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06-02-2023, 12:01 AM
Hi Guys
You need to look at places like Digikey and Mouser for the non-tube-specific parts - which is pretty much everything in an amp other than the tubes and sockets.
470uF-16 or 25V is 10mm diameter and 10-16mm tall as a radial-lead 10khr part. Tiny.
These caps have to be a high value because the portion of the pot resistance they work against can be very low.
For example, say you increase Rk by two times as you did, going from 150R to 300R. If you want to limit the range of adjustment so that the tube currents can never exceed the stock value, then the range resistors R10,11 should equal the pot value. So, with 25k pots R10,11 should be 25k each. Say you had 10k pots, then R11,12 would be 10k each. In both cases, the portion of the pot between the wiper and ground can sweep to zero and may in practical terms be only 1k or so.
At 1k, to have an AC ground with a lower cutoff frequency than the coupling caps of the amp might mean using at least 10uF. That works out to 16Hz, and flat to 160Hz. That seems "good" but we need it to be better. 100uF reduces the frequency roll-off to 1.6Hz. Anything lower is still an improvement and we want this roll-off to be at least a decade away from the lowest roll-off in the amp. 470uF brings it down to 340mHz. We could use 1,000uf (1mF) and cut that in half.
You can measure the pot resistance to ground with the power off and see what time constant you get with 50uF caps. That is an off value in modern manufacturing, except as large cans, you more typically see 47uF then 56uF in the E24 value range. 50 would be part of the older E12 range.
We are using low-voltage caps here.
It is good to make voltage measurements of the stock circuit first. The bias voltage Vk in a 2x EL84 amp is typically <15V. So doubling Rk to potentially have double the Vk at the stock current BUT with a limit of voltage across the pot to be half, means we can use a 16V cap. A 25V cap gives us some leeway.
Remember: Electrolytic cap voltage ratings are "working voltage" meaning you can operate the cap at its rating. Using a 100V cap at 15V for a few years means that the cap "becomes" a 15V cap as it reforms to the voltage applied. If it regularly sees 100V but mostly 15V it will likely retain its 100V forming.
You need to look at places like Digikey and Mouser for the non-tube-specific parts - which is pretty much everything in an amp other than the tubes and sockets.
470uF-16 or 25V is 10mm diameter and 10-16mm tall as a radial-lead 10khr part. Tiny.
These caps have to be a high value because the portion of the pot resistance they work against can be very low.
For example, say you increase Rk by two times as you did, going from 150R to 300R. If you want to limit the range of adjustment so that the tube currents can never exceed the stock value, then the range resistors R10,11 should equal the pot value. So, with 25k pots R10,11 should be 25k each. Say you had 10k pots, then R11,12 would be 10k each. In both cases, the portion of the pot between the wiper and ground can sweep to zero and may in practical terms be only 1k or so.
At 1k, to have an AC ground with a lower cutoff frequency than the coupling caps of the amp might mean using at least 10uF. That works out to 16Hz, and flat to 160Hz. That seems "good" but we need it to be better. 100uF reduces the frequency roll-off to 1.6Hz. Anything lower is still an improvement and we want this roll-off to be at least a decade away from the lowest roll-off in the amp. 470uF brings it down to 340mHz. We could use 1,000uf (1mF) and cut that in half.
You can measure the pot resistance to ground with the power off and see what time constant you get with 50uF caps. That is an off value in modern manufacturing, except as large cans, you more typically see 47uF then 56uF in the E24 value range. 50 would be part of the older E12 range.
We are using low-voltage caps here.
It is good to make voltage measurements of the stock circuit first. The bias voltage Vk in a 2x EL84 amp is typically <15V. So doubling Rk to potentially have double the Vk at the stock current BUT with a limit of voltage across the pot to be half, means we can use a 16V cap. A 25V cap gives us some leeway.
Remember: Electrolytic cap voltage ratings are "working voltage" meaning you can operate the cap at its rating. Using a 100V cap at 15V for a few years means that the cap "becomes" a 15V cap as it reforms to the voltage applied. If it regularly sees 100V but mostly 15V it will likely retain its 100V forming.
