The complete 6SN7 preamp

[Joel]

I've been asked to show the power supply I'm using with my preamp design posted in the "tube preamp" discussion - so here is the circuit first:

[Joel]

and the power supply...

[Original Burnedfingers]

Thanks Joel


Your power supply circuit is quite interesting. I was wondering ...
a choke is not needed? Would there be an advantage of having a choke and a RC network?

Its another newbee question..

Thanks
Joe

[dhaen]

burnedfingers said:

I was wondering ...a choke is not needed?

The power supply, as it stands, is perfectly adequate as long as it doesn't run into distortion.
This is because the average current of a class A stage is constant.
However, as soon as you start getting any asymetry, the current varies.
As a matter of fact you can check exactly how much 2nd harmonic distortion a class A stage has, by measuring the change in consumed current.

So, the answer to the question is: It depende how loud you want to play it.
If you want to get the max out; go for a lower impedance choke smoothed circuit.
If you want to economise, and don't want it loud. Use resistors.

Cheers,

[Joel]

Exactly what John said.

But you'll find that, whether used as a linestage, or to drive headphones - you will never need to push the tubes in this. There is plenty of gain, and a low enough output impedance to handle either task. A choke will only add cost... to this project. (I really do love chokes people! I swear. But you just don't need one all the time)

[SY]

No, you probably don't need a choke there. But you might want to bypass that last filter cap with a film cap of a uF or so. A scope or some headphones and a cap will tell you if that's effective.

I didn't read the other discussion- why use a 6SN7 in the first place for this application? It's a great high voltage driver that can source some current, but for a CF intended to pass a couple of volts at most, a higher gm tube with higher mu might be a better choice from the standpoints of source impedance and distortion. Don't misunderstand me, a 6SN7 will work OK, it's just not optimal.

[fdegrove]

Hi,

I agree higher gm is what makes a CF sing.

Even wouldn't mind a penthode there.

But you might want to bypass that last filter cap with a film cap of a uF or so. A scope or some headphones and a cap will tell you if that's effective.

Personally IMO,it's tricky to implement.
You often get the effect"Who stole the bass?" when bypassing.

Cheers,

[SY]

Well, if there's low frequencies moving around on the rail, you'll be able to spot it with the scope/phones. Though I don't see any reason why this should be greater with a bypass in place; the main effect of a bypass cap is seen at frequencies where the elytic is starting to get inductive.

I agree that a pentode would work well as the CF. For triode purists, a high gm triode (or a pair paralleled as Joel is doing with the 6SN7) will do nearly as well, but one generally has to accept 7 or 9 pin envelopes.

[dhaen]

the main effect of a bypass cap is seen at frequencies where the elytic is starting to get inductive.

Agreed. Though electrolytics can introduce distortion. They are a long way away from an ideal capacitor - BTW no voodoo here.
IMO it is essential to have a large enough resovoir; that probably means it's going to be a bit inductive - so bypass it. But in doing do remember; youve made a parallel tuned circuit at "some" frequency. Best to avoid 'lytics if you're a puritan.
Best buy some shares in a PIO company.

Cheers,

[Joel]

Originally posted by SY
I didn't read the other discussion- why use a 6SN7 in the first place for this application?

Because I have a shoebox full of them.