robert casey wrote:
Been playing around in simulation with a SRPP
circuit using a triode on bottom and a pentode
up top. Reason for the pentode is so I can filter its
screen supply to get better power supply ripple
rejection. Otherwise it would be a triode
connected pentode. Simulations suggest that
I can get about 50dB power supply rejection with
the pentode with a well filtered screen supply.
You are correct about this. The B+ supply sees a high Ra so little ripple
current
flows, but the fixed screen voltage makes the rest of the tube act like a
trioded tube.
The bypassing and filtering of the G2 supply might need to be well considered
to keep
its hum lo lest the problem be trasnsferred to the G2 instead of anode..
The same type of pentode could be used trioded for the bottom tube.
Then for simplicity, the two Rk could be equal.
But I like using a fixed bias supply to the top tube G1 via a 1M R,
then have say 10k between top k and bottom a and cap couple bottom a to top
g1,
which gives the least 3H, usually a lot lower 2H, more gain closer to U,
and a lot lower Ro from the top k output.
Its then a bootstrapped follower, not SRPP, but the BF is better imho.
( better music ).
The G2 of the bottom pentode can also be connected to the top tube cathode
to make the Ro and distortion even lower, rather than just have
the bottom tube connected with G2 to A.
Say the triode is half a 12AU7 and pentode a 6AU6,
or a 6AU6 triode connected on the bottom, and
another 6AU6 up top.
6AU6 is a nice tube...
There are millions laying forgotten in draws, boxes, on shelves around the
world.
Patrick Turner.
Made a new 3f4 spice model for the sub-mini
sharp cutoff pentode 5840 while doing this.
Don't think I'll use 5840's, but here it is.
************************************************** *********
*Vacuum Tube submini pentode (assumes K tied to g3)
.SUBCKT X5840 A S G K
*
* Calculate contribution to cathode current
*
*the number at the right end determines sharpness of knee
Bat at 0 V=0.636*ATAN(V(A,K)/9)
*the URAMP(V(S,K)/# mostly determines peak plate current, grid line
spacing nearly constant
*the number at the right end determines slope of grid lines (plate
resistance)
Bgs gs 0 V=URAMP(V(S,K)/28.8+V(G,K)*0.75+V(A,K)/4000)
*the exponent sets the linearity of grid line spacing, and big impact on
peak plate currrent
Bgs2 gs2 0 V=V(gs)^ 1.5
Bcath cc 0 V=V(gs2)*V(at)
*
* Calculate anode current, grid line spacing adjust and peak plate current
*
Ba A K I=2.585E-3*V(cc)
*
* Calculate screen current
*
Bscrn sc 0 V=V(gs2)*(1.1-V(at))
Bs S K I=0.550E-2*V(sc)
*
* Grid current (approximation - does not model low va/vs)
*
Bg G K I=(URAMP(V(G,K)+1)^ 1.5)*50E-6
*
* Capacitances
*
Cg1 G K 3.5p
Cak A K 2.9p
Cg1a G A 0.0155p
.ENDS X5840
************************************************** *******
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