ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

RS-Deluxe OT question

5/20/2003 4:16 PM
Matt	RS-Deluxe OT question You know how you think you understand something but then a question comes up, and then you're not so sure? There is/was a discussion on another forum about this so I thought I'd ask here. I'll just state "my understanding" (or mis-understanding) of this and hopefully someone can verify it or shoot as many holes in it as possible. I just did a cut&paste from another post: There are 3 separate secondary windings in the JTM45 OT. This is different than an OT with tapped secondaries (like a later Plexi OT) where they are all common. Of these 3 separate windings, 2 are 4 ohm and 1 is 8 ohm. Connecting both 4 ohm secondaries together in series makes it an 8 ohm secondary. So now you have 2 each 8 ohm secondaries. Wire them in series for 16 ohm and parallel for 4 ohm. I'm guessing that this is where the "trick" to set your impedance selector on "16ohm" when using an 8 ohm cab came from. It also could be the reason for the myth to always use the highest impedance setting (and choose an appropriate cab). On the old RS OTs, it doesn't look like you can set it at 8 ohms AND USE ALL THE WINDINGS. You're basically "not using" half the windings when going 8 ohm. But all of this is based on what I read in TUT3 (Kevin, if you read this and I'm wrong on this, a big "I'm sorry!"). That's the post. As for what actually SOUNDS best, that may be a different matter. PS - I understand that the secondary impedance is just a reflected ratio of the primary but for the sake of simplicity, I went ahead and put the values in as if the primary load was correct. Any help appreciated. Matt

5/20/2003 4:16 PM

Matt

RS-Deluxe OT question

You know how you think you understand something but then a question comes up, and then you're not so sure?

There is/was a discussion on another forum about this so I thought I'd ask here. I'll just state "my understanding" (or mis-understanding) of this and hopefully someone can verify it or shoot as many holes in it as possible. I just did a cut&paste from another post:

There are 3 separate secondary windings in the JTM45 OT. This is different than an OT with tapped secondaries (like a later Plexi OT) where they are all common. Of these 3 separate windings, 2 are 4 ohm and 1 is 8 ohm. Connecting both 4 ohm secondaries together in series makes it an 8 ohm secondary. So now you have 2 each 8 ohm secondaries. Wire them in series for 16 ohm and parallel for 4 ohm.

I'm guessing that this is where the "trick" to set your impedance selector on "16ohm" when using an 8 ohm cab came from. It also could be the reason for the myth to always use the highest impedance setting (and choose an appropriate cab). On the old RS OTs, it doesn't look like you can set it at 8 ohms AND USE ALL THE WINDINGS. You're basically "not using" half the windings when going 8 ohm.

But all of this is based on what I read in TUT3 (Kevin, if you read this and I'm wrong on this, a big "I'm sorry!").

That's the post. As for what actually SOUNDS best, that may be a different matter.

PS - I understand that the secondary impedance is just a reflected ratio of the primary but for the sake of simplicity, I went ahead and put the values in as if the primary load was correct.

Any help appreciated.

Matt

5/20/2003 4:49 PM
Bruce /Mission Amps	Good question. I'm not sure, but it doesn't feel like it should be a linear thing. In other words just putting the two "equal zed" 8 ohm taps in series, doesn't mean that the new zed is 16 ohms. Bruce

5/20/2003 6:57 PM
MattM	The secondary impedance thing sadly is not this easy: Two 8Ohm windings in parallel result in a 8Ohm winding with more current handling capability (How could the current in the secondary winding know if it is flowing in two wires ore in one thicker wire? ) Two 4Ohm windings in series give a 16Ohm winding. It is a bit tricky, but if you keep in mind that the impedance ratio is the turns ratio squared, that one thick wire can be replaced by two parallel thinner wires with no effect at all (except some advanced stuff like easier winding, different winding capacity etc.) and do a bit of math, it becomes clear. HTH ;Matthias

5/20/2003 6:57 PM

MattM

The secondary impedance thing sadly is not this easy:

Two 8Ohm windings in parallel result in a 8Ohm winding with more current handling capability (How could the current in the secondary winding know if it is flowing in two wires ore in one thicker wire?

) Two 4Ohm windings in series give a 16Ohm winding.

It is a bit tricky, but if you keep in mind that the impedance ratio is the turns ratio squared, that one thick wire can be replaced by two parallel thinner wires with no effect at all (except some advanced stuff like easier winding, different winding capacity etc.) and do a bit of math, it becomes clear.

HTH
;Matthias

5/20/2003 7:11 PM
Matt	Two 4Ohm windings in series give a 16Ohm winding. Just want to make sure that's not a typo. Is that supposed to be "two 4Ohm windings..." or "two 8Ohm windings..."? I don't really understand the rest of what you're saying but I probably just need to think about it a little more. Thanks for the responses, please keep 'em coming! Matt

5/21/2003 10:00 AM
Dave H	It’s not a typo Matt. It doesn’t work like resistors. You have to think of what happens to the turns ratio. The impedance ratio is the square of the turns ratio. Two 4ohm windings would both have the same number of turns. If you put them in series the number of turns is doubled so the impedance is multiplied by 4 (because 2 squared = 4). Two 4ohm windings in series would therefore be 4ohms x 4 = 16ohms. Here’s a test for you What would be the impedance of three 2ohm windings in series? Unlike resistors only windings with the same impedance should be connected in parallel because windings with different impedances will generate different voltages. Two 8ohm windings will both have the same number of turns. If you put them in parallel it’s effectively the same number of turns but with thicker wire. The number of turns hasn’t changed so the impedance doesn’t change. It’s still 8ohms. HTH Dave

5/21/2003 10:00 AM

Dave H

It’s not a typo Matt. It doesn’t work like resistors. You have to think of what happens to the turns ratio. The impedance ratio is the square of the turns ratio. Two 4ohm windings would both have the same number of turns. If you put them in series the number of turns is doubled so the impedance is multiplied by 4 (because 2 squared = 4). Two 4ohm windings in series would therefore be 4ohms x 4 = 16ohms.

Here’s a test for you

What would be the impedance of three 2ohm windings in series?

Unlike resistors only windings with the same impedance should be connected in parallel because windings with different impedances will generate different voltages. Two 8ohm windings will both have the same number of turns. If you put them in parallel it’s effectively the same number of turns but with thicker wire. The number of turns hasn’t changed so the impedance doesn’t change. It’s still 8ohms.

HTH
Dave

5/20/2003 8:08 PM
sr	you do use all 3 windings in all 3 load setups with the RS. The 'myth' is just that the best way to maintain consistant leakage inductance (eg: high freq response) is to use the whole winding, and that still holds true TTBOMK. Traditional tapped OT's have (theoretically at least!) different freq response at each tap, with 4 being the worst, 16 the best. Can you hear it? I dunno. Also it is considered good practice to take your -fb tap from the same tap the speaker is on (not critical, but nice) and again, it's often on the highest tap. The original JTM45 amps which used the RS Spares OT were hardwired for 16 ohms (had no switch.. very Henry Ford!) I'm sure folks were buggering around with bad load imps prior to this though.. (wonder what a Champ sounds like through a 12", or a 4x12? or .. You're basically "not using" half the windings when going 8 ohm. How do you figure? All 3 windings are hooked up no matter which imp you choose. Whether it's 4, 7 or 15 they are all hooked up. Terminals are: A - F. 3 Ohm: A-C, B-E, D-F terminals are A & F 7 Ohm: A-B-D C-E terminals are E & F 15 Ohm: C-D E-F terminals are A & B

5/20/2003 8:08 PM

you *do* use all 3 windings in all 3 load setups with the RS. The 'myth' is just that the best way to maintain consistant leakage inductance (eg: high freq response) is to use the whole winding, and that still holds true TTBOMK. Traditional tapped OT's have (theoretically at least!) different freq response at each tap, with 4 being the worst, 16 the best. Can you hear it? I dunno.

Also it is considered good practice to take your -fb tap from the same tap the speaker is on (not critical, but nice) and again, it's often on the highest tap.
The original JTM45 amps which used the RS Spares OT were hardwired for 16 ohms (had no switch.. very Henry Ford!) I'm sure folks were buggering around with bad load imps prior to this though.. (wonder what a Champ sounds like through a 12", or a 4x12? or ..

You're basically "not using" half the windings when going 8 ohm.

How do you figure? All 3 windings are hooked up no matter which imp you choose. Whether it's 4, 7 or 15 they are all hooked up.

Terminals are: A - F.
3 Ohm: A-C, B-E, D-F terminals are A & F
7 Ohm: A-B-D C-E terminals are E & F
15 Ohm: C-D E-F terminals are A & B

5/20/2003 9:50 PM
Bob Predaina	Matt, I've been following a thread like that on the LP Forum. If you're reading the same board I am, there are three issues going on with that fellow's amp: 1) correctly wiring up the "RS" type OT, 2) properly wiring the OT for negative instead of positive feedback, and 3) changing the value of hte NFB series resistor to compensate for the secondary impedance change. on OTs that provide multiple taps, its possible to change speaker impedance without moving the tap from which the NFB signal is taken. this means that you can change the value of the speaker cab's impedance without having to move the NFB tap or change the value of the NFB series resistor to compensate. On "RS" type OTs, changing the pegboard connections (or the wire leads) changes the secondary impedance of both the speaker tap and the NFB tap, so recalculation of hte NFB series resistance is mandatory to maintain the same gain in the feedback loop. in addition to the recommendations that have been made about positive feedback, I think that adjusting to provide the correct value of the NFB series resistance will solve the fellow's problem.

5/20/2003 9:50 PM

Bob Predaina

Matt, I've been following a thread like that on the LP Forum. If you're reading the same board I am, there are three issues going on with that fellow's amp: 1) correctly wiring up the "RS" type OT, 2) properly wiring the OT for negative instead of positive feedback, and 3) changing the value of hte NFB series resistor to compensate for the secondary impedance change.

on OTs that provide multiple taps, its possible to change speaker impedance without moving the tap from which the NFB signal is taken. this means that you can change the value of the speaker cab's impedance without having to move the NFB tap or change the value of the NFB series resistor to compensate.

On "RS" type OTs, changing the pegboard connections (or the wire leads) changes the secondary impedance of both the speaker tap and the NFB tap, so recalculation of hte NFB series resistance is mandatory to maintain the same gain in the feedback loop.

in addition to the recommendations that have been made about positive feedback, I think that adjusting to provide the correct value of the NFB series resistance will solve the fellow's problem.

Page 1 of 2

Next>

Last Page>>