01-01-2021, 03:37 AM
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
NGW Mark 2D - December 202001012021.pdf (Size: 491.27 KB / Downloads: 5)
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
