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Old 19th May 2008, 03:36 AM   #1
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Default SimpleSE goes Push Pull

I got put on a killer project at work recently so I have had no tube time. The boss asked me how much time it would take to do a particular project and I said 2 to 3 months. He said I had 3 weeks, I negotiated for 4. I have 3 weeks left, so I won't be here much. Most of my friends at work have been laid off, so I can't complain.

I was suffering some serious tube withdrawals, so I decided to spend my afternoon blowing up some tubes instead of mowing the lawn (we are having record breaking heat). I gathered up a couple of 22 volt sweep tubes, which I planned to wire into a SimpleSE board. I connected a SimpleSE board to a Fluke 407D, and put in my trusty pair of well worn 6L6GC's to test everything out. Then I had a brain flash. Why couldn't I make a push pull amp out of the SimpleSE board? It turns out to be pretty easy, but as with all simple experiments the snow ball started rolling, and my experiment that started at lunch time just finished (10PM) and I never touched the sweep tubes.

The first attempt at making a P-P amp involved using a driver transformer with a center tapped secondary, The UIS-1 from Triode Electronics. The center tap goes to ground and the hot leads go to each channel inputs. I connected a P-P OPT up to the OPT terminals of both channels. When driven with a signal generator the results were good. 20 watts in triode mode, 35 in UL. I connected this up to my CD player, but it sounded bad, real bad. My CD player didn't like being connected up to the transformer, so this experiment was over. I went into the kitchen and got lunch.

I thought about it while eating, and then tried this. I decided to feed the CD player into one channel like normal. Then I made a voltage divider out of the grid resistor on that channels output tube to drive the other channel, sort of a simple paraphase splitter. I replaced the 220 K grid resistor with a 200K resistor and a 20K trimmer pot in series, and wired the wiper to the second channels input. It worked! It worked GOOD!
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Old 19th May 2008, 03:57 AM   #2
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I was using an 6K ohm OPT that I got from Handwound Transformers (the Lucas guy that ripped everybody off) that has never really sounded good, but I didn't care if I blew it up. I tried triode mode. With 450 volts of B+ I could get about 18 watts at clip. The measured distortion was low, 0.5% at 1 watt and 1.3% at 10 watts. I took out the Chinese 6L6GC's and installed EH KT88's. Power went up to 23 watts at clip. I switched to UL mode. I could get 40 watts at clip. Distortion was 0.2 % at 1 watt, and under 1$ at 20 watts. OK, I cranked the power supply all the way up, 552 volts. Clipping happens at 62 watts. The distortion numbers were even lower. I tried cathode feedback, which lowered the distortion even more, 0.165% at 1 watt, and 1% at 40 watts. The output impedance measured about 2 ohms without any GNFB.

I connected up the CD player and speakers. It didn't sound too good. I disconected the CFB. OK, much better. It sounded pretty good, much better than I expected. Good solid bass, but clipping sounded pretty gross, then again why do you drive a 60 watt amp into clipping?

I have been exploring several different P-P amp designs that will eventually become the SimpleP-P. I really want to come up with a simple P-P amp that sounds a lot like a SE amp with the bass, punch, and dynamics that a P-P amp can deliver. I know that It can be done, because I have an amp with that sound, but it is not simple or low cost (300B P-P). This amp has the simple part down, but the sound is not there....yet.
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Old 19th May 2008, 04:01 AM   #3
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One more picture of the mod.
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Old 19th May 2008, 04:04 AM   #4
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OK, now I am curious, and I haven't blown anything up yet. Could I manage to rewire the front end of a SimpleSE into a traditional P-P input stage? Didn't sound too hard. Just rip out a few parts, sky wire in a few parts, and now I have a typical input stage, feeding a split load phase splitter. The measured performance was similar to the previous mod, except the output impedance was about 3 ohms, I don't know why. I played with this for a while, but I couldn't make it sound as good as the simple mod. Again CFB sounded gross, and triode sounded better than UL. Time for something really wild!
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Old 19th May 2008, 04:18 AM   #5
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Now that I had made a mess out of a perfectly good working SimpleSE board, I decided to try an idea that has been percolating in my brain for a while. I have recently re-read the paper written by O.H. Schade of RCA in 1938 called "Beam Power Tubes" after reading an article in Audio Xpress by Pete Millett. Both of these articles extoll the virtues of applying local feedback from the output tubes plate to its grid circuit. The claims are triode characteristics with pentode power output. For this to work you need a driver with a relatively high output impedance. This usually implies a pentode. My board has a dual triode. I remembered a post (#54 in this thread "magic" phase splitter for SimpleP-P
that showed a LTP / mosfet cascode. I decided to wire one of these up and apply some feedback to it.
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Old 19th May 2008, 04:35 AM   #6
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It only took me a few minutes to blow the whole thing up, so I got to do it all over again. This is really an experiment in its initial stages, since it has been alive for about 2 hours. I am running the amp in pentode mode. It makes stupid levels of power, over 75 watts at clip. 4% distortion at 70 watts. 0.2% at 1 watt, output impedance is under 2 ohms without GNFB.

OH, It ROCKS! This amp plays everything that I throw at it very well, and verry LOUD! I played all of my usual test music and it sounded great on all of it. If this thing survives my usual torture tests, and it can be reliably and repeatably built, this could be the SimpleP-P. With 450 volts the power is about 40 watts, with 550 volts power is 75 watts. At 70 watts the distortion is 4.12%. When you pound it into clipping it just gracefully compresses. It never gets nasty. This picture was taken at an input level that should have produced about 90 watts.

I plan to develop this design further, as my schedule allows.
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Old 19th May 2008, 06:15 AM   #7
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Quote:
Originally posted by tubelab.com
I have recently re-read the paper written by O.H. Schade of RCA in 1938 called "Beam Power Tubes" after reading an article in Audio Xpress by Pete Millett.
Any chance you have a link to one, or both, of these, or something postable?
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Old 19th May 2008, 07:06 AM   #8
chrish is offline chrish  Australia
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G'Day TubeLab,

I have had a pretty crappy couple of days, and your quote:

"It only took me a few minutes to blow the whole thing up"

gave me a laugh and cheered me up!

I was wanting my next big project to be a pair of monoblock octal PP amps and it looks like this will be it!

If you end up designing a dedicated board - all octal would be nice.

Look forward to seeing how your experiments progress.

Cheers,

Chris
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Old 19th May 2008, 01:19 PM   #9
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This is very exciting!

Since I made my decision to wait for the Simple PP design, I have been following your posts closely. It must be thrilling to make discoveries like this. Experimentation is such an inexact science, so many tries and few successes, but when you do succeed or know you are on the right path it can be exhilarating.

Keep up the good work! I can't wait to hear more.

-Steve
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Old 19th May 2008, 02:59 PM   #10
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"Beam Power Tubes" is here: http://www.one-electron.com/Misc_Docs.html
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