tubes @ low voltage, inverted operation and underheated cathode

[GFR]

I wonder how it sounds 

http://www.pat2pdf.org/patents/pat20040257158.pdf

Found the reference on the Prodigy forum, supposedly related to VOX "cooltron" pedals.

http://www.voxamps.co.uk/products/pedals/cooltron.htm

http://www.prodigy-pro.com/forum/viewtopic.php?t=5064

PS.

Some patents (applied for) by Randall Smith:
http://www.pat2pdf.org/patents/pat20030169106.pdf
http://www.pat2pdf.org/patents/pat20030179893.pdf
It would be funny if it wasn't sad...

[Dirk_Hendrik]

Fumio Mieda, also known for inventing te univibe, is the nowadays head honcho at the R&D facilities of Korg. One can expect he knows what he's doing.

[GFR]

Nice!

I guess the "servo" part they refer to in the Vox page is to guarantee the thing biases (at this low voltage and current any variation must be troublesome).

Want to know how it all works?

Normally, tubes are used at high voltages and this restricts their use in battery-powered devices such as guitar effects pedals. Historically, attempts have been made to use tubes at low voltages in what has been dubbed “starvation” mode, but these circuits don’t allow the tube to function in an acceptable manner. The tube simply cannot be biased to a satisfactory operating condition. Plus, the necessary heater (filament) current is too high to deliver acceptable battery life. Imagine how annoying it would be to have to change batteries every ten minutes during a gig!

Here’s what makes the COOLTRON system tick. The COOLTRON circuit basically operates by using two very special signal paths. The first is the “servo circuit,” which provides the right conditions between the plate of the tube and the grid. This achieves controllable, stable parameters for the tube and provides the correct operating conditions for the tube to function as it would if run at a higher voltage. The second circuit is a patented power supply that provides a low voltage, low current supply to the heater elements in the tube. Since the tube is now running at such a reduced supply level, the anode current is much smaller than normal. This means that the amount of heat required at the cathode to achieve sufficient cathode current emission is much smaller â€" hence the ability to run the heaters at a lower level and for COOLTRON pedals to run for 16 hours on 4 x AA batteries!

The COOLTRON system provides true tube sonic performance at very low voltage and at reduced heater current â€" ideal for inclusion into guitar effects pedals. COOLTRON guitar effects pedals offer valuable tube type sonic performance.

[changes]

so is there any full schem on how to use this method cause i think i didn't get it right from the drawings...

[JimRayden]

Cooltrons certainly have their own tone. Like a huge growl. I absolutely love it. Actually, that's the tone I've been looking for the last few years. Something like a really loose n' smooth high gain growl. GROWL, man!

Anyone know a (tube or tranny) pedal that achieves those tones?

--------------
Jimbo

[JimRayden]

Bump. 

-----------
Jimbo

[GFR]

I'm trying to guess how the cooltron connection works, here's what I think:

The grid is positive, there's a grid->cathode current. Since the cathode is so cool and the grid voltage is so low, this current is very small (there are few emitted electrons and the attraction to the grid is weak).

In a "normal" inverted operation, the plate would be negative and repel these electrons so with the plate more negative there's less grid current, and with it less negative there's more grid current. Put a resistor at the grid and that's an inverting amplifier (with gain less than one and low output Z).

Now, in the cooltron, the plate is positive and so it conducts. Since it can't make more electrons leave the cathode (as it's so cool), it "steals" some electrons that would go to the grid. When it's more positive it steals more electrons, so the grid current lowers. With a grid resistor it's a non-inverting amplifier, with a high output Z.

I'll try to watch the grid current x grid voltage curves in a scope today...

[GFR]

I did some tests.

First, set the heater to 2.5 VDC, cathode grounded the cathode and used a floating 10Vpp AC source in series with a 1K resistor (one terminal of the source to the grid, the resistor to the cathode/ground) to measure grid current x grid voltage in a scope (XY display). A second DC source was used to set the plate-cathode voltage. The tube was an old japanese Philco 12au7, ironically labelled as "cool chassis TV set tube" .

The curve had a "pentode" shape. Changing the Vak moved the Ig (in the constant current region) up/down, but very little (too small gm).

Then I tried increasing the heater voltage. As the heater got hotter, the current increased (rapidly) but also the curves changed to a "triode" shape. gm was still small.

After that I tried the circuit as described in the patent text, k to ground, grid to 100k to +5V, input at the plate. By changing the plate bias, I couldn't get the grid to bias at 2.65V like in the patent. It set at ~1.5VDC no matter what the plate was set (makes sense given the results of the curve tracing experiment above). The gain was very low - ~0.1 instead of the 3.5 in the patent. The linearity was very good, and it could handle plate voltages even greater than 5V without visible distortion. But as you approached 0V and negative plate voltages, there was some squashing (but not clipping). It was non-inverting as the patent said.

I will try some other tests. Now I think the servo function is to set the heater voltage, not the plate voltage. I'll try to run the tube even cooler until I get the same bias as the patent and then see if the gain increases (and see if the distorion is more evident).

[transient]

I want to try it too, but the damn circuit requires 3 different voltages (4 if you add a clean-boost stage after it, as the tube output volume will be low)

Let us know about your progress

.
e

[GFR]

OK.

I tried the circuit again, with a 100K "grid load" to +5V. I slowly changed the filament voltage until it biased at ~2.6V grid voltage. You have to do it slow because it takes some time for the cathode to cool/heat. It biased with 2.0V on the filament. It is very sensitive, 100mV up or down will change the bias a lot.

Now the circuit showed a gain of ~1.5, non-inverting, with a very obvious non-linearity. The bottom of the wave gets squashed (even when the input is not very high). As you turn the input up, the bottom gets more squashed, eventually the top starts to squash too, but the edges are always very rounded, even with a couple of volts in the input. The transfer curve is assymetric, "S" shaped, with very soft "knees".

You NEED to fine tune the filament voltage to get it working. I don't know if a preset adjustment would be enough or if it has to be a servo like in the VOX ads. If you go for the servo, remember that there's the thermal delay...

[oiseau]

brilliant.