Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PeaceMaker on July 09, 2012, 11:24:04 pm
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Hi All, it's been a long time... I hope everyone is doing well.
Anyways, I have a Pignose G40v (40watt tube amp- 2 x 6L6 and 3 x 12AX7) that has a 4ohm out and 8ohm out and I want to run a 4ohm Speaker on the 4ohm out and a 8ohm Speaker on the 8ohm out at the same time?... Will this be ok, or will it damage something?
Thanks for the help; Love You,
PeaceMaker
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No, unfortunately. Amp manufacturers should really be more clear about this, since most people are not as smart as you to ask.
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> 4ohm Speaker on the 4ohm out and a 8ohm Speaker on the 8ohm out at the same time
This is equivalent to 2 ohms on the 4 ohm tap, or 4 ohms on the 8 ohm tap. Too-heavy loading. The G40V is such a hot-rod that it will probably bust a gut (or a tube) trying.
However.... with that setup, 4 ohms on the "8" tap and 8 ohms on the "16" tap would give proper loading. (If that amp has a "16" tap, of course.)
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PRR meant 8 ohms on the 4 ohm tap, and 16 ohms on the 8 ohm tap.
If you attach 2 loads, on their "correct" tap (4 ohms on 4 ohm tap, etc), the two loads are in parallel. The reflected impedance on the primary will be half what it was intended to be.
So attach double the marked load to each of two taps, and you will arrive at the proper primary impedance.
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... I have gut understanding of the approach taken to putting the 8 ohm speaker on a 16 ohm winding, and putting a 4 ohm on the 8 ohm, but my brain tells me check this approach out. So, I am asking what is theory behind this being able to work? ...
Normally, this is suggested when someone has four output tubes and wants to run two output tubes for reduced output power.
But the output transformer has a winding ratio between primary and secondary which reflects a load attached to the secondary to appear, to the primary, as an appropriate load for the output tubes present. When we change the number of output tubes attached to the primary, a different load is optimum.
If you think of the output tubes as a conduit for current, halving the number of tubes is equivalent to doubling the resistance. Instead of 4 parallel paths for current flow, there are only two (or you might say 2 vs. 1 if thinking in terms of one side of the push-pull output section).
Since the OT has no "impedance" of its own, only a turns ratio between sections, if you want the primary to appear like double the impedance, you need to attach double the speaker impedance to your secondary tap. So that means if you pull 2 output tubes out of your Twin, you want to attach 8 ohms of speaker load to the 4 ohm secondary tap. That will use the OT's turns ratio to reflect double the impedance to the primary.
My calcs indicate, if the speakers were wired in parallel, (not plugged in the 4 and 8ohm respectively,) the equiv ohms is about 2.67, something you probably do not want to put on the 4 ohm tap. However, if you were to wire them in series, then I think you could safely use either the 4 ohm or 8 ohm windings, at a loss of efficiency. I would use the 8 ohm tap.
Right on the calculated impedance, but why mismatch even farther to attach ~2.7Ω to an 8Ω tap, rather than use the 4Ω tap, which is a closer match?
Regardless, PeaceMaker's question implied a different situation, such as someone wiring 4Ω and 8Ω jacks that didn't originally exist. Maybe something like the jacks on some Dr Z amps in lieu of an impedance select switch.
So can you plug into two different jacks at the same time, and load two different taps? Sure, but the loads are effectively in parallel. If each load is on its correct tap, it's all the same as a pair of 8Ω speakers on the 8 ohm tap. That would be a 4Ω load, so the implication is we need to attach our loads to a tap expecting half the load we're actually applying (8Ω speaker on 4Ω tap, 16Ω speaker on 8Ω tap). That way, the total reflected load to the primary still lands where it's supposed to.
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Wow, thanks everyone for the humongous help (I'm impressed)! :icon_biggrin:
So, if I run a 8Ω speaker on the 4Ω tap, and a 16Ω speaker on the 8Ω tap it will work with no problems?
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WHat HBP said, not what I typed. (Thanks for the catch!)
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Wow, that's pretty neat!
So with a 8K:4/8/16, an 8 on the 4 would reflect 16K and a 16 on the 8 would reflect 16K and by using both at the same time you would reflect 16K paralleled with 16K for 8K.
Sounds like it makes sense, but it "feels" wierd to use two different taps at once.
Way to think outside the box. :worthy1:
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Now, does anyone have all the needed components to try it?... Or, has anyone ever tried it?... Need input! :dontknow:
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Now, does anyone have all the needed components to try it?... Or, has anyone ever tried it?... Need input! :dontknow:
What do you need?
An OT with multiple secondary taps, two 1/4" jacks, and spekaer loads wired up for the appropriate impedances.
In the end, it won't do anything different than having two 12" speakers instead of one, of mixing a 15" and a 10" speaker. Otherwise, the amp's operation is exactly the same.
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Well, I have a 8ohm 10" Speaker (in the amp) but I don't have a 16ohm Speaker ready to go (I have a 12" 16ohm in a box- that I just replaced with a new 4ohm Speaker :BangHead:). :w2:
Also wondering if power comes out of the two taps to the speakers at the same time? or that maybe when you plug in a speaker to one tap it turns off or disengages the other tap like some devices?
Just trying to find out if I should go through the effort... I don't want to do any extra work just to find out it doesn't work after all. :dontknow:
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Plug a into cord in each jack. Test for resistance between the tips. If each jack goes directally to the Xformer and they're both "on" you should read the resistance of the winding between the 4 and 8 ohm taps. If I had to guess I'd say that each tap goes to the tip of each jack. There probally is switching going on IE one tap goes to the switch on one jack, to the switch on the next jack to the switch on the next jack so when there is nothing plugged in it grounds one tap but when you plug into any one of the jacks it breaks connection.
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Also wondering if power comes out of the two taps to the speakers at the same time?
Have you ever seen or used a power transformer with a bias tap on the high voltage winding? Same idea here.
When the primary (connected to the wall) is energized in our power transformer, the ratio of the windings causes voltages to appear on the secondary (or secondaries, high voltage, 6.3v, 5v). If we have a bias tap on the high voltage secondary, it is most often a single wire at some mid-point on one side of the high voltage winding. If the voltage needs to be higher, the tap is moved closer to the end of the high-voltage winding, if lower it's moved closer to the grounded center-tap.
But voltage and current are available from both the high-voltage winding and its bias tap all the time.
The same situation applies to the OT secondary taps, though we think of them in terms of impedance and not voltage/current. But if we changed our thinking to be voltage/current taps (review ohm's law), then if power output is constant the difference between taps is highest voltage/lowest current on the 16 ohm tap, less voltage/more current on the 8 ohm tap and lowest voltage/highest current on the 4 ohm tap. Any/all can be used at any time.
... or that maybe when you plug in a speaker to one tap it turns off or disengages the other tap like some devices? ...
Only if you create a switching scheme to cause this (which can be done, but not what you say you're trying to do).
I think I've read people assert that this happens in some amps, but they may be assuming it happens. When I look at a Dr Z manual (http://www.drzamps.com/docs/Maz%2018-38%20NR%20%20Manual.pdf), they don't warn that only one jack works at a time. If I owned a Dr Z amp with multiple impedance jacks, I'd ask them (or look at the jacks) if there is any switching.
However, some note that Mesa does use a switching scheme to connect only one impedance tap at a time.
But if you're building this into an amp, the most likely way you'll wire it up doesn't do any switching.
Just trying to find out if I should go through the effort... I don't want to do any extra work just to find out it doesn't work after all. :dontknow:
I wouldn't suggest doing it unless you already have a jack for each impedance tap, or honestly have no other way to connect additional speakers to the amp, especially if you have to go buy speakers to have the right impedance to make the setup work. Sounds like your case is trying to figure out how to make use of the extra speakers you have hanging around.
It's really up to you to answer if this is a "better" solution than simply having a cabinet with multiple speaker wired for a single, total impedance.
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Jeff, I ordered another speaker cable so I can do the test you suggest... Thanks.
HotBluePlates, actually I'm not trying to build an amp (I wish I was that smart) I'm just trying to utilize what I have... I figured I have a Pignose G40v (40watt tube amp- 2 x 6L6 and 3 x 12AX7) that has a 4ohm tap and 8ohm tap (it came that way) and I have a 4ohm speaker and a 8ohm speaker that I thought I could just plug in and it would work but I wanted to check with you to make sure I wasn't going to blow anything up... Which from the look of things that's exactly what I was about to do... Thank "YOU and EVERYONE"! for heading off that disaster (I would of cried).
drgonzonm, thanks for bringing that up... I was wondering if there would be any sound degradation, anyone know?
Thanks again everyone for helping me figure this out.:)
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... After all, the O/T has inductor qualities. ...
This is like saying there is a phase shift from one leg of a capacitor to the other, because it has capacitance. Unless you can have both phases present at the same point, it isn't possible to tell if it's shifted.
To detect a phase shift, you have to have reference to something else of a different phase.
Ever been to a big concert with a giant video screen? The image on the screen moves slightly later than the dude playing onstage. After all the video equipment in between the dude and the screen, there's a slight delay which is the same as a phase shift. But as long as you look only at the screen (or the stage) you don't know the difference. There's no other reference to compare against.
Same happens with speakers wired in different polarities. If you wire all speakers one way, or the other, you can't perceive absolute phase. As soon as you have two different phases at one place, one time, you can detect a difference.
I hear what you guys are saying, but isn't there a phase shift between the 8 ohm tap, and the four ohm tap.
No.
I explained why phase is irrelevant unless you have differing phases at one place at one time. I'm not convinced that even happens in a single OT secondary.
That's because the way a transformer works is based on volts-per-turn. A power is applied to the primary which results in a certain number of volts per primary turn. There must be the same number of volts per turn on the secondary, due to the nature of a transformer. While the relative phase of voltage versus current is changed (as with all reactive components) as compared to some other point in the amp, the relative phase of voltage on the secondary 4Ω tap is the same as the phase of voltage on the 8Ω tap.
You have a water hose (whole secondary winding) connected to a spigot outside your house. You punch several holes (taps) along the length of that hose. You turn the spigot on and off repeatedly to apply water pressure; the individual holes all squirt water or not at the same time, not alternately.
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> You have a water hose
If the hose were long enough, the spurts would be sequential.
The goal in an audio transformer is that the reactances do "not" affect performance across the audio band. This calculation does not depend much on the secondary impedance.
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PeaceMaker are you asking because you have two speaker cabs of different impedences or is it because there is only one jack per tap on the amp?
Radio shack sells Y-adapter so you could plug two 1/4" cords into it and it has one 1/4" plug out. http://www.radioshack.com/product/index.jsp?productId=2103712 (http://www.radioshack.com/product/index.jsp?productId=2103712)
I'm glad you asked the question because I never knew you could do this. Pretty interesting. I always thought if you had 4/8/16 ohm taps and 8 ohm speakers you could only run one(8ohm), two(4 or 16 ohm),or four(8 ohm). Now I know how to run three without changing the load. Good trick to know in a pinch if I ever blow one of my speakers in my 4X10" cab.
I'm just wondering if you asked to know if it was possible or because you didn't have two 8 ohm or two 16 ohm jacks on the amp and wanted to use multiple cabs. Sometimes the simplest solutions are overlooked.
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> You have a water hose
If the hose were long enough, the spurts would be sequential.
I debated adding another element to the scenario: the hose is capped at the other end, so that pressure along the entire hose varies directly with the spigot.
In the end, the real goal is a realization that the separate taps of a transformer winding are not discrete items, but parts of a whole. Further, that output voltages are present at unused taps, as well as those actually being used.
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I do this very thing all the time. It works great. Mixing cabinets. I just finished a build with this in mind. It is a combo Princeton Using a multitap Deluxe Mercury ot and A deluxe PT. The cab alone is 2,10's, 8 ohms. I have a made 2 single 12's extensions. Run the 8 ohm in the 16 tap and the 4 in the 8. Works very well. That is without using the cab speakers. I plan to add a series parallel stereo which would allow me to use all of them.
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> the hose is capped at the other end, so that pressure along the entire hose varies directly with the spigot.
If you stretch this hose analogy too far, it rips.
In a transformer, you don't have a spigot at one end. Energy is induced in _every_ turn. More like one of those porous "soaker" hoses deep under water (or in a pressure tank).
There "are" issues with multi-tap windings. The super-fussy hi-fi winders either wind for one specific impedance or scatter a dozen part-windings thoughout the build which you series-parallel to suit your impedance. However this is about eking-out the last 10 degrees of phase margin at 55KHz so the NFB don't go nuts. But any full-range loudspeaker will be hundreds of degrees of phase-shift above 5KHz, and any two such speakers never match phase even within a hundred degrees. Even worse if you are not ON-axis. So the different (even same but built in different batch) speakers are far more phase-shift than any transformer thing.
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Plug a into cord in each jack. Test for resistance between the tips. If each jack goes directally to the Xformer and they're both "on" you should read the resistance of the winding between the 4 and 8 ohm taps. If I had to guess I'd say that each tap goes to the tip of each jack. There probally is switching going on IE one tap goes to the switch on one jack, to the switch on the next jack to the switch on the next jack so when there is nothing plugged in it grounds one tap but when you plug into any one of the jacks it breaks connection.
Hi everyone and thanks for all the input...
I received the speaker cable that I ordered and I'm now ready to do the above test... Do I do this with the amp on (with no speakers attached)?... And is there anyway that I could damage the amp or myself trying this?... And what should I be reading from the ohm meter?
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Amp off with no speakers. It's a harmless check. Probably gonna read about zero ohms.
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Amp off with no speakers. It's a harmless check. Probably gonna read about zero ohms.
Thanks sluckey,
If it's only going to read about zero ohms how will I know if I'm getting juice coming through both taps at the same time?
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You'll know because both speakers will be making a beautiful noise.
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You'll know because both speakers will be making a beautiful noise.
The point of the test is so I can find out if this will work without having the proper ohmed speakers attached (I don't have the speakers ready yet, and I don't want to get them ready if it won't work- that's the reason for the test)? :w2:
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I don't think that particular test will give you any info. What's involved with getting speakers ready? I thought all you had to do was hook 'em up?
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I don't think that particular test will give you any info. What's involved with getting speakers ready? I thought all you had to do was hook 'em up?
Well, I just put a new speaker in my 1x12 cab that I'd have to undo which I'd rather not do, especially if it's not going to work (that's why I'm trying to find out if it will work another way- that's why I'm doing this test).
Also, I thought that I was to do the test with the amp on?... Will that damage something?
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I do this very thing all the time. It works great. Mixing cabinets. I just finished a build with this in mind. It is a combo Princeton Using a multitap Deluxe Mercury ot and A deluxe PT. The cab alone is 2,10's, 8 ohms. I have a made 2 single 12's extensions. Run the 8 ohm in the 16 tap and the 4 in the 8. Works very well. That is without using the cab speakers. I plan to add a series parallel stereo which would allow me to use all of them.
Hi Ed_Chambley, and Everyone,
Well I finally tried it and it does work and I'm very pleased with the results... If there is any sound degradation it's to slight for me to notice and everything seems fine (been using it alot).
I now have a 8 ohm 10" Speaker running on the 4 ohm tap and a 16 ohm 12" Speaker running on the 8 ohm tap... That's "22 inches of Tonal Speaker Bliss"!!!... And I owe it all to you- Thanks Everyone!
Take care, thanks again, Love You,
PeaceMaker
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Glad to help!
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Glad it worked to your liking. Now, I wish there was a way to put the wet on one speaker and the dry on the other without a stereo amp or 2 amps. I can dream.