I'm in the planning stages of a 6AQ7 PP stereo amp with 6DJ8s up front. I'm using the transformers from an old Nutone stereo/intercom, and I'm planning to follow the power section schematic. It calls for a 500k volume pot, and that's a rare beast these days. I should say that the original front end in the Nutone had 2 x 12AX7s per channel. The new circuit will use one 6DJ8 per channel. Do I really need a 500k pot? I'm used to matching pots to pickups on guitars, and the pot affects tone. Would that be the case here?
Guitar Pickups are usually designed to feed a high input impedance, typically a megohm or more, and a low-impedance load reduces the high-frequency response of the pickup .
Other sources like CD players, pre-amps, iphone, mp3-player, etc. usually have low impedance and 500k is a bit on the high side for a triode 6DJ8 input which has noticable input capacitance due to miller effect, which could limit high frequency response. A 500k pot is also more susceptible to pick up hum.
A 100k pot is therefore usually more practical.
Other sources like CD players, pre-amps, iphone, mp3-player, etc. usually have low impedance and 500k is a bit on the high side for a triode 6DJ8 input which has noticable input capacitance due to miller effect, which could limit high frequency response. A 500k pot is also more susceptible to pick up hum.
A 100k pot is therefore usually more practical.
Yes, for a home audio system don't go above 100k for the pot, unless the first tube stage is a cascode.
If all your sources are solid state only, I'd even use a 10k pot.
If all your sources are solid state only, I'd even use a 10k pot.
Did you really mean 6AQ7 or should that read 6AQ5?
A 500K pot is on the high end for VC on the signal input. At its mid-point it looks like about 125K if the input signal is at very low impedance. And higher for anything else. That 125K + needs to drive the Miller effect of the first tube in the amp, sounds like 6DJ8. So the HF content of the signal is limited.
A better choice for the gain control on a tube amp is 100K. And readily available.
Those 500K pots were common in Public Address amplifiers as was One Meg.🙂
Respect what the original designer asked for.
Not sure how deep you searched but Mouser alone has 16 different models to choose from, matching your needs, just select shape , size, shaft and pins.
https://www.mouser.com/Passive-Comp...xtZ1yzvm31&Keyword=500k+potentiometer&FS=True
Selection terms were:
EDIT: and REALLY you *should* have posted the schematic here.
NOBODY answering (me included) knows *where/how* is that 500k pot connected.
Most seem to assume it´s straight at the input (or they wouldn´t be talking music source impedance).
Me? nothing in your question says so, so I´ll wait for the schematic first.
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