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Mods to Preamp with 7025/LND150 Cascode Input Stage & LND150 TS Driver
#1
Hi Guys,

It sure is quiet here. Where is everyone?

I need some advice on how to calculate the output and input impedances of an LND150 Depletion Mode MOSFET and the ouput impedance of one in a Cascode with a 7025.

I have done some Mods to an Albion TCT 35 (designed & manufactured by Steve Grindrod; former engineer for Marshall and Vox).
Attached is my schematic of the Preamps with my Mods.

The owner didn't like the Drive sound very much, so he used the Clean channel with pedals instead.
The idea was to do minor mods to the A channel (Clean) to sweeten the tone and change the B channel (Drive) to emulate the Overdrive sound of a Mesa-Boogie Mark 2C.
I have done the mods and it sounds good.

The interesting part of this amp is the 7025/LND150 Cascode Input stage (shared by both channels) and the LND150 Driver for the B channel Tone Stack.
I am not up to speed with the LND150, so I was surprised to see a voltage gain of nearly 140 from the Input stage.
100mV Input to 13.8V output, which required the gain pot to be kept very low to get anything like a clean sound (dirty Clean).

The original circuit had a 470pF coupling cap (C51) to the B channel, followed by 100k (R81) - 470k (R80)/bypassed with 2n2 - 1M Gain pot with a 100pF Bright cap.
I changed C51 to 1n, which suited the Mods better than the 470pF and more capacitance produced too much Bass. 
I added the 2M2 resistor and removed the 2n2 bypass and the Bright cap, to attenuate the signal into the Gain pot; which is workable now.

I don't know how to calculate the output impedance of the Input Cascode and therefore the load on the cascode output, coupled with the following circuits and how much effect this has on the Voltage Gain. It will be good to know this, for future reference; and I need to increase my Solid State knowledge.
I am also wanting to know how to calculate the input impedance of the LND150 Tone Stack Driver to ascertain how much load it and the Tone Stack apply to the V2B Valve Stage.
It must be quite a heavy load because this stage is only showing a Voltage Gain of approximately 12, or there is something else affecting the gain.
A 7025/12AX7 with a 270k Ra and an unbypassed 3k3 Rk should produce a Voltage Gain of about 34/30dB into a 1M load.

I forgot to add the Tone Stack circuit.
It is the same as the A channel, but with the following changes in values: Slope R= 33k. Treble Cap= 470pF. Bass & Mid Caps= 22nF. Bass Pot Dropping Resistor= 10k.

All info and help much appreciated.
Happy New Year to you all and I hope you guys aren't freezing in Canada and the Northern Countries; we could use a bit of your cool here in hot, steamy and rainy Gold Coast Oz.

Cheers, Noel


.pdf   NGW Mark 2D - December 202001012021.pdf (Size: 491.27 KB / Downloads: 9)
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#2
Hi Noel

Note that the input stage is not a cascode; rather, a standard common-source gain stage with a current-source load. The tube contributes nothing to the gain per se and does not limit it either - the gain is all from the LND150. Output impedance is essentially the same as for a cathode-follower as 1/gm - pretty low even with a 12AX7. This means you can change how the signal is attenuated into the lower channel and reduce some potential for noise.

The alternate way to pad the gain and drive controls - any level control - is to add a shunt resistance across the pot to work against the series R feeding the pot. The lower part of the divider is then the prallel of R-shunt and the pot, where R-shunt can be used to swamp the ot's value out of the divider calculation in situations where you may be constrained to using high-value pots. For example, a 1M pot parallel with 100k is 90k net. If R-series is 90k then the divider ratio is 0.5 - signal-max cut in half. If R-series is 900k, then the ratio is 0.09 (1 / 11). You could make R-shunt much lower, say 33k, then R-net is 32k. Now R-series can be reduced in value to 320k for the high attenuation of 1/11 - a bit lower noise.

Was the input stage stock, or did you modify it to be as it is? I suspect a straight 12AX7 stage would be fine here, both gain-wise and noise-wise, and it might improve control sweep utility.

In both channels the Drive control does not go to zero, with signal feed-through allowed by R75 (upper) and R82 (lower). The lack of grid-stops for most stages is a bit worrisome, but as you say, one channel is copied from Mesa practice.

The mosfet source follower depicted uses a voltage divider to set the gate voltage and thus the source voltage for the output. This makes deciding on the source current independent of the mosfet parameters. The divider sets Vg and Vs at B+/2, about 165V. You set R15 to 47k so current is 3.5mA and Pd is 577mW - maybe a bit hot for the LND150.

Input impedance of the divider-bias follower is simply the parallel combination of the divider elements. Since both are 1M, then the net is 500k, making the effective load on V2B 175k. The mosfet gate impedance approaches infinity, same as a triode, so it can be ignored. Output impedance is device-dependent and will be 1/gm.

Personally, I do not like bright caps on level controls as it makes the tone versus sweep irregular. The two channels have lots of gain and either can easily be an overdrive channel. They are out of phase though, which some people care about and some do not.

It's usually sunny here even in winter, so winter is quite bright Smile

Have fun
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#3
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.


.pdf   Albion TCT35 HG iss3 circuit diagram.pdf (Size: 243.16 KB / Downloads: 9)
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