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Old 25th August 2017, 03:41 PM   #191
RhythMick is offline RhythMick  United Kingdom
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Originally Posted by Rod Coleman View Post
The Capacitor that bypasses the bias voltage should be located near to the triode that it biases, and connect to the local ground.

The bias network resistors/poti should probably be locally mounted too. There's no problem running the supply to different places - use screened cable is you are worried about pickup.
Something like this Rod ?

The pots and caps will be located right next to the triodes - I'm just getting my head around the loops so I keep the current flows separate.
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Old 26th August 2017, 10:27 AM   #192
Rod Coleman is offline Rod Coleman  United Kingdom
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Yes, looks OK, though having the trimmer/resistors there to is better still.
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Old 13th September 2017, 08:30 AM   #193
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Default Calculation of capacitor to bypass the Fixed Bias trimmer

How does one calculate the value of the capacitor needed at this point ?

So in the attached example, the bias sees the 5k resistor + a variable resistance from the pot (up to 5k), so between 5k and 10k. Is it simply

1 / (2 * pi * R * F)

where R is 5k to 10k
and F is desired frequency corner (-3db)

eg 1 / (2 * pi * 10k* 5) = 3uF ?
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Old 13th September 2017, 10:04 AM   #194
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... or does the cap "see" the AC path through the pot and the "top" 3k6 to the B- rail in parallel with the path through the "bottom" 5k to ground ?
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Old 13th September 2017, 01:59 PM   #195
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Quote:
Originally Posted by RhythMick View Post
How does one calculate the value of the capacitor needed at this point ?
You'd want to look at the amount of ripple that you see at the pot wiper. You can also put a cap from the 3.6K resistor/pot junction to ground to provide some additional filtering. Here's a decent (short) page to read through about these types of filters:
Low Pass Filter - Passive RC Filter Tutorial
If you've done any work with loudspeaker crossovers, this will look very familiar.

Now, when it comes down to putting a cap from wiper output to ground, this is certainly something you can do, but if you make it too big, you'll have sluggish bias adjustments since the cap has to discharge when you turn the bias pot. I think you could get away with as much as 470uF here, but I don't see much of a reason to go beyond 47uF.
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Old 13th September 2017, 03:50 PM   #196
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The bias rail is LCLC filtered with very high inductance chokes. I'm expecting <10 uV ripple on the bias. I'm not sure I even need a cap there, but at most 20uF. That's based on the 2 AC paths in parallel and it's whether that's right that I wanted to check.
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Old 14th September 2017, 11:40 AM   #197
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Those tutorials are excellent and I've seen that one before - thanks for the link though. My question is really whether the resistance in this case should be the parallel paths for the AC fluctuations - the path to ground (via 5k and 50% of the trimmer) and the path to B- (via 3k6 and 50% of the trimmer).
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Old 14th September 2017, 02:05 PM   #198
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So just to elaborate... at the midway point of the wiper we have

8k7 to ground, in parallel with
8k2 to B-
which gives 4k2. So for 2Hz corner I would need 19uF.

Am I right ?
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