cascode gain stage

[phsyconoodler]

Is a high-gain cascode gain stage going to work if put in front of the existing input stage on a typical Fender amp.AB763 style?
What I mean is,can I just interrupt the input,place the cascode stage before it and then graft the output of the cascode into the input of the existing grid?
  I would use a gain control as well.I'm not well versed on cascode stages.Any insight would be appreciated!
Like this one.

[tubeswell]

IIRC, in Merlin's pre-amp book, he says that the gain of a cascode is not as high as if the same two triodes were cascaded. However that might be what you want?. I wouldn't mind hearing a soundbyte of your finished product when you are done. From what I gather a 6SN7 or an ECC88 are supposedly the optimum tubes for a cascode (in terms of maximising the space between the grid curves) FWIW.

[Fresh_Start]

Funny, I read the section on cascodes in Merlin's first book yesterday.  He has a strong preference of a 12AU7 instead of a 12AX7 based on distortion characteristics.

There's no reason you couldn't use a cascode as the first stage, but it's going to have one heck of an output signal for the next triode to handle unless you knock it down a lot.

There is another version of the cascode circuit which uses grid leak bias for the upper triode.  Apparently adds some compression which might be nice.

Assuming you have 2 triodes to play with, an alternative might be two triodes in series.  You could have all kinds of fun biasing one hot, the other cold, switching cathode bias caps in and out, etc.  Also relatively low noise which obviously is good for the input stage.  Just a thought...

Cheers,

Chip

[phsyconoodler]

Well if the gain isn't too high,adding it to an existing gain stage might be just fine?I'm talking about simply plopping this cascode stage right in front of the existing input.i would use a gain control so it doesn't hit the next stage too hard.
I'll just have to try it and see if it gives me what I want.
  I don't want to disturb the existing AB763 circuit except to add this stage before it.Will it work?

[stingray_65]

Is a high-gain cascode gain stage going to work if put in front of the existing input stage on a typical Fender amp.AB763 style?
What I mean is,can I just interrupt the input,place the cascode stage before it and then graft the output of the cascode into the input of the existing grid?
  I would use a gain control as well.I'm not well versed on cascode stages.Any insight would be appreciated!
Like this one.

I'm unsure where I've read it, but it is my understanding that a tradional cascode stage like circuit 2 in your attachment is prefect before a high loss tone stack.

Please, if I'm incorrect someone let me know.

Ray

[RicharD]

What... is this cascode week at Hoffman?  My Janitor thread has a cascode circuit too.

The 3rd circuit is not really a traditional cascode but a SRPP symmetrical totem tole.  This circuit is in TubeCad but it calls Ra = Rk (your R's 2 & 3). 3rd circuit results (from TubeCad):
AV = 80
Zin = 58k
Zout = 11k
I = .5mA
Vbias = -.77
Vthreshold = .33
Vout max = +/- 62V
That Vthreshold looks so low you could easily spank the circuit with a single coil pickup.

The 2nd circuit is a text book cascode amplifier.  This circuit is in TubeCad as well.  It spits out higher resistance values for the voltage divider of the upper triode but pluggin in the rest of the values it returns the following:
AV = 142
Zin = 824k
Zout = 89k
I  = .32mA
Vbias = -.7
Vthresh = .14
Vout max = -20 / +32 V
Again vthreshold is unusable.  The 200V B+ appears to be whats killing these circuits.  Think about it.  Starting from B+ you have RA, Rp1, Rp2, and Rk in series.  Vtube max for a 12AX7 is 330V.  You could easily feed this circuit with 600VDC and not put the tubes in harms way.

The first circuit is a self biased cascode.  Again from TubeCad:
AV = 142
Zin = 824k
Zout = 90k
I = .28mA
Vbias = -.71V
Vth = .18V ouch
Vout max = -25/+28

Now let's see what this same circuit does at 450V.  I'm gonna change Rk to 1k too to get current up.  The damn tube is plate starved.

I didn't jack with gain, Zin or Zout. 
I = 1.23mA, Vbias = -1.23V 
Vthresh = .8V which is still low but usable.
Vout = -112 / +123

There's no problem sticking any gain stages together like Legos, just be mindful of your isolation caps and RL which is not shown.  I think Weber's B+ voltage is easily 50% lower than it should be.  He also has his A's & B's reversed on the triodes.  Personal pet peeve.  I see a lot of novel gain stages for geetar running plate starved so maybe that's the goal. 

-Richard

[DummyLoad]

12AU7
350V B+
Rk 1K5
Ra 33K
Ck 25uF
Rload 500K
Ibias 3mA
AV 54
Vo-p-p ~180Vp-p
Vthresh 1.75V
Zout 26K
WRa 300mW

6SN7
350V B+
Rk 1K33
Ra 33K
Ck 25uF
Rload 500K
Ibias 3mA
AV 65
Vo p-p ~180Vp-p
Vthresh 1.5V
WRa 300mW

[phsyconoodler]

Thanks for all the help on this one guys! Now to go do something with it!

cheers!

[RicharD]

We'll be lighting up the Janitor this evening.  I should have some hands on telemetry late tonight.  It's almost exactly what Dummy Load posted for the 6SN7 except for a higher B+.