PI with negative voltage reference instead to be connected to ground ?

[kagliostro]

In a tread in an italian forum where a guy was asking about the use of an unused 30v winding as to obtain a "balanced PI" (??? I don't understand what he was meaning)

one other member of the forum suggested to use the winding as to obtain a negative voltage and to use it as reference for the PI as to increase the dynamic (I think he mean the swing)

as an example he attached this schematic



I didn't remember to have seen a similar solution applied in a guitar amp

do you think there is a chance for the negative reference of the PI to be interesting in guitar amps ??

Many Thanks

K

p.s.: The source of the schematic :
http://www.angelfire.com/electronic/funwithtubes/Amp-Phase.html

[HotBluePlates]

In a tread in an italian forum where a guy was asking about the use of an unused 30v winding as to obtain a "balanced PI" (??? I don't understand what he was meaning)

one other member of the forum suggested to use the winding as to obtain a negative voltage and to use it as reference for the PI as to increase the dynamic (I think he mean the swing)

That is a variation of the long-tail inverter.

The bigger the tail resistance, the better the balance between outputs. In this case, the resistor has been replaced with a transistor constant-current source, which passes current but simulates a very large resistance.

A given current for the triodes to pass, through a very large resistor, creates a very large voltage drop. You might need more total supply voltage to allow for plate resistor drop + triode voltage + tail resistor drop than what any of the other stages require. The easiest way to give the long-tail "more supply voltage" is to use the B+ and add a auxiliary negative voltage supply.

I didn't remember to have seen a similar solution applied in a guitar amp

do you think there is a chance for the negative reference of the PI to be interesting in guitar amps ??

Guitar amps never seem to strive for perfect balance; they often use relatively small tail resistances compared to, say, an oscilloscope. That said, I haven't measured a guitar inverter to see if there's any significant imbalance (a fast way to do it might be to measure the 2 outputs on a 2-channel scope, and use the scope's ADD function).

Older guitar amps don't have a negative voltage supply, other than for bias. Because the tube grids don't draw current under normal operation, the bias supply may not be capable of maintaining its voltage with significant current draw (I don't know, as again, I've never tried measuring that).

If you're interested in trying this variation in a guitar amp, then making a negative voltage supply shouldn't be too hard.

[kagliostro]

Many thanks HotBluePlates

My interest is about the knowledge about thing that I was missing

so I higly appreciate your explanations about this argument

K

[HotBluePlates]

Can someone familiar with solid-state explain to what current the 2N3904 is biased?

Nevermind; read some of the article and I get it.

Overall, the biasing of the transistor is for ~3mA of collector current, so each triode gets 1.5mA. 1.5mA * 100kΩ = 150v, and the 12AX7 cathode is only going to be a volt or 2 above ground. That idles the 12AX7 plates at about 1/2 the supply voltage, and probably approaches the maximum clean output for the long-tail.

[kagliostro]

 

 

K

[PRR]

> what current the 2N3904 is biased?

Figure the base voltage. 3K and 1K, it's 1/4 of the 20V supply, so 5V.

Subtract base-emitter drop, assume 0.6V. 4.4V.

4.4V must appear across the 1.6K emitter resistor. 2.75mA.

The "1K" is really shown as 1.1K. We can use smaller thumbs, or approximate the voltage and current 10% higher. Pretty-near 3mA.

Note that the base bias divider current is larger than the transistor current. We do not need any particular current-gain in the transistor. If the bias current is much less than transistor current, we should include the effects of base current loading. That can be messy. The scheme shown is "wasteful" but ~~4mA 20V is "nothing" in a tube amp.
___________________________________

> I didn't remember to have seen a similar solution applied in a guitar amp

Who has spare windings? Or if so, who has the pennies to put in a diode and capacitor? Such silly stuff is only in DIY.

Although, in guitar amps: most BIG amps have a Negative Bias supply. Usually this is low current. However some early Mesa Boogies used the bias supply to feed transistor things.

I've been even sillier. Half-Watt P-P triode output stage (headphone amp) on 300V. The long-tail driver was +300V to the plates and -300V to the long-tail. A simple 100K resistor forced essentially perfect balance.

You find a similar thing in the popular tube op-amp module. In one model the inputs could swing +/-100V common-mode with little error.

In the plan you show: I do not see a reason for the 470K 0.1uFd on the right side tube grid.

[HotBluePlates]

> what current the 2N3904 is biased?

Figure the base voltage. 3K and 1K, it's 1/4 of the 20V supply, so 5V.

Subtract base-emitter drop, assume 0.6V. 4.4V.

That's what I eventually realized: base voltage is known due to divider, emitter voltage is less by a diode drop, and resulted emitter voltage tells you the collector current, close enough.

Is it painfully obvious I don't play with transistors much? 

In the plan you show: I do not see a reason for the 470K 0.1uFd on the right side tube grid.

Those could just be a piece of wire to ground, right?

I get the impression based on the rest of the guy's page (where he derives several versions of this circuit) that these might be placeholders for a contemplated a.c. connection to feedback.

[DummyLoad]

Those could just be a piece of wire to ground, right?

yessir. 

I've been even sillier. Half-Watt P-P triode output stage (headphone amp) on 300V. The long-tail driver was +300V to the plates and -300V to the long-tail. A simple 100K resistor forced essentially perfect balance.

how did it sound?

getting silly with tubeCAD. with a 6SN7 i was able to tweak to around .7%. i was able to get even better with a 12AT7 - .5% - both sims with 300V B+ and -300V B- at ~3ma and 100K for the tail R - tail R would need to be 1W or better.
 
--pete

[kagliostro]

Many thanks to all for the interesting answers

K