Tell me about mini tubes

[trad3mark]

Hello all,
So i'm planning a little preamp build. I was planning on doing it with a single 12AX7, but, if possible, using smaller tubes would be great. So i've a couple of little questions about them before i go investing in any.

Firstly, voltages. So i know that 12AX7's need high voltage at the Anode, and either 6 or 12V for the heater. So how do the miniature ones fare up on this one? Do they need just as much voltage? I know about the tube numbers, ie 6 series tubes need 6V at the heater etc etc, but what about for the anodes? do i need to give it as much voltage, or do they require less? (or more?) Is there a way, looking at data sheets, that I can tell?

Secondly, gain. In setting up a gain stage on a 12AX7, the cathode resistor I'd be using is a 2.2k. It's being based on a favourite amp of mine, that uses 2.2k at the cathode, and 220k at the anode. So in setting up a gain stage with smaller tubes, do they have the same curves that a similar stage would suffice?

They're the only questions I have right now. I've been looking about on the web for more info, but i'm really confused. I've done a lot of reading, including reading back over ValveWizards common gain stage (that's just awesome reading) but i'm still a little confused about the mini tubes.

Cheers all.
N

[iccaros]

Welcome to the world of tubes

So building a tube pedal or a preamp for your amp? The difference is the types of compromise  you have to make, a Pedal in my opinion, should play well with other pedals in your chain, while a preamp needs to work with your amp.

For a pedal it is normal to look at lower voltages for or use voltage multiplier. Some tubes need higher voltages to get nice linear output, but when placed with lower voltages can overdrive at much smaller signals.

As for Submini tubes, they work the same as any other tube and you will have to pull the datasheets on the tubes you want to use to see what heater voltages it needs.. Google is your friend here.

If you search for Risk's stuff, he has done a lot of work with Subminis and just looking at his projects should give you a lot of information.

but general rule as I think this works.. Again, I am married so I know I am wrong most of the time..


Biasing from an amp normally does not have the same sound in a pedal, but could be a good place to start.. : Amps have more things effecting sound than just the tube bias, some of these things get tossed when a being transplanted into a pedal. The coupling between preamp and power section is one thing, the way the power supply is setup ect...


If you understand the ratios between anode voltage and cathode resistor you can normally achieve a like load-line at lower voltages, so for a 12ax7 @ about 80volts on the anode I hear it act more linear, but with less volume on the output. Adjust in the bias will allow me to get the same sound as if I had 150volts on the anode, Again in life you always give up something, in this case volume 

Submini tubes like the 6112 have similar characteristics to a 12ax7, but are not the same... More like 5751 to me..  and list a max of around 150 volts on the anode, and in my High Octane clone using subminis I see about 75volts on the plates, but get like sound as I do on a HO with 12ax7.

using a single tube and expecting it to make any amp sound like your favorite amp is well.. expecting a little too much in my opinion.  Matter of fact, I do not see how even cloning an entire preamp will make any amp sound like another as other factors play into what the total sound is. This is why I do not like emulation to an amp, but do not mind it strait to the recorder.

[trad3mark]

Welcome to the world of tubes

So building a tube pedal or a preamp for your amp? The difference is the types of compromise  you have to make, a Pedal in my opinion, should play well with other pedals in your chain, while a preamp needs to work with your amp.

Well... The plan is to basically have a pedal, in a big box. It'll have 2 triode gain stages, but i'd use it as the last pedal before the amp. It'd go into a fairly clean amp. The idea is to get a very specific driven sound, but from a pedal. Basically, I'm looking for that old vintage driven tone you get from the likes of Silvertones, Selmers and old Gretch amps. I did consider getting a new amp to do this, but i've never come across a modern/currently available amp that comes close to that sound. So, I figured i'd make a pedal that produces a similar, somewhat authentic sound to get that. So yeah, a pedal.

For a pedal it is normal to look at lower voltages for or use voltage multiplier. Some tubes need higher voltages to get nice linear output, but when placed with lower voltages can overdrive at much smaller signals.

I was planning on going for ~100-125V, powered by a Nixie SMPS, as per some of frequencycentral's builds. I made one before, and it worked well.

As for Submini tubes, they work the same as any other tube and you will have to pull the datasheets on the tubes you want to use to see what heater voltages it needs.. Google is your friend here.

If you search for Risk's stuff, he has done a lot of work with Subminis and just looking at his projects should give you a lot of information.

but general rule as I think this works.. Again, I am married so I know I am wrong most of the time..

Yup, rick's been another source of getting this right.

Biasing from an amp normally does not have the same sound in a pedal, but could be a good place to start.. : Amps have more things effecting sound than just the tube bias, some of these things get tossed when a being transplanted into a pedal. The coupling between preamp and power section is one thing, the way the power supply is setup ect...

If you understand the ratios between anode voltage and cathode resistor you can normally achieve a like load-line at lower voltages, so for a 12ax7 @ about 80volts on the anode I hear it act more linear, but with less volume on the output. Adjust in the bias will allow me to get the same sound as if I had 150volts on the anode, Again in life you always give up something, in this case volume 

Submini tubes like the 6112 have similar characteristics to a 12ax7, but are not the same... More like 5751 to me..  and list a max of around 150 volts on the anode, and in my High Octane clone using subminis I see about 75volts on the plates, but get like sound as I do on a HO with 12ax7.

using a single tube and expecting it to make any amp sound like your favorite amp is well.. expecting a little too much in my opinion.  Matter of fact, I do not see how even cloning an entire preamp will make any amp sound like another as other factors play into what the total sound is. This is why I do not like emulation to an amp, but do not mind it strait to the recorder.

So perhaps it'd be better off for me to stick with a 12AX7, put it in a bigger box, and give it 100V at the anode? I know Silvertones gave 100-125V at the anode, so it might be an idea to more or less copy their stuff? If even just to breadboard it and experiment.