choke question

[rockstah]

so I'm looking at a Francesca and a plexi.

i see similar circuit past the first three gain stages between the two.

one has a choke and one doesn't.

could i simply disconnect the choke in a plexi without harming the amp?

could just add a choke to the Francesca?

i have studied both circuits and cant seem to find any reason i couldn't do either of these things. to either amp.

seems to me like Ken just left the choke out of the express and marshalls just happens to have one?

someone set me straight if I'm not on the right track here.
...i dont know much about chokes.

Mark

[Nigel Tufnel]

so I'm looking at a Francesca and a plexi.

i see similar circuit past the first three gain stages between the two.

one has a choke and one doesn't.

could i simply disconnect the choke in a plexi without harming the amp?

could just add a choke to the Francesca?

i have studied both circuits and cant seem to find any reason i couldn't do either of these things. to either amp.

seems to me like Ken just left the choke out of the express and marshalls just happens to have one?

someone set me straight if I'm not on the right track here.
...i dont know much about chokes.

Mark

If you're gonna get rid of the choke in a plexi then you will want to put a resistor in place of it. You could also substitute a choke for the 1k 25w in the PS of a wreck. Neither one is right or wrong, let your ears decide, or your hands...imo it changes the feel as much if not more than the tone. Old traynor bassmasters were a very marshall/bassman kinda circuit but they used a 470 ohm resistor instead of a choke and they're tons of fun.

[billyz]

The 1K 25 w resistor in the Wreck acts as an Inductor to smooth out the power supply ripple. It will hum loudly without it. Also, the 1k Resistor does drop the voltage more than a Choke will. But it is a lot less expensive too.
The feel is different too.

[skyboltone]

The 1K 25 w resistor in the Wreck acts as an Inductor to smooth out the power supply ripple. It will hum loudly without it. Also, the 1k Resistor does drop the voltage more than a Choke will. But it is a lot less expensive too.
The feel is different too.

Well, sorta. It's an RC network rather than an LC network. The math is different, the results are similar. But an R is not an L.

[rockstah]

so i could replace that 1k 25watt resistor with a choke and on a plexi i could use that 25watt 1k resistor in place of the choke without causing any harm to the amps?

[fperron_kt88]

Very interesting question! I was tempted to play with this R versus choke thing as well.

In the circuits you are referring to (a 50W plexi versus an express), I see both elements located at the same place in the schematics. So your assumption seems very good at this point: both are used as part of the power supply filtering and they both provide current to the screen nodes down to the preamp tubes.

They will both produce a voltage "sag" that reduces the ripple for the screens and preamp stages (some would say "regulation", I really much prefer "filtering"...). Their "sag" will both respond to the current drawn, albeit in a slightly different way.

I always assumed the designer choice between pure R versus choke based on: price, sound and touch responsiveness. For the sound and touch part, I bet this is pretty much like playing with different values of screen filtering in a plexi indeed. Things I have not done personally yet, so I will defer to the more experienced builders here...

From my understanding though, a few simple physics assumptions could be used just to get a better idea of how it might be:

-The R is just an R and will sag 1000V per ampere. At 25W dissipation, this accounts for a max draw of around 150mA (or a max "sag" of 150 volt) across the part. We can also assume this behavior to be consistent across the guitar frequency range...

-The choke is a winding that has an R (probably around 100 Ohms up to more than 1K in some cases: http://www.hammondmfg.com/153.htm) plus an inductive part. So the R in the choke works exactly like the pure resistor. Then the inductive portion reacts to the *change* in current. So the deal is: the total effect of the choke is frequency dependent. Plus it can saturate (yup, like an OT) when current draw is higher. If this limit is ever reached... Dunno...

So, in a similar amplifier, all else being equal (probably a good assumption in the case of an Express vs a 50W plexi), I would bet that the choke would yield higher screen and preamp voltages than the R at idle. I would also bet that the touch response of the amp would be softer with the R (more sag), stiffer with the choke. I would expect the differences to be more noticeable with the amp dimed and on chunky chords low down on the neck.

As far as swapping the parts safely between these amps, I cannot say really, but I would keep an eye on the max current specified for the choke as a start... I would also watch for the B+ change for screens and preamp tubes downstream.

Very curious about your findings with this...

[Jana]

Hi, this is my first post, go easy on me!

"-The R is just an R and will sag 1000V per ampere. At 25W dissipation, this accounts for a max draw of around 150mA (or a max "sag" of 150 volt) across the part. We can also assume this behavior to be consistent across the guitar frequency range... "

the resistor is after the point of the B+ supply that provides the voltage for the plates of the power tubes. Thus, there will not be the levels of current draw that you mention. The current draw across this resistor will be approximately 8 to 10 milliamps. Going from memory here.

This is the reason that the choke is in this position - between the B+ tap for the plates and the screens. The large majority of power is consumed by the power tube plates, everything else is minimal in comparison. By having it in this position, the huge voltage drops are avoided.

thank you, I now return you to your regularly scheduled program.

[fperron_kt88]

Hello Jana (don't fret about your number of posts, I am *not much* older... I don't even know if I have reached the official status to welcome people here, but I can certainly welcome you in this thread!!!)

I should have added that the 150mA is not a typical current draw value, just the absolute max that the designer expected to see in there in sizing the resistor. Since Rockstah wanted to swap parts...

My point really was that the resistor will be very stiff in responding to current draw. Ohm's law is what it is.

I might have added as well that I expect most of the sag in this R to be short lived (during hard pick attack, perhaps) while the power tubes would draw current through the screens: essentially a transient condition... The other class A stuff downwards should remain pretty calm current wise I fully concur.

Does that make sense?

[bnwitt]

The express PI filter is not as efficient as the one in the Plexi. That is because it has a resistor in the fiter array and not a choke. A choke or inductor in the PI filter performs better ripple reduction than a simple resistor because the voltage is converted to a magnetic energy field by the choke and is available to pour its electrons back into the supply when the voltage drops and the field collapes. A resistor cannot do this. As fperron_kt88 correctly states in his post, the choke or resistor in the PI filter does not affect the power tube output, as the position of the choke is down stream from the OT/power tube plate B+ node.

The typical Marshall Fender setup with the choke will deliver more constant B+ to the power tube screens and the preamp section as opposed to the resistor PI filter in the express.

Smaller wattage amps and simple single channel amps typically do not have a choke in the power section as less power is demanded. In higher power amps (and more complex multichannel amps) chokes are used to deliver a steadier DC voltage supply to the power tube screens and preamp section.

Choke or no choke is really a tonal choice in most cases though not everyone will noice it in a big way. Once you get past the 50 watt level however, you'll definitely want a choke in the power supply. This is also true for amps with multiple channels and tube driven effects which use a lot of triodes in the preamp section.

You can set up your plexi or express amps with the 1k 25 watt resistor and the 10 henry choke on a switch and go back and forth to see which one you prefer. Make sure you go to standby before doing this.

The power sections of the two amps you mention have other differences beyond the choke. The Marshall has six 50uf filter caps compared to the Express' three 40ufs and three 20ufs. These two amps are really built for different jobs. The Marshall was built to deliver power power power and the Express was built to deliver tone tone tone. If you alter either one's power section to be more like the other you will end up with a hybrid of sorts.

For more on chokes see Randall Aikens site:

http://www.aikenamps.com/Chokes.html

[Jana]

woohoo, post number 2


yes, that makes sense. The 150 ma would never be seen across that 1K resistor though (unless there were the magic smoke billowing from the amp).

to answer Rockstah's question, yes, you can put the 1K 25 watt resistor in a plexi in place of the choke. What may happen, however, is that the hum increases since there is considerably less "smoothing" of the power supply ripple for the preamp stages. Will you blow anything up or etc.? No.

If you want to keep the benefit of the choke as a ripple smoother but also wanted the increased sag of the 1K resistor, use both (in series). I would put the choke first, then the resistor feeding the screens and everything else down stream.

My guess is that there will be a bit more compression with the resistor in there. When a note is hit hard the screens will draw a bit more current, then the voltage will drop a bit across the 1K which will in turn lower the screen voltage which lowers the output of the tube. All this will be subtle, but it should be there. Let us know if you try it and how the amp feels. I suspect this is one of the "secrets" to the twreck "bloom."

Naturally, large filter capacitors will minimize this effect. For best results, use the smallest value filter caps that you can get away with without the amp motorboating or humming like crazy.

[sliberty]

so i could replace that 1k 25watt resistor with a choke and on a plexi i could use that 25watt 1k resistor in place of the choke without causing any harm to the amps?

Cause harm?

Some amps have power supplies with a choke, and others have power supplies with a resistor instead. Switching from one type to the other type will not in itself cause any harm. They both still provide smoothed DC power to the rest of the circuit. But, you do have to check some voltages to make sure that no harm results. If your Marshall was delivering 420 volts right after the choke, and if you decide to replace the choke with a resistor, you should make sure that the voltage right after the resistor (in place of the choke) is not significantly different from 420 volts. it might go up or down a bit, but if it swings too much (particularly up), you could have problems. I don't expect that this would be the case, but you should check for it. But, if the modified power supply still provides near 420 volts, I can't see any reason that harm could be caused. [420 was just an example of course]