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....
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....
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.
Thanks for the advice. so what you're saying is that at 150 volts the grid 2 of 6p15p will only draw 2ma?
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.
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.
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.
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.
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?
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.
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...
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.
i used the 6P15P pentode to drive a 6C33 set.....http://www.diyaudio.com/forums/tubes-valves/216206-my-russian-set.html
Hey Guys,
Does the voltage applied to grid 2 cause the tube to conduct? Or is that simply the cathode resistor? Jim, you say you ended up with an 85ohm cathode resistor, was that for each tube or for both together?
The regulator glows and regulates just fine. But the outputs don't seem to draw much current. I'm assuming that is a bias problem then?
Does the voltage applied to grid 2 cause the tube to conduct? Or is that simply the cathode resistor? Jim, you say you ended up with an 85ohm cathode resistor, was that for each tube or for both together?
The regulator glows and regulates just fine. But the outputs don't seem to draw much current. I'm assuming that is a bias problem then?
think of grid2 in a pentode as the plate in a triode, there are 2 ways to increase plate current in a triode, one is by increasing plate voltage, and the other by decreasing grid1 voltage,(also lowering the value of the cathode resistor).........
can you post the scheme so we can look at it?
can you post the scheme so we can look at it?
Just an observation, my experience with the RH807 is that regulating the screen voltage will have absolutely no impact on the overall sound quality. It seems the schade feedback is to strong an influence on overall sound.
Shoog
Shoog
Applying a positive voltage to g2 of a pentode causes the tube to conduct. Using a cathode resistor to apply a positive voltage to the cathode reduces conduction, because in effect it applies a negative voltage to g1. The bigger the cathode resistor the less the tube conducts. Getting this right is called biassing.mr2racer said:
Thanks Again Guys,
Before I added the VR150 I used a 10k resistor to supply grid2. The tubes were at 28ma with the 270ohm cathode resistor. Then I added the gas regulator and conduction almost stopped, 15ma. Then I tried one 250 cathode resistor for both tubes, even worse 19ma total. Another member, whose opinion I respect, suggested regulating the 6P15P grid2 with a VR150 so I thought I would give it a try. If for no other reason than to investigate gas regulators.
I understand bias and load lines. I just wasn't sure about the voltage on grid2 and conduction. I spoke with Steve Deckart on Friday and he said he runs grid2 on 6P15P at 250 - 270 volts without a problem.
So I have two options; scrap the VR, up the voltage on G2, and fix the hole in the chassis? or, change the cathode resistor to a much lower value as suggested in a post above? Can anyone tell me which is more likely to sound better?
(I'm still learning about how pentodes work.)
Before I added the VR150 I used a 10k resistor to supply grid2. The tubes were at 28ma with the 270ohm cathode resistor. Then I added the gas regulator and conduction almost stopped, 15ma. Then I tried one 250 cathode resistor for both tubes, even worse 19ma total. Another member, whose opinion I respect, suggested regulating the 6P15P grid2 with a VR150 so I thought I would give it a try. If for no other reason than to investigate gas regulators.
I understand bias and load lines. I just wasn't sure about the voltage on grid2 and conduction. I spoke with Steve Deckart on Friday and he said he runs grid2 on 6P15P at 250 - 270 volts without a problem.
So I have two options; scrap the VR, up the voltage on G2, and fix the hole in the chassis? or, change the cathode resistor to a much lower value as suggested in a post above? Can anyone tell me which is more likely to sound better?
(I'm still learning about how pentodes work.)
Learn how pentodes work. Then design your output stage. At present you are just groping around in the dark. Alternatively, just copy whatever the valve datasheet suggests.
Low g2 voltage compensated for by small g1 voltage can give you the desired anode current, but then you will get trouble from g1 grid current on signal peaks.
Low g2 voltage compensated for by small g1 voltage can give you the desired anode current, but then you will get trouble from g1 grid current on signal peaks.
I personally would replace the front end with a suitable small signal pentode and regulate the g2 of that with the VR150. i have done exactly that on a Tabor clone (PP Schade) and it works a treat.
Others may not agree.
Shoog
Others may not agree.
Shoog
In this case I'm just modifying an old Motorola console chassis into an RH84. I've done this with another Zenith. Designing from scratch is something I'm just getting into. In this case though I don't want to rebuild an already almost finished amp. So, if I have the two options posted in #57 above, you would recommend not using the VR150?
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