Tube pream in a pedal

[Oddvar]

Hi.  I am trying to build a 12au7 tube preamp.  Does this look as if it could be working?

[sluckey]

yes

[Oddvar]

Thanks.  Will 12 or 9 volt dc be enough you think

[sluckey]

Must be 12 volts to heat the tube properly.

[eleventeen]

You are going to run the tube off a 12 volt B+?


I guess I have seen stranger things but that seems like not enough volts to run the tube.

[silverfox]

Chandler Tube Driver, and the car radio of the 60's is another 12 volt use.

silverfox.

[sluckey]

The chandler tube is operating on 25v

[eleventeen]

Yeah, I'd consider that 25 volts B+. (12.5 + 12.5) Oddvar's schemo has 12 volts grand total B+ *AND* 100K, 220K plate resistors. I am for sure not saying it's impossible. Yes, car radios obviously ran off 12 volts (or maybe even 6 volts) but they had vibrators which switched the battery volts thru an inductor > rectifier > filter, like a switching power supply, and thereby created more like 75 or 100 volts or maybe more.

[Willabe]

Chandler Tube Driver, and the car radio of the 60's is another 12 volt use.

The Chandler is getting it's gain/drive from the op amps not the tube.

[Ritchie200]

I've built several PAIA Stack-In-A-Box with the 12AX7 "starved" at 45-50v on the plates (cant remember exactly).  This is also opamp driven.  It does sound pretty darn good (to me) BUT it is a stand alone design with specific parameters driving the tube.  In other words, you have a whole support system specifically for that tube.  The other thing I've found is it has to be at the front of the signal chain.  It has a great effects loop and it will drive anything after it pretty well.  However, if you push the front end it gets really flabby and not very musical.  It could be the driver stage OR it could be the tube reacting as well as it can at the front end of it's operating curve. 

How will this pedal react to being driven?

Another question I have is a 12AU7 does not have much gain normally.  With plates that low, if it works would it not end up being WAY below unity gain?  I would think even a 12AX7 would have a hard time getting to unity at those voltages?

If you are just looking for a "pedal" to give you the tube warmth and not necessarily a boost (ala 12AU7), why not use Doug's toroidal with a wall wart and get it up and working in it's normal operating voltage?  Then you could swap out tubes to taste. 

Jim

[fossilshark]

I built a 12au7 preamp that ran off 12v, it has an exact gain of 2 when tested on the oscilloscope. Sounds really good clean i havnt tried to overdrive it though.

[HotBluePlates]

You are going to run the tube off a 12 volt B+?

I guess I have seen stranger things but that seems like not enough volts to run the tube.

https://www.youtube.com/watch?v=gY6FjykVSxg

Matsumin Valvecaster

The youtube clip of this pedal sounds a bit muddy; maybe yours will sound better with the tone control at max.

You may also consider shaving bass going into the gain stage by using a smaller coupling cap.

[terminalgs]

the car radio of the 60's is another 12 volt use.

Not exactly.


automobile tube radios were powered by 6v or 12v DC batteries (everyone knows that, of course),  and they had 6av6 and 6v6s or 12v6's etc.  the battery powered the heaters with DC.  the plates on those tubes had 200VDC+ and an output transformer to drive speakers.  They used a vibrator which was a mechanical coil that operated a make/break contact which set up an AC waveform (I guess sawtooth or squarewave..) that went to a step up transformer. VAC appeared on the secondary which was rectified with a 12X4 or a 6X5 into DC B+. 

[kagliostro]

automobile tube radios were powered by .......

That is correct, but there were also tubes build to be used with 12v B+

12U7 tube


12K5 tube




http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

but those 12v tubes are difficult to be find

Franco

[PRR]

> They used a vibrator

UNTIL a power transistor was developed cheaper than 6AQ5 and a vibrator.

For just a couple years we had 12V on plates of RF, IF, and first-audio tubes. That 1st-audio was one of several very strange types which could make "real" power (0.010 Watts) with just 12V on hand. By stretching the design gain of the transistor Power stage we got several Watts out.

These were not impressive radios either radio or audio. Real soon we got transistor throughout the audio section, and pretty soon tolerable RF/IF transistors. Still not as good as the best 250V-vibrated radios. But a couple bucks cheaper and said "TRANSISTORIZED!" on the front, real space-age.

[HotBluePlates]

... there were also tubes build to be used with 12v B+ ...

It's clear from the video the circuit will work (it's the exact same circuit Oddvar is asking about), and perform its intended function.

I don't know if there's a better circuit for building a tube distortion pedal with a 12vdc supply (or 18-15v, given others report hum/noise unless a higher voltage is regulated down to the 12-15vdc desired).  It would take some investigation about how the tube performs at that low voltage/current...

[silverfox]

I did a search on, "tubes low plate potential" and this site seems pretty informative for starters if you want to pursue this concept further: http://www.junkbox.com/electronics/lowvoltagetubes.shtml

silverfox.

[kagliostro]

Nice score Silverfox  


I know the Cpmpactron page of that site, not the low voltage tube section

Thanks for sharing

Franco