Vox Tonelab ST High Voltage Mod?

[kingkorg]

Hi people.

I was wondering if i could use a separate power supply and raise plate volatage to something like 200v.

This is tube section of the unit. It uses 12ax7 runing at 30v.

In case i raise voltage, i should change rating of C125 to something that can handle high voltage, and re-bias the tube.

Is there something else i should do?

 

[PRR]

If you do that, the tube will be "dead clean" for guitar (or ADC) levels. No creamy vacuum distortion. You might as well bypass it. (As I have done; but on a recording channel, not a guitar tone-box.)

 

[Bo Deadly]

You would need to change a lot to get it to work at the higher voltage and make sense.

The triodes are apparently trying to mimic a long tail pair phase inverter used to drive output tubes in a power amp. Note that the cathode bypass capacitor is labelled C82 NU presumably for "Not Used" and explains is why the output of the anode of the left triode isn't even used.

When that LTP circuit is overdriven, it rounds off the peaks. The transfer function is an S shape instead of squared off edges. It's a wave shaping device. The circuit driving it is also an LTP that probably has the same effect except at some point the two would be compounded.

If I were trying to do what I think you're thinking, I would first look at it on a scope, on the audio spectrum analyzer and record noise levels carefully. Maybe run the analyzer so that it records spectra as it steps through increasing levels (the QA400 is great for this). I would hookup the function generator to put in a triangle wave and look at what happens to the triangle shape on the scope as the LTP is overdriven. My guess is it would be rounded off into a sine wave and then into a square wave.

Armed with that knowledge, I would try to think about adjusting all of the resistors and capacitors around the triodes to account for changing the supply voltage. The bias is going to change. Not sure what the values would need to be of course. I would just have to play around with it. Or do a quick model in LTSpice actually.

As for the power supply, you might actually find a 200+V SMPS. Since it's only 1 triode, the SMPS only needs to be ~1W. There might actually be an SMPS like that. A 1W DC/DC converter for 200V could be the size of a deck of cards. Just feed it the 12V from the heater supply and viola you have 200V.

 

[kingkorg]

Thank you guys, that was more than helpful.

I'll play around with it and report back!

 

[Kingston]

It's 12AX7. At 200VDC it will be linear in that circuit, 12AU7 would be, too, and you can't reach the [harsh] clipping point from the driving circuit.

But you will certainly learn a lot implementing that.

 

[kingkorg]

I did the test as squarewave sugested. And it does behave the way he described.

However the unit has multilayer pcb and smallest possible smd components, surrounded by tall electrolytics. Modding is close to impossible unfortunately.

One thing that comes to mind is trying out some of those subminiature tubes that operate at full potential at 30v.

 

[PRR]

> tubes that operate at full potential at 30v.

Tubes is tubes, electrons are electrons. There is a small glitch in that conventionally processed tubes may go off-bias when B+ is less than Mu (the contact potential is OTOO 1V). This can be mitigated by unusually LOW impedance drive (stupid in all-tube work but trivial with opamps) and, today, buying a box of every brand tube on the market and selecting for happy result, then buying a boat-load before they change the recipe. (The 30V series are just processed with more care than 100V-300V types.)

You want a high-voltage tube, build a tube amp with 400V in it. Don't dink around with SMD boards and cheap parts made for cellphones instead of Real Amplifiers.

 

[Kingston]

One thing that comes to mind is trying out some of those subminiature tubes that operate at full potential at 30v.

From the original designer point of view the tonelab set up performing at full potential. It's meant to be a gimmicky overdrive effect. It's not at 30VDC to save money or to gimp the performance in any form, but to serve as an effect.

As PRR already suggested, taking the thing up to 200VDC is practically the same as bypassing the whole tube. You will not hear it anymore.

 

[Kingston]

I propose a far more interesting experiment. One that actually fits the tonelab PCB.

Insert a  variable PSU voltage dropper between R162/R162 and the 30VDC source.  You can do this with a single pot and an electrolytic with an optional limiter resistor so the thing will play between maybe 5-30VDC. This will take the design straight into high end stompbox territory. I think you will be quite surprised with the fuzzy characteristics of 12AX7 at very low voltages.

 

[kingkorg]

Now that's a cool sugestion.
That point in the circuit is very tricky to tamper with, as the track to those two resistors are coming from inside the pcb (multilayer). So it's very tricky to remove those 1mm smd resistors, or put something between them and 30v supply.

To be clear, i am not attempting to turn this unit into real tube amp, or anything like that. Just explore options...

By the way i love ToneLabs for what they are, i have also TonelabLe which changes the bias and topology of the circuit from A to AB and turns on and off the tube part. I believe it uses both triodes and changes arrangement. It has a dummy load after the tube which emulates Celestion speaker and is supposed to emulate amp-speaker interraction.

Tube or no tube, these are very good sounding units, especially if used with real speaker instead of speaker emulation part.

 

[kingkorg]

I made it today. I just disconnected anode and cathode pins from the tube board and made a small board with regular size R161 R162 R38 C124 and C125.

I used PSU from a tube mic for 140v, and used a pot to drop the voltage and experiment.

First of all, the tube is used as output stage of an amp, and doesn't overdrive the signal that much at all. All the distortion comes from digital modeling before it. I tried 140V, 30V which is stock, and down to 1.7V.

Biggest difference was in the output off course. Even at 1.7V the tube didn't saturate too much. It gained some extra sizzle in the high end, but did not turn into fuzz or anything like that.

At 140V it gained some low mids, but also at some spots i heard some nasty clipping. I believe it was starting to clip the next stage because of significantly higher output. Will try to address that later. Cool character change anyways.

Well at 30V it sounds stock...

I did not play with bias resistor, nor R161/162. I will do that tomorrow. 

I have some DC-DC converter i bought some time ago, it goes up to 400V so i'll try that as well.