🚀 go-pugleaf

npstewar@eos.ncsu.edu (NATHAN PHILLIP STEWART) writes:


>1) What exactly is the relation between plate voltage and the level where output
>   distortion begins. As far as I understand it now, the higher the plate 
>   voltage, the higher output level before distortion sets in. Is this the
>   relationship? Is it more complex than this? (of course it is)

Roughly speaking, yes you're right. However for a more accurate answer you
have to look at the characteristic curves and the load line which will give
you the points where the amp will start to clip, however, unlike a preamp
stage, a power amp has a dynamic load line which makes rather more complex
to work out accurately but, broadly speaking, the higher the HT voltage (is
this B+ in the US?) the higher the output level before overload distortion
sets in.


>3) About measuring plate voltage, it occured to me that if the impedance of 
>   my voltmeter is not high enough I could fry my voltmeter and quite possibly
>   more (me for instance.)  Is this a real concern, or do Multimeters generally
>   have a high enough impedance that I don't need to worry.

In general there's no real need no worry about your multimeter loading down
the reading. If it's a digital one it'll have an input impedance of about 
10 megohms!! If it's an old fashioned moving coil type meter, you *may* have
to be a bit more careful. If it's rated at 20kohms/volt or better, it'll be
ok.
What you *will* have to watch, particularly if your hanging it off the HT
voltage or the anodes of the power stage is that the probes or the meter
can actually handle the *voltage*. It's usually stamped on the case it's
max dc input level but that's only for the meter circuitry *not* the probes
so make sure the probes are well insulated.

>4) If I want to leave my 6L6's, but drop their plate voltage (This depends on
>   the answer to the first question), without changing my transformer, can this
>   be done.  (Since my carvin uses some FET EQ circuits and reverb buffering,
>   I can't just sub a lower power transformer.)

In short, no unless you fit some sort of additional transformer inside the
amp to drop the HT while maintaining the filament voltages etc.

Dave
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