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In Reply to: Did you measure the B+ posted by DaveInVA on July 28, 2006 at 08:56:17:
Kinda-Sorta... I use a variac to equalize the B+ by measuring the plate voltage of the driver tubes, to avoid connecting the meter to 450VDC. It should be within a few %, but I'll double check.The sonic difference is much greater than what I usually expect from a change in B+ ... not like the sort I hear when switching between a 5U4G and GB, which is enough to require re-biasing.
Follow Ups:
If you were using a variac you might have been running some of the tubes at lower heater voltage/dissipation/emission. And unless you are acounting for the voltage drop across the rectifier when the output tubes are drawing suffient current for an audible output you really are comparing "persimmons and kumquats." As you seem to be a bit "voltage shy" you could - with a known signal level from a audio oscillator, fer instance, - monitor output tube cathode current as a surragate for plate voltage(all other parameters being unchanged). But the driver plate voltages, if accidentally encountered, will "disturb" you just about as much as the 450VDC will. Having conducted more current though my body over the years than I wish to relate the "thresholds" seem to be "discernable current (as low as 12V when my skin is wet up to about 90V)" then "line current" then anything less than about 700 and then anything that you can actually draw an arc from (which leaves a nifty little brown burned hole in your skin which seems to take forever to fade away).
I'm mostly 'voltage shy' here because I can measure the ~160VDC driver plate voltage easily in these amps (with speakers connected), and measuring the B+ necessitates turning them on and off to avoid any loud "pops". Mostly a convenience issue...But anyways, I checked it out... I measure 432-433VDC with either, given 110VAC input. So, really, no discernible difference whatsoever. This is in contrast to the 5U4G, where I do measure a bit lower and compensate accordingly.
So, I'm now fairly confident that whatever differences I hear are due to the rectifiers' AC harmonic signature, or their response to a dynamic load. I am just surprised that a rectifier could have such a profound effect, especially with a choke input power supply.
But did you measure the voltaged under load? For all practical purposes the output stage where all the current goes. Hmmm, unless you've got a Class A amp - can't remember. If so then your observations may be right on - but you're AB then you need to measure the voltages with an adequate signal on the output stage.
Good point there ... the amps are class A Williamson amps so no worries, so the load is pretty much constant. I'm convinced that it has some different dynamic properties, or switching behavior. Maybe I can dig up a pair of 5AS4As of different make to see if what I hear is consistent with the type in general, or just these particular Tung Sols.
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