Simple Tube Boost Circuit

[Jim Hagerman]

For you DIYers, here's a simplified tube boost pedal you can make using very few parts. NOT FOR COMMERCIAL USE! One knob boost circuit adds warmth to your tone. Operate anywhere from -9V to -12V, although you may have to select tube for best gain. Similar to a Matsumin but more tame.



Does not work with 12AX7. Use a 12AU7 or 12BH7 type.

[merlinb]

NOT FOR COMMERCIAL USE!

lmfao

[duck_arse]

for -9V to -12V operation, your LED indicator is backwards. that's assuming it's a bypass indicator, and not supply fail indicator.

[merlinb]

for -9V to -12V operation, your LED indicator is backwards.

The whole circuit will not work with -9V and will barely work at all with +9V. Tbh I think it is a troll, I don't believe this is the real Jim Hagerman.

[Jim Hagerman]

for -9V to -12V operation, your LED indicator is backwards.

Look at the wiring for the DC power jack. Notice center tap goes to circuit ground.

[antonis]

Look at the wiring for the DC power jack. Notice center tap goes to circuit ground.

And LED's Anode to -9V..
So we have a back-biased LED, don't we..??

Also, that 100μF cap doesn't very comfortabe..

[Jim Hagerman]

And LED's Anode to -9V..
So we have a back-biased LED, don't we..??

Also, that 100μF cap doesn't very comfortabe..

Look again. Power supply rail is 9V positive wrt circuit ground. LED functions properly as shown. You are welcome to use a larger filter capacitor, but 100uF fed by 10k makes for a very quiet B+ rail.

You can spice things up with more components, circuit shown is minimalist, easiest to build.

[Jim Hagerman]

This is what output signal (500mV/div) can look like with a good tube, 100mV input:

[antonis]

Power supply rail is 9V positive wrt circuit ground.

Just in case you're not a troll, -9V means NEGATIVE..!!!

As for the 100μF cap, it's not the size but the polarity..

[Ksander]

https://en.m.wikipedia.org/wiki/Brandolini%27s_law

[PRR]

Jim's drawing is correct but unclear.

[drdn0]

I mean, I really don't see how this is any different (or better) than any of the Valvecaster variants that have existed forever.

Throwing 'not for commercial use' on a post is also sort of funny

[GibsonGM]

There is nothing proprietary here, other than the actual drawing, which is of course copyrighted by its creator.

[duck_arse]

wrt op stated text

Operate anywhere from -9V to -12V

I looked again.

alright. at 280% normal size, I see the circuit diagram is poorly drawn/labelled. what about the text, how do we fix that?

[amptramp]

With a 9 volt rail and 10K cathode resistors and 100K plate resistors, you cannot get more than 8.18 microamps per stage.  I doubt you can get much amplification out of that with a 12AU7.

[Clint Eastwood]

At first glance you would think we should be very gratefull for this fantastic gift. However..
I see at the bottom of the first post a link to a commercial website. If Jim Hagerman wants to sell things like an ordinary 2 watt amplifier combo for almost 1000 dollars, that's his business. But should he be allowed to use this forum for his linkbuilding?

[Jim Hagerman]

With a 9 volt rail and 10K cathode resistors and 100K plate resistors, you cannot get more than 8.18 microamps per stage.  I doubt you can get much amplification out of that with a 12AU7.

You are close! Plate current is roughly 20uA for each stage (2V bias across 100k plate load). See video posted above showing very strong output signal, more than most pedals. With a selected tube you can easily get 20dB gain.

[PRR]

cannot get more than 8.18 microamps

I can't follow your math. Maybe arthitis in counting fingers? I get closer to 80 than to 8 microAmps.

VOLTage gain in tubes is little affected by operating current, long as it can drag the load.

I'm not up to boiling some electrons today. Or even squinting 12AU7 low-I data. I do have a spice model I trust +/-50% or so. It says 50uA and gV=3, for the one stage. Say 100uA and gV>7 for two stages, which is just about enuff to clip guitar.



EDIT: I see Jim got different numbers from actual trial. My model is, if anything, pessimistic at low current and voltage. 20dB in two is essentially 3.1 per stage; the DC currents disagree and I may not care.

[Jim Hagerman]

My model is, if anything, pessimistic at low current and voltage.

Very nice, promising results. A few things you may want to try: add a 1uF bypass to cathode resistor, gain will go up. Add 100k loading through a 100nF, gain will go back down. Also, add maybe 20k source impedance from generator? This is important because a starved tube no longer behaves nicely as grid current goes through the roof.

From what I've seen, a tube operated at proper high voltage will have a cathode-to-grid current ratio (Ik/Ig) of about 20,000. When greatly starved (as shown), that ratio drops to about 20. This is why you see unusual values in the circuit; it's to deal with grid current and its effects on bias. I'd be surprised if any mathematical tube model accurately portrays such characteristics.

Is there a way to take into account 9V on heater? I get about 130mA, or roughly 62% of normal heater power. Hence, electron emission will also go way down, gm drops, rp goes up. Fortunately, our plate current is 1/1000th of what this tube can normally do, so 9V heater is a nice compromise. It may also greatly increase tube life, but that is debatable.

Anyhow, with a strong tube I get a stage voltage gain of about 3 at 9V and 4 at 12V. We're in the same ballpark!

[Jim Hagerman]

Here's what I measure with a real tube. Nice clean gain at low signal level, but then it starts to compress and finally clip at high input signals. Axis are in mV. This is why bass players love it so much.