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Input impedance experimentations, aiming to build an A2 stage
Input impedance experimentations, aiming to build an A2 stage
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Old 16th July 2018, 06:46 AM   #21
Elerion is online now Elerion  Spain
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Quote:
Originally Posted by Gnobuddy View Post
Credit where due: George (Tubelab_com)
Well, yes, I supposed that. I just meant to say that it was you who recommended me to look for it. In reality, I already knew of this possibility, just I didn't get the occasion to work on it yet. This just accelerated the process :-)

I did tried the grid stopper resistor, and, as expected, it worked, but THD climbed from 0.4% to 3%, and that's the reason I wouldn't use it.

Re-reading some books, I recalled the grid-cathode zener, which I definitelly will include in my design. Thanks smoking-amp for pointing it out.

About the drain resistor, as I'm winding my own transformers, I'll just probably make a separate supply for the MOSFET, and only if that is not enough, include the resistor. A thin wire will be enough for the winding (~ 0.2mm) and it won't take too much space in the bobbin.

Last edited by jazbo8; 16th July 2018 at 07:39 AM. Reason: typos
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Old 16th July 2018, 05:02 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by smoking-amp View Post
...plus a Zener from cathode to grid...
Just a reminder, the zener will need an ordinary silicon diode in series to make sure it doesn't also clamp negative signal swings at (-0.7 V) or so. I got bitten by that once in a different application, many years ago, when I forgot that Zener diodes actually conduct in both directions!

The diode should be fast enough for full-bandwidth audio, and able to cope with the maximum negative peak voltage.

Vishay's 1N4148 datasheet specifies 75 volts max reverse voltage, which may be enough, depending on the drive signal.

-Gnobuddy
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Old 16th July 2018, 05:20 PM   #23
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Elerion View Post
This just accelerated the process :-)
Glad I could help.
Quote:
Originally Posted by Elerion View Post
I'm winding my own transformers
Impressive! As a boy, I've taken apart commercially made chokes (for old-fashioned long fluorescent lights), and in the process, realized I could never pack the laminations as tightly myself when re-assembling. I also lost some blood to those sharp-edged metal laminations.

Quote:
Originally Posted by Elerion View Post
I'll just probably make a separate supply for the MOSFET, and only if that is not enough, include the resistor.
I used a small, off-the-shelf 12.6-0-12.6 V transformer to build a small power supply for the MOSFET drivers. It generates +/- 40 V rails by using the entire 25.2V winding, half-wave rectified for each polarity.

Thinking about this now, maybe I should only use half the transformer winding for the positive rail, which should leave me with something closer to +20 volts / -40 volts.

The output valve I have in mind is a 12AB5 (same as 6V6, but in a smaller 9-pin envelope.) The datasheet shows positive G1 voltages up to +15 volts, so perhaps a +20V rail is really all that's necessary, if the grid stopper resistors are small enough.

The other possibility is to use something like 1k - 1.2k grid stoppers. 15 volts at the grid, around 25 V max across the grid stoppers at 20 - 25 mA grid current. The additional distortion might turn out to be a good thing for my purposes (electric guitar), though I will have to try it and see what it sounds like.

Tubelab George uses +/- 160V rails on his MOSFET driver in one of his current threads. I wonder if he incorporates additional protection against driving too much power into the output valve control grids.

-Gnobuddy
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Old 16th July 2018, 07:00 PM   #24
Elerion is online now Elerion  Spain
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Originally Posted by Gnobuddy View Post
I wonder if he incorporates additional protection against driving too much power into the output valve control grids.
Seems not.

From here: Limiting the grid current of PowerDrive

"The PowerDrive circuit (as concieved) can indeed feed the grid of a tube as much current as it can eat. Under normal music conditions transitions into the positive grid current regions should occur only on transient peaks, unless you run your amp into clipping a lot. These conditions should not cause excessive G1 dissipation since the time in the grid current region is very low. It should be noted that it might be possible to damage a tube by long term sine wave testing at or above the clipping level since the tube will see grid current on every cycle. I have not seen this happen on conventional G1 drive"

I just simulated the zener+regular diode circuit.

It makes the mosfet and tube grid happy, but not the diodes! If still using 70V for the V+, you can get 4A current during the overload situation, which seems like it can kill all the zeners I have at hand. A series resistor reduces the effectiveness, so you end up not knowing which device will die
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Last edited by Elerion; 16th July 2018 at 07:24 PM.
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Old 16th July 2018, 08:05 PM   #25
Elerion is online now Elerion  Spain
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I almost forgot. A 100 ohm drain resistors seems to work perfect.
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Old 17th July 2018, 02:55 AM   #26
smoking-amp is offline smoking-amp  United States
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IRFH5020

Using a 20 Amp Mosfet is asking for trouble, not just with the grid1 or Zener or diode, but input capacitance too. But as you found, a Drain resistor can limit the current without deteriorating the grid drive performance.
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Old 17th July 2018, 06:16 AM   #27
Elerion is online now Elerion  Spain
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Originally Posted by smoking-amp View Post
Using a 20 Amp Mosfet is asking for trouble, not just with the grid1 or Zener or diode, but input capacitance too.
No, I'm planning to use a STP11NK40Z, 5.5 A, and less than half the input capacitance (< 1nF) that the one I used in the simulation (just because I wanted a rough approximation, I didn't spend the time to look for a more precise model).

I've got IRFU9310, 1.1A, about 300 pF input capacitance, but it can barely dissiapte 1 W without heatsink. So I decided to use the bigger one.

By the way, how does a smaller mosfet help grid1 or zener or diode?
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Old 18th July 2018, 02:38 PM   #28
indra1 is offline indra1  Indonesia
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Originally Posted by Elerion View Post
... I've got IRFU9310, 1.1A, about 300 pF input capacitance, but it can barely dissiapte 1 W without heatsink. So I decided to use the bigger one....
I think a 5.5A part is not going to perform as good and using a heatsink is recommended. If a lager part is needed or you want to avoid IRF p-channel kink, try something like FQP2P40 still available from Digikey.
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Old 18th July 2018, 06:06 PM   #29
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Elerion View Post
It makes the mosfet and tube grid happy, but not the diodes! If still using 70V for the V+, you can get 4A current during the overload situation
Well, sure, you can't just put a 24V Zener across a 70 V DC power supply and expect anything to be happy! A drain or source resistor to limit current flow is necessary. Or an active current limit, like the short-circuit-protection circuits often used in solid-state power amp output stages.

I think it may be much simpler to simply limit the maximum AC signal coming in to the gate of the driver MOSFET. That is a high-impedance location with small signal currents, and easy to limit. By comparison the source of the same MOSFET is a low-impedance node capable of much higher currents - which is exactly why it makes a good grid driver for an output valve in A2 mode!

Just an idea - the attached schematic shows a MOSFET driver with a bipolar junction transistor added as a current limiter, to protect the delicate control grid of the output valve.

In the image, (R3+D1) represents the input of the valve being driven. The 60 V negative supply V3 biases the "grid" to (-60 V). Vgrid (the drive voltage to the grid from the MOSFET) can swing from (-60 V) up to (0 V) without any grid current flow, but if it swings positive, D1 conducts, and grid current starts to flow, and the MOSFET is now driving a load of about 1 kiloohm instead of infinity.

The problem we have been discussing is that, left to itself, the MOSFET can now drive up to nearly 70 mA of current into the grid, and this will quite likely fry the grid. (Assuming the input of the valve acts like roughly a 1k resistance once grid current starts flowing.)

This is where Q1 and R2 come in. R2 is quite small, and unlikely to cause much THD due to non-linear grid current flow through it. However, if enough current flows through it, the voltage drop across R2 will turn Q1 on, clamping the gate voltage of M1 and not allowing it to increase any more.

With values shown, this happens at about 16 mA in the LTSpice simulation, where you see the "knee" in the curve. R2 should probably be a 100 ohm trimpot in practice, to allow setting the maximum grid current. Note that R1 plays a role - as the MOSFET is driven harder and harder, but the gate voltage is clamped by Q1, more and more current will flow through R1. This will then flow through Q1, and into the grid of the output valve.

This is why the plot shows the grid current rising slowly past the 16 mA clamp limit, as the gate of M1 continues to be driven more and more positive, all the way to +70 V in this simulation. However, it doesn't increase beyond 21 mA in the simulation.

Note that the plot shows I(R3) - the current flowing into the grid of the output valve - as a function of V1, the signal driving the MOSFET gate.

In practice, one would turn R2 to a low value, then slowly turn V1 up until the output valve is in full clipping, and then just a smidge more. Now adust R2 so that it limits the grid current, but not enough to bring the valve out of clipping - there is still enough grid current for full output, but not enough to destroy the valve.

Adjusted this way, the current limiting should never become audible, because the output valves will be deep into clipping before Q1 steps in to protect them.

Since R2 is so small (a few tens of ohms), I don't think it will have a significant effect on THD. However, if this is a worry, by adding a second transistor (PNP), R2 can probably be moved to the drain side of M1. R2 will turn on the PNP transistor, which in turn will turn on Q1. The current limiting will still work as described here.

An even simpler solution is to simply wire a Zener diode (with series silicon diode) to limit the maximum positive signal voltage at the gate of M1. Choose the Zener voltage to give full drive to the output valve, without driving the grid so far positive that it is in danger of overdissipation and damage. R1 is still needed, to avoid damage to the Zener diode.

-Gnobuddy
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Old 25th July 2018, 07:26 AM   #30
Elerion is online now Elerion  Spain
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Thanks Gnobuddy, I played around with all your suggestions, and I agree. I just didn't understand how some other's driver circuits just don't have any current limiting mechanism, even though being powered by high dual voltages. So instead of looking too much for elaborate solutions, just to hide my lack of understanding, I tried to understand as much as possible the un-protected circuits.

My conclusion so far is that other simple approaches just rely on the fact that music is not a constant high level input, so the average power in the output stage remains on the safe side. But feeding a sine wave input might be asking for trouble, and so a very high (maybe accidental) input level.

So, I'm building a prototype monoblock for real world testing, and I want to use dedicated +/- supply for the mosfet stage, and a small drain resistor. Do some testing, to see how it works under sine wave conditions, and probably adding zener or bipolar for current limiting afterwards, just because they are cheap.

But the chassis is not a big one. I'm using a 20x25 cm aluminium sheet, to fit inside a wooden structure. I plan to place the mosfet stage on a perforated circuit board in the free area between the noval and octal socket, and the two transformers at the back. They will occupy most of the free area at the back. And I need three rectifiers with their filtering. Suggetions on layout/placement?
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