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Old 9th December 2012, 04:55 PM   #61
DF96 is offline DF96  England
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Use a VR150 if 150V is an appropriate g2 voltage and you want to stabilise it. If not, don't use a VR150. My instinct would be to drop the VR150.
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Old 9th December 2012, 05:34 PM   #62
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I thought that would be best too. Thanks as always.
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Old 9th December 2012, 06:09 PM   #63
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Save it for a good preamp.

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Old 9th December 2012, 08:48 PM   #64
kevinkr is offline kevinkr  United States
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You could try something like a VR150 and VR75 in series for screen voltage supply before you call it a day.. Just make sure you have adequate voltage headroom to get the gas tubes to conduct.
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Old 9th December 2012, 09:54 PM   #65
AJT is offline AJT  Philippines
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the 6P15P likes 150v on its screens.....it's a video pentode...i would use a 6BQ5 or 6P14P instead...
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Old 10th December 2012, 01:42 PM   #66
JimW is offline JimW  United States
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There are mfg curves for both 150 and 200 volts on the screen of the 6np15 (sv83). With 150 volts, the cathode r has to be really small, on the order or 75 - 85 ohms per tube, and the bias voltage will be in the 3-4 volt range. With 200 volts, the bias resistor will be in the 135-150 ohm range, with bias voltages around 4.5-5. I have one set up with 200 (actually 210, slightly over) from a pair of stacked vr-105's, and I believe shoog is correct that regulating the screen has minimal effect on this design, but I had the parts and space, so I did it anyway, plus the vr tubes look cool. I used this tube in a "legacy" 300B amp, and used the simple resistor divider network, and it worked well. Just need to ensure that the screen voltage is low enough that the plate will not dip below it at maximum drive. I had 135 on screen, and about 235 on the plate.
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Old 16th December 2012, 07:47 PM   #67
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Thanks Everyone,

I've made a lot of progress. I tried it without the VR tube by using a voltage divider to get the G2 voltage down. It drew a little more current but not enough. I think it was 18ma per output. So I put the VR back, LOVE the purple. and put a 5watt 1k pot in parallel with the 250ohm cathode resistor and twiddled it. I came up with a cathode value of 100ohms. The outputs are drawing 33ma and the screens 4.1ma which seems about right. But now I have too much gain, and distortion. Is 6P15P more sensitive than EL84? The RH84 has positive (?) feedback through that 100k resistor. I might try disconnecting that to see what happens? Any thoughts?
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Old 17th December 2012, 02:08 PM   #68
JimW is offline JimW  United States
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The 6n15p does have higher gain. Because of this, I used a higher value feedback resistor (it is negative feedback, btw), my driver tubes (5695) have 25k plate loads, and I am using 150k feedback resister, works very well. I am using a pair of vr105 tubes stacked, and they are supplying voltage to both the driver and screens of the 6n15p. My setup has 300 volts on the plates, 200 on the screens, and about 35 ma per tube with a 135 ohm cathode resistor. Driver tube has 100 volts on the plate, running at about 6ma per section, through a 250 ohm unbypassed cathode resistor.
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Old 17th December 2012, 08:05 PM   #69
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Jim,

You've got me confused. I'm assuming its negative feedback because its attached to the plate? And using a [I]higher[/I ]value resistor gave you more feedback? And is the feedback through the "Rfb" resistor? And lastly, is it damping the preamp tube or the output tube?
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Old 18th December 2012, 01:35 AM   #70
AJT is offline AJT  Philippines
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because your 6p15p screen is 150v instead of 300v for the 6P14p types, you need less voltage to drive your grid...
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