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.
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.
Attachments
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.)
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.
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.
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.
Looking at the datasheet, with maybe 50pF in (Xc~150k@20k) there seems to be some room to move with this resistor.
So, never can one be totaly happy. As I said I was really happy with the amp, then suddenly I started hearing scratch and hiss from one channel (right). I swapped tubes right to left and the noice moved. OK - as I was still using the cheapo sino-tubes I got with the kit and I had better stuff on the shelf I put in a couple of 6n23p-ev and a quad Genalex N709/El84 that I´ve had for ages. Wow, total bliss, or as they say; audio nirvana. The detail; singers breathing, fingermovements on the instruments, and the bass, deeper, tighter. BUT...when the music stopped it wasn´t dead quiet. There is a faint hiss from the left channel, faint, but it wasn´t there from the start. Any ideas , where and what should I look for? And hypothetically, can cheap faulty output-transformers be noicy? ( not saying it´s that, just wondering) A bad connection somewhere, could that be it?
And just to be shure, I have tried with my tpa3116, it´s not the speaker.
And just to be shure, I have tried with my tpa3116, it´s not the speaker.
Correct for this design.
However, for parallel single ended designs, lack of individual grid stoppers can easily lead to motorboating.
Hiss and squeaks? Using higher quality OPT's eliminated this in the instances I encountered it. Sorry if this is not the answer you might want to hear...
Ian
Correct. I know this since I have corrected it a few times (parallel single ended output only) where the builder decided to spare a 1k grid resistor after the coupling capacitor.... Not something you would expect to see in push-pull designs such as this one though.
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I Googled "Motorboating" but found only one hit, this:
https://www.youtube.com/watch?v=iDeXlQIvqco
According to my experience this is not motorboating but someting else.
My idea of "original" motorboating is much lower frequency oscillation. Something like this at about 1.30min....:
https://www.youtube.com/watch?v=vS_dnFVYGpA
Anybody have same opinion ?
https://www.youtube.com/watch?v=iDeXlQIvqco
According to my experience this is not motorboating but someting else.
My idea of "original" motorboating is much lower frequency oscillation. Something like this at about 1.30min....:
https://www.youtube.com/watch?v=vS_dnFVYGpA
Anybody have same opinion ?
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Seems to be much dissagreement on these terms. wikipedia states "Squegging in a vacuum tube audio amplifier is called motorboating because it sounds in the loudspeaker like a diesel engine in idling rotational speed."
In any case, we know what it is and how it comes about being. no point in discussing this further.
bub bub bub bub bub bub bub....
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