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Old 6th December 2012, 05:21 AM   #41
AJT is offline AJT  Philippines
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screens are like plates in a triode, to increase current you have to increase plate voltage...

another way to increase screen current is to decrease grid bias...but watch out that you do not exceed plate dissipation max of 12 watts for the EL84....
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Old 6th December 2012, 09:30 AM   #42
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The regulator is doing precisely what it is supposed to do: regulate the screen voltage. Reducing the series resistance merely increases the regulator current. If you want more screen current then you have to raise screen voltage. That means either using a different regulator or two in series to make up the voltage you want.
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Old 6th December 2012, 10:24 AM   #43
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Thanks for the advice. so what you're saying is that at 150 volts the grid 2 of 6p15p will only draw 2ma?
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Old 6th December 2012, 11:23 AM   #44
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So what I really need to do is manipulate the 6P15P cathode resistor?
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Old 6th December 2012, 11:31 AM   #45
DF96 is offline DF96  England
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At 150V screen and at whatever grid bias you have used it is clear that your 6p15p only draws 2mA. I have no idea whether this is to be expected, or why you regard this as significant.

If your design requires 150V on the screen, then you have it. If you want more than 2mA screen current then you need to increase screen voltage or make the grid less negative, with the ratio between the two changes being given by the 'inner mu' which is often given on pentode data sheets.
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Old 6th December 2012, 12:14 PM   #46
JimW is offline JimW  United States
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Default sv83 with 150 volts on screen

If you are using the stock rh84 circuit, the cathode resistor is 270 ohms. The sv83 will need a much lower value if you wish 30 -35 ma plate current. I did the very thing you are talking about, and if I recall correctly, I would up with about a 85 ohm cathode resistor. Bias voltage on the finals will be low, on the order of 3 volts or so, but with the low screen voltage, the gain of the sv83 rises, so it will still make decent power. As a side, svetlana also had curve for that tube with 200v on the screens, which is what I would up using, make it a little closer to el84 bias arrangement.
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Old 6th December 2012, 12:16 PM   #47
AJT is offline AJT  Philippines
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Quote:
Originally Posted by mr2racer View Post
Thanks for the advice. so what you're saying is that at 150 volts the grid 2 of 6p15p will only draw 2ma?
and the plate current will also be lowish, what is your cathode current? if we take it that screen current is about 1/5th of plate current, then you could be running 10mA of plate current, is this correct?
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Old 6th December 2012, 11:23 PM   #48
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Yes, total current through the tube is 15ma. And that includes the 2ma for the screens. I have 12ohm resistors in the screen supply so I can measure the current there. I think Jim is right the cathode resistor is choking the amp down too much.
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Old 6th December 2012, 11:30 PM   #49
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yes, decreasing the cathode resistor increases cathode current, the plus here is that you will need lower drive voltage....at 15mA and 300volts plate voltage your tube dissipates 4.5watts, your tube is capable of 12 watts so you still have much room to work on...is that a pp amp or a SET? if set, then close to 12 watts is ok, if pp then 70 to 80% is just right...
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Old 7th December 2012, 12:55 PM   #50
JimW is offline JimW  United States
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Hi Mr2racer, since your intended use is as a driver for a pair of tubes (SE, parallel I am assuming?) you could use the sv83 as an input tube, regulate the screen at 150 like you are proposing, use a suitable power resistor to set the plate voltage around 250, and a suitable cathode resistor to pass 10-12 ma through the tube. This is the set up T. Loesch used on his "legacy 300B" design, and it does indeed work very well as a gain/driver stage. I used this set up (although with a simple resistor divider for the screen voltage) for several years, and can testify to its merit. If you are going to drive a push pull pair, then the RH amp could be used with a properly gapped interstage transformer to drive a pair of triode strapped output tubes. Lundahl makes some that can be gapped up into the low 20's (ma plate current) that are designed for phase splitting.
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