12AU7 in cascode configuration

[georgehifi]

Hi guys what is the approx output impedance of a 12AU7 in cascode configuration. Also what is it's max current output.

Thanks Georghe

 

[SY]

Just about any cascode will have an output Z roughly equal to the plate load resistor. The max peak current out will roughly equal the idle current, but the distortion with that kind of load will be unacceptably high. In a low distortion circuit (for a 12AU7, not a linearity champion!), the max current swing should be held to about one tenth of the idle current.

 

[DF96]

Roughly (mu^2)/gm in parallel with the anode resistor. In most cases the anode resistor will dominate. There was one celebrated GEC naval receiver which used 12AU7 cascode as the RF amplifier, but for audio there are almost certainly better ways of doing whatever it is you want to do.

 

[kevinkr]

I'd recommend a 6DJ8 in cascode over the 12AU7A for linearity and noise performance, and unlike the 12AU7A it was actually designed with cascode operation in mind.

 

[revintage]

Hey Georghe,
How much gain do you need and what is the application?

As others have told you 12AU7 is next to no, no.

Raw gain of a E88CC is a few dB over 55dB. You can easily get over 40dB gain with acceptable low THD.

If you need low Zout you can always add a follower.

 

[georgehifi]

Sorry guys I'm not making one, a tube guru just said to me yesterday this would be around 33k output impedance and maximum of 10mA current output. Just trying to find out. It says it's 12AU7 in cascode configuration, directly connected to the output rca's (probably via coupling cap) and that it has switchable gain of 6db or 12db.
Concert Fidelity | CF-080 LSX
He uses it into a power amp that is 20k input impedance, this then would be a bad impedance match.
Cheers George

 

[revintage]

As the machine is called hybrid I can guarantee it has a sand buffer with very low Zout, not 33k. So it is probably no problem for your friend.

About the circuit low gain also indicates local feedback, maybe through the upper grid and an unbypassed cathode resistor. Still one wonder why 12AU7 was choosen and what a cascode is needed for when total gain is 6/12dB?

 

[georgehifi]

As the machine is called hybrid I can guarantee it has a sand buffer with very low Zout, not 33k. So it is probably no problem for your friend. ?

But it says that the 12au7's are directly connected to the output rca's somewhere in the blurb.

Cheers George

 

[revintage]

Hey George,

Have tried to find Zout in the blurb, but nothing. But I read there is a sand input stage and the cascode at the output. So 33k Zout could be true!

Weired deLuxe!

 

[SY]

This may come as a shock, but... hope you're sitting down... is there a fainting couch nearby?...

....sometimes ad tear sheets are not technically accurate.

 

[revintage]

Hey SY,

, actually the sheet didn´t say anything at all. Found the awkward info in a review.

This is my idea how a cascode, if it for some reason must be, should be used:

 

[DF96]

If you want 6 or 12dB gain, you don't use a cascode.

If you want lowish output impedance, you don't use a cascode. (NB. in a preamp you will want low output impedance, unless you are incompetent).

If you are serious about valves, you don't hang them out horizontally at the back of the chassis, even with fancy roll-bars. This is asking for poor contact as the valve slowly drops out of the socket, and the risk of 'beverage contamination'.

 

[Tax]

If you want 6 or 12dB gain, you don't use a cascode.

If you want lowish output impedance, you don't use a cascode. (NB. in a preamp you will want low output impedance, unless you are incompetent).

If you are serious about valves, you don't hang them out horizontally at the back of the chassis, even with fancy roll-bars. This is asking for poor contact as the valve slowly drops out of the socket, and the risk of 'beverage contamination'.

I have a borrowed CF-080LSX preamp in my system. Yes the valves hang horizontally at the back but they use an extremely good socket that has a GRIP on the valve that I have not experienced in any other valve amp before. It is not easy to even 'intentionally' put a valve in a take one out without a bit of brute strength.

 

[DF96]

This means risk of damage such as glass cracking around the pins. Modern valve bases often grip a bit too tightly.