To choke, or NOT to choke....

[axpro]

Hey guys, as none of you know (seeing as i have no responses on my other post )

My next project is a high power Princeton Reverb. using the Weber kit.

I just noticed last night, as i was wondering about putting a choke in my other project, that the princeton doesn't come with a choke (though the Deluxe reverb does)

Question is... shouldi add the choke?!?!?

None of the amps i built in the past used one... i did have a few amps with chokes (not my build though)

It is going to be my "Go out and play" amp. a sort of "do everything" build. as such i want it to have as little noise as possible, and great parts... whats the concensus? are chokes worth the trouble or not?

[Steve Conner]

The choke filters hum off the power supply. This makes for a quieter amp with less hum, and possibly less of the modulation that people call "ghost noting".

Downside is, it costs money, takes up space and adds weight.

Some amps use a big, expensive choke to filter the entire B+. Other amps use a smaller, cheaper one to filter only the supply for the power tube screens and preamp tubes. This is a nice compromise for high-powered amps, but the Princeton is small enough that you can probably just filter the whole thing and be done with it.

You may need an extra filter cap, as you need one before the choke and another after it.

I think a choke is nice to have in every amp, but in a little SE amp like the Champ, it's a must-have. SE amps all tend to hum, but a choke can wipe out the hum completely.

[axpro]

Hmmmm, you are selling me on it!

Cost isn't much of an issue (I sold off my dual rectifier half stack, and a few guitars, so the addition of a choke on my weber order is pretty small.

but now you have me thinking i should put one on my champ!

[echuta13]

I use a choke in all of my SE builds (for the most part). I usually use a Bassman style choke to add a little filtering to Champ/Princeton builds with a 40u/20u/20u/20u cap arrangement. Amps are VERY quiet.

[tubeswell]

Chokes clean up the sound - both from a hum-removal aspect (more ripple removal in the B+), and from a higher-headroom aspect (less voltage drop in the filter because of much less DC resistance in the choke than a resistor). So you should have a quieter (noise floor), chimier, and slightly-cleaner sounding amp - all other things being equal. Having said that, they make more difference in a single-ended amp, because the hum in a PP amp is to some extent cancelled-out in the opposite sides of the OT primary winding anyway

[imaradiostar]

Let me first say I agree with the other things said here- a choke removes a lot of ripple with little effort.

In the old days a choke was probably a lot cheaper than big filter caps.

If you're not averse to twist-lock can caps you can easily create an RC filter that'll be cleaner than a choke for way less money and there will be no need to avoid mounting it too close to other transformers. Of course the capacitance can get out of hand pretty quickly.

How will it sound? Well, the screen voltage will likely be a little bit lower- usually not a problem since it's excessively high on most guitar amps. This means theoretically less power but it's probably not going to be noticeable in the real world.

The real difference will be in the authenticity of the amp. Light plate and screen filtering is sometimes thought to be part of the vintage sound. I usually don't care for it and I have a bunch of caps and large resistors around so I tend to skip the choke on many designs.

On a push-pull amp I usually end up with the screen dropping resistor being pretty small (1k or under) with a large cap at the screen node. After that a larger value resistor (over 1k) usually drops the voltage to the phase inverter and preamp. Once you're downstream a bit from the power section you can safely add a bunch of capacitance without hurting a tube rectifier. This way the preamp still has nice ripple-free power.

Any ripple in the power supply should mostly cancel out of a push-pull output section anyway- thus the use of a choke in a simple single ended design. It makes for much cleaner power without the voltage drop.

One other thought- If RC was good enough for Ken Fischer then it's good enough for me!

There are, of course, other ways to skin this cat. Steve told me that a screen voltage regulator I was planning for an amp basically amounts to a capacitance multiplier- a resistive divider is connected to a reasonably sized cap so that there is a voltage drop and it forms a reference voltage for the mosfet. Because there is so little current draw from the resistive divider the result is very clean power. This is probably a better plan for trying to use up old iron that produces excessively high b+ voltages than it is for your situation.

I guess I said a whole lotta nothin!

jamie