From:
         
         Heiko Frost   (heiko_frost@hotmail.com)
   Date:
         10/28/2001 1:52 AM
 Subject:
         Low Voltage Power Tubes, High Voltage Preamp Tubes?!



Hey Guys,

Kind of a dilemma I'm facing on my amp. It's rather similar to a Top Boost AC-30 with 4xEL84's. I plan on running the EL84's with 320-325 volts
on the plates as they sound best around that range. However, this limits the upper range of where I can run the preamp tube plate voltages. I tend to
like preamp tubes that are driven somewhat harder. I have not had much luck using a standard series power supply rail even using the smallest
dropping resistors. Can anyone here recomend an alternate idea? I'm thinking about paralleling the power supply rail ala Matchless amps. Also, as
this directly relates, can anyone here confirm whether the Trainwreck rocket does this or uses seperate taps on the Power Tranny for the preamp
tubes? That idea seems rather complicated to me but got me thinking when I saw it meantioned in the build section. Thanks,

-Heiko Frost 


   From:
         
         Bruce /Mission Amps   (missionamp@aol.com)
   Date:
         10/28/2001 2:11 AM
 Subject:
         Re: Low Voltage Power Tubes, High Voltage Preamp Tubes?!



How much supply voltage do you want to the plate load resistors?
Obviously you can't have more then the max B+ rail without some serious mods, but if you are looking for +315vdc to the preamp stages, you
already have it.
Why use series dropping resistors for the preamp stages anyhow?
It sounds like all you want is power supply node isolation and a threesome of preamp tubes aren't going to draw much more then 4ma- 8ma total, so
just parallel them off the screen supply and use bigger value filter caps. 
That way each tube has it's own small value isolation resistor with minimal current flowing so there will be very low voltage drop across those isolation
resistors.
If you were using 12AX7s, each would draw around 2ma across say a 1k5 isolation resistor connected to the screen supply filter cap and there
would only be a 3 volt drop from the supplied screen voltage.
But I think you know all this.
Bruce 


   From:
         
         Heiko Frost   (heiko_frost@hotmail.com)
   Date:
         10/28/2001 3:17 AM
 Subject:
         Re: Low Voltage Power Tubes, High Voltage Preamp Tubes?!



Hey Bruce,

Thanks for the info, it was exactly what I needed. Curious though about how high I should go with the preamp filter cap values? I was using 40uF for
the OT, and 20uF for each each other B+ rail using the series setup before. Are we talking about 50-100uF for each preamp tube power node?
Thanks again man,

-Heiko Frost