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Old 6th February 2012, 12:56 AM   #501
Salas is offline Salas  Greece
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Why you can't adjust the single one? It has a fixed R4 resistor?
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Old 6th February 2012, 01:00 AM   #502
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I can. Before i fire it all up again, I will have the second one built. Have to wat on a few parts as I burnt though my extra supply. Learned a lot screwing up and listening to sage advice. Being a newb to tubes and in general, I just wasn't sure of what to look for. I know now. Will be out of town so want get to do much.
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Old 6th February 2012, 01:02 AM   #503
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Quote:
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Check post #1, is it OK?
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Old 19th February 2012, 04:14 PM   #504
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This is phase inverting preamp and has to be wired with output signal to rca ground and output ground to rca signal
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Old 19th February 2012, 05:05 PM   #505
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Fired it up. All is good with a few exceptions. I need totrim B+ to 340 from 370. I have added two parallel 100R resistors on one filament cs and the voltage is only at 5.9V. Biggest problem is i have oscillation. I assume this is tube grid resistor related and not PSU related, but not sure. Wouldnt expect it to change with B+ change, but we will see.
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Old 19th February 2012, 05:12 PM   #506
Salas is offline Salas  Greece
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6.1V would be a bit better. Does it take about 45sec to glow heaters enough?
You don't have Vo trimmers? Why 370VDC and not 340VDC setting? 1k grid resistor installed?
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Old 19th February 2012, 05:16 PM   #507
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It does take about 30 seconds or so. I set 340 using 10k load resisyor in setup. Difference is simply due to difference in load. I can trim that out, no big deal. 1K is there, apparentlty not enough. Close as can be. I have some 1.5 k and 2.7 k. Ill remove 100r an dreplace with 220 to bring up filament some more. The other is 6.08. So much for matched.
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Old 19th February 2012, 05:18 PM   #508
Salas is offline Salas  Greece
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Check with oscilloscope to be sure oscillation ain't coming from B+ route first.
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Old 19th February 2012, 05:34 PM   #509
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I am using 170 instead of 117. Wouldnt it show up on B+ if measured. I have oscilloscope, but am new to using it.
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Old 19th February 2012, 05:43 PM   #510
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This is phase inverting preamp and has to be wired with output signal to rca ground and output ground to rca signal
Doing that is likely to cause noise problems, pickup of r.f. from A.M. radio, ham, C.B. radio etc. and trouble if there are ground loops, unless there's a output transformer providing isolation. If you need to flip the phase it is otherwise much better to reverse the speaker leads instead. Remember to do it on all of them if bi-amping or tri-amping.

I had a very strange thought on importance of absolute phase yesterday. The releases of air from voice and horn instruments generally have a waveform that spikes a large amount in one direction, and is wider but lower (same area under the curve, so no d.c.) in the other. Individual tube stages have distortion that generally gives more compression on the cutoff side of the waveform due to falling transconductance at low current (it's different, only near zero as crossover distortion if at all, in push-pull). Lower distortion will result (and with more of the natural spectrum maintained), if the phase (taller peaked side) of the material agrees with the more capable direction of our amps. It may actually be worth having phase inversion switches. Previously I only thought absolute-phase mattered on things like cannons and snare drums. It feels different to be pushed back by explosive pressure than to experience a burst of vacuum. A tight snare drum whack is a different experience when backwards.

Although I believe that voice and horns always peak in the same direction, it is very likely that many of our recordings have the phase flipped somewhere along the line. All it takes is one mike cable reversed, so we can't count on sources to be correct or even consistent. A.M. broadcasters often use automatic phase-flippers since the transmitters have an absolution limit on negative peaks, but only an arbritrary one (set 25% higher by the F.C.C. in the U.S.) for positive peaks.

Last edited by riccoryder; 19th February 2012 at 06:07 PM.
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