Mods to Preamp with 7025/LND150 Cascode Input Stage & LND150 TS Driver

[Murrayatuptown]

Hi Kevin,

Thanks for explaining this.
I have scanned the complete Albion schematic and attached it for your reference.

Is this a normal level of gain for this type of circuit and if so, why would you want that much signal slamming the grid of the next stage?
I had considered that removing C3 may cut the gain a fair bit, but it is a radial electro and is glued to the PCB; too much trouble to remove.
The Input Stage is stock and I am surprised they didn't just use a normal 12AX7 stage.

I was concerned about resistor noise with the high series R and I intended reducing R and shunting the Gain pot, once I found a suitable divider ratio, but there is no perceptible noise, so I left it as is.
The out of phase channels don't pose any problems and the channel switching removes the non operating channel - without any bleed through.

R82 was also 100k and I changed it to 10k because it allowed too much OD, even with Drive at 0.

Grid Stops were a worry to me also, but surprisingly, there was no indication of any problems, but I added the 330k stopper to V2B to cut some Treble and add a bit of insurance. I may add some stoppers, to the stages that have none, as a safeguard.

The Source Follower driving the Tone Stack is stock, as is all of the A channel; apart from R8, which I changed to 3k9, because we preferred the tone.
Would it be wise to increase R15?

With the effective load of 175k on V2B, I figured a gain of 21 and I measure an actual gain of less than 12. What do you think?

I am not fussed on Bright Caps on pots either and I may remove C61, although I haven't noticed any undesirable effects from it. Maybe the series 470k negates a lot of the treble bleed?

The A channel does have a good OD, but it is used purely as a Clean channel only. VR1 & VR2 provide useful tone variations; I like controls.

Thanks again for all your help and we are having fun, despite the pandemic.

Cheers, Noel.

This is another interesting input circuit. I, too, initially wanted to say "call cascode", but there are so many variations of vertical/totem-pole things we don't see very often, I easily confuse them.

I understand a JFET or MOSFET current source with a glass triode...but a glass current source for a MOSFET...it's Albion's amp...they can do what they want...there was a half 12AX7 available, no extra real estate or filament current, but...but...

I poked around online while my registration was processing...

I no longer see cascode. Not enough parts for a cascode party.

I did see something interesting (if only from an armchair, if not a workbench)...John Broskie/TubeCad's depiction of a vertical FET-triode casCADE, with the triode upper device using its cathode as a 2nd input.

I should know better than to ask "why?" in audio, whether hifi or guitar...because the designer wanted to, and/or liked the way it sounded.

I'll just be quiet & keep reading...

But one more thought...since the OP's question was about analysis of a cascode, I thought of Ulrich Neumann's Solo hybrid amp.

He starts out discussing an example of a popular Vox amp that cascoded a JFET & triode, then ends up with a LND150 on top of a JFET (J113, I think). In the book, he thoroughly covers how to handle noise, JFET deviations from desired parameter values, and how much gain it can deliver.

Since his example IS a cascode, it has the typical high output impedance, probably needing a follower of some kind to buffer it.

Murray