Resistor to choke switch question?

00jett · 11-01-2009, 11:53 PM

I recently added a choke to my dsl 50 and thought the different feel of the amp was really cool, but decided to make sure I noticed that much of a difference by switching back to the resistor and listening close to the difference. The truth is i really like both feels of the amp. For some stuff it was nice to have that super tightness feel, and for other stuff the loose smooth sound of the resistor was nicer. So my question is, would someone know if it possible to have a switch to select between the two units to literally change the feel of the amp when suited? I figure it wouldn't be hard but want to check some opinions on this first.

Wilder Amplification · 11-02-2009, 04:00 AM

The way I would do it would be through the use of a relay, which would require a separate supply to power the relay coil. A footswitch could supply a ground to the relay coil.

Personally, I'd use a relay with contacts rated for at least 2 amps.

Here's the relay contact schematic you'd use for such a mod.

jcmjmp · 11-02-2009, 08:14 AM

The above always leaves the resistor in though.
A simple a/b switch could be implemented, as long as you switch while the amp is turned off, just like switching between a SS and Valve rectifier.

PaoloJM · 11-02-2009, 08:23 AM

Quote:

Originally Posted by jcmjmp

The above always leaves the resistor in though.
A simple a/b switch could be implemented, as long as you switch while the amp is turned off, just like switching between a SS and Valve rectifier.

No it doesn't. Follow the path in both the open and closed positions.
This way would be better as it leaves a path for storage energy in the choke to go, in theory anyways

jcmjmp · 11-02-2009, 08:50 AM

Quote:

Originally Posted by PaoloJM

No it doesn't. Follow the path in both the open and closed positions.
This way would be better as it leaves a path for storage energy in the choke to go, in theory anyways

What I meant was... its not a "true" bypass. Not that it matters, maybe it does.

If you turn off the amp between the switching, you shouldn't have to worry too much about stored charge since the DSL has a bleeder resistor anyways.

00jett · 11-02-2009, 08:56 AM

Wow thanks!

Wilder Amplification · 11-02-2009, 09:39 AM

Quote:

Originally Posted by jcmjmp

The above always leaves the resistor in though....

What I meant was... its not a "true" bypass. Not that it matters, maybe it does.

You've piqued my interest...exactly how does this work? Please enlighten us.

Buggs.Crosby · 11-03-2009, 08:33 PM

Jon
I assume the amp would have to be shut down to make the switch correct?

because i swapped the tranny and choke at the same time i cant say how much of a change just the choke alone made and would not mind trying this to a/b them....P.S new Valves are here just need time to swap them

***Adwex*** · 11-03-2009, 11:37 PM

I'm not 100% clear on what function the choke serves, and what are the advantages. My amp has one, but I know not all do. I remember from physics that a coil, or inductor was known as a "choke", and it resists change in current flow. Does it serve a similar electrical purpose in an amplifier, and if so, what effect does it have on the audio signal...and more importantly, the sound/feel?

Can anyone educate me a little? I'm an Engineer, but not an EE, or an amp tech.

Wilder Amplification · 11-03-2009, 11:50 PM

Quote:

Originally Posted by Adwex

I'm not 100% clear on what function the choke serves, and what are the advantages. My amp has one, but I know not all do. I remember from physics that a coil, or inductor was known as a "choke", and it resists change in current flow. Does it serve a similar electrical purpose in an amplifier, and if so, what effect does it have on the audio signal...and more importantly, the sound/feel?

Can anyone educate me a little? I'm an Engineer, but not an EE, or an amp tech.

Basically what happens is that you have a "ripple" coming from the rectifier circuit, which is basically the positive pulses from the transformer that the rectifier passed through while blocking the negative pulses. These pulses must be filtered into a constant voltage, which is the job of the filter caps.

The first stage filter caps that power the plates of the output valves filter out most of it, but you still have some by the time it gets to the choke. You don't hear it in the output section because 1/2 the output valves are running out of phase from the other half so the 120Hz ripple frequency gets cancelled out of the audio signal. But some of it is still there, which is why you have multiple filter sections in an amplifier.

As the ripple voltage rises positive, it causes a magnetic field to build around the choke coil as the voltage rises. When the voltage falls back toward zero, the magnetic field collapses around the coil and when it does, it induces a counter-voltage into the coil to pick the voltage across the choke coil back up. The voltage across the choke offsets the ripple voltage, thus eliminates some of the ripple.

It also provides a small amount of DC resistance (around 75-110 ohms) between the plate supply and the screen supply.

The "sag" resistor does not provide any filtering at all like a choke does because it is a "non-reactive" component. It does not build and collapse a magnetic field like a choke does. It only provides isolation between the plate and screen to prevent "intermodulation distortion" caused by the plate circuit of the output valves trying to steal current from the screen filter caps. Electrons from the screen filter caps will see the screen as the easier path as compared to having to flow through the sag resistor to get to the plate, and likewise for the electrons supplied by the first stage plate filter caps...they'll see the plate circuit as the easier path rather than the sag resistor and screens.

It is called a "sag" resistor because it is typically a higher value than the DC resistance of a choke. Due to its higher value, it exhibits a higher voltage drop to the proceeding stages more than the voltage drop of a choke as the amp circuitry pulls current through it, which we all know as "voltage sag", or simply "sag".

Advantages to running a choke over a sag resistor is for one, less voltage sag at the screens, phase inverter & preamp as well as more filtering provided by the choke that the sag resistor cannot provide, which will reduce the likelyhood of "ghosting" (i.e. where you hear "ghost harmonics/notes" around the fundamental/original note).

Randy, yes I would recommend switching the amp into Standby mode before making the switch between the sag resistor and the choke.

MartyStrat54 · 11-04-2009, 12:18 AM

Bravo. Hell of a great explanation. You know Jon, you seem to know a lot about valves. Oh...we're talking about valve amps aren't we?

+1

Procter2812 · 11-04-2009, 08:33 AM

Quote:

Originally Posted by MartyStrat54

Bravo. Hell of a great explanation. You know Jon, you seem to know a lot about valves. Oh...we're talking about valve amps aren't we?

+1

He's the Budda/Gandhi of Valves

jcmjmp · 11-04-2009, 09:11 AM

Quote:

Originally Posted by Wilder Amplification

You've piqued my interest...exactly how does this work? Please enlighten us.

What do you want to know?

jcmjmp · 11-04-2009, 09:20 AM

Quote:

Originally Posted by Adwex

I'm not 100% clear on what function the choke serves, and what are the advantages. My amp has one, but I know not all do. I remember from physics that a coil, or inductor was known as a "choke", and it resists change in current flow. Does it serve a similar electrical purpose in an amplifier, and if so, what effect does it have on the audio signal...and more importantly, the sound/feel?

Can anyone educate me a little? I'm an Engineer, but not an EE, or an amp tech.

A choke opposes changes in current, similarily to how a capacitor opposes changes in voltage.

With a rectified voltage source, that is somewhat close to DC, the choke adds a different level of filtering, on the current, to keep it as stable as possible.

The problem is that if the PT can't keep the current flow steady, the voltage will drop anyways, which is why it is important to have a proper PT, although some sag can be desirable in some applications. The modern Marhsalls have under rated transformers IMO, but they get by.

The difference with a resistor is that as current flow increases, the voltage will drop accross the resistor and sag quickly, but this can be mitigated with different values of resistors. In the end, a resistor is a big compromise.

In terms of sag, the difference between a choke and a resistor could be analogous to having a tube rectifier or a solid state (diode) rectifier.

00jett · 11-04-2009, 10:46 AM

Quote:

In terms of sag, the difference between a choke and a resistor could be analogous to having a tube rectifier or a solid state (diode) rectifier.

I have the switch on my amp now that lets me chose between the choke or the resistor and it really reminds me of the difference in the duel rectifier amps. One setting makes the amp nice and tight feeling and the other is more bouncy, it is nice to have both side by side.

Wilder Amplification · 11-04-2009, 12:02 PM

Quote:

Originally Posted by jcmjmp

What do you want to know?

Exactly how my circuit above is not a "true bypass".

jcmjmp · 11-04-2009, 12:19 PM

Quote:

Originally Posted by Wilder Amplification

Exactly how my circuit above is not a "true bypass".

A true bypass circuit would disconnect the resistor from the cct instead of having a wire in parrallel to bypass it.

Like I said though, it may or may not make an audible difference.

jcmjmp · 11-04-2009, 12:20 PM

Quote:

Originally Posted by 00jett

I have the switch on my amp now that lets me chose between the choke or the resistor and it really reminds me of the difference in the duel rectifier amps. One setting makes the amp nice and tight feeling and the other is more bouncy, it is nice to have both side by side.

Yeah. I have a dual rectifier setup on my 2550 Silver Jubilee clone. The difference is noticeable when playing louder though. At bedroom/practice volumes, I can't hear the difference.

Wilder Amplification · 11-04-2009, 12:26 PM

Quote:

Originally Posted by jcmjmp

A true bypass circuit would disconnect the resistor from the cct instead of having a wire in parrallel to bypass it.

Like I said though, it may or may not make an audible difference.

Fact is...it won't. Why? Because the switch is the easier path. The switch creates a zero resistance path AROUND the component, which is also known as a "bypass".

Proof of this...measure voltage around the choke while the switch shunts it out and you'll see no voltage drop because the switch is a zero resistance path. Same goes for when the switch shunts out the sag resistor. No voltage drop across either component with the switch shunting it out = no sag caused by that component = no tonal difference.

jcmjmp · 11-04-2009, 12:36 PM

Quote:

Originally Posted by Wilder Amplification

Fact is...it won't. Why? Because the switch is the easier path. The switch creates a zero resistance path AROUND the component, which is also known as a "bypass".

Proof of this...measure voltage around the choke while the switch shunts it out and you'll see no voltage drop because the switch is a zero resistance path. Same goes for when the switch shunts out the sag resistor. No voltage drop across either component with the switch shunting it out = no sag caused by that component = no tonal difference.

Hey, all I know is that sometimes, theory and practice can be 2 different things.

A lot of the tone sucking pedals use a wire-bypass method but experience shows that this kind of setup can load the signal and suck tone. Case in point: The Dunlop Wah. Wire bypass works in theory on this pedal, but in practice, it doesn't do a good job. A switch is not zero resistance (probably close enough though) in reality and it also adds some capacitance in the path.

Like I said in an earlier post... it may not really affect anything as far as tone goes. The differences might not be detectable to the human ear, especially at the voltage levels.

Wilder Amplification · 11-04-2009, 12:38 PM

Quote:

Originally Posted by jcmjmp

Hey, all I know is that sometimes, theory and practice can be 2 different things.

A lot of the tone sucking pedals use a wire-bypass method but experience shows that this kind of setup can load the signal and suck tone. Case in point: The Dunlop Wah. Wire bypass works in theory on this pedal, but in practice, it doesn't do a good job. A switch is not zero resistance (probably close enough though) in reality and it also adds some capacitance in the path.

Like I said in an earlier post... it may not really affect anything as far as tone goes. The differences might not be detectable to the human ear, especially at the voltage levels.

Not the same.

On a wah pedal, the wire bypass only bypasses the signal to the output jack, but you still have a load to ground imposed on the signal by the shit in the negative feedback loop on a wah because the input is still connected. When you leave the input connected while only bypassing the output jack to the input jack on a pedal, the input of the circuit will still pose a load to ground on the signal path.

This circuit is COMPLETELY different and cannot be related to a pedal bypass for this reason. Apples & oranges.

MajorNut1967 · 11-04-2009, 12:40 PM

Quote:

Originally Posted by Wilder Amplification

Basically what happens is that you have a "ripple" coming from the rectifier circuit, which is basically the positive pulses from the transformer that the rectifier passed through while blocking the negative pulses. These pulses must be filtered into a constant voltage, which is the job of the filter caps.

The first stage filter caps that power the plates of the output valves filter out most of it, but you still have some by the time it gets to the choke. You don't hear it in the output section because 1/2 the output valves are running out of phase from the other half so the 120Hz ripple frequency gets cancelled out of the audio signal. But some of it is still there, which is why you have multiple filter sections in an amplifier.

As the ripple voltage rises positive, it causes a magnetic field to build around the choke coil as the voltage rises. When the voltage falls back toward zero, the magnetic field collapses around the coil and when it does, it induces a counter-voltage into the coil to pick the voltage across the choke coil back up. The voltage across the choke offsets the ripple voltage, thus eliminates some of the ripple.

It also provides a small amount of DC resistance (around 75-110 ohms) between the plate supply and the screen supply.

The "sag" resistor does not provide any filtering at all like a choke does because it is a "non-reactive" component. It does not build and collapse a magnetic field like a choke does. It only provides isolation between the plate and screen to prevent "intermodulation distortion" caused by the plate circuit of the output valves trying to steal current from the screen filter caps. Electrons from the screen filter caps will see the screen as the easier path as compared to having to flow through the sag resistor to get to the plate, and likewise for the electrons supplied by the first stage plate filter caps...they'll see the plate circuit as the easier path rather than the sag resistor and screens.

It is called a "sag" resistor because it is typically a higher value than the DC resistance of a choke. Due to its higher value, it exhibits a higher voltage drop to the proceeding stages more than the voltage drop of a choke as the amp circuitry pulls current through it, which we all know as "voltage sag", or simply "sag".

Advantages to running a choke over a sag resistor is for one, less voltage sag at the screens, phase inverter & preamp as well as more filtering provided by the choke that the sag resistor cannot provide, which will reduce the likelyhood of "ghosting" (i.e. where you hear "ghost harmonics/notes" around the fundamental/original note).

Randy, yes I would recommend switching the amp into Standby mode before making the switch between the sag resistor and the choke.

Feckin brilliant! You can come over and boff the neighbors cat!
In all reality perfectly explained A++ I'm getting a little tired of that fly countering what other people say, when he can't even trace a signal path!

11-01-2009, 11:53 PM	#1 (permalink)
00jett Senior Member Join Date: Jan 2009 Posts: 133	Resistor to choke switch question? I recently added a choke to my dsl 50 and thought the different feel of the amp was really cool, but decided to make sure I noticed that much of a difference by switching back to the resistor and listening close to the difference. The truth is i really like both feels of the amp. For some stuff it was nice to have that super tightness feel, and for other stuff the loose smooth sound of the resistor was nicer. So my question is, would someone know if it possible to have a switch to select between the two units to literally change the feel of the amp when suited? I figure it wouldn't be hard but want to check some opinions on this first.

11-02-2009, 08:14 AM	#3 (permalink)
jcmjmp Senior Member Join Date: Nov 2008 Posts: 1,341	Re: Resistor to choke switch question? The above always leaves the resistor in though. A simple a/b switch could be implemented, as long as you switch while the amp is turned off, just like switching between a SS and Valve rectifier.

11-02-2009, 08:56 AM	#6 (permalink)
00jett Senior Member Join Date: Jan 2009 Posts: 133	Re: Resistor to choke switch question? Wow thanks!

11-03-2009, 08:33 PM	#8 (permalink)
Buggs.Crosby Member Join Date: May 2009 Posts: 90	Re: Resistor to choke switch question? Jon I assume the amp would have to be shut down to make the switch correct? because i swapped the tranny and choke at the same time i cant say how much of a change just the choke alone made and would not mind trying this to a/b them....P.S new Valves are here just need time to swap them

11-03-2009, 11:37 PM	#9 (permalink)
*Adwex* Super Moderator Join Date: Sep 2006 Location: Long Island, NY Posts: 1,631	Re: Resistor to choke switch question? I'm not 100% clear on what function the choke serves, and what are the advantages. My amp has one, but I know not all do. I remember from physics that a coil, or inductor was known as a "choke", and it resists change in current flow. Does it serve a similar electrical purpose in an amplifier, and if so, what effect does it have on the audio signal...and more importantly, the sound/feel? Can anyone educate me a little? I'm an Engineer, but not an EE, or an amp tech. __________________ http://www.youtube.com/MarshallLesPaul

11-04-2009, 12:18 AM	#11 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Resistor to choke switch question? Bravo. Hell of a great explanation. You know Jon, you seem to know a lot about valves. Oh...we're talking about valve amps aren't we? +1 __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

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