How does the choke affect the sound compare to a resistor?

[Alan0354]

I notice some amps from Marshall ( JCM900) and Mesa do not use choke. Marshall use 100 ohm resistor and Mesa use a few of the 680 ohm or so in series to make up about 2.5K resistor in place of a choke. How does this affect the sound? Notice, they all have a resistor in series with the screen grid.

[Alan0354]

Anyone? The reason I ask is because I am doing power scaling where the HV for power tubes/PI are under adjustable HV. But the preamp are using constant +B. When I use the choke for the screen grid and PI, I can hear a slight 60Hz hum ( no noticeable of higher harmonics). I trouble shoot down to the +B line. If I increase the filter cap of the first stage to 40uF, that lower the hum a lot. Then I move the choke over to the preamp side, that cured the hum all together. So the hum is from the +B.

So my choice is:

1) Increase the filter cap of the first stage to say 60uF.
2) Move the choke over to the preamp side and use a resistor to separate between the OT and the screen grid/PI.
3) Put the second choke so both side have a choke.

Obviously the second option is cheapest. That't the reason of this post. I did try the resistor of about 3.5K. It is not easy to A/B compare and I am not sure I trust my ears. That's why I want to hear from you guys.


BTW

On the side note, I build this amp from scratch by the OD channel is exactly the same circuit as I have in the Bassman prototype. But it sounds different.......better!!!! The Bassman is very messy, took a lot of tries to get there. I even went from 3 stages to 4 and all different schemes. The signal lines ended up crossing and jump over and it's a mess. When I build into the KMD combo, I started out clean, I planned the layout and even drilled the holes for the preamp tubes, signal paths are very clean and propagate down nicely. It sure get rid of some funny sound that I could not explain before.

There goes to show the layout is a major part, you never know the final product until you have it build. I bet if I put into a pcb, things might change. You never can be sure until you put it in the final form. When the signal path is high impedance in hundreds of kohm or meg ohm, they talk to each other!!!!

[Loogie]

Well, I'm no expert, but I've been experimenting with a single ended amp that can be triode strapped. The first one I built used a resistor for the entire B+. No matter how much capacitance I added to the power supply it hummed (at least a little) when I switched it to triode. So I figured I would try a choke which took care of the hum. The two amps are very close in terms of voltages and component values -- and I can't really hear a difference. I can't imagine there aren't differences and I'm sure the more discrimminating ear could sense the difference, but I can't. And if there are differences how would they be described? I think you should add the choke to the preamp and then devise a way to switch it out with a resistor. Let *your* ears be the judge.

[Alan0354]

I don't think they should sound different in triode mode. You might get more sag with the resistor than the choke at high volume. I am more concern in pentode mode. The choke can hold the DC a lot better than the resistor and the screen grid only sag according to the screen resistor. With resistor in place of the choke, it will drop extra voltage when the screen draw extra average current. So the screen will sag more with resistor instead of choke.

But this is all hot air as I am guessing it too.

It is not as easy to switch the choke in and out as it can spark when opening the choke. That's the reason I ask before I go to this step. Also, the effect only happens when you crank the amp all the way. It's hard for me to do this at home. So I need to rely on people that have experience before.

[loudthud]

I tried the choke-resistor experiment in an amp. I wired the choke and resistor in series and connected a switch to short one or the other. It didn't make much difference because the main filter was on the big side. Don't read too much into this, the amp was solid state

In amps that have smallish main filters, the screen node sags quite a bit when ripple on the reservior node suddenly increases. Depending on the size of the screen filter cap, the screen node can exhibit ringing (0.1 to 1Hz) to big transients. This actually sounds nice because it punctuates the notes of a lead passage. The screen node jumps around alot but doesn't show any 120Hz ripple. The screen node in a Trainwreck is slower but is low in ripple because the main filter is large.

[Alan0354]

I am trying 500 ohm resistor with 40uF screen filter cap. Then I connect to the screen grid with a 1K 5W resistor. Noise is fine, I yet to crank it up.

Hopefully the 500 ohm will make the voltage at the 40uF cap stiff enough without sagging too much when a large signal hits. Then just let the 1K screen resistor do the fast sagging and fast recovery.

[J M Fahey]

Since you are using power scaling, a very big name for a simple variable +B power supply (that's what it is,no more, no less) , using a choke vs. a resistor becomes MUCH less relevant to sound.

The sagging/compression comes from varying grid current through resistor or choke DCR dropping available voltage.

The difference comes because DCR is quite different between them.

To see it from a simplistic point of view:
a) suppose you compare a 500 ohms resistor to a choke with a DC winding resistance of 200 ohms and such an inductance that at 100/120 Hz it shows an impedance equivalent to 1000 ohms.

b) the resistor will have "500 ohms" as far as DC voltage drop is concerned and will have "500 ohms" as far as ripple killing effect (combined with later filtering cap) so it will always show "500 ohms".

c) the choke will behave very different: for DC it will show "200 ohms" so it will have far less DC voltage drop with varying screen current; and for AC/ripple it will show "1K" , killing ripple much better.

d) so plain resistor is just ... a plain resistor while a choke is a "super/improved/magic resistor" low value so supply is stiffer (good) and high value so as to be an improved hum killer, that's what justifies its use, cost, expense and inconvenience (weight, space, cost).

In the old days it was cheaper to wrap some wire around an iron core instead of expensive treated wet aluminum foil inside an aluminum can (electrolytic caps) so they used a choke and those wimpy 8uF caps; today it's cheaper and better (or at least the same) to use a relatively low value resistor (to approach choke DCR values) and much higher filter caps (50/100 uF) to filter hum out as well as before.

But ... but .... so Leo (and Jim and Hartley and Randall and .... ) tried to design as cheap as possible? (within design goals, of course) ... ?
Yup, you got it.

So people hunting for some elusive mystical design secret while studying old manuscripts by the candlelight (ok, ok, yellowing musty stained old paper schematics often the onscreen scanned .jpg version ) sometimes are missing the point.

[Loogie]

I couldn't hear a difference in either mode. I should have made that clear.

[Alan0354]

Since you are using power scaling, a very big name for a simple variable +B power supply (that's what it is,no more, no less) , using a choke vs. a resistor becomes MUCH less relevant to sound.

The sagging/compression comes from varying grid current through resistor or choke DCR dropping available voltage.

The difference comes because DCR is quite different between them.

To see it from a simplistic point of view:
a) suppose you compare a 500 ohms resistor to a choke with a DC winding resistance of 200 ohms and such an inductance that at 100/120 Hz it shows an impedance equivalent to 1000 ohms.

b) the resistor will have "500 ohms" as far as DC voltage drop is concerned and will have "500 ohms" as far as ripple killing effect (combined with later filtering cap) so it will always show "500 ohms".

c) the choke will behave very different: for DC it will show "200 ohms" so it will have far less DC voltage drop with varying screen current; and for AC/ripple it will show "1K" , killing ripple much better.

d) so plain resistor is just ... a plain resistor while a choke is a "super/improved/magic resistor" low value so supply is stiffer (good) and high value so as to be an improved hum killer, that's what justifies its use, cost, expense and inconvenience (weight, space, cost).

In the old days it was cheaper to wrap some wire around an iron core instead of expensive treated wet aluminum foil inside an aluminum can (electrolytic caps) so they used a choke and those wimpy 8uF caps; today it's cheaper and better (or at least the same) to use a relatively low value resistor (to approach choke DCR values) and much higher filter caps (50/100 uF) to filter hum out as well as before.

But ... but .... so Leo (and Jim and Hartley and Randall and .... ) tried to design as cheap as possible? (within design goals, of course) ... ?
Yup, you got it.

So people hunting for some elusive mystical design secret while studying old manuscripts by the candlelight (ok, ok, yellowing musty stained old paper schematics often the onscreen scanned .jpg version ) sometimes are missing the point.

Yes, that's the concern I have.....that the choke has low DCR but reactance increase with frequency. So the choke present higher impedance at 60, 120Hz than a nominal value resistor. BUT a choke presents lower DCR at low frequency. When you hit the guitar hard, the envelope of the signal is low frequency, that's when the choke can hold the DC better.

That's what I decided to do yesterday, by decrease the resistance and monitor the hum to make sure I don't increase hum when lowering the resistance. I settle with 500 ohm 15W resistor ( have one) where I don't hear increase in hum. I think that is low enough value. I use a 40 uF filter cap for the screen, so I think I am good on this now.

Ha ha, it would be funny to have two chokes!!!

Thanks

[tedmich]

sure it makes no difference in sound but FINALLY you can dial in the "no difference"!!

http://www.mercurymagnetics.com/page...multichoke.htm

now if we only had MIDI choke switching! I'm sure Mesa has a patent on this...

[Justin Thomas]

"Did you know that upgrading your tired old or wimpy reissue stock choke -- or adding a Mercury choke to an amplifier that doesn't have one* -- will actually extend the life of your power transformer and rectifier?"


Hmmmm... I didn't know that a choke will make my PT last longer... I guess I should start offering upgrades for all those BF/SF Champs & Princetons, so they don't burn out those crappy old Schumachers.

Why didn't we hear about this in The Immortal Amp Mods? I feel cheated! My amp doesn't have a choke, and those mods were supposed to help my transformers last longer!

And, why would Mercury be selling chokes under the guise of saving your PT and making it last longer anyway? Wouldn't they WANT to sell more PTs?

Sorry, I saw that line & it set me off...

Justin

[g1]

"Introducing the first variable choke!" except it's not really variable, I would call it switchable. Is an export PT with switchable taps for primary voltage a "variable" transformer?
"Replacing that resistor with a choke is a low-cost upgrade" lol, like double the cost of a resistor, or more?

[Alan0354]

To change the subject a little. Is Mercury Magnetics really better?

I was on an end user forum, they think MM transformers are superior and they consider it's upgrade changing the Fender SF to MM transformers. I personally do not care for brands of PT, I am using Classic Tone OT, so far, I have no complain. I choose Weber PT for Deluxe because it's the only one spec for 150mA current, AND it has CT 540VAC and 680VAC. It is designed for both SS and tube rectifier and you choose the taps accordingly. But for me, I do SS rectifier, I can use the 680VAC for 6L6 option. It is the highest current 6V6 PT I can find. Also, it has 5A 6.3VAC and 3A 5VAC secondary. I use the 5VAC to get DC for relays. It's only $42.

[hylaphone]

I've used a couple and cannot even begin to justify the cost.
Besides, it's about as subjective as your choice of speakers...
I am a large fan of the Classic-tone transformers, they are domestic, affordable and honest..

To change the subject a little. Is Mercury Magnetics really better?

[Alan0354]

I've used a couple and cannot even begin to justify the cost.
Besides, it's about as subjective as your choice of speakers...
I am a large fan of the Classic-tone transformers, they are domestic, affordable and honest..

I made a mistake, I meant I use Classic Tone OT in my design. I am happy with it.

Correct me if I am wrong, It is my believe the bigger the OT, the harder it is to get compression. For guitar amp, bigger might not be a good thing. Those MM or Harmon trannys are huge.