How low can you go?

[Andy Le Blanc]

RCA tables for R-C amps start at 90v, Sylvania tables at 100v.

I haven't invested noodle time with this one, have tried successfully
running grounded grid stage at less than 100v, at plate resistor not the plate.
I assume that the heater voltage is more critical to the operation.
Has anybody found a practical lower limit to plate voltage?

[David Root]

I think it was Bruce at Mission Amps in Denver reported that he built a harmonica amp with 35V on the preamp plates that worked very well.

Now good harp amps are not like guitar amps and frequently use 12AY7s and even 12AU7s for lower gain and easy overdrive, so I wouldn't try that in a guitar amp, but I have to think that around 35V has to be about as low as you can go!!

[Andy Le Blanc]

Whats that called? Anode starvation?

running the device for the opposite effects, less headroom and more
harmonic distortion and color.

vrs. a higher plate V for more linearity

[Firestorm]

I suppose it depends on how big a signal you're trying to pass (and how much gain you want). Some Ampegs (and Traynors too, IIRC) coupled the tremolo oscillators through a pair of not quite parallel triodes (opposite plates and cathodes connected) with no DC voltage applied anywhere. Apparently, the signal swing alone is enough to get the electrons moving. Of course, that isn't an RC coupled amp, per se. It might be fun to wire a triode as a mu amp, put a fixed signal in and watch what happens to the output as you dial down the voltage.

[Badside]

There is a popular pedal design on the internet that runs a 12AU7 at 12V (sometimes even 9V!). It's very dirty obviously, no headroom.

[Andy Le Blanc]

I've already played with that tremolo, it utilizes the tube as a diode, not for gain.
Fun alternative to couple a LFO into a circuit, wiggly...

I remember in an old O-scope book, it had pictures of real scope traces,
state of the art at the time of printing, it pictured "real" operational conditions
as represented by the scope trace. It showed typical low and high voltage and bias
conditions represented by the visible distortion in the trace.
Goes back the the basic's, the usable undistorted gain for a given set of
conditions applied to the device.

The starting place for most hobbyist is from working examples, like old fenders,
instead of demonstrating the solutions to basic questions.

Like in that trem coupling, you can use the basic diode nature of the tube
and set up the grid as a valve to regulate current, end up with a way to
vary the plate voltage, I've noodled with that, haven't installed it into an amp
with any practical intents. So... there are working examples of production
amps that use a plate supply V of 35v applied to a pre., place to start.
Any other examples? Wasn't there some info on the Steve Bench page?

Got a link to that 12v pre?

[Badside]

Got a link to that 12v pre?

And yes it works! Search for "ValveCaster" on Youtube

[img:606:357]http://www.beavisaudio.com/Projects/Val ... Caster.gif[/img]

[Andy Le Blanc]

Fun.... that vr1, looks like a neat way to control gain with a zero bias stage.

How long does the battery last running the fil.?

wasn't there a takimine guitar pre that did something like that?

[Zippy]

RCA tables for R-C amps start at 90v, Sylvania tables at 100v.

I haven't invested noodle time with this one, have tried successfully
running grounded grid stage at less than 100v, at plate resistor not the plate.
I assume that the heater voltage is more critical to the operation.
Has anybody found a practical lower limit to plate voltage?

This is the territory that one gets into using scaling schemes such as VVR.

[Badside]

Fun.... that vr1, looks like a neat way to control gain with a zero bias stage.

How long does the battery last running the fil.?

wasn't there a takimine guitar pre that did something like that?

What I don't get is the absence of a bias resistor on the 2nd stage...
Unless this is not the final schematic.

The thread over at the DIY Stompbox forum is 84 pages long so I didn't take the time to go through it haha.

As for battery life, I don't believe anyone building those is using a battery. The 9VDC is mostly to power it from any basic pedalboard supply.

I have yet to build one but I'm saving a 12V 1A wall-wart specifically for this.

The bias control on the first stage is indeed an unusual way to control gain, but it also allows you to dial in the character. The voltage and current is low-enough to not worry about putting DC on this put. It'll probably scratch a bit, but only when you're moving it.

[Andy Le Blanc]

That ones a hoot, it works, couldn't resist.

[Andy Le Blanc]

The first stage has a combination bias (the gain control adds more bias V),
The second stage uses grid leak or zero bias if your used to seeing a bias resistor.
A charge builds up on one side of the cap and "leaks" to ground through the
grid resistor. Old radio method. The signal in provides some additional bias v too.
The distortion figure on this is "bad", the sylvania table list it as the most
gain you can get before 5% distortion. Now thats harmonic distortion not the
hard clip you might be thinking of. The clean is never clean, it is "colored".

The battery works, dont know how long it'll last with the heater.

[Badside]

Hahaha, now that's some nice looking prototyping!

Wait, you actually did it all in 2 hours?

I guess that's the beauty of this circuit. Too simple not to try!

[Andy Le Blanc]

It's the practice, 20+ years, that one was fun.

Wicked no brain-ah.

[jaysg]

I thought all of the Chandler/Real Tube Drivers were low voltage affairs. Certainly, power scaling shows that all this stuff can work fine. When car radios used 6V6s, they must have run off of 12V...or 24V with a doubler.