4P1L

JamesD · Post by **JamesD** » Fri Aug 23, 2013 11:21 am

Just noticed on Polish data sheet (VM) that suppressor(G3) can be run at up to +15V - so much the same as we discovered and this increases power out. Used like this for rf use mainly (30MHz).

James

pre65 · Post by **pre65** » Fri Aug 23, 2013 11:36 am

So, what is the "best" configuration of 4P1L (as a pentode) to drive a GK-71 ?

I'm starting to draw out the "specifications" for my build.

IslandPink · Post by **IslandPink** » Fri Aug 23, 2013 11:44 am

This is the driver I'm intending to use, Phil - same as on my 300B amp right now :
http://www.audio-talk.co.uk/phpBB2/view ... &start=165

The cathode resistor can be 200R , I changed slightly to that on latest build . Best to have a 2W part on the 200R, I think it dissipates 0.5W or a bit more , as it has 45mA going through it .
Take the cathode connection from the filament centre ( is it pin 9 ? ) to reduce interaction of the signal with the filament supply .

JamesD · Post by **JamesD** » Fri Aug 23, 2013 12:56 pm

By way of a contrast with Marks design (and its a good one!). ere is an alternative design using the things we have discussed in various threads - i.e. just a resistor dropper for G2 and to elevate G3 above the cathode... Its unproven so the design is for illustrative purposes only - to promote discussion and to get someone off their backside to build it (me! )

Damn forgot the attachment!!! lets try again... and updated to version 2 due to a mistake on the drawing...

JamesD · Post by **JamesD** » Fri Aug 23, 2013 12:59 pm

One of the things I want to try is to use filament bias with the above circuit but I haven't worked out all the thinking on that with the latest debates about filament operation...

J

pre65 · Post by **pre65** » Fri Aug 23, 2013 1:22 pm

I see from readings Ronald's writings that it is also possible to vary the filament voltage and see what effect that has on the curves.

IslandPink · Post by **IslandPink** » Fri Aug 23, 2013 5:29 pm

Probably more worthwhile on triode operation , though it does reduce Gm at some point. For pentode I think the next worthwhile tweak - which I tried but wasn't successful with - is shifting the G3 voltage up a bit from the cathode , see Paul's curves shown further back in this thread.
Out of interest what is the supply voltage you intend to use, Phil ?

James - are resistors between the filament supply and the ends of the filament there just to drop from 5V to 4.2V , or do they have other secret purposes ?

JamesD · Post by **JamesD** » Fri Aug 23, 2013 7:42 pm

Damn! A mistake! Updated to version2. The volt drop is in the CMC (might need external resistors too)

J

IslandPink · Post by **IslandPink** » Fri Aug 23, 2013 11:38 pm

Is there any money in being your circuit diagram beta-tester ?

JamesD · Post by **JamesD** » Tue Aug 27, 2013 7:31 pm

There's untold riches in understanding my mistakes

A lifetime of experience blossoming through the insights gained

A.....

Thats enough of that nonsense...

But money - not so much

How long can I blame the after effects of the general anesthetic? Am I out of time yet?

IslandPink · Post by **IslandPink** » Tue Aug 27, 2013 10:18 pm

Milk it for all it's worth !
More importantly, did you have a lovely bank-holiday weekend up on t'hill ?
I trust you had tea on the lawn ?

JamesD · Post by **JamesD** » Tue Aug 27, 2013 11:40 pm

We did have tea on the lawn - and dinner too

Hope you had a lovely one up in the hills?

And I got to remove half of the packing cases from my office... soon I'll have room for the Quasars soon...

Been thinking about a PSE 4P1L amplifier driven by 6BR7 with partial feedback on the WAD2A3PSE chassis that I have... except I like the Sovtek 2A3s in that amp....

There are a couple of other options too - but the restricted B+ rules out GK-71s!

J

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