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Old 7th December 2012, 10:32 PM   #51
AJT is offline AJT  Philippines
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i used the 6P15P pentode to drive a 6C33 set.....My Russian SET
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Old 9th December 2012, 02:57 AM   #52
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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?
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Old 9th December 2012, 03:11 AM   #53
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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?
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Old 9th December 2012, 08:55 AM   #54
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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.

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Old 9th December 2012, 01:33 PM   #55
DF96 is offline DF96  England
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Quote:
Originally Posted by mr2racer
Does the voltage applied to grid 2 cause the tube to conduct? Or is that simply the cathode resistor?
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.
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Old 9th December 2012, 02:36 PM   #56
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mr2, you probably need more voltage on the screen...
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Old 9th December 2012, 04:15 PM   #57
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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.)
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Old 9th December 2012, 04:21 PM   #58
DF96 is offline DF96  England
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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.
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Old 9th December 2012, 04:25 PM   #59
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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
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Old 9th December 2012, 04:42 PM   #60
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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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