DH>>>     I was once told that very high plate voltages will tend to
DH>>>     choke off the high harmonics.  Zat so, do you know?
DH>>It minimises the electron cloud.  The number of electrons which come
DH>>off the cathode are pretty much dependent on the temperature of the
DH>>filament, not the voltages applied.
DH>>If you run under low plate voltage, you don't attract all of them away
DH>>and you will get a cloud forming between the cathode and the first
DH>>grid.  This is also called self-biasing.  If the grid is negativly
DH>>charged, it will block the flow of electrons to the plate and this
>     Ooh!  Ooh!  Ooh yeah!!!  Thank you!!!  You just solved a puzzle I've
>     been puzzling about for years!  I built an amp very similar to a
>     1956 Fender Champ (mainly, lower plate voltages) that sounded _much_
>     better than the modern stuff.  I had a suspicion that lower voltages
>     (about 110V on the 12AX7) had something to do with it.  But it sounds
>     to me as though running the tube at 45V pushes it off the steep end of 
>     some linearity curve, and accentuates the musical-type distortion.
Right - the design using the 45 volts was pretty neat and good 
sounding.  www.paia.com if you have web access - I think the 
schematic is posted there or else off one of their links...
Their deisgn was pretty clever too in that they derived the 45 volts 
from an AC wall wart and a voltage trippler circuit.
About as simple as you can get!
If you wanted to get fancy, you could make a variable supply.  You 
would want one based on a regulator rather than just using a variable 
resistor since you do want the power supply to have as low an 
impedance as possible.  If you use a high impedance, the supply will 
"dip" every time you run a signal through it.  This -may- be musical 
but it doesn't sound like it would be from just thinking about it. 
More like the compressor from hell...
>     This solves a problem I've been mulling over for some years.
>     I built a larger experimental guitar amp with the passive tone
>     controls removed.  Yet, the amp still has the tube stage that
>     recovers the signal loss from the tone controls -- so the preamp
>     gain was way too hot.  Without that gain stage, the gain was much
>     too low, but with it the gain was too hot.  I fixed it with a 3 meg
>     resistor, but it didn't sound right.  Now it's obvious that the thing
>     to do is reduce the plate voltages drastically, so the overall gain
>     is nominal but I'll get this effect you've mentioned.  Gotta try it!
Actually, the gain is fixed by the components in the circuit and not 
the voltage supplied to the circuit.  If you reduce the voltage, you 
will still have the signal be too hot, it will just bottom out when it 
runs out of plate voltage.
Check a local used book store for tube books - many times you can get 
Radio Amateur books from the 50's and 60's which still have lots of 
useful info on designing tube stages.  The holy grail is a copy of the 
RCA Radiotron Designers Handbook but these things are scarce!
>     Questions:
>     Fender-type amps are set up for a gain of 40 in preamp stages.
>     Can I assume this would not need to be changed?
>     If you started with a single 12AX7 preamp stage with standard
>     voltages (100 - 150), and replaced it with two parallel 12AX7
>     stages at much lower voltages, so that the voltage gain was
>     the same, can you predict any behaviors or problems that might
>     come up?  It seems obvious that the current would be higher.
The gain is set by the components of the circuit.  If you want to 
change the gain, you will need to redesign the circuit.
>     In 12AX7 preamp stages, I'm assuming that the self-bias
>     created by the electron cloud is fundamentally different
>     (much more non-linear) than that created by the cathode bias.
>     Could you therefore reduce the cathode bias and get the same voltage
>     gain with more warmth, or would that be a quick way to make a
>     stupid mistake?
Cathode bias works the other way - you want to nudge the tube out of 
this self-biasing area and into a linear area.  You can change the 
bias but...  If you go too far the other way, you will drive the tube 
into hard conduction and it will burn up quickly.  Less chance of this 
with a low plate voltage but you would want to get a good digital 
multimeter and keep it in series with the plate to measure current 
flow as you were working with it.  Remember that the test leads will 
be at least 45 volts above the rest of the circuit - more if you are 
using higher plate voltages...
I am really happy with a Fluke meter - one of those $80 yellow 
rubberised units which has fallen onto the cement floor more times 
than I care to think about.
Some cash but worth absolutly every cent!
>     Power tubes (pentodes) can be switched to triode operation,
>     resulting in a lot less ouutput and a lot more nice greasy
>     50's distortion.  IIRC, this is because the two extra grids
>     are involved in reducing the electron cloud.  Is it therefore
>     possible that you could incrementally add this effect by
>     reducing the voltage to the suppressor grid rather than just
>     switching it out?  IOW, if the plate is at 300V and the suppressor
>     grid is at 275V, what might you get if you reduced the suppressor
>     voltage to 200V?
My understanding is that you are just changing the operating 
conditions of the circuit.  Extra grids are not going to change the 
flow of electrons, they just provide convenient "handles" fo this 
flow at varying degrees.
>     Sorry to be a pest with all these questions.  But one of the
>     things I learned in working with tube guitar amps is that
>     unlike hi-fi tube amps, linearity is not a design goal; rather,
>     if guitar players like it, you got it right.
>     Apparently Leo Fender understood this.  When he built a prototype
>     amp, he didn't go by what the o-scope said.  He would send it out
>     on the road with some of his favorite professional players, and
>     their opinions told him whether or not to put in production.
You bet!
Also, today ( Monday ) is Les Paul's birthday!
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