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Circuit Ideas for the 815 (martian tube)

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jaymanaa

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Joined 2006
#1
Hello, I've been searching around for a few days trying to find experiments with the 815. A search here kept me busy for a couple hours this morning, but mostly the 815 just got a passing mention in threads where other transmitting tubes were being discussed. The thing that stands out to me (as a potential problem) is the 175v max screen rating. I have however, been a fan of George/tubelab's experiments for long enough to know that many times ratings are extremely conservative. I struggle with circuit design and theory, but love to experiment with tubes. My most recent success was a screen drive sweep amp that closely followed the Berhning 230 (I think) 6JM6 job. It was my first serious effort into the world of "non-audio intended" TV tubes, and I am officially hooked. I was expecting something "ok" fidelity wise, but the amp absolutely blew me away. I can't stop listening to it. So, now I'm terribly excited about "oddball" tubes, and the 815 is pretty odd. I like George's idea of the 7193 (basically a 6j5 with dual caps) for a driver as well. Here's my problem in a nutshell. I usually find a proven circuit, then go from it to do my experimenting. The idea of designing a circuit from the inputs on, is totally overwhelming to me. I'm hoping that maybe you gentlemen could either point me to some experimentation with the 815, or help me to get a "starting point". Any help at all would be greatly appreciated. I know it takes time and effort to give this sort of help over the net, but that approach has been the most helpful to me. I have secured a few 815s and have lots of different iron and bits and pieces to play with. Thank you all, very kindly in advance for any and all help you can give, to making one of these dudes sing. Jay
 
#4
Dude,

The RCA Data Sheet shows an AB2 condition set that looks good: 500 V. on the plate, 125 V. on g2, and -15 V. on g1. The set uses an 8 KOhm, end to end, O/P trafo for approx. 54 W. of O/P. Edcor's model CXPP100-MS-10K is pretty close, for an off the shelf item.

Use Bandersnatch's "exolinear" setup for the bulk of the essential NFB. Use Tubelab's "power drive" between the LTP plates and the 815 control grids. IMO, the IRFBC20 is well suited here. Either a Mullard circuit or very high gain in the LTP (cascode or pentode) generates the drive voltage needed.

JMO, follow in Stu Hegeman's footsteps and use a few dB. of GNFB.
 
#5
Hey jaymana.

I struggle with the theory too. you said "designing from the inputs on" It is better to design from the speakers back. I have read this in several places and it does makes sense.

I hope this helps.

Cheers.
Rolf.
 
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