Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on February 26, 2023, 10:51:14 am
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Hi all, I decided to build up the preamp ive been working on in ltspice. It seems almost impossible to get the voltages to match the schematic by modifying R1, R2, and R3 in the schematic. Even adjusting the plate resistors its very tricky and some of the resistor values just dont seem correct. I am trying to get about 1vrms output on C13 in the simulation (the waveform shown) wich i have managed to do. any suggestions on where to go from here? should I just trust these resistor values?
The simulation is missing the 10k resistor right off the 370v in the schematic and I need this resistor for the filter caps. Should I modify the plate resistors to get the right voltage with the 10k installed?
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It is hard to tell the way it shows (13 digit?!) voltages on top of values, but isn't the sim R31 the wrong value from the schematic?
That's an extremely ugly waveform (perhaps cuz wrong R31 plus gross overdrive). And you don't need signal to set-up DC voltages on preamp stages.
The DC voltages are not critical! Throw a chain of 10k droppers and play it.
OK, 8-digit display.
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PRR i was hoping to attract your attention. i Realized you were one of the ones helping me design the transistor power amp in 2019 :)
So just to be clear: you have faith in this version of the circuit? yes ill have to mod the drop resistors slightly. I assumed the basically square waveform was because its a guitar amp and you aint getting a pure sign wave out of a guitar amp.
can you please state any concerns you have about this circuit + do you think I should just send it and order all the parts for this iteration of the schematic? am I in over my head?
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It is hard to tell the way it shows (13 digit?!) voltages on top of values, but isn't the sim R31 the wrong value from the schematic?
That's an extremely ugly waveform (perhaps cuz wrong R31 plus gross overdrive). And you don't need signal to set-up DC voltages on preamp stages.
The DC voltages are not critical! Throw a chain of 10k droppers and play it.
OK, 8-digit display.
Yes R31 is one of the resistors giving me weird values. So I should just build it and ignore DC voltages?
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> giving me weird values.
I can't see it from here. How "weird"? Like HP Lovecraft? Or like 1K when should be 100K?
Use your numbers.
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its mostly in the drop resistors. I was told a 10k right off the rectifer + the filter caps would be ideal for filtering. But when i have a whole 10k right off the recitfier i need to change the drop resistors lower then in the schematic to get the right dc voltages.
PRR you said to just build this with 10k drop resistors and figure the rest out later, do you think I should just do that? I am only worried about turning a chassis into swiss cheese by modifying it too much. but i am open to experimentatioin.