Reducing EL84 screen voltage while keeping plate voltage high

[Mike_J]

Trying to figure out a way to reduce screen voltage to an EL84 and at the same time keep the plate voltage high.  The reason I thought of this is the AC30 I did a rebuild on had screen voltages that I know are too high for modern tubes (about 350VDC).  From web searches it seems some report favorable results with their AC30 plate voltage around 370VDC.  Was wondering if I could wire the plate solid state rectified and the screen supply through a 5Y3, or another rectifier tube with a large voltage drop, to reduce the voltage to the screens by about 50VDC.


The plate voltage was about 357VDC through a 5AR4 which supposedly drops about 10 volts which is why I thought solid state should get the voltage to about 367VDC.  Placing a 5Y3 in the power supply should drop the screen voltage to about 317VDC I would think.


My questions are:
1. Is this something I could do?
2. Is it something I would want to do?
3. How would I go about doing it?


Any suggested alternatives would be appreciated.  I tried increasing the screen resistor from 100 to 1K ohms and it reduced about 6VDC which is what put me on this quest.


Thanks
Mike

[sluckey]

VVR for the screens only. Replace the pot with a fixed voltage divider. Low cost and easy to do.

[jjasilli]

Trying to figure out a way to reduce screen voltage to an EL84 and at the same time keep the plate voltage high. 
 
This requires something special, as sluckey points out.  The screen draws little current at idle, so a mere series screen resistor drops little screen voltage (Ohm's Law) at idle .  Also, google:  "reduce screen voltage"


If you use rectifiers with higher impedance it will drop both plate & screen voltage, and they will remain about the same at idle. 

[PRR]

Screens alone on a vacuum rectifier won't drop much.

What Sluckey says. Even back in 1990 (when high-volt transistors were expensive!), that's what I did (except NVVR: Non-voltage variable reg, pre-set on the bench.)

[Willabe]

The plate voltage was about 357VDC through a 5AR4 which supposedly drops about 10 volts which is why I thought solid state should get the voltage to about 367VDC.

I don't think you'll hear much if any difference in ~ +/- 10 dcv's at the EL84 plates.

I tried increasing the screen resistor from 100 to 1K ohms and it reduced about 6VDC which is what put me on this quest.

Dropping the screen dcv's is a good idea, if their too high. The 100R on the screens does not provide much protection the 1K screen R will and it is dropping a few more dcv's in the right direction. (Higher value screen R's can cause more compression in the amps sound. Some like it, some don't. You might have to try different values, you can go even higher, if you want.)

I'd try leaving that 1K screen R AND do what Sluckey suggested.        

[Mike_J]

Thank you to everyone for your input and suggestions.  Looked through my Kevin O'Connor books and unfortunately I don't have the one that covers the VVR.  Have performed a web search.  So far have figured out that an N-channel mosfet (IRFP460 or another) and a zener diode is needed and that heat is its enemy so layout is important.  Not sure what zener diode is needed to reduce voltage 30 or 40 volts.  Most I have seen are 12 volt?


Haven't found a layout that looked as though it would apply to an AC30 screen supply.  Also, saw some layouts with a 1 meg pot but not sure how I would construct a fixed voltage divider.  Again, any suggestions, advice or nice well thought-out layout diagrams for this application would be appreciated.


Thanks
Mike

[sluckey]

Search Hoffmans forum for VVR. Lot's of info, schematics, layouts, pics right here. The simple circuit can be built on a simple terminal strip.

[Mike_J]

Search Hoffmans forum for VVR. Lot's of info, schematics, layouts, pics right here. The simple circuit can be built on a simple terminal strip.

Found a lot of information from Tubenit and others which resulted in questions. 


1.  Does the 12 volt zener diode mean only 12 volts can be dropped with this circuit?  Sorry for the level of ignorance but I would like to understand this a little better.


2.  For the AC30 do I put the entry of the VVR after the choke and exit the VVR circuit into the screen electrolytic cap which feeds the individual screen resistors?


3.  I think when you said to use a fixed voltage divider, does that mean I could use a 1 meg pot and adjust it to get the voltage I want and measure each side and replace the pot with two resistors of the measure values and install appropriately?


4.  Since the screen voltage would be fixed, albeit lower, would I just change the dropping resistors to the PI and preamp tubes to get the appropriate plate voltages for those tubes?


Thanks
Mike

[printer2]

Search Hoffmans forum for VVR. Lot's of info, schematics, layouts, pics right here. The simple circuit can be built on a simple terminal strip.

Found a lot of information from Tubenit and others which resulted in questions. 


1.  Does the 12 volt zener diode mean only 12 volts can be dropped with this circuit?  Sorry for the level of ignorance but I would like to understand this a little better.


2.  For the AC30 do I put the entry of the VVR after the choke and exit the VVR circuit into the screen electrolytic cap which feeds the individual screen resistors?


3.  I think when you said to use a fixed voltage divider, does that mean I could use a 1 meg pot and adjust it to get the voltage I want and measure each side and replace the pot with two resistors of the measure values and install appropriately?


4.  Since the screen voltage would be fixed, albeit lower, would I just change the dropping resistors to the PI and preamp tubes to get the appropriate plate voltages for those tubes?


Thanks
Mike

The zener is there only to protect the mosfet. That is where I would put it. Yes. You may not need to change much, and yes you could if need be. You mentioned about heat earlier, not much current running through the VVR as compared to doing the plates, also not much of a voltage drop. Just think of the VVR as a resistor, the current times the voltage is the power being dissipated.

[Mike_J]

You mentioned about heat earlier, not much current running through the VVR as compared to doing the plates, also not much of a voltage drop.

Thank you for your response.  How would you attachi the mosfet given a 50 volt drop from the screen supply?  I have seen videos where the mosfet is attached to the chassis with some paste to dissipate heat.  Is that necessary in this instance or could I just attach it to a terminal strip without attaching it to the chassis (with the mica sheet as an insulator)?  I have also seen some schematics with what looks like additional power capacitors. Any idea why?


Thanks
Mike

[kagliostro]

Screens consumption is low so may be a resistor divider can do the job

but +1 for Sluckey, with a VVR you can surely solve the problem

Franco

[Mike_J]

Attached is my attempt at a schematic for reducing the screen supply voltage with a VVR.  Any comments would be appreciated. 

[Willabe]

I think the zener goes in between the mosfet and R?

And I think you have to isolate the VVR from the preamp B+. Need a 1N4007 diode and another B+ filter cap after it to smooth out the pulsating dcv caused by the 1N4007, say 22uF?

   

[Mike_J]

I think the zener goes in between the mosfet and R?

And I think you have to isolate the VVR from the preamp B+. Need a 1N4007 diode and another B+ filter cap after it to smooth out the pulsating dcv caused by the 1N4007, say 22uF?

 

Thanks Willabe.  Attached a modified schematic with the diode and filter cap included.  Is this how you thought it should be done?  All the schematics I have seen show the zener diode after the 10R/5W resistor.  Not sure which is correct.


Thanks
Mike

[2deaf]

You don't need the diode between the Choke and the Drain.  You only need the diode after the Source and the 10 ohm resistor when there is more filtering over there.  The Zener diode goes from the distal end of the 10 ohm resistor to the Gate as originally shown.

[Mike_J]

You don't need the diode between the Choke and the Drain.  You only need the diode after the Source and the 10 ohm resistor when there is more filtering over there.  The Zener diode goes from the distal end of the 10 ohm resistor to the Gate as originally shown.

Thank you for your response.  Do you think I need to add more filtering after the 10 ohm resistor?  I added the diode and a filter cap between the choke and drain on the latest schematic.  Should I have filtering in both places? 


Thanks
mike

[2deaf]

I don't see why you need more filtering for the screens than the amp originally had, but maybe I'm missing something. 

[Mike_J]

I don't see why you need more filtering for the screens than the amp originally had, but maybe I'm missing something.

That is what I thought.  Already have a filter cap for the screen supply on the right side of the choke but I don't understand the mechanics of a lot of this.


Very thankful to you for the formula for the fixed voltage divider and your schematic.  Was going to use a one watt trimmer but fixed resistors would be better.  Did the calculation for 357 plate volts and a desired 300 volt screen supply and came up with 89K.  Can I use a 91K 1/2 watt resistor and a 470K 1/2 watt resistor for this application.


Thanks
Mike 

[PRR]

> desired 300 volt ... 470K 1/2 watt resistor

300V across 470K is over 2/3rd mA.

2/3rd ma (0.00064A) times 300V is 0.190 Watts.

Assume things may get 20% over-voltage. 0.276 Watts.

Double that. Unless you expect a lot of 20% over-volt time, half-Watt appears OK.

You should look at resistor voltage rating. Many are rated 300V MAX. No instant death, but increased drift and shorter life. You are on-the-edge. If you want it to live "forever", you'd go to a larger (higher V rated) resistor, or two resistors series.

Screen current for a pair of hard-worked EL84 can be over 24mA (or 2mA idle, which is why a passive divider is often difficult). 57V at 24mA is 1.4 Watts. A TO220 package "can" dissipate over 1 Watt, but it will burn your hand, and IMHO is perhaps over-hot if you offer more than 30 day warranty (or don't *enjoy* fixing your own DIY work). It really wants some place to put heat.

Note that a metal-tab device has the tab at 357V! You need good insulation before you "bolt to chassis". There are plastic-encased devices now, and this is probably a better idea.

Consider about 1K 2W-5W between +357V and Drain. By dropping about 24V this takes some of the heat load off the MOSFET. More important, in a dead-short (cap-surge), the current can't go to "infinity" but "only" to 700mA, and the resistor will pop before the MOSFET. This does not replace any K resistors AT the screen grid pins.

[Mike_J]

>
You should look at resistor voltage rating. Many are rated 300V MAX. No instant death, but increased drift and shorter life. You are on-the-edge. If you want it to live "forever", you'd go to a larger (higher V rated) resistor, or two resistors series.

Screen current for a pair of hard-worked EL84 can be over 24mA (or 2mA idle, which is why a passive divider is often difficult). 57V at 24mA is 1.4 Watts. A TO220 package "can" dissipate over 1 Watt, but it will burn your hand, and IMHO is perhaps over-hot if you offer more than 30 day warranty (or don't *enjoy* fixing your own DIY work). It really wants some place to put heat.

Note that a metal-tab device has the tab at 357V! You need good insulation before you "bolt to chassis". There are plastic-encased devices now, and this is probably a better idea.

Consider about 1K 2W-5W between +357V and Drain. By dropping about 24V this takes some of the heat load off the MOSFET. More important, in a dead-short (cap-surge), the current can't go to "infinity" but "only" to 700mA, and the resistor will pop before the MOSFET. This does not replace any K resistors AT the screen grid pins.

Think I will go with 1 watt 470K metal film to be safe.


I really want to do this once and do it right.  If you can recommend a part number for a plastic cased device that would be better could you please list it?


Like the idea of 1K - 2 to 5W resistor between the +357V and Drain.  Will change the screen resistors back to 100R from current 1K if they are in series. Maybe your statement that it does not replace the resistors at the screen grid pins covers this and I don't quite understand the relationship. Have seen some reports of too much sag with much more than 1K screen resistors.  Have no idea how much validity there is to it and whether lower screen voltage results in sag whether it is from a resistor or a mosfet.


Thanks
Mike

[Mike_J]

I talked to Mouser.  Tech there thought the 1K resistor before the drain was a good idea to help against surges hitting the drain at start up.  He said he has seen values from 20R to 1K with 470R probably being most common.  He didn't seem at all concerned about the amount of screen current hitting the mosfet otherwise.  He said it is engineered for six times what four EL84s will throw at it.  Don't know if he is right.  He was also saying it can handle over 300 degrees Fahrenheit which would be a torch inside the amp.  Don't want anything close to that temperature inside a chassis which is what PRR was alluding to.

Thanks
Mike

[PRR]

> Tech there thought

I'm impressed. I ass-umed distributor staff were drones, but this one has enough clues.

The *screen* current may be 24mA, 0.024A. Perhaps 0.050A if things go pretty wrong.

The smallest MOSFET with enough case-size for the heat is probably rated 4A. About 80 times more than you need.

The possible problem is turn-on with caps at zero voltage. If we take 50 Ohms total supply impedance, 350V/50r is 7 Amps, beyond the MOSFET's rating. With 1K in the loop, only 0.35 Amps, easy.

Leave those stock screen resistors as-is! Taking them out is like blocking-up the springs on your Jeep with solid metal from axle to frame. No "give". 500r, 1K, I do not care (I don't have exact experience). But the 1K before the semi-regulating MOSFET does not let the screens "sag" when you hammer the plates to the max. when you work these tubes much over 250V, you need screen resistors that will sag.

[2deaf]

I don't get it.  How is 7 amps going through the MOSFET?  It would handle it, BTW, being rated at 14A continuos at room temperature and 8.7A boiling (sea level).

[Mike_J]

Thank you for the replies. Was concerned about the longevity of the mosfet but I think PRRs advice is very sound. I very much appreciate your input as well 2 deaf. It is good to test answers for no other reason than to arrive at the correct conclusion.

Thanks
Mike

[2deaf]

I think PRRs advice is very sound.

I would say. 

Every forum seems to have their resident authority, but PRR and possibly Merlin are in a class by themselves.  Any time PRR says something to you, pay very close attention.

[Mike_J]

I think PRRs advice is very sound.

I would say. 

Every forum seems to have their resident authority, but PRR and possibly Merlin are in a class by themselves.  Any time PRR says something to you, pay very close attention.

Agreed

Thanks
Mike