bias capacitor

[ed]

another naive question guys:

can somebody advise what's the best type of capacitor in the C? position. I think I need to trap the - dc content but not affect the ac load...am I looking at this the right way?...If I use an electrolytic backwards then aren't I putting it in the wrong way round with respect to the signal?

[andrew Ivimey]

hm, well here's some one with the understanding of a beginner in some matters but this is a much simpler version of how a VT4C is biased in Ongaku.

I used an electrolytic never imagining I could use anything else. In the original Ongaku diagram (I found on the net) the polarity was the wrong way round. I spotted that and felt very chuffed.

[Mike H]

Can be a 'lytic, positive end would be to ground.

Assuming 'bias' is negative Volts.

[pre65]

Can be a 'lytic, positive end would be to ground.

Assuming 'bias' is negative Volts.

I would assume, cos there is a cap in the input line.

[ed]

Can be a 'lytic, positive end would be to ground.

Assuming 'bias' is negative Volts.

Thanks Mike...I've had an eletrolytic in for the last 3 or 4 years but I suddenly had doubts regarding my existing question and the 'backwards' thing. I was thinking about how Pete Millet uses poly foil and thought maybe I had got it wrong........

[Mike H]

Can be a 'lytic, positive end would be to ground.

Assuming 'bias' is negative Volts.

I would assume, cos there is a cap in the input line.

I only put that in, 'cause just in case someone came along saying "but what if it's positive bias"

[ed]

I'm beginning to wish I hadn't looked at this now........I think I took a lot for granted when I first put this circuit together as I don't know how I arrived at the value for the cap in the first place.....

can anybody suggest how to get the time constant for that cap because I don't see a load resistor to work with.....and hence the pass frequency

or suggest a place to read....I havn't been able to find anything with my usual(bad) search phrases, and RDH and Jones seem pretty quiet on the subject

[Mike H]

It would be versus the grid leak resistor value? Whatever that is. Plus, whatever it is that supplies the negative bias will be in parallel with the cap.

[ed]

ahhhh thanks Mike....

just to clarify, using my circuit below. I've taken the average R to be half the 20k pot plus the 10k, say 20k in all. Which I make 0.79uf cap for 10hz at -3db. Does that sound anywhere near right?

[Mike H]

I guess so, I'd make it a whole round 1uF for convenience

How is the -120V derived, is it smoothed at all, if not, cap may need to be larger to minimize hum ripple from that direction?

[ed]

this is a new one, as yet untested...ignore the 100k, it's shown on the other diagram......The Tx is two RS 2x18v, all wired in series....I have found that the RS txs are rated for 230v and give consistently good results when fed 250+ volts...together with a lower load than the rating I'm expecting between 110v and 120v.

I'm also playing with CLC because I'm curious what effect the bias ripple has on the output.

[Mike H]

According to that you can easily get the maximum minus Volts there is available, do you need that much?

Now the cap value is looking like what's the best value for minimum ripple from this bias supply.

[ed]

According to that you can easily get the maximum minus Volts there is available, do you need that much?

yep

Now the cap value is looking like what's the best value for minimum ripple from this bias supply.

don't understand this one though!....I thought the cap question was answered before, providing my sums were correct..the pie filter caps in the bias supply are all sorted. Well, as sorted as I really need because the comparison was between this and the previous half wave rectifier with an RC filter, so I was curious to see if there was a difference.

[Mike H]

Edited

I forgot about the 100k resistors on the preset wipers, OK so this isolates the pi filter from the grid leak decouple cap, as an RC filter, so as you were.