replacing jfet with a miniature tube in a preamp

[mofed]

Hi,

I've been building some of Runoffgroove's amp simulators recently and I really like them (Supreaux Deux, Azabache).
I use them as a preamp before a little power amp wich feeds a regular 12 inch guitar speaker.
But I was starting to wonder if I could replace some of the jfets in those circuits (wich should be acting like a tube stage in the cicuit, read: http://www.runoffgroove.com/fetzervalve.html) with actual tubes. (12au7 miniature tubes)
I guess they would behave differently (more volume, softer clipping, bla bla bla) but could I make this work?
Has anyone tried this before?

Mo

[tubegeek]

Has anyone tried this before?

Well, those circuits were tube circuits to begin with, so I guess you could say yes.

[deadastronaut]

. as someone who has a couple of mini tubes collecting dust , but never messed with tubes,  i,d find the actual
process of subbing fets interesting..subbed. 

[midwayfair]

12AU7 requires a high plate voltage. Look at a real plate voltage preamp circuit -- like any of the guitar amps the ROG circuits are based on -- to see what's involved in powering a tube circuit. Then look at the data sheet for your favorite FETs and see what they can take. You will probably not want to mix and match those components.

[Groovenut]

Since the 12AU7 is a lower gain tube, it doesn't require quite as much plate voltage to sound good (hence the good response to the Valvecaster). That said, tubes like the 12AU7 have a sweet spot for plate voltage. In 12A*7 tubes this appears to be above about 100 volts at the plate. This is where they start acting and sounding like what most expect tubes to sound and act like. So you have to decide what plate voltage you want the tubes running running at, then generate that voltage.

Most jfets don't like to run at high voltages (there are exceptions) and have 35 to 50 volt maximums. So if you decide to mix the two techs, you'll have to have a split B+ rail.

the next issue is, unlike jfets, tubes require a filament/heater supply. The 12A*7 series can run on AC or DC at 12.6V/150mA or 6.3V/300mA

This complicates the circuitry a fair bit more than just using solid state devices, but it's not impossible. Nixie tube type switching power supplies can take care of the split heater/plate voltage issue. 12VDC wall supply for heater, then the switching power supply converts to 150-350VDC for the plates. See the GTFO project http://www.diystompboxes.com/smfforum/index.php?topic=93936.msg809161#msg809161

That same 12VDC supply could easily be used to power the SS devices.

Since a good deal of the ROG circuits used the same values as their tube counterparts, you may be able to just swap in the tube once the power supply is dealt with.

[amptramp]

There is a way to power tubes quite easily for small circuits.  Use a wall wart that puts out AC at the voltage required for the filaments.  Then use a filament transformer (or another wall-wart) inside the enclosure connected so it takes in the 12 VAC and puts out 117 VAC.  You can rectify this and get enough voltage to run the tube at over 100 volts.

[tubegeek]

There is a way to power tubes quite easily for small circuits.  Use a wall wart that puts out AC at the voltage required for the filaments.  Then use a filament transformer (or another wall-wart) inside the enclosure connected so it takes in the 12 VAC and puts out 117 VAC.  You can rectify this and get enough voltage to run the tube at over 100 volts.

Just don't tell the little filament transformer that you are using the secondary as the primary, whatever you do - it'll get confused.

[Groovenut]

There is a way to power tubes quite easily for small circuits.  Use a wall wart that puts out AC at the voltage required for the filaments.  Then use a filament transformer (or another wall-wart) inside the enclosure connected so it takes in the 12 VAC and puts out 117 VAC.  You can rectify this and get enough voltage to run the tube at over 100 volts.

The only issues I've ever had with using the back to back transformer method is AC hum. There's usually not enough room in a small enclosure to get away from the transformer and it's associated AC radiation. Twisting the filament wires will help reduce the radiation from the wires but the transformer itself is the issue. I've only had moderate luck with grounded shield plates (pseudo Faraday cage).

That plus the physical size of the transformer have made the switching supply more attractive to me in most cases.

my two cents..