Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on May 16, 2023, 12:49:10 pm
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i have asked this before and was not really answered. Let's say my V1 preamp tube plate reads 300V and the plate resistor is 100k. i need to get the V1 plate to 200V. What would be the difference between changing the plate resistor value (say to 220k or 330k) vs lowering the B+ feeding the plate resistor to accomplish the target plate voltage? Is there a tonal difference
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i have asked this before and was not really answered. Let's say my V1 preamp tube plate reads 300V and the plate resistor is 100k. i need to get the V1 plate to 200V. What would be the difference between changing the plate resistor value (say to 220k or 330k) vs lowering the B+ feeding the plate resistor to accomplish the target plate voltage? Is there a tonal difference
Easy experiment imho.
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Wouldn‘t your scenario mean you got 400V supplying that tube? Anyways I remember your question and if I remember correctly lowering the supply voltage or increasing the plate resistor leads you to the same technical result was PRR‘s answer. My experience being, I do not need max voltage in the preamp supply, amp sounds better to my taste with 200 - 250V in the power supply.
twimc.
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I would lower the B+.
With respect, Tubenit
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okay well i increased the plate resistor to 220k. My B+4,5 is over 400v.
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You don't say what tube you're using, so I'll guess a 12AX7. Also, I find that almost everything I change in a design effects the tone, so the question is whether it is better tone or not.
In my exp., 100k load res drops about 100vdc off the plate at V1. So, a B+ of 300vdc will end up at around 200vdc on the plate. But you have 300v at the plate even with the 100k res.
There is an important relationship between the plate and the plate resistor (usually called the load resistor). If you look at high gain designs, they sometimes have a 120k load resistor on V1, going as high as maybe 220k. In addition, the cathode resistor is often increased in conjunction. The effect is to give greater gain and much more "overdrive" effect. Usually, this means less headroom and more crunch, although whether the crunch is good or not is subjective.
In technical terms (which I am cretainly not an expert on - no, that's not a typo :icon_biggrin:), I believe there is a sort of Ohms Law trade off going on between plate current and plate voltage. A higher value load resistor reduces voltage but in exchange allows more current, which can then overdrive the next stage. But I am guessing here. But there is certainly a match that needs to be made between the plate and resistor, depending on what tone you're looking for.
In any case, 300v on a 12AX7 plate seems higher than usual - I am more comfortable around 160-200v on a 12AX7. I don't know why - probably just my limited experience. But does it sound how you were hoping? If not, drop the B+ at the power filter section with a larger dropping resistor, rating it for the expected wattage/heat dissipation.
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the V1 (and V2) on the ab763 Super Reverb uses 100k plate resistors. The B+4 feeding those is over 400vdc (415vdc on mine) (see attached doghouse). So the plates are seeing 300vdc
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i have asked this before and was not really answered. Let's say my V1 preamp tube plate reads 300V and the plate resistor is 100k. i need to get the V1 plate to 200V. What would be the difference between changing the plate resistor value (say to 220k or 330k) vs lowering the B+ feeding the plate resistor to accomplish the target plate voltage? Is there a tonal difference
Lowering the B+ that's feeding the V1 plate resistor is the correct way to do this. Changing the plate resistor is the wrong way to do this. When you change the plate resistor you are also changing the gain of the stage.
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change out the 4.7k v dropping resistor with a 10k 3w and see what transpires. Could use other values over 4.7K too.
What plate V are you seeking for V1? or did I miss it.
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cant do that as it also feeds V2 which is my vibrato channel leaving stock. I will add another filter cap and dropping resistor on the board and have a B+5 for V1
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Lower the corresponding cathode resistor to gnd. The current will raise and voltage will drop
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...as mentioned, "tube type", "gain"
I found the math wasn't fun any more so I found datasheets with lookup tables, now they are fun :icon_biggrin:
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okay so i added the B+5 and swapped the plate resistors to 100k's. Amp is quieter and some additional headroom but overall much better. In order to get the B+5 down to 320 volts i had to use a 66k dropping resistor. Quite a large resistor coming off the B+4 node to get this correct.
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okay so i added the B+5 and swapped the plate resistors to 100k's. Amp is quieter and some additional headroom but overall much better. In order to get the B+5 down to 320 volts i had to use a 66k dropping resistor. Quite a large resistor coming off the B+4 node to get this correct.
The current for V1 is probably only about 1.5mA. And you dropped the voltage from 415 down to 320, or 95V. So, 95v and 1.5ma calculates to 63K. All good.