EL34 screen grid voltage regulator

[nexy_sm]

Hi all,

I am finally back to my old project of restoring and old Diastone amp based on Fender Reverb Deluxe amp, but with EL34 in the power stage. The original topic I started was here: https://el34world.com/Forum/index.php?topic=22560.msg241052#msg241052

The HT after the rectifier is about 516V, so I would like to reduce screen grid voltage a bit, to something like 400V. I this forum, I have found one approach similar to the linear regulator using a FET. Here are schematics:




Regarding this I have a few questions:
1. What should I expect from the screen grid in terms of load characteristics. Does it only consumes 0-5mA?
2. Why the second schematic from above uses diodes at the circuit's input as well as on the output?
3. Why do they protect gate-source from breakdown and how can that happen? Is g2 on power on a capacitor which is going to be loaded?

Thank you

[tubeswell]

The biggest problem is making sure there is enough capacity in the regulator to cope with the demands of peak screen current. 5mA is not  peak screen current in an EL34.

[2deaf]

1.  The screen grid on an EL34 can consume over ten times what the first schematic specifies.  The STP6NK90Z can conduct way more current than even multiple EL34's could consume.

2.  Beats me.  I'm confident it will work fine without the input diode.

3.  The 10R resistor and the 12V zener diode form a crude current limiter.  They are not there to restrict the gate-source voltage.  The 12V zener without the 10R on the hand drawn schematic likely resulted from a lack of understanding of the function of the two components.  The STP6NK90Z has built-in zener diodes that prevent gate-source over voltage.

The second question in #3 is not clear to me.  G₂ usually has a capacitor associated with it, but I don't know what these guys did.


I have attached an idea for a Deluxe Reverb style power supply that reduces the screen voltage.  The filter capacitor for the screens is at the gate of the MOSFET in order to take advantage of the capacitance multiplier effect.

[Colas LeGrippa]

Use an 0C3 as screen voltage regulator

[PRR]

> The HT after the rectifier is about 516V

At full load? Or UN-loaded?

516V no-load is likely to be closer to 450V with power tubes hot, and everybody works EL34 in that ballpark. Keep It Simple.

1N4007 diodes cost pennies and can save dollars. The sneak-paths around an amp at power-up, power-down, and OVER-drive are hard to figure out for-sure, that MOSFET Gate oxide is super fragile, play safe.

[2deaf]

Use an 0C3 as screen voltage regulator

Que es eso?

[kagliostro]

Que es eso?

http://www.r-type.org/exhib/aaa0450.htm

http://www.r-type.org/pdfs/gl874.pdf

Franco

[DummyLoad]

Use an 0C3 as screen voltage regulator

Que es eso?

glow tubes dude!

https://frank.pocnet.net/sheets/141/g/GL874.pdf

[nexy_sm]

@PRR Actually, you are right, I didn't take into consideration resistance of the secondary of the power transformer. So, open secondary voltage would be 516V.

The biggest problem is making sure there is enough capacity in the regulator to cope with the demands of peak screen current. 5mA is not  peak screen current in an EL34.

You probably mean the transistor should be able to dissipate quite some watts when loaded? What is the excpectable value of the peak current in the screen grid of EL34?

1.  The screen grid on an EL34 can consume over ten times what the first schematic specifies.  The STP6NK90Z can conduct way more current than even multiple EL34's could consume.

2.  Beats me.  I'm confident it will work fine without the input diode.

3.  The 10R resistor and the 12V zener diode form a crude current limiter.  They are not there to restrict the gate-source voltage.  The 12V zener without the 10R on the hand drawn schematic likely resulted from a lack of understanding of the function of the two components.  The STP6NK90Z has built-in zener diodes that prevent gate-source over voltage.

The second question in #3 is not clear to me.  G₂ usually has a capacitor associated with it, but I don't know what these guys did.


I have attached an idea for a Deluxe Reverb style power supply that reduces the screen voltage.  The filter capacitor for the screens is at the gate of the MOSFET in order to take advantage of the capacitance multiplier effect.

My third question is about what kind of load is the screen grid on power on? Is it of a capacitive nature?

In your schematic:
1. Why do you use diodes after the transistor? In which situation can happen that current flows in the opposite direction?
2. Why do you use 22uF on the gate, for filtering reasons? and why is a diode there?

[2deaf]

My third question is about what kind of load is the screen grid on power on? Is it of a capacitive nature?

In your schematic:
1. Why do you use diodes after the transistor? In which situation can happen that current flows in the opposite direction?
2. Why do you use 22uF on the gate, for filtering reasons? and why is a diode there?

The screen grid has very low capacitance and is of no consequence at power-up.  The usual filter capacitor associated with the screen grid is of great importance concerning the inrush current at power-up, however.  There is no filter capacitor at the screen power node in my schematic and there is a 10K current-limiting resistor in series with the P.I. filter capacitor, so inrush current through Q1 is not an issue.  Accordingly, I have removed the 10R/12V zener current-limiting network.

1.  Consider the case when the standby switch is first connected to ground.  There is no Gate current, so the Gate is at 0V.  C4 is at 400V, so the Gate-Source voltage would initially be 400V without D2.  The STP6NK90Z has internal zeners to protect the Gate-Source junction, but using a feature that is meant for emergency protection as an integral part of the power supply design seems like poor form to me.  Also, the STP6NK90Z is way overkill for this application and somebody might just substitute in a lessor part with no internal protection.

D3 prevents the lingering charge on C5 and C6 from energizing the screen grids when the standby is connected to ground.  This way the standby takes full effect quickly.

2.  The capacitance on the Gate gets multiplied by the gain of Q1 when it appears at the Source.  This way the filtering for the screen node is way more than if C3 had been placed at the Source, instead.

If you turn the power off, the still hot plates will drain C1 and C2 rapidly and bring the Drain of Q1 towards 0V.  If the Standby switch is in "play" mode, C3 will discharge relatively slowly.  This will result in the Gate becoming positive with respect to the Drain.  I don't want the Gate positive with respect to the Drain, so I put D1 in to prevent this.

         

[2deaf]

glow tubes dude!

Thanks, Pete and Franco.  I actually have a 0A3 that I have kept all of these years just because it looks cool.

It's a little hard to see how a 105V tube voltage regulator would be connected to achieve the goals of the OP. 

[kagliostro]

I was not suggesting to use and 0D3, only showing what it is

I'm not sure but this schematic




seems something I've draw some time ago (to feed a 1625 G2 ??)


Where did you find it ?

Franco

[nexy_sm]

@2deaf
Thank you for clarification. I understood your idea except the thing that C3 get multiplied by the amplification of Q1. In my opinion this is a voltage follower, and in this case with common drain topology, voltage gain should be close to 1. How I see this, is that C3 holds gate at the stable voltage, with less variations from the rectifier and since changes in gate voltage are filtered out, the changes in the source voltage will also be suppressed.

@kagliostro
I found it somewhere here, on the forum.

[2deaf]

I understood your idea except the thing that C3 get multiplied by the amplification of Q1. In my opinion this is a voltage follower, and in this case with common drain topology, voltage gain should be close to 1.

It is not the voltage gain of Q1 in a particular circuit.  It is the forward current gain that is inherent in Q1 itself.

[DummyLoad]

why do el34 screens need a regulated ps?


--pete