Semi starved plate tube dilemma

Zebb

Member
2009-10-24 12:04 am
London
In short, does anybody know how easy it would be to adapt a starved triode pre-amp valve stage with a 12v (single sided) HT, to a 30v HT with a double sided (+/- 15v) existing PSU? (So a positive anode and a negative Cathode), and what biasing method would be best? And would there be any benefit?

Why?: I want to add a mild 'crunch' over-drive valve stage to my (home brew) solid state guitar practice amp as an over-drive effect.

From some experimenting I found the 'best' crunch tone I got for this purpose was from the circuit variants in Merlin Blencowe's article on low voltage triodes. http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf (pics included below (if they get through)).

Problem is, these are based on a 12v single ended supply with a biasing method specific to that arrangement. But the rest of my circuitry uses bi-poler +/-15v supplies which gives me the option of a 30v HT, which I would have thought was an advantage, but maybe not. Also, his designs are focused on minimising distortion, while my needs are specifically for (light) distortion (of the right kind).

Can anybody advise? As Mr. Blencowe points out, all the usual load line charts and calculators assume high voltage HT and triodes behave very differently at these low voltages, so it's left to real world test and measurements which are beyond my abilities/facilities.

I'm thinking of a combination of these two circuits (if the pics get past the moderators) (adding the cathode follower part of circuit 2 to to the output of circuit 1), but with a bipolar supply.
 

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Zebb

Member
2009-10-24 12:04 am
London
If you want distortion, high voltage is NOT your friend.

Run the "12V" amp on single 15V, no big difference.

Running cathodes on a negative rail can work, but is an un-fun complication.

So run the valve section from the positive 12-15v+ rail only (cathode resister to earth), but run the op-amp (impedance buffer) from bipolar rails while the capacitor (C1) blocks the DC, right?

12v to 30v will greatly change the characteristics of the triode. A normal 6922 works great with 42v B+, let alone space charge effects on tubes designed for low voltage.

I have to use ECC82 or ECC83 as thats allI have

With a coupling capacitor before the stage and one after, the only one knowing there is 30V across the plate and cathode (and resistors) is your mother. Don't forget to put in a grid leak resistor.

But how do I apply bias?

Also, grid leak is to bias the grid which is ok with high HT but doesn't work for low voltage, hence the 'pull up' grid resistors instead.

To be honest I don't quite get this which is why I'm here.
 

Printer2

Member
2010-04-02 6:34 pm
The grid of the triode acts as a diode when the input goes positive. because the grid go to low impedance when this diode conducts you have a high impedance feeding a low impedance, most of the voltage is developed in the source. The triode still conducts and amplifies just fine in the positive region if the source has a low impedance and can supply the current. This is what the opamp does, it buffers the signal and when the signal goes positive the clamping action of the diode in the triode does little to the signal. So no clipping of the positive peaks of the signal. Not much distortion.

But if we run at 12V the triode has a limited dynamic range, rather than a cathode bias voltage of 1.5V allowing us to pass a clean signal of 1.5V(roughly) we might have a bias of 0.5V or less. Doesn't quite sound the same, fizzy to my ears. But that is with a 12AX7. With a 12AU7 biased up properly a larger signal swing makes it through the tube before chopping off the top half. This is why people use the 12AU7 in the Valvecaster. Mind you the 12AU7 does not have as much gain, in comes the opamp.

So pick a flavor, grid biased or pull-up method. Or go fixed bias, use a voltage divider to adjust the bias point with the opamp feeding the grid through a coupling cap. Or just thinking about it now, why not try the Fig 9 circuit as is fed by the opamp? There is a cap on the input of it, if I paid it more attention I would not have typed as much.
 

Gnobuddy

Member
2016-03-01 4:10 pm
Problem is, these are based on a 12v single ended supply with a biasing method specific to that arrangement. But the rest of my circuitry uses bi-polar +/-15v supplies
Why fix what isn't broken? The circuit in your first image (Fig 4) requires no changes at all. Simply replace the "+12V" with your +15V rail. Nothing else is needed.

You don't indicate where heater power is coming from in the Fig 4 circuit. If it's from the +12V rail and not a separate heater power supply, you'll need to make one more change: add a resistor in series with the 12V heater, so that 12.6V appears across the heater, and the other 2.4V (of your 15V) are dropped across the resistor.

This may require a little trial and error, as the heater resistance changes as it warms up, and its ratings aren't exact. But the starting point is easy: the 12AU7 heater is rated at 300 mA and 12.6V. So we need a series resistance that will drop 2.4 volts when 0.3 A flows through it; Ohm's law tells us that its value will be eight (8) ohms. The nearest standard value is 8.2 ohms.

Incidentally, this resistor should be rated at at least 2 watts in order to have an adequate safety margin; the actual power dissipated in it once the heaters have warmed up, will be about 0.7 watts.

As for the circuit in your second image, you need only one change: the same 8 ohm (or 8.2 ohm) resistor in series with the heater.

In either case, I'd suggest starting with that 8 or 8.2 ohm resistor, then measuring the actual voltage across the heater pins of the tube, and tweaking the resistor accordingly. If there's less than 12.6V across the tube heater pins, lower the 8 ohm resistor a tad; if there's more than 12.6 V across the heater, raise the resistor a tad.

Fortunately, 2W resistors are not expensive. Chances are that one of these three values - 6.8 ohms, 8.2 ohms, or 10 ohms - will get you close enough.


-Gnobuddy
 

Zebb

Member
2009-10-24 12:04 am
London
Thanks for this Gnobuddy.

Actually these aren't mine, they're simplified theoretical circuits from Merlin Blencowe's article on Triodes at Low Voltages.

http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

They assume the heater power to be derived from the same 12v+ supply (as in top right of Fig. 9).

I don't have a problem with devising a heater supply. My dilemma is with biasing. Fig. 4 has an impedance buffer opamp that is powered by a single sided +12v supply, which means, I suppose, that the signal idle would have to be biased to +6v as well as the +2.6v for grid biasing. But my circuit would be fed by opamps with a bipolar supply of +/-15v each side, so the signal idle will be at 0v, which must affect the biasing.

My other dilemma is that I could derive a tube HT from the 30v available from my PSU (+15v to -15v). But would that have any advantage as it would make grid biasing even more complicated.
 

Gnobuddy

Member
2016-03-01 4:10 pm
Actually these aren't mine, they're simplified theoretical circuits from Merlin Blencowe's article on Triodes at Low Voltages.
Yes, I got that from your first post. :)
My dilemma is with biasing. Fig. 4 has an impedance buffer opamp that is powered by a single sided +12v supply, which means, I suppose, that the signal idle would have to be biased to +6v as well as the +2.6v for grid biasing.
Trust me, I understand the circuit, and there is no dilemma! :)

The op-amp isn't biased to +6V plus +2.6V. It's just biased to +2.6V. The voltage is indicated on the schematic you posted.
But my circuit would be fed by opamps with a bipolar supply of +/-15v each side, so the signal idle will be at 0v, which must affect the biasing.
Firstly, the TL071 is a single op-amp; so you can run that one op-amp from a single supply rail: use your 0V and +15V rails. Simplicity itself. Other op-amps elsewhere in your circuit can still be run off the +15/0/-15 bipolar rails.

Secondly, if you do run that TL071 from a bipolar supply, that changes nothing whatsoever. Use R3 to set the op-amps (+) input at +2.6V, and its output will follow. The op-amp doesn't care if its negative rail is 0V, or -15V!

If you still don't see it: the op-amp is wired as a unity-gain voltage follower. The output voltage is identical to the voltage on the (+) input - the non-inverting input. Set that to +2.6V, and the output will also be at +2.6V.

This will be true as long as the op-amp has enough power supply voltage for its output to settle at +2.6V. The op-amp doesn't care if you feed it +/- 12V, +12V and -15V, or +12V and 0V.

Note that the 12AU7 will still be wired as in Blencowe's figure, between 0V rail and +15V rail. (NOT to the -15V rail.)

My other dilemma is that I could derive a tube HT from the 30v available from my PSU (+15v to -15v). But would that have any advantage as it would make grid biasing even more complicated.
This is a bad idea, unless you want to throw out all of Blencowe's hard work, and start over doing your own research.

So, keep it simple: forget about the -15 V rail as far as the 12AU7 is concerned. Wire the 12AU7 between 0V and +15V, as Blencowe showed. Add the series resistor to let the heater work on +15V. And that solves the entire problem. No more dilemmas, except what snack to munch on while you solder the thing up. :D


-Gnobuddy