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Help with preamp/driver/output setup

allears

Member
2008-03-04 9:03 am
I could use a little guidance as someone fairly new to tubes . I've built a guitar amp by copying a schematic but now I'm heading up the learning curve and trying to create something similar , a clean low -power amp , from bits of old radios etc. I have a 6p25 output valve and OT (same spec as Pen 45 only 6.3 volts heater) and the schematic for the radio it came from , now I want to add a different driver/preamp stage . However , it's hard to find information about matching preamp valves and output valves - can someone enlighten me or point me towards a link ? I have 12ax7 and EF86 available , but I'd like to know how you go about deciding which valve to use . Can you do it by basic valve data ?Or by lengthy calculations /Tubecad . From looking at schematics I never see 12ax7 with Pen45 type valves , so I presume there is some mis-match . Also , as an aside , I find a lot of stand-alone preamp designs , but not stand-alone power amp , is there a reason for this or have I just missed them?
Anything that can point me in the right direction much appreciated.
 
allears said:
However , it's hard to find information about matching preamp valves and output valves
That may be because there is no such thing. You need to get the voltage gain roughly right, but there are lots of ways of doing that. You decide what you want to do, then you choose/design a circuit to do it. Designing the circuit runs in parallel with choosing the appropriate valves for that circuit.

6P25/PEN45 are two generations of valves apart from ECC83 so this may be why you rarely see them in the same circuit.

There are lots of stand-alone power amps. What you won't see very much is stand-alone output stages, as the norm is to put a voltage amplifier on the same chassis so the unit can be driven at line level signal voltages.