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Old 2nd June 2009, 08:41 AM   #1
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Default Big Boss: a simple and basic el84 PP amp in progress

The time has come for me to finally jump in and build me a tube power amp. I've got nearly a dozen preamps kicking around, so I figure I'm ready for something bigger. Up until now I've been using low-gain preamps with no-gain SE mosfet buffers (huge szekeres circuits, basically) but finally decided to build my own integrated tube amplifier. I have quite a pile of tubes, but settled on el84 power tubes, as I can do with 10 ~ 15w easily. As for my circuit, I decided to go like this:

Basic grounded cathode
Split-load phase splitter (both will be either 6cg7 or 12at7, depending on whether I want to use cathode feedback or not))
El84 output stage in pentode mode (with edcor budget trafos, as they work hard for the money)

I'm shooting for a B+ of around 320v. I will use a ez81(6ca4) rectifier. As for regulating my screen currents, I was thinking of using another el84 or half a 6cm7 as a cathode follower, with a 1meg pot at g1 to set output voltage and therefore idle current, will this work well in this application? I've used this type of setup as an adjustible PSU for a couple Aikidos with good results. The only silicon in this amp will be the leds under the tubes

A couple other things. With a 10k plate-to-plate trafo, and 120 ohm cathode resistor, what formula(e) should I use to determine a good value of bypass capacitor?

Any help in getting the values and ideas down will be greatly appreciated!
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Old 2nd June 2009, 12:54 PM   #2
SY is offline SY  United States
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Stop. Do not pass go. Do not collect $200. Get yourself a copy of Morgan Jones's "Valve Amplifiers." Study carefully the design steps for the Bevois Valley amplifier. This will save you much random futzing and smoke-releasing.
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Old 2nd June 2009, 03:54 PM   #3
rknize is offline rknize  United States
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+1

You are going for a very well-tested, tried-and-true amplifier design. MJ uses his "Bevois Valley" project to teach each stage of the design process for an amp like this in great detail. It is an EL84 PP design using a split-load "concertina" phase splitter. It is probably well worth your time to pick up this book and use that chapter to walk you through the design process for your amp. You will learn a great deal along the way. I know I did...even though I didn't build that amp, the same steps apply to similar PP designs.

I have the 2nd edition that I picked-up about 8 years ago, which does not really talk much about SE power amplifiers. MJ is not very fond of them and concentrates mainly on PP designs. However rumor has it that he reluctantly goes through a simple SE PA project in the 3rd ed. I've been meaning to pick up the 3rd ed, but I end up balking at the price given that I already have the 2nd ed and his 3rd ed seems to be split into 2 books.
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Old 2nd June 2009, 03:58 PM   #4
athos56 is offline athos56  United States
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Quote:
Originally posted by SY
Stop. Do not pass go. Do not collect $200. Get yourself a copy of Morgan Jones's "Valve Amplifiers." Study carefully the design steps for the Bevois Valley amplifier. This will save you much random futzing and smoke-releasing.
Or do what I do, and only build stuff that other people have done the hard work on already. And buy Morgan Jones, it helps you not sound like a bonehead when you ask questions (not that you did, I learned this on my own...)
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Old 2nd June 2009, 07:12 PM   #5
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The bevois valley is pretty much spot on to what I had in mind. The one thing that I noticed though, was that he used separate cathode resistors for each output tube. Are there any marked advantages/ disadvantages to this approach? Is there anywhere in the book that he discusses this?
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Old 2nd June 2009, 10:04 PM   #6
SY is offline SY  United States
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Separate resistors will tend to equalize idle currents for less-than-perfectly matched tubes. Disadvantage is parts count.

If you'd rather use a 12AT7/ECC81 than MJ's ECC88/6DJ8, you can graft on the input stage from the Red Light District amp. Either will work just fine.
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Old 3rd June 2009, 03:24 AM   #7
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I have no problem with extra parts, so I'll keep 'em separated. Also, I think I'll do as you said for the 12at7's. Should I change the resistor values, or could they stay the same? I have plenty of 100k's ,1k's, and 47k's that would work.
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