and sound the best... so I don't think the amount of glow automatically means better sound.

Jeffj85 · September 15, 2009, 11:14:12 AM

Thanks for the speedy reply, but I'm not getting any sound or glow from the tubes. with the 6-14 vdc.

Ripthorn · September 15, 2009, 11:46:02 AM

You have to be careful how you wire the heaters. If you wire them in parallel, they are looking for 6-6.6V AC or DC. If you wire them in series, they are looking for 12-12.6V AC or DC. If you have too little voltage or current, they will sound fuzzy or may not produce any sound at all. Also, after putting the voltage on the heaters, you need a plate voltage in order to get any amplification. You will need to build at least a quick and easy gain stage to see if the tube really works well. You could just do a cathode follower for that, or a cathodyne phase inverter. Or you could just do the first gain stage of the valvecaster. Keep in mind that there are two separate triodes in those tubes, so don't wire the plate and grid of one and the cathode of the other, it won't work. Get a datasheet and make sure all your connections are correct.

By the way, welcome, and tubes are awesome, that's all there is to it.

Renegadrian · September 15, 2009, 01:56:56 PM

Quote from: Ripthorn on September 15, 2009, 11:46:02 AM
By the way, welcome, and tubes are awesome, that's all there is to it.

Yeah, tubes are fun!!! Tell us more on how you power them...If they receive 12V, they oughta work!!!
I got several, some are like light bulbs and some are barely lit - my EH 12au7 falls in the last scenario, but I am jumping for happiness for this one...

frequencycentral · September 15, 2009, 02:03:22 PM

Re the 12AU7. Pins 4, 5 and 9 are the tubes heater terminals. Nothing to do with the 'sound' bit of the circuit, and need to be understood before you'll get anywhere.

There are two way to heat the tube:

Method #1.

12.6v / 150ma: Pin 4 to ground, pin 5 to 12 volts, pin 9 no connection

Method #2.

6.3v / 300ma: Pins 4 and 5 to ground, pin 9 to 6.3 volts.

Method #1 is the easiest and consumes less juice. 12v is close enough, it doesn't have to be 12.6v exactly. The 12 volt supply needs to be from a filtered (to prevent hum) and regulated (to keep the voltage constant) source. And your power supply needs to put out AT LEAST say 20% more milliampres than the heater requiement of the tube.

Once you get the heater powered correctly everything else should fall into place!

maarten · September 15, 2009, 06:54:17 PM

Hello,

when testing you measure 36 volts on the heaters? Either you used the wrong pins for this (see Rick's message for the correct pin numbers), or you misinterpreted the data - it should measure 6,3 volts. I doubt whether your tester would be able to up the voltage on the heaters to 36 volts.

As your tubes glow in the tester, the heaters themselves probably are in good shape, which leaves the question: what do you use to supply the heaters with? Batteries would probably give you not enough current, and even if you are using a wall wart it may not give you the 300 mA needed at 6,3 volt.

Maarten

Jeffj85 · September 15, 2009, 07:42:15 PM

Well thank you all for your help, as it turns out I've made a tube-noob mistake with the pin-out. I will now hang my head in shame as I jam out on my guitar and new valve caster overdrive porto.

thanks again.

amptramp · September 16, 2009, 11:38:15 PM

I would be interested in seeing what you come up with for a 12K5 design. This is a space-charge grid tetrode which has the first (control) grid, G1, wired to +12.6 VDC and the input goes on G2, the screen. It was designed for car radios which used a lineup of tubes for small-signal stages and a power transistor output.

The 12K5 has a 12.6 VDC 400 mA filament, an absolute maximum plate voltage of 30 VDC and is usually specified to run with 12.6 VDC on the plate, -2.5 volts on the signal grid (G2) 12.6 volts at 85 mA on G1 and has an Rp of 600 ohms, transconductance of 9mA/V, 40 mW output and a rated load of 800 ohms on the output. The plate current is typically 8.0 mA. Naturally, for a tube used in a car radio, the voltage fluctuations on the 12 volt line could go from 11 VDC to 15 VDC steady-state (not including spikes and surges) and the device was supposed to work without problems within this range.

This tube should be relatively non-linear (which is a plus in this application) and should be usable with a standard line transformer as the plate load. If you use a standard 70.7 volt speaker line transformer on the 5 watt tap, you will get 1000 ohms load, close enough for this kind of work. There are a lot of telephone transformers with a 600 ohm impedance which would also be usable. Sounds like a neat project, but it may run hot if it is confined in a small box.

Renegadrian · September 17, 2009, 06:23:01 AM

I believe you already know the Sopht Site - http://www.sophtamps.ca/mambo/
He used that tube in his projects.

amptramp · September 17, 2009, 10:20:56 AM

Thanks, Renegadrian - I had not seen that site before and it was interesting to see that the performance and distortion levels were about what I expected.

puretube · September 17, 2009, 04:15:52 PM

Quote from: Renegadrian on September 17, 2009, 06:23:01 AM
I believe you already know the Sopht Site - http://www.sophtamps.ca/mambo/
He used that tube in his projects.

Slajeune`d ?

Jeffj85 · September 18, 2009, 09:02:54 AM

Quote from: Renegadrian on September 17, 2009, 06:23:01 AM
I believe you already know the Sopht Site - http://www.sophtamps.ca/mambo/
He used that tube in his projects.

This was actually the first tube project I've attempted. I'm more than likely going to take a crack at this one again and maybe instead of the tl072 opamp, I'm going to use the 12au7 in its place. My problem now is if i run it at 12vdc from a wallwort I'm getting alot of noise, I've read some were on the forums that if i make pins 123 of the first triode the follower it will reduce the amount of hum. Looks like I have some work to do tonight.

Ripthorn · September 18, 2009, 09:28:46 AM

Is your wall wart filtered and regulated? For tubes, a filtered and regulated supply is a must for low noise, otherwise, you can get heater noise going on.

Jeffj85 · September 18, 2009, 06:03:30 PM

I think it is filtered I'm using the power section of dano's tube cricket I think that's a filter I could be totaly wrong though. And I'm regulating the voltage with a 7809 regulator but it seems to knock the 12vdc Down to 8.89 vdc instead of the 9vdc. Strange.

puretube · September 18, 2009, 06:14:27 PM

Quote from: Jeffj85 on September 18, 2009, 06:03:30 PM
I think it is filtered I'm using the power section of dano's tube cricket I think that's a filter I could be totaly wrong though. And I'm regulating the voltage with a 7809 regulator but it seems to knock the 12vdc Down to 8.89 vdc instead of the 9vdc. Strange.

Not strange, at all : almost within about 1% tolerance...

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General 12v tube question.