How Much Current is Too Much?

[erix]

I'm wokring on a design for a booster and, since I don't own a tube amp, I'm in a quandry re: input current on the first tube.

How much is too much? What happens if I feed a tube 250mA from a pedal? 500mA?

I know this circuit works wonders on audio but I don't want to blow up my friends' amps with my shiny metal boxes...

thnx,
erix

[Mike Burgundy]

tubes are electrically more rugged than solid state. That said: with 250mA(AC) and an input impedance of several M, what do you think? I'd say there's a little too much voltage on the input - more than a battery-operated device could produce. A tube draws very, very little current into the input, and is a voltage amplifier.
If your design works well as an audio (hifi?) booster without distorting the next preamp stage, no worries.
But howcome all those mA? Are you pushing DC current into the next stage? That won't work on tube amps 'cause of the blocking caps. I'm a little confused by the question. Could you be a bit more specific?

[RedHouse]

Most Tube amps (Marshall, Fender, Vox types) don't have blocking caps on the input, more typically there's a 68k resistor that is all that stands between your effect and the tube grid.

I would ask as Mike did, why all the current, and additionally why aren't you using a blocking cap on the output of your effect?

[erix]

Ahh - hadn't thought about the output cap... :oops:

The thing is basically the same as the schem on this app note.
http://www-s.ti.com/sc/psheets/sboa031/sboa031.pdf
I've used it for several audio projects (into headphones) and it's a very powerful sound.
I was planning on making a simple op-amp distortion box using a dual opamp and having the second side of the opamp switched (position 1 = single opamp, input of opamp switched to ground, position 2 = parallel opamps) for a semi-bogus two-channel effect.
Seems like if I had the input and ouput caps large enough I could take advantage of the increased power and headroom that the circuit provides.

ok,
erix

[R.G.]

In general, it's not possible to simply feed current into a tube input - you usually don't have enough voltage available to do that.

The input of a tube looks like an open circuit in the negative going direction, up to several hundred volts, where the tube arcs over inside.

In the positive going direction, most triodes look like about a 5K to 10K resistor. Feeding 250ma into a 5K resistor requires 5K*0.25 = 1250V, which I don't think those opamps will do as an output.

High output current is only possible in circuits with voltages that can drive the loads to those currents. Alternatively, high currents are only possible where the load resistances are low enough to eat that much current at the available voltages.

Ohm's law is an inescapable part of Mother Nature's rules on how electronics work. No designer can afford to be unaware of how to use it. If they are, they're just easter-egging parts.

...oops, forgot... or just searching for their own beautiful truth no matter what "they" say.

[puretube]

in short: an (input-) tube is not a "load".

[erix]

in short: an (input-) tube is not a "load".

There's the answer I was looking for. Thanx puretube!

hoppy,
easterbunnnyerix