Choke question

[daz]

A friend who heard my EL34 amp wants one badly, so i told him I might sell him mine cheap. The reason i was willing is there are a lot of little logistic changes i want to make now that it's sounding perfect. different layout, pot and switch placement, etc. So i think i may do this and build myself a second one that's new and clean with not extra holes and other experimental "Oops's. So heres the question....i used a 20H hammonds choke. The exact specs are as follows: 20H, 100ma, 181 ohms, 600VDC. I later found it's way more choke than i need and i wanted opinions as to what i should get this time.
The amp is a marshall style el34 50 watt with 436 volts at the beginning of the B+ and a 32/32uf can with three 20uf nodes for the PI and pre and a high gain preamp. Is there a value i should look for thats not going to sacrifice tone and will be as much as i need? here are some hammond choke specs for a few i think would work at least voltage wise....

159M 15H, 100ma, 256 ohms, 500VDC
159P 10H, 125ma, 155 ohms, 500VDC
159Q 7H, 150ma, 100 ohms, 500VDC
159M 15H, 100ma, 256 ohms, 500VDC
159S 4H, 225ma, 65 ohms, 500VDC

[tubeswell]

If you want the choke for the screen supply node, I reckon the 4 Henry one will do fine. The 5G9 I built a few months back uses a 4H choke in that position, is fixed bias, B+ of 373, uses 40uF at the reservoir and 20uFs in the screen and preamp supply.

[daz]

It would be located between the 2 sides of the 32/32uf can with only the OT center located before the choke if thats what you mean.

[tubeswell]

Yep - Between the OT Primary and the Screen(s)

Like this sort of one

http://store.triodestore.com/ch90ma4hfe.html

[mooreamps]

Just having a choke filtered screen and pre-amp power is always a good choice. But it it come down to picking one, given the current load on this one, a 4 Henry should do you just fine.

-g

[mac1amps]

So what had been the affect of using the 12H choke?

[daz]

Thats what i'd like to know. But i asked when i first got the 20H and i got 2 answers...1)-none, 2)-it may make the amp sound stiff. I think #1 is the correct answer because i tried replacing it with a resistor and the amp didn't get less stiff feeling. Just rattier. I still wonder tho what you'd hear is you A/B'd various sizes. But you can't do that w/o spending a lot unlike resistors and caps.

[mooreamps]

So what had been the affect of using the 12H choke?

In the cold light of day, probably would cost more dollars, yet the net benefit of additional ripple reduction might not be audible.

-g

[CitizenCain]

What is the difference between running a choke vs a resistor?

[Enzo]

The choke is an inductor. The inductor is more or less the series equivalent of a cap. Caps are wired across rather than in series, of course. The choke will add more filtering effect than a resistor.

[daz]

Ok, then heres yet another novice question. What are the audible effects of too much ripple?

[mooreamps]

What is the difference between running a choke vs a resistor?

The difference is better voltage regulation. A choke with a low series resistance is going to keep the voltage up on the screens and on the pre-amp. For the screens, a bigger drop in voltage may not have a significant sonic impact. But, to my ear, running the pre-amp at a too much reduced high voltage may make it tonally too dark sounding. In fact, later on I'm going to substantially up the high voltage on my pre-amps just for this reason.

Now, personally I select chokes between the 3 and 5 henry range, because I also use an old timers trick of by-passing the choke with about a 0.5 uF to 4.7 uF cap for improved 120 cycle ripple rejection. I still the high voltage regulation of using a low series resistance choke, but gain the benefit of better filtering. I've not nailed down the exact value of the choke by-pass cap as yet. I'm still taking data on this one.

-g

[Enzo]

Ripple on the B+ comes out as hum. In a well balanced push pull output stage, the plate ripples cancel, but through the rest of the amp they won't. Same on the bias supply, ripple there makes hum.

[mooreamps]

Ripple on the B+ comes out as hum. In a well balanced push pull output stage, the plate ripples cancel, but through the rest of the amp they won't. Same on the bias supply, ripple there makes hum.

Yes, it does. However, in a single-end amp, there is no common mode cancellation for 120 cycle ripple on the plates. But, I'm not sure this is a problem since as I look at the plate curves for the pentode power tubes, I don't see much if any delta change in plate current as a function of the delta change in plate voltage. Now, I realize this may look all very good on paper.. But this is my thinking...

I've only read a few other opinions where others believe you should do choke filtering for the PA plate rail. But I also know just by virtue of running the plate voltage through the primary coil of the output transformer, you already get as much as 9 Henrys of series inductance along with the plate filter cap(s). Maybe this is why the plate rail is not usually choke filtered, just a thought. . . .

-g

[Enzo]

SIngle ended amps do hum more than p-p if using simple filtration. putting the choke before the power tube plates does smooth it out and reduces hum considerably.

COnsider the 5E1 CHamp. By the 5F1 they cheaped out and drew the plates off the first node.

Look at Princetons as late as 5F2 while they were still single ended.

5E7 Bandmaster draws plates after the choke even though it is push pull.

Later versions of these amps reverted to plate off the first node. ANother example of Leo Fender going for "good enough" rather than good as can be.

I have a Fender service bulletin somewhere here where they tell all us techs not to "repair" the hum in the Champs, it is normal. This was a later version with less filtration and no choke.

But since daz opened this thread by stating his amp was a 50 watty Marshall style amp with EL34s, I had to assume his was push pull.