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Old 10th January 2013, 10:22 AM   #21
Jaap is offline Jaap  Netherlands
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Why does the lower tube not have 100K from grid to ground ?
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Old 10th January 2013, 10:37 AM   #22
jane is offline jane  Norway
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Quote:
Originally Posted by 6J7 View Post
Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything !
There is no input signal to the lower tube so there will be no output signal either..
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Old 10th January 2013, 01:15 PM   #23
6J7 is offline 6J7
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Quote:
Why does the lower tube not have 100K from grid to ground ?
Lower grid is directly connected to ground, why adding a 100K in //

Quote:
There is no input signal to the lower tube so there will be no output signal either..
It seems that I'm too stupid to understand how a two tubes splitter works, I listened to the left channel like this and the right channel not yet modified (with input transformer). No bass on left channel and strange sound, like out of phase.
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Old 10th January 2013, 02:23 PM   #24
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Quote:
Originally Posted by 6J7 View Post
Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything ! tube seems to be good, led on, plate voltage ok. But no signal at all, tried to change the probe, same result.

Here is what I did.
You have built a Long Tail Pair phase splitter and then removed the Long Tail by using LED's. The input signal for the bottom pentode should come through the cathode not the grid1. In many LTP's you can simply ground the second Pentode grid (but you lose half the gain). There is no way for the input signal to communicate to the second Lower Pentode. I would advocate using at least a stiff resistor to emulate a long tail (though a CCS is much better). Look at how I achieved a LTP phase splitter with input transformer in my Tabor to understand how you could achieve the same without having to introduce a negative rail or an input capacitor.

Shoog
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Old 10th January 2013, 02:24 PM   #25
sajti is offline sajti  Hungary
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Quote:
Originally Posted by 6J7 View Post
Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything ! tube seems to be good, led on, plate voltage ok. But no signal at all, tried to change the probe, same result.

Here is what I did.
Take away the LEDs from the cathode of the EF80! Connect together the cathodes, and apply resistor to the ground. It will be slightly better.
But the best solution would be ccs in the cathode, like the attached picture.

Sajti
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Old 10th January 2013, 02:30 PM   #26
mwiebe is offline mwiebe  United States
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I’m not sure what’s wrong, the topology works. The LEDs do ground AC which would have an impact on how it sounds, but it should still work.

Have a look at some other similar designs.

JC Morrison’s 6CK4 P-P
http://web.archive.org/web/201012051...er/6ck4-pp.gif

Eric Kingsbury’s Music Machine
6V6 Amp Audio Circuit; Parts Expo

Kevin Kennedy’s 6CK4 P-P
http://www.kta-hifi.net/projects/amp.../6ck4amp_c.pdf


El Cheapo a slight variation with feedback,
up-to-date El Cheapo schematic

One version of Gary Pimm’s Tabor,
http://www.pimmlabs.com/web/Tabor3s.gif

Hopefully you will see something that will make a difference.
Matt
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Old 10th January 2013, 02:35 PM   #27
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Your input transformers make life massively more straightforward.

Shoog
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Old 10th January 2013, 02:49 PM   #28
mwiebe is offline mwiebe  United States
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Shoog,
I used to build with input phase splitting transformers or autoformers, or interstage versions of the same. I kept hearing so many good reports of Kingsbury’s Music Machine that I finally had to try it. So I pulled out the input transformer and now my favorite P-P amp uses that input topology and I couldn’t be happier. But input splitting does work better, vastly better, with very high inductance on the input tube cathodes.
Matt
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Old 10th January 2013, 02:59 PM   #29
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Quote:
Originally Posted by mwiebe View Post
Shoog,
I used to build with input phase splitting transformers or autoformers, or interstage versions of the same. I kept hearing so many good reports of Kingsbury’s Music Machine that I finally had to try it. So I pulled out the input transformer and now my favorite P-P amp uses that input topology and I couldn’t be happier. But input splitting does work better, vastly better, with very high inductance on the input tube cathodes.
Matt
I always use the Pink Mouse LED referenced CCS circuit for mine now.
The great thing with using CCS to bias up both your input stage and your output stage is that when you get the load resistor values right, everything is always self adjusting regardless of what tubes you plug into it. My last two amps have been DC coupled and universally stable. Its magic of the best kind.

Shoog
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Old 10th January 2013, 06:03 PM   #30
6J7 is offline 6J7
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Thanks all for your comments.

I tried again with input transformer but only 4K across secondaries and led's were replaced by unbypassed resistors. Sound has changed, now it is "slow" - not dynamic at all - with boomy bass, more "tubey" but always this unpleasant forward treble. I'll try with bypassed Rk.
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