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.
Say the triode is half a 12AU7 and pentode a 6AU6,
or a 6AU6 triode connected on the bottom, and
another 6AU6 up top.

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

************************************************** *******

Behold, robert casey signalled from keyed 4-1000A filament:

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. Say the triode is half a 12AU7 and pentode a
6AU6, or a 6AU6 triode connected on the bottom, and another 6AU6 up top.

Thaks Robert!

/me model addict :-)

I did the SRPP triode/pentode thing (6GH8A) and found great PSRR by tying
the screen bypass cap to ground, rather than cathode. Is that the way
you've gone?

Thaks Robert!

/me model addict :-)

I did the SRPP triode/pentode thing (6GH8A) and found great PSRR by tying
the screen bypass cap to ground, rather than cathode. Is that the way
you've gone?

Hi Gregg, that's what I did, bypass the screen to ground.
I figured that instead of triode connecting the pentode,
I'd make the circuit "think" that the top tube was
a triode with its plate connected to a well filtered
supply.

With screen connected to AC ground, the top valve is in triode mode.
Yes, you avoid the ripple. Choice of resistor and cap quite crucial
though.

cheers, Ian


"robert casey" wrote in message
nk.net...


Thaks Robert!

/me model addict :-)

I did the SRPP triode/pentode thing (6GH8A) and found great PSRR
by tying
the screen bypass cap to ground, rather than cathode. Is that the
way
you've gone?

Hi Gregg, that's what I did, bypass the screen to ground.
I figured that instead of triode connecting the pentode,
I'd make the circuit "think" that the top tube was
a triode with its plate connected to a well filtered
supply.

don't know if you are too interested in this, but John Broskie, at
www.tubecad.com has been playing around with injecting psu noise into
SRPP ccts, and similar, and he too has come up with massive
reductions in noise

have a look.

kind regards

bill ramsay

"bill ramsay" wrote in message
...
: don't know if you are too interested in this, but John Broskie, at
: www.tubecad.com has been playing around with injecting psu noise into
: SRPP ccts, and similar, and he too has come up with massive
: reductions in noise
:
: have a look.
:
: kind regards
:
: bill ramsay

yep, the 'art of balance'
Rudy

Ian Iveson wrote:

With screen connected to AC ground, the top valve is in triode mode.
Yes, you avoid the ripple. Choice of resistor and cap quite crucial
though.


"critical" in that I have to create a tuned circuit, or
just that more R and more C is better, and that I need
to select R to get a reasonable screen voltage, and then
C to get better effective ripple filtering?

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

************************************************** *******

Gregg wrote:

Behold, robert casey signalled from keyed 4-1000A filament:

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. Say the triode is half a 12AU7 and pentode a
6AU6, or a 6AU6 triode connected on the bottom, and another 6AU6 up top.

Thaks Robert!

/me model addict :-)

I did the SRPP triode/pentode thing (6GH8A) and found great PSRR by tying
the screen bypass cap to ground, rather than cathode. Is that the way
you've gone?

Another way is to have a CCS transistor to supply the top tube G2 current,
and bypass the G2 to the K with a 100 uF.

This then makes the top tube work in true pentode cathode follower mode,
with good supply noise rejection, and the load
imposed by a screen supply dropping R is avoided since it is normally coupled
to the
B+ via the Cbypass.
Alternatively, have a higher B+ so a larger value dropping R can be used.
The EG2 should still be well below Ea for the top tube.

Patrick Turner.

Behold, robert casey signalled from keyed 4-1000A filament:

Ian Iveson wrote:

With screen connected to AC ground, the top valve is in triode mode.
Yes, you avoid the ripple. Choice of resistor and cap quite crucial
though.


"critical" in that I have to create a tuned circuit, or just that more R
and more C is better, and that I need to select R to get a reasonable
screen voltage, and then C to get better effective ripple filtering?

There's a "point of no returns" with C. In the case of my 6GH8, I used an
R of 330k and any C above 2.2uF was moot.

--
Gregg t3h g33k
"Ratings are for transistors....tubes have guidelines"
http://geek.scorpiorising.ca

Behold, Patrick Turner signalled from keyed 4-1000A filament:



Gregg wrote:

Behold, robert casey signalled from keyed 4-1000A filament:

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. Say the triode is half a 12AU7
and pentode a 6AU6, or a 6AU6 triode connected on the bottom, and
another 6AU6 up top.

Thaks Robert!

/me model addict :-)

I did the SRPP triode/pentode thing (6GH8A) and found great PSRR by
tying the screen bypass cap to ground, rather than cathode. Is that the
way you've gone?

Another way is to have a CCS transistor to supply the top tube G2
current, and bypass the G2 to the K with a 100 uF.

Yes, I tried that too. CCS makes for fantastic PSRR.

--
Gregg t3h g33k
"Ratings are for transistors....tubes have guidelines"
http://geek.scorpiorising.ca

"robert casey" wrote in message

"critical" in that I have to create a tuned circuit, or
just that more R and more C is better, and that I need
to select R to get a reasonable screen voltage, and then
C to get better effective ripple filtering?

There is a conflict of requirements. Your arrangement hopes to
improve on the option of simply adding extra LC filtering for the
whole stage. Instead, you use an extra LC filter on the screen
alone. Because the screen draws much less current, you can use a
larger value resistor, and therefore a smaller value capacitor, for
the filter.

Taking this to extremes, you could reduce the ripple at the screen
at 100Hz hum frequency to 100th with a 1 meg resistor and a cap of
150nF. But for a screen current of say 1mA, this arrangement would
drop 1kV. At the other extreme, 10k would drop just 10V, but you
would need a cap of 15uF for the same attenuation.

The triode will behave as if its anode were at screen potential, so
if you are matching AC characteristics for cancellation of
harmonics, you should look at the triode characteristics at the
screen voltage you end up with.

All of the above could be solved by using a current source for the
screen, as I see Patrick suggests. But then there other
matters...(and equally you could use a voltage regulator for the
whole supply if your going to involve SS).

Normally in triode mode you can use a lowish value screen resistor,
or none at all. The non-linear effects of screen current are then
insignificant or non-existent.

One common reason for regulating screen supplies to pentodes is to
minimise the effect of screen current. A low impedance supply
reduces the voltage distortion caused by the current. Your choice of
cap value and resistor should take this into account.

Also, the mechanism for producing the even harmonic distortion that
you consider desirable in the top valve, also produces a DC offset
that will not be passed however big the cap is. This offset depends
on the amount of 2H signal current drawn by the screen multiplied by
the value of the screen resistor. Onset and recovery time depends on
the cap size. Maybe none of this is significant because any sensible
values of cap and resistor will make it vanishingly small?

Many amps using pentodes for voltage amplification don't seem to
care much about these things. I suspect they take the screen
impedance at
the operating point and divide it by10 to get the required
resistance of the screen supply, and then choose a decoupling cap to
suit, with cost in mind.

The screen impedance used for calculation should be the minimum it
reaches with the amp at full power. That could be considerably lower
than at the operating point, so you need a lower impedance supply,
and a correspondingly bigger decoupling cap.

All this is equally true of your grounded-screen triode-mode
pentode. Also, because it is on the top of a totem-pole, you have
the high voltage to consider.

Assuming you don't want electrolytics in series, you will be using a
polyprop cap, probably 600V. It could be cheaper to regulate the
voltage to the anode, and use the normal triode connection.

cheers, Ian

Well, time to put the spice and computer to bed and build the real
circuit. The pentode over a triode is not a new idea and it does work.
The trick is to get it to work correctly. I use an SRPP driver
stage in the power amps I build and also as the first gain stage in a
guitar amp I designed and built. This particular stage will pass a
perfect square wave from around 8hz to over 80Khz. It is quiet and
resists overload better then a single triode gain stage.
Bill Bittle
Tube amp tech at:
http://home.alltel.net/wbittle


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.
Say the triode is half a 12AU7 and pentode a 6AU6,
or a 6AU6 triode connected on the bottom, and
another 6AU6 up top.

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

************************************************** *******

"Wbittle" wrote in message
...
: Well, time to put the spice and computer to bed and build the real
: circuit. The pentode over a triode is not a new idea and it does work.
: The trick is to get it to work correctly. I use an SRPP driver
: stage in the power amps I build and also as the first gain stage in a
: guitar amp I designed and built. This particular stage will pass a
: perfect square wave from around 8hz to over 80Khz. It is quiet and
: resists overload better then a single triode gain stage.
: Bill Bittle
: Tube amp tech at:
: http://home.alltel.net/wbittle

....i find _ nothing _ at that adress ??
Rudy

You bet, because I typed it in wrong.. Sorry about that.
My address is:
http://home.alltel.net/wbittle1

I always forget the 1.


Ruud Broens wrote:
"Wbittle" wrote in message
...
: Well, time to put the spice and computer to bed and build the real
: circuit. The pentode over a triode is not a new idea and it does work.
: The trick is to get it to work correctly. I use an SRPP driver
: stage in the power amps I build and also as the first gain stage in a
: guitar amp I designed and built. This particular stage will pass a
: perfect square wave from around 8hz to over 80Khz. It is quiet and
: resists overload better then a single triode gain stage.
: Bill Bittle
: Tube amp tech at:
: http://home.alltel.net/wbittle

...i find _ nothing _ at that adress ??
Rudy