tube cascode help

[HTH]

I've been messing with valve/tube OD pedals for a while and have been messing with a cascode configuration (like the AX84 Firefly input stage) this weekend.

However, with just the cascode section plugged into my plexi, I'm getting only a very slight increase in gain when I was expecting quite a considerable gain boost.

I'm sure the wiring is okay, any ideas? (or am I expecting something that the cascode will never deliver?)

EDIT: oh, here's the link to the firefly schematic... http://ax84.com/media/ax84_m276.gif

[Jazz P Bass]

Cascode pedal

Heck of a pedal!
Do you measure 8.8V at the cathode output , as per the schematic?

[HTH]

Heck of a pedal!
Do you measure 8.8V at the cathode output , as per the schematic?

I'm only using the first bit of the schematic (V1), the cascode bit, not the full schematic.

[Steve Conner]

According to Valve Wizard,

"Total Gain: The gain of the cascode is equal to:
A = 1 / (1/gm1 * Ra2) + ((ra2 + Ra2)/Ra2) * (1/ mu1 (mu2 + 1))
When using two identical triodes this can be simplified to:
A = gm1 * Ra2
as was calculated earlier."

A 12AU7 has gm = 0.0022A/V so you'd expect a gain of 0.0022 * 100000 = 220. That's not a "slight increase" by any means, so your circuit probably isn't working right.

[HTH]

According to Valve Wizard,

"Total Gain: The gain of the cascode is equal to:
A = 1 / (1/gm1 * Ra2) + ((ra2 + Ra2)/Ra2) * (1/ mu1 (mu2 + 1))
When using two identical triodes this can be simplified to:
A = gm1 * Ra2
as was calculated earlier."

A 12AU7 has gm = 0.0022A/V so you'd expect a gain of 0.0022 * 100000 = 220. That's not a "slight increase" by any means, so your circuit probably isn't working right.

I've attached a schematic of whats in my OD pedal along with voltages. They look pretty close to the valve wizard website's voltages.

Any ideas why I'm not getting any distortion?

[J M Fahey]

Hi, I've never built a cascode, I don't like them, I think they belong in TV or oscilloscope circuits, not in Guitar Amps (by the way, *no* major guitar amplifier used them, ever, at least that I know of), but anyway, I do understand triodes. Considering this as two triodes in series, a common cathode one and a common grid one, I see very strange voltages in your schematic.
To begin with, the currents are consistent: 30V across the top 100K translate to approximately those .66V across the 1K8 cathode resistor. My calculator says .54V but that is irrelevant, we deal with real world measurements here.
*But*, -.66V is *nothing* to bias a 12AU7 into those 0.3mA flowing there.
In practical terms, I'd consider the lower triode unbiased and saturated.
Now onto the upper triode: with no cathode resistor, it shows -40V bias (69-29) WTF?
EDIT: now that I peek at the FireFly schematic: V1 is a 12AX7, not 12AU7, what did you use?
2)Considering you have +210V there, what's wrong with a single triode, 100K plate, 2k2//10uF cathode?

[Steve Conner]

it shows -40V bias (69-29) WTF?

I think the voltage discrepancies are caused by the meter loading down the upper grid. The grid voltage isn't 29V when it's not being measured.

I have no idea why it doesn't work. Unless maybe the 12AU7's gm is specified at a high current, and gets a lot lower at the current you're running.

The best tube for cascodes is the ECC88/6DJ8. If you're feeling heretical, you can make a hybrid cascode, replacing the bottom triode with a transistor or JFET.

[HTH]

thanks for the responses.

just to make things clear, I'm using an ECC83/12AX7 - I want LOTS of gain.

[HTH]

Considering you have +210V there, what's wrong with a single triode, 100K plate, 2k2//10uF cathode?

nothing at all - I tried that (except a 1uF cathode bypass) with the triodes in the ECC83 cascaded (alot like the preamp of a tweed champ). I couldn't get enough gain to get a clean/crunchy amp cooking.

[J M Fahey]

Hi Steve.
I agree, of course, there *is* a measurement error here, no doubt, but typical cheap digital multimeters now have around 10Meg. input resistance, so they will still load and change the measurement, but 40V seems too much, I would think perhaps 4 or 6 volts.
Probably measuring cathode-grid voltage would be somewhat more accurate.
Another discrepance is that the lower triode , with 1k8 bias resistor (admittedly small), drops around 70V , and the upper one, *no* bias, drops around 200V !!! .
If they were different triodes, well .... , but they both live in the same bottle .
EDIT: Hi HTH. A typical 12AX7 with cathode capacitor will provide around 50X real worls gain.
2 of them cascaded (not cascoded he he) then provide 2500X raw gain !!, enough for the most rabid heavy/gothic/thrasher/industrial out there.

[Steve Conner]

I agree! If you can't get enough gain to blow your brains out by putting two cascaded triodes in front of your amp, something is wrong.

I once did a similar mod where I added an EF86 stage to the front of one of the channels in an old Selmer. It didn't give "rabid" levels of gain, but it turned it into quite a nasty piece of work.

JM: The upper grid leak resistor is bootstrapped by the cathode, so the impedance at the grid is probably a lot more than 1M.

[loudthud]

What kind of cap are you using for the 1uF at the grid of the upper tube? I would expect more voltage at the cathode of the upper tube, perhaps a leaky cap is holding it down.

[HTH]

What kind of cap are you using for the 1uF at the grid of the upper tube? I would expect more voltage at the cathode of the upper tube, perhaps a leaky cap is holding it down.

Its an electrolytic cap. The voltage at that point eventually settled at 29v, but started off up close to the cathode voltage (69v).

I'll try replacing that cap tomorrow and see what it does. The circuit is very simple, so there must be something VERY simple screwing this up.

[Jazz P Bass]

Boost Pedal

Kind of curious that the second tube on tne schematic is labeled Gain.
That tells me that the first section was engineered as a buffer amp.
Why not build the whole circuit?

[HTH]

Kind of curious that the second tube on tne schematic is labeled Gain.
That tells me that the first section was engineered as a buffer amp.
Why not build the whole circuit?

I only want to use one ECC83/12AX7 in my OD pedal as thats what its wired up for right now. Plus, everything I've read on cascodes says that I'll get lots of OD, so I at least want to get this working.

As for building the whole circuit, in a sense my signal path into my amp (plexi clone) has a similar signal path if you follow it down the chain >> cascode OD pedal >> two gain stages then CF & PI in plexi.