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Old 10th December 2011, 03:06 AM   #1
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Default 6CB6 Spice Model wanted

Hi folks,

I am looking for a "pentode connected" 6CB6 Spice Model, did somebody have one for me ???

I was thinking about using a 6AU6 but this tube is less linear than the 6CB6 and is plate to cathode capacitance is too high, then if the load resistance is high, there is a big treble loss ...

Thanks in advance.

Alain.
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Old 10th February 2012, 08:45 PM   #2
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Found these lurking on my hard drive:
Code:
*
* Generic pentode model: 6CB6
* Copyright 2003--2006 by Ayumi Nakabayashi, All rights reserved.
* Version 3.01, Generated on Wed Mar 22 17:20:00 2006
.SUBCKT 6CB6 A G2 G1 K
BGG   GG   0 V=V(G1,K)+0.7350712
BEP   EP   0 V=URAMP(V(A,K))+1e-10
BEG   EG   0 V=URAMP(V(G1,K))+1e-10
BEG2  EG2  0 V=URAMP(V(G2,K))+1e-10
BM1   M1   0 V=(0.031445788*(URAMP(V(EG2)-1e-10)+1e-10))^-2.20504
BM2   M2   0 V=(0.40485393*(URAMP(V(GG)+V(EG2)/18.926098)+1e-10))^3.70504
BP    P    0 V=0.0046867337*(URAMP(V(GG)+V(EG2)/46.747967)+1e-10)^1.5
BIK   IK   0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.010960467*V(M1)*V(M2)
BIG   IG   0 V=0.0023433669*V(EG)^1.5*(V(EG)/(V(EP)+V(EG))*1.2+0.4)
BIK2  IK2  0 V=V(IK,IG)*(1-0.4*(EXP(-V(EP)/V(EG2)*15)-EXP(-15)))
BIG2T IG2T 0 V=V(IK2)*(0.78046782*(1-V(EP)/(V(EP)+10))^1.5+0.21953218)
BIK3  IK3  0 V=V(IK2)*(V(EP)+21225)/(V(EG2)+21225)
BIK4  IK4  0 V=V(IK3)-URAMP(V(IK3)-(0.0024945513*(V(EP)+URAMP(V(EG2,EP)))^1.5))
BIP   IP   0 V=URAMP(V(IK4,IG2T)-URAMP(V(IK4,IG2T)-(0.0024945513*V(EP)^1.5)))
BIAK  A    K I=V(IP)+1e-10*V(A,K)
BIG2  G2   K I=URAMP(V(IK4,IP))
BIGK  G1   K I=V(IG)
* CAPS
CGA   G1  A  0.02p
CGK   G1  K  3.9p
C12   G1  G2 2.6p
CAK   A   K  1.9p
.ENDS
Code:
*
* Generic triode model: 6CB6T
* Copyright 2003--2006 by Ayumi Nakabayashi, All rights reserved.
* Version 3.01, Generated on Wed Mar 22 17:20:00 2006
.SUBCKT 6CB6T A G K
BGG   GG   0 V=V(G,K)+0.7350712
BEP   EP   0 V=URAMP(V(A,K))+1e-10
BEG   EG   0 V=URAMP(V(G,K))+1e-10
BM1   M1   0 V=(0.031445788*(URAMP(V(EP)-1e-10)+1e-10))^-2.20504
BM2   M2   0 V=(0.40485393*(URAMP(V(GG)+V(EP)/18.926098)+1e-10))^3.70504
BP    P    0 V=0.0046867337*(URAMP(V(GG)+V(EP)/46.747967)+1e-10)^1.5
BIK   IK   0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.010960467*V(M1)*V(M2)
BIG   IG   0 V=0.0023433669*V(EG)^1.5*(V(EG)/(V(EP)+V(EG))*1.2+0.4)
BIAK  A    K I=URAMP(V(IK,IG)-URAMP(V(IK,IG)-(0.0024945513*V(EP)^1.5)))+1e-10*V(A,K)
BIGK  G    K I=V(IG)
* CAPS
CGA   G    A 2.6p
CGK   G    K 3.9p
CAK   A    K 1.9p
.ENDS
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Old 10th February 2012, 08:51 PM   #3
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If anyone has a 6CB6 PSPICE model I would love to have one. I've tried converting the ones above to use in MicroCap but have been unsuccessful so far...made my head hurt.

Thanks in advance!
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Old 10th February 2012, 08:57 PM   #4
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And another one:
Code:
*
* Generic pentode model: 6CB6
* Copyright 2003--2006 by Ayumi Nakabayashi, All rights reserved.
* Thanks to Tooru Kuroda.
* Version 3.01, Generated on Wed Mar 22 17:20:00 2006
.SUBCKT 6CB6 A G2 G1 K
.param GG  {(V(G1,K)+0.7350712)}
.param EP  {URAMP(V(A,K))}
.param EG  {URAMP(V(G1,K))}
.param EG2 {URAMP(V(G2,K))}
.param M1  {((0.031445788*EG2)^-2.20504)}
.param M2  {((0.40485393*URAMP(GG+EG2/18.926098))^3.70504)}
.param P   {(0.0046867337*URAMP(GG+EG2/46.747967)^1.5)}
.param IK  {(U(GG)*P+(1-U(GG))*0.010960467*M1*M2)}
.param IG  {(0.0023433669*EG^1.5*(EG/(EP+EG)*1.2+0.4))}
.param IK2 {((IK-IG)*(1-0.4*(EXP(-EP/(EG2+1e-10)*15)-EXP(-15))))}
.param IG2T {(IK2*(0.78046782*(1-EP/(EP+10))^1.5+0.21953218))}
.param IK3 {(IK2*(EP+21225)/(EG2+21225))}
.param IK4 {(IK3-URAMP(IK3-(0.0024945513*(EP+URAMP(EG2-EP))^1.5)))}
.param IP  {URAMP(IK4-IG2T-URAMP(IK4-IG2T-(0.0024945513*EP^1.5)))}
BIAK A  K I=IP+1e-10*V(A,K)
BIG2 G2 K I=URAMP(IK4-IP)
BIGK G1 K I=IG
* CAPS
CGA G1 A  0.02p
CGK G1 K  3.9p
C12 G1 G2 2.6p
CAK A  K  1.9p
.ENDS
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Old 10th February 2012, 10:48 PM   #5
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Quote:
Originally Posted by cogsncogs View Post
If anyone has a 6CB6 PSPICE model I would love to have one. I've tried converting the ones above to use in MicroCap but have been unsuccessful so far...made my head hurt.

Thanks in advance!
Thank you very much cogsncogs,

I will try them with my SIMetrix simulator right now to see if they work as they are and give some new's about that later. Precises pentode models are hard to find, it is more complicated than a triode because there is also the action of the screen grid.

But I also have problems with some triodes models, they work well in most circuits but not in others, like the cathode follower using the simulator "current source" instead of a resistor, both made with the CurveCaptor program and 8 Koren parameters ... I have to find out why the cathode voltage raise to severals negative kV ...

Alain.
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Old 10th February 2012, 11:40 PM   #6
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Quote:
Originally Posted by Alain Poitras View Post

But I also have problems with some triodes models, they work well in most circuits but not in others, like the cathode follower using the simulator "current source" instead of a resistor, both made with the CurveCaptor program and 8 Koren parameters ... I have to find out why the cathode voltage raise to severals negative kV ...

Alain.
Try putting a diode eg. 1N4007 from (anode) grid to (cathode) cathode. Not elegant, but it should run then.
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Old 11th February 2012, 02:36 AM   #7
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Quote:
Originally Posted by cogsncogs View Post
Try putting a diode eg. 1N4007 from (anode) grid to (cathode) cathode. Not elegant, but it should run then.
Tanks a lot for the tip, I will try that this weekend !
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Old 11th February 2012, 03:01 AM   #8
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I just try the triode connected 6CB6 model and it seem to work very well.
I made those curves with my SIMetrix simulator :

Click the image to open in full size.

Unfortunatly, I look in many 6CB6 datasheet and I don't find any triode connected curves or data, so I have no way to compare them except measuring a real physical average 6CB6 ...

Click the image to open in full size.

I quickly made this small preamplifier circuit to test the 6CB6 triode model, of course not the best that can be done. It will be much better with no bypass capacitor and a current limiter or a big inductor instead of the 36K anode circuit resistor.

I will next try the first "pentode connected" model with a similar code and come back with the results.
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Old 11th February 2012, 07:15 AM   #9
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Looks great, just a little glitch in the zero bias curve showing some current with no plate voltage. Some comparison can be made at certain points to the published pentode curves, like along the axis where plate voltages match a screen voltage that curves were provided for. One also has to add the values from both the screen and plate current curves for triode. To get an idea what actually makes a real-world improvement, some non-ideal things added can be helpful, like a source resistance for the generator (a tube preamp might be as high as 100K), input cable capacitance, maybe 100 pf, and some sort of input capacitance for the next stage. The miller effect can make the input capacitance large in triode stages. With those added you'll see the side effect of more high frequency rolloff as you go higher with the plate load impedance. RCA published many circuits with the 6CB6. The 6EW6 is a bit newer and has about 50% higher transconductance. Older lower gm types included the 6AG5 and 6BC5. If you want to try a type to see what large gm changes do to distortion, the 6BA6 is what's called remote-cutoff (as opposed to sharp). Basically the the pitch of the grid wires is non-uniform so that as bias goes negative, a smaller area of the grid allows flow, reducing current and gm. That's used for gain control (by bias) in receivers and works fine for small signals, but introduces considerably more distortion in large signals. I'm a little surprised to see the attention these tubes are getting as triodes with 12AX7s working so well. These really shine as pentodes. The gain of triodes is lower since the plate acts like an input (falling voltage lowers attraction and thus current), but that negative feedback can be reduced by some positive feedback (see input stage of Dynaco ST-35 amp) or by getting rid of the voltage variation (transistor or tube added in series between tube and load - cascode mode. I like to call it the tetrode/pentode mode for triodes, as that makes people think about the tube behavior more). As pentodes the 6CB6 was widely used as a gain stage at about 45 MHz (after a tv tuner down-converts the desired channel to a fixed intermediate frequency a.k.a. I.F. band for filtering and gain) In pentode mode, distortion can be lowered by feedback - unbypassing the cathode is common. However that linearizes cathode current which isn't quite the same as plate current. You can see from the curves that the plate current falls off, sometimes even gets kinks, at lower plate voltages. A closer look shows that much of that error is due to cathode current being robbed by the screen. Feedback in the cathode circuit doesn't normally remove screen grid related nonlinearity. A floating screen supply with the negative return to the cathode instead of ground is one akward way to avoid the screen error. Another is to feed the screen with a current source (set low enough to limit screen power/current to a safe value), and then use a zener or other shunt regulator from the screen back to the cathode NOT GROUND to regulate voltage to the desired value. Then the total of the shunt and screen current in the cathode resistor is just the clean DC from the current source, so it won't affect AC feedback or distortion. Note also, that the current sum is still a constant even if the screen current rises to where voltage regulation is lost and screen voltage decreases. That's sometimes needed in output pentodes for protection (and is neglected in many designs). As can be seen from the datasheet curves, pentode screen current goes way up at very low plate voltages. That happens when an amp is clipping, and notably is a problem with light loads, like when a speaker is at the bass resonance frequency.

Last edited by riccoryder; 11th February 2012 at 07:35 AM. Reason: Yogi: I see Boo boo
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Old 12th February 2012, 09:17 AM   #10
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Quote:
Originally Posted by riccoryder View Post
Looks great, just a little glitch in the zero bias curve showing some current with no plate voltage. Some comparison can be made at certain points to the published pentode curves, like along the axis where plate voltages match a screen voltage that curves were provided for. One also has to add the values from both the screen and plate current curves for triode. To get an idea what actually makes a real-world improvement, some non-ideal things added can be helpful, like a source resistance for the generator (a tube preamp might be as high as 100K), input cable capacitance, maybe 100 pf, and some sort of input capacitance for the next stage. The miller effect can make the input capacitance large in triode stages. With those added you'll see the side effect of more high frequency rolloff as you go higher with the plate load impedance. RCA published many circuits with the 6CB6.
Yes the models curves made with SIMetrix look nice and are very usefull when drawing load lines because the simulated circuit voltages and currents are exactly the same.

When I make a "final" circuit and like to know the exact frequency roll-off with a particular source impedance, I make a special circuit at the input. But for a fast simulation like that, I just put a 50K volume control at the input and verify the frequency at 100% and 50%, the worst position, it is not perfect but give a good idea of what is going on ...

Here is an example involving source impedance and Miller effect :

A member of this forum design this pre-amplifier and use a 10pF at the input ... Of course, everybody tell him it was wrong ... But I simulate is circuit to see "for myself" what was the "exact" effect of this capacitor on high-frequency roll-off with a 50K source output impedance.

Click the image to open in full size.

As you can see, to simulate a 100mV RMS input from a 50K source impedance, I use a 4,34228uA "peak" current source in parallel with a 50K resistor and a DC blocking 100000 farads capacitor ... This give a steady input voltage all over the frequency range, it is impossible to do that with a simple series resistor at the input because it make a voltage divider with the input "capacitive impedance" of the preamplifier ...

The input impedance of the amplifier is "about" 100K, the output impedance of the source is 50K, both in parallel = 33,333K ... To get 100mV RMS at the input at any frequency, since the current source is peak current, 0,1 x 1,4142 / 33333 ohms = 0,000004242 A or 4,4242uA ...

To get exactly 100mV, I have to ajust the current at 4,34228uA because the four parallel triodes also have a input impedance ... This method is very cumbersome, that's why I don't do that very often, it is much simpler and faster to make the simulation with a volume control at 100% and 50% assuming the output source impedance is 0 ohm.
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