Basic knowledge.

[pre65]

I do know a bit about the theory of valve circuits, but there are gaps in my knowledge that confuse me. So I'd like to seek help. Some of these questions may seem too basic for one who has built so many amps, but that's me, the one and only me.

1) When one is seeking data to bias the grid of a driving valve, is the bias needed the same as the likely max input signal ? ie if the max input volts is 2v (CD output voltage) does one bias the grid at -2v ?

2) If so, what would be a good input voltage for a healthy input signal from a pre amp, mainly fed from a DAC.

If this knowledge is available from an external source that's easy to read a link would be appreciated. If it's in MJ third edition, what section ?

[Nick]

Ok, first part IMHO is you need to go back to the loads lines and see why you need what headroom you need for a given input,

second part, remember 2v from a CD player is 2v RMS, so the peak voltage is 2 * 1.414 or 2,828

third part, try and keep away from the zero volt grid line (see part one), so in this case I would suggest -3v bias at least for the input directly from a CD.

[pre65]

Thanks Nick.

More by luck than judgement the 12HG7 I've just installed in the PSE pentode amp are biased at -2.8v, sounds near enough.

I'll try and find that DTB tutorial on load lines, it's on the forum somewhere.

[Mike H]

Yep.

Ergo you couldn't run such a CD player directly into an ECC83, for example, 'cause its DC bias Volts is am very tiny

It would clip like a clipping clippy thing.....

[Mike H]

Appendix ~ depends on the source impedance, feasibly possible it might be driven into A2

[pre65]

Question 3. This came up when talking to a friend the other week.

When using an anode choke (like I do on 112a pre) is there any reason to have a CCS between the PSU and the choke.

We both thought there was a reason that would prohibit such a configuration, but could not find what it was.

[Nick]

No. There is no reason why you would, and one big reason why you shouldn't

To work out what the reason is, read up about inductors. (hint: what happens if the current through a inductor is constant)

[pre65]

Ah, so in effect, with constant current the choke ceases to function as a choke.

[Dave the bass]

I'll try and find that DTB tutorial on load lines, it's on the forum somewhere.

This one Phil?

http://www.audio-talk.co.uk/phpBB2/view ... =loadlines

DTB

[pre65]

I'll try and find that DTB tutorial on load lines, it's on the forum somewhere.

This one Phil?

http://www.audio-talk.co.uk/phpBB2/view ... =loadlines

DTB

Cheers Dave, I've saved it in my "hi-fi" favourites section so I can have a good read.

[Nick]

Ah, so in effect, with constant current the choke ceases to function as a choke

The choke will always be a choke, and so will always function as a choke.

[Mike H]

<imagination mode>
If choke is supplied via a CCS, it's current can't change, so it can't make any AC signal Volts across itself. Or not a lot
Would be mostly like a CCS with a resistor in series, and the CCS is doing most of the work.
</imagination mode>

[IslandPink]

V = L*dI/dt is what Mike said .

The choke will always be a choke, and so will always function as a choke.

But if a choke fails to act like a choke, in the woods, and nobody sees it fail to act like a choke, did it .... no sorry you are correct, Nick

[Nick]

Yep, I was sort of hoping Phil would take the hint and go and try and read about just what a inductor is.

Resistor: current is proportional to voltage

I = E / R

Capacitor: current is proportional to the rate of change of voltage

I = C dv/dt

Inductor: voltage is proportional to the rate of change of current

V = L di/dt

[pre65]

[quote="Nick"]Yep, I was sort of hoping Phil would take the hint and go and try and read about just what a inductor is.

I did, but I did not explain it very well.