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Old 15th May 2013, 09:03 PM   #1
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Default DC coupled 6P15P

Hey Anyone,

I want to build a DC coupled amp. It would be a SE amp using a 6GK5 to drive a parallel pair of 6P15p outputs. (If this is even possible that is) The idea came from the fact that the 6GK5 runs well in the 135 to 150 volt range. Which is about the right voltage for 6P15P grid 2. My question is would applying 150 volts DC to grid 1 be possible? And if that can be done would it make it unnecessary to apply voltage to grid 2? Or would the DC have to be applied to both grid 1 & 2? I'm assuming there would need to be a grid stopper in place to limit current? I have all the parts to build this amp. And will only DC couple it if it would possible. Of course I could build it without the cap first, assuming it wouldn't damage the tubes. Any help would be much appreciated.

Kevin
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Old 15th May 2013, 09:28 PM   #2
DF96 is offline DF96  England
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It is possible to apply 150V to g1 of a 6P15P. The valve probably won't survive such rough treatment, though. g1 and g2 are not the same thing so you can't swap their voltages around. I think you need to do some more reading about how pentodes work.

Some valves can have g2 drive. I don't know if that is one of them. In some cases (very beefy valves) a feed for g1 (via a resistor to limit current) can also be done. Generally this is only for valves designed for Class C or AB2, as they can cope with g1 grid current.
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Old 15th May 2013, 10:12 PM   #3
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Thanks DF96,

So driving grid 2 with the AC input is possible? That thought had occurred to me. It would be simple enough to try. If it doesn't work I could just add a coupling cap to grid 1. I could also try DC coupled to grid 2 withAC coupling to gird 1? Or two inputs to the tube might really muck things up?

The thing about being an amateur is you don't know what won't work.

Kevin
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Old 16th May 2013, 12:52 AM   #4
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Probably not going to work, see Tubelab's comment re EL84 (which is similar to the 6P15P other than the internally connected G3)...

Screen Drive EL84
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Old 16th May 2013, 01:32 AM   #5
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jazbo,

Thanks for the thread. Quoting Tubelab,
Quote:
You want to look for the lowest maximum screen grid voltage rating that you can find. This implies good screen sensitivity.
Maximum screen grid voltage on 6P15p is 150 volts. I know from experience that 6P15P is verysensitive much more than EL84. Also, Anatoliy tells me that on 6P15P grid2 is denser than EL84. So it might just work. Its worth a try.

Kevin

Last edited by mr2racer; 16th May 2013 at 01:37 AM.
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Old 16th May 2013, 01:55 AM   #6
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Sure, nothing beats actually trying it out.
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Old 16th May 2013, 01:19 PM   #7
DF96 is offline DF96  England
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If you can afford to lose a few valves, give it a try. I suspect it won't work too well or last too long as the 6p15p (~EL83) is not intended for this. Better to use a sweep tube/line output valve as they are more robust.
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Old 16th May 2013, 01:49 PM   #8
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The big advantage of screen drive is in push-pull, where you can run very low quiescent current with low distortion. In SE, you need an extra stage (cathode follower to supply grid current) plus more gain. But it can be done - see: Tube-Town Germany - Hot Stuff Cool Sounds

Such a scheme to stabilize DC operating point is worth investigating. Mullard 3-3 also uses DC feedback: http://vintageradio.me.uk/amplifier/mullard33_2.jpg
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Old 17th May 2013, 02:35 AM   #9
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Tom,

It seems the trick will be to not exceed the current capability of 6P15P's screen grid. If I run two outputs in parallel driven by a single 6GK5 won't the current of the preamp tube be split between the two output's screens?

Kevin
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Old 17th May 2013, 05:40 AM   #10
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Quote:
Originally Posted by jazbo8 View Post
Probably not going to work, see Tubelab's comment re EL84 (which is similar to the 6P15P other than the internally connected G3)...

Screen Drive EL84
It is similar, but not exact: 6P15P has denser G2, so G2 drive will work better than with EL84.

Here I have some data, including curves from "Not for public" book:

www.wavebourn.com • View topic - 6p15p data ?????? 6?15?
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