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Old 18th April 2015, 08:55 AM   #1
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Default "bevois valley" in triode

So, I built this;

From a chinese kit of the taobao for $178. Basically the Bevois valley without the CCS and a simpler ss power supply. Anyway, one channel motorboated, and I couldn´t figure out why. ( not strange, I don´t really know what I´m doing) I tried to up the front gridstoppers with another 1k. Even tried a couple of 0.1uf caps up front, in case my signal wasn´t clean. ( got a Aleph L clone as a pre ) So I sort of gave up for now. Then I started reading about these triode-strapped push pull amps. El Cheapo, Poinz musical machine and Tubecads simple el84 pp. And i figured..wth. Got rid of the front caps (of course) dished the feedback and the UL taps and strapped 100r 2w(kiwames as are the rest) between screen and plate.
Thats all I did, and--heureka, it works..and, it sounds fantastic. Quiet, no hum and fantastic soundstage, a little less clean than my tripath, but some would call that warmer. All in an hours work. Feeling a bit proud about it I couldn´t resist posting it here.
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Old 18th April 2015, 09:15 AM   #2
nec3 is offline nec3  Turkey
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Circuit diagram is original or after correction you made ?
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Old 18th April 2015, 10:02 AM   #3
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It was probably the negative feedback loop the wrong way around. That will cause positive feedback and instability.
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Old 18th April 2015, 11:07 AM   #4
DF96 is offline DF96  England
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Grid stoppers (or lack of them) very unlikely to cause motorboating.
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Old 18th April 2015, 12:10 PM   #5
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Schematic posted is the original. However I haven´t really changed anything except omitting the feedback and the screen tap, and putting 100r between screen and plate. Though I must admit that I didn´t remove the extra 1k on the grids of the ecc88s so they now have 1.33k in front of them. However, at the moment I am so pleased that I won´t do anything more, leaving as it is for now. Resistors as I said kiwames except cathode and powerline vishay 3w. Coupling caps Mundorf ZN. Cathode bypass Elna cerafine. The 10w 100r in the supply is the generic chinese that came with the kit. (could be replaced by a choke in the future.)
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Old 18th April 2015, 12:14 PM   #6
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Btw - how does one connect the feed back loop the wrong way? The grid stopper thing was something I found in another thread about oscillatition in a Beavois valley...
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Old 18th April 2015, 12:41 PM   #7
DF96 is offline DF96  England
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Feedback loop can be inverted simply by inverting the OPT output winding.

Grid stoppers prevent HF oscillation. They have no effect at LF and subsonic.
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Old 18th April 2015, 02:02 PM   #8
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Quote:
Originally Posted by DF96 View Post
Feedback loop can be inverted simply by inverting the OPT output winding.
or OPT input leads!
The feedback seems a little too much to my eyes anyway, I prefer a little feedback to no feedback.
If you have enough power, good on you.
personally I do not believe super resistor brands do anything here.
I suppose slit-foil ones are less noisy, in very sensitive areas, but that's all.
Capacitors are another story, my favorites signal caps are cheapo Russian K40y-9's.
If you have no noise (hum etc.) you do not need a choke.
Next thing you can do, is change the circuit to fixed bias, you eliminate the cathode capacitors/resistors, get better balance (if you use adjusters for each tube) and more power.
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Old 18th April 2015, 07:40 PM   #9
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Quote:
Originally Posted by bunkhokum View Post
...However I haven´t really changed anything except omitting the feedback and the screen tap, and putting 100r between screen and plate....
You haven't changed anything except...
You have actually changed everything.
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Old 19th April 2015, 08:59 AM   #10
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I've had a Leak Stereo 20 for most of my life, during which it's been through so many changes I've lost count. By far the best version of it uses 6S4 output tubes. This is a triode, so no feedback necessary. I actually use these in PPP - eight in total. I rewired the OPT to be permanently on the 4 ohm wiring, a fix which Morgan Jones notes.

I then started to play around with the input tube. ECC40 has been pretty good but some day I'll replace this with a 3A5 in filament bias, with a Hammond 124B (50% nickel) as plate choke. I've used this combination before and it sounds great.

There's a much better sound waiting to be got out of this classic old circuit.
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