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Old 28th June 2007, 12:29 PM   #1
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Default Cascoded 6N6P

hi all,

I have a bunch of 6N6P, they are really nice tubes, quite linear and powerful.
I'm thinking to use them to drive on 6AS7G (6N13S) tube, with both halves paralleded.

Two 6AS7G tubes will be used to form a PP output stage (actually a long-tailed pair, like THE ONE made by SHOOG).
The output HT will be of about 200V.

As you know I will need a really large signal to drive the 6AS7G tube (about 100Vp-p), so I need to build two preceeding stages with 6N6P to give the amp a sufficient gain.
I was thinking to realise a LTP with a CCS in the tail, using a 6N6P cascode for each leg of the LTP (two 6N6P tubes in total).
Can this be done?
This solution should give me enough gain to drive the 6AS7G.

Another solution is to simply build a LTP using one 6N6P (one triode per leg) and then drive a common-cathode, active loaded (CCS) interstage.
Of course in such a case I will have three stages, and feedback will became more difficult to manage.

I'm quite afraid about interstage transformer, what is your opinion?

Ciao,
Giovanni
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Old 28th June 2007, 02:08 PM   #2
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I can't answer all your questions, specifically the cascoded LTP Q. But have you thought of using a PP CF out of one of the 6n6p's after the gain stage? This would give a very low source Z to the finals which is a good thing. I've read that lots of the low rp tubes like the 6as7 are gasey and should be best driven from a source Z of 600R or lower.

I would look at Morgan Jone's glass palace front end, which you could substitute the 6n6p in lieu of the 6J5's. This is should work very well because reportedly the 6n6p has quite good triode to triode matching inside the envelopes which makes for good differential stages.

Then there is PP Aikido. You can cascade two PP Aikido stages with bipolar supply (so the second gets high B+) but that is a lot of tubes, although with low distortion it might be preferred to a simpler front end.

You can do a split load phase splitter into CF's driving the output tubes which helps keep the balance of the split load because of the buffer the CF provides. The gain in front of the split load can be a cascode if you like. You can even aikido-ize this setup. JB has provided all the pieces, you just need to add them together.

Plenty of choices. I am designing a PP 6C33C which is similar in a lot of ways so I have thought about the choices some. The glass palace is somewhat like your idea of a cascode LTP, except it is split into two LTP's cascaded instead. This is probably advantageous for a number of reasons but probably needs the bipolar supply.
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Old 28th June 2007, 05:51 PM   #3
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It can be done and work; but I personally left cascodes a long time ago for frequency conversion applications on UHF.
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Old 29th June 2007, 08:53 AM   #4
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If you go with the CCS in the cathode with the increased voltage which you are suggesting, then you will need to find a different chip to the LM317. The LM317 is only good to about 40V and since you will be putting about 100V across it, it will likely blow. Consider something from the IXY series CCS chips or a simple descrete CCS.

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Old 29th June 2007, 09:10 AM   #5
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The 6N6P looks like a great tube for driving 6AS7s, I've never tried the 6N6P tube, looks tempting.
The 6AS7 will need more like 100Vp, or 200Vpp, with b+ at 200V. With the high gm of the 6N6P you should get good gain in a cascode config, but enuff? Also beware the Vk-heater limits, (or use seperate heater windings).
Personally I believe a low rp tube like the 6AS7 should be run at lower voltages and higher currents...
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Old 29th June 2007, 10:35 AM   #6
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Quote:
Personally I believe a low rp tube like the 6AS7 should be run at lower voltages and higher currents.
I agree totally. I also think this configuration will sound better. It also makes the driver easier to setup.
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