12AT7,12AU7 Ultra linear cascode

[ppl]

I have been playing with various cascoded input Driver stage for my 777 Amp and so far, the ultra linear cascode has performed the best in this Amp. I also tried self-biased cascoded SRPP and Conventional voltage divider cascode from B+ to ground and returned to this Ultralinear version.

This drives a triode strapped EL34 or 6550 operating at about 350 Volts B+ with cathode feedback for about 3-5 watts very well with plenty of dynamics and with the EL34’s a 3D Holographic midrange without a lot of euphoric 2ed harmonic to contend with.

Attached are the actual voltage measurements of various 9 pin Dual triodes of which the 12AT7 sounds the best. I would like your people view on optimization of this circuit for use with the 12AT7 AS WELL as the 12AU7.

[sulcorebutia]

Hi PPL,

Maybe you can post the circuit files here so that everybody can discuss together? I have been working on some 6sn7 Ultra linear cascode, result is not very impressing when compared to others 6sn7 topology. So, it is interesting to see the implementation on 12au7/12ax7 etc.

[ppl]

Originally Posted by sulcorebutia

Hi PPL,

Maybe you can post the circuit files here so that everybody can discuss together? I have been working on some 6sn7 Ultra linear cascode, result is not very impressing when compared to others 6sn7 topology. So, it is interesting to see the implementation on 12au7/12ax7 etc.

thanks for your interest. Circuit Files?? the schematic and measured voltages are shown on the attachment on my previous post.

i surprised you get poor performance from a 6SN7 UL cascode that Tube has worked Great with a 47K plate load in My previously Posted Circuit, However could you elaborate more..

[AmadeusMozart]

FWIW I did see the other day a tube - but cannot think about the name of it - that had half a 12AX7 and half a 12AU7 in one envelope (from memory).

That would be nice to use the 12AX7 for the amplfier and the 12AU7 for the cathode follower part.

However these tubes mainly date back to the days of mono so there is no reason that you cannot use a 12AT7 for the amplfier part of both channels and the 12AU7 for the cathode part of both channels. That would allow you to get the filament at cathode potential for the 12AU7 and as such you can go to voltages far higher than the Vkf breakdown. Would certainly open a new frontier to explore.

AM

[sulcorebutia]

Originally Posted by ppl

thanks for your interest. Circuit Files?? the schematic and measured voltages are shown on the attachment on my previous post.

i surprised you get poor performance from a 6SN7 UL cascode that Tube has worked Great with a 47K plate load in My previously Posted Circuit, However could you elaborate more..

If a particular tube's UL cascode works great, I would think of this circuit has either lower output impedance, high input impedance, higher mu, lower distortion or better PSRR etc when compared to grounded cathode, common cathode, grounded grid, white follower or any other topology one can think of. So, I would like to learn more about the advantage of UL cascode(50% of output signal feed back to the upper triode grid).

To be more direct, I wasn;t impressed with the result of UL cascode (50% feedback) in 6sn7. And I want to learn more in the case of 12at7/12ax7 etc.

[sulcorebutia]

The following ltspice circuit runs 47K plate load on a 6sn7 UL cascode, biased around 4.1mA plate current for both tubes. 2V pp 200Hz input yields around 42V pp 200Hz output. The Mu is 21, which is similar to what an one stage 6sn7 grounded cathode can give you.

[ppl]

my previous circuit has evolved so my latest version in its complete Amp is attached. Works quite well however is on the verge of instability.

[atmasphere]

Originally Posted by ppl

my previous circuit has evolved so my latest version in its complete Amp is attached. Works quite well however is on the verge of instability.

No doubt. You have a feedback loop nested within another- sort of a recipe for instability.

[revintage]

Except for the distorting driver, it will, as Atmasphere says, surely be instable. If going the cascode route why not go for high Gm-tubes?

The UL connection itself is not so bad even if it doesn´t do so extremely much for linearity but it helps.

Unfortunately you´ve got all wrong as you can´t have the capacitor from the upper grid to ground! No feedback at all that way. See sulcorebutia´s circuit above, it is correct.

[TheGimp]

Older NE-2s may not conduct in total darkness.

Some activation energy is needed to initiate conduction and ambient light normally provides this.

Back in the 80s this problem was identified, and Krypton gas (low level radioactivity) was added to the neon gas to insure proper operation.

If you have a stash of older neon bulbs they may have this characteristic (poor ignition in dark).