Fender Showman Question
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Matthew Dawson
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Fender Showman Question
I have a Fender Showman with an 8 ohm output. Can I run a 4 ohm speaker off of it? It has an extension speaker out and I assume this speaker would be 8 ohms as well, making a 4 ohm total load. Do I need to put some kind of jumper in the extension speaker jack?
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Michael Brebes
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Usually the two speaker connectors are tied together, unless it is a later version that might have dual impedance outs. But usually it's just 8 ohms and you shouldn't really run a 4 ohm load without danger of blowing the output transformer.
Michael Brebes
Instrument/amp/ pickup repair
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Jon A. Ross
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"you shouldn't really run a 4 ohm load without danger of blowing the output transformer"
Huh?!? Those double negatives always confuse me!!
My first question would be how do you know its a 8ohm Showman? Fender didn't exactly advertise the difference between the S.M. and the Dual S.M. until they started including the word Dual on the front panel('66?). The N.F. resistor size is one way to tell or the output trans if one knows the model number.
To answer your question more directly, no harm will come to an 8 ohm Fender loaded with 4 ohms.
JAR
Huh?!? Those double negatives always confuse me!!
My first question would be how do you know its a 8ohm Showman? Fender didn't exactly advertise the difference between the S.M. and the Dual S.M. until they started including the word Dual on the front panel('66?). The N.F. resistor size is one way to tell or the output trans if one knows the model number.
To answer your question more directly, no harm will come to an 8 ohm Fender loaded with 4 ohms.
JAR
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Rick Johnson
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Matthew
I think your Showman amps is 8ohm, the Dual Showman
is 4ohm and I think the Dual Showman Rev
is 4ohm. In the early '60s some blonde Showman
amps had 2-15's in the smaller cabinet.
I'm not sure if they were 8ohm or 4ohm.
That speaker cabinet didn't stay in production
very long and are pretty rare. They switched to the single 1/15.
I agree with Michael
if you ran 2-8ohm speakers in the Showman
that would be a 4ohm load
The Showman can handle a 4ohm load, however
I would not run anything else with the 4ohm speaker.
Fender amps can tolerate a 100% mismatch.
Rick
www.rickjohnsoncabs.com
I think your Showman amps is 8ohm, the Dual Showman
is 4ohm and I think the Dual Showman Rev
is 4ohm. In the early '60s some blonde Showman
amps had 2-15's in the smaller cabinet.
I'm not sure if they were 8ohm or 4ohm.
That speaker cabinet didn't stay in production
very long and are pretty rare. They switched to the single 1/15.
I agree with Michael
if you ran 2-8ohm speakers in the Showman
that would be a 4ohm load
The Showman can handle a 4ohm load, however
I would not run anything else with the 4ohm speaker.
Fender amps can tolerate a 100% mismatch.
Rick
www.rickjohnsoncabs.com
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Ken Fox
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Good read from Duncan amps:
Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?
For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.
For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.
Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.
For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.
Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?
For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.
For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.
Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.
For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.
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Lefty
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Rick Johnson
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Lefty
Thanks, I didn't know the
speakers were 16ohm.
Does both speakers have the
metal tone ring like the 1/15?
Those are really nice cabinets.
Rick
www.rickjohnsoncabs.com
Thanks, I didn't know the
speakers were 16ohm.
Does both speakers have the
metal tone ring like the 1/15?
Those are really nice cabinets.
Rick
www.rickjohnsoncabs.com
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Danny Hall
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Ken's reference to the Duncan material is right on the money. I might add, for clarity, that if the input of the amp is unplugged, then the input is shorted and the pre amp puts no voltage into the power amp stage. Hence no voltage is developed across the plates of the amp through the primary of the output transformer. Very unlikely in that condition to damage anything.
Most well designed amps have a shorting speaker jack in the primary, (not extension) speaker position. Here's why:
In general a tube produces more voltage gain when it is terminated into a higher impedence. Up to a point when voltage passed through the internal resistance of the tube creates more current (hence power) than the tube can dissipate. If the tube is working into NO impedence, then no voltage is produced. That's why Ken says he'd much rather see his amp work into a short than an open condition.
An output transformer is an impedeance matching device. It takes the speaker impedence of (say) 8 ohms and transforms it into (say) 3600 ohms where the pair of tubes BY DESIGN are at their most efficient and least distorted operating point. But you can operate that pair of tubes at some other point on the curve without damage (as Ken and Duncan say) until you exceed the current and power limitations of the plates and screens of the selected tube. This relationship between voltage developed and terminated impedence is why it is almost always safe to go LOWER than the OT is designed for but somewhat problematic if you go HIGHER than the OT and tubes are designed for. It's called "reflected impedeance". If the OT has a turns ratio that leads to a 3600 ohms to 8 ohms match, then, when you hook up the 4 ohm load you now have the tubes working into 1800 ohms. But also they are working in a different, perhaps less "straight" part of the operating curve for the tube. Because the tubes are working against LESS impedence, they develop less voltage which means less current and power (wattage) is produced. But of equal importance, because you are now at a different point in the curve, it WILL SOUND DIFFERENT.
There was a discussion a few weeks ago on finding the best impedeance to terminate an amp into and I failed utterly to communicate these points. Perhaps here, with Ken jumping in ahead of time, it will be easier to understand.
Most well designed amps have a shorting speaker jack in the primary, (not extension) speaker position. Here's why:
In general a tube produces more voltage gain when it is terminated into a higher impedence. Up to a point when voltage passed through the internal resistance of the tube creates more current (hence power) than the tube can dissipate. If the tube is working into NO impedence, then no voltage is produced. That's why Ken says he'd much rather see his amp work into a short than an open condition.
An output transformer is an impedeance matching device. It takes the speaker impedence of (say) 8 ohms and transforms it into (say) 3600 ohms where the pair of tubes BY DESIGN are at their most efficient and least distorted operating point. But you can operate that pair of tubes at some other point on the curve without damage (as Ken and Duncan say) until you exceed the current and power limitations of the plates and screens of the selected tube. This relationship between voltage developed and terminated impedence is why it is almost always safe to go LOWER than the OT is designed for but somewhat problematic if you go HIGHER than the OT and tubes are designed for. It's called "reflected impedeance". If the OT has a turns ratio that leads to a 3600 ohms to 8 ohms match, then, when you hook up the 4 ohm load you now have the tubes working into 1800 ohms. But also they are working in a different, perhaps less "straight" part of the operating curve for the tube. Because the tubes are working against LESS impedence, they develop less voltage which means less current and power (wattage) is produced. But of equal importance, because you are now at a different point in the curve, it WILL SOUND DIFFERENT.
There was a discussion a few weeks ago on finding the best impedeance to terminate an amp into and I failed utterly to communicate these points. Perhaps here, with Ken jumping in ahead of time, it will be easier to understand.
The Last of the World's Great Human Beings. Ok, well maybe one of the last. Oh alright then, a perfectly ordinary slacker.
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Jon A. Ross
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I never heard of that either (16 ohm JBL's in a Dual/Double Showman cab). I can tell you that only the single spkr cabs had the tone ring (Omy fave Showman cab!).Rick Johnson wrote:Lefty
Thanks, I didn't know the
speakers were 16ohm.
Does both speakers have the
metal tone ring like the 1/15?
Those are really nice cabinets.
Rick
www.rickjohnsoncabs.com
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Danny Hall
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So Matthew, if you want to retain your amps crystal clarity and tall headroom, convert both speakers to 16 ohms, parallel them, and plug them into the primary and extension jacks without change. Weber makes a JBL clone called the Californian that sounds terrific. You could probably get enough for your JBL to buy two and have money left over for a good dinner.
Those are wonderful amps.
Those are wonderful amps.
The Last of the World's Great Human Beings. Ok, well maybe one of the last. Oh alright then, a perfectly ordinary slacker.
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Lefty
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Rick,
The Double Showman was made only a couple of years (1963 and 1964 I believe). The 1964 model had black face cosmetics.
Basically a two 15 cabinet without tone rings. The Showman had 1 -15 JBL D130F 8 Ohm and the double Showman (that predated the Dual Showman) had 2-JBL-D130F 16ohm speakers. This allowed them to use the same exact head. Mine uses the 7355 power tubes that were only used for a couple of years. There is some interesting stories concerning this on the internet.
James: Do your research on the internet.
regards,
Lefty
The Double Showman was made only a couple of years (1963 and 1964 I believe). The 1964 model had black face cosmetics.
Basically a two 15 cabinet without tone rings. The Showman had 1 -15 JBL D130F 8 Ohm and the double Showman (that predated the Dual Showman) had 2-JBL-D130F 16ohm speakers. This allowed them to use the same exact head. Mine uses the 7355 power tubes that were only used for a couple of years. There is some interesting stories concerning this on the internet.
James: Do your research on the internet.
regards,
Lefty
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Jon A. Ross
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Lefty
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The story I read was that Leo Fender got a good buy on those tubes. They were used in a lot of the old Harmon-Kardon and other tube amps from the period.
I am pretty much a 6L6 man myself, but I can't tell a big difference with the 7355. I will say that amp is very loud. The speakers have a reinforcement foil tape (silver) that keeps the JBL dust cover from loosening up.
Lefty
I am pretty much a 6L6 man myself, but I can't tell a big difference with the 7355. I will say that amp is very loud. The speakers have a reinforcement foil tape (silver) that keeps the JBL dust cover from loosening up.
Lefty
Last edited by Lefty on 28 Feb 2010 5:46 am, edited 1 time in total.
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James Quackenbush
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Jon A. Ross
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Lefty
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Matthew Dawson
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Thank you everyone for the responses. Sorry I haven't been able to reply sooner. I think I have solved all my impedance problems (I hope!) by picking up a Weber Z Matcher. Anyone have any experience with one of these? To make things clear, I have just a Showman head and a 4 Ohm cabinet with a 15" Weber California.
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Rick Johnson
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Lefty
I got some original 1/15 Showman cabs
and the tall 2/15 from '67 but
I must say that rig looks killer.
I feel sorry for whoever has to carry that
beast, let alone get it up into a van.
Nice Stuff.
Rick
www.rickjohnsoncabs.com
I got some original 1/15 Showman cabs
and the tall 2/15 from '67 but
I must say that rig looks killer.
I feel sorry for whoever has to carry that
beast, let alone get it up into a van.
Nice Stuff.
Rick
www.rickjohnsoncabs.com