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Author
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Topic: Fender amp impedance mismatch problem
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Bob Carlucci
Member From: Candor, New York, USA
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posted 07 April 2005 09:06 AM
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I have a Vibrosonic[cut down to a head] on the way here that I bought from a forumite... Now as we all know The Vibro is designed to run at an 8 ohm load,unlike many Fenders which run at 4 .. Both of my speaker enclosures at this time are 4 ohm.. I have a 1x15 and 2x12[2x12 8 ohm in series] .
anyway, I really don't want to INSTANTLY go swapping speakers if I can help it. So the question I am asking is this.. Will it REALLY do any harm to the amp to run it at a 4 ohm load??.. I read somewhere that Fender amps were designed to operate at an impedance mismatch at up to 300% without damage... I HAVE done this in the past and never had a problem... just wondering if anyone knows for certain.. I do understand there may be somewhat more distortion etc, but I'm thinking more along the lines of massive fireballs hurtling out of the input jacks!.... bob |
Ken Fox
Member From: Ray City, GA USA
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posted 07 April 2005 09:28 AM
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Is that the 135 watt or 100 watt Vibrosonic?The cab with two 8 ohms would be 16 ohms in series, a 4 ohm load if they are in parallel. Assuming your two cabinets are 4 ohms each, they would present a two ohm load to the Vibrosonic 8 ohm transformer tap.(if it is a 100 watt head). A 135 watt Twin is a different animal. It has switched from an 8 ohm tap to the 4 ohm tap when an extenion is plugged in. You can pug an open jack in the extension and the main jack will be a 4 ohm tap, allowing you to run one of your 4 ohm cabs safely. |
Bob Carlucci
Member From: Candor, New York, USA
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posted 07 April 2005 10:13 AM
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Ken.. you are correct of course, my 2x12 is wired parallel, It is 4 ohm.. I think you misunderstand however,, I will not be running the amp with two 4 ohm cabs . Just one at a time.. I just happen to have 2 different cab types of the same impedance. I was just wondering about the impedance mismatch doing any harm to the head. I THINK its a 135 with an ultralinear transformer as it HAD a master volume.which was disconnected, and I think all the MV Vibros were 135, but I am not certain. I am not sure if a "true" blackface mod can be done to these heads without changing the transformers,but I do know they can be "sweetened up" while still keeping the big headroom. The seller is sending schematics and descriptions of the amps mods.. I just don't want to do any damage with a 4 ohm load. I used a Blackface Showman head which was 8 ohms in 4 ohm cabs for years and never had a problem.. later went to Dual Showman heads which are 4 ohms.. I don't notice much difference... they all sounded good. Anyway, just wondering if the impedance mismatch is going to possibly do any damage?? .. Personally I doubt it, but there are a lot more knowledgable minds on the subject than mine and thats why I'm asking... bob |
Ken Fox
Member From: Ray City, GA USA
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posted 07 April 2005 10:20 AM
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The biggest problem with impedance mismatch is the fly back voltage that results from it. That causes some hefty spikes on the plates of the tubes and can cause them to wear out pretty fast. My old Peavey mace has two diodes on the outputs to absorb these spikes. This same issue has been discussed many a time here in the electronics sections. General consensus from most people is a mismatch is tolerable. Even my old Peavey Mace was intentionally mismatched from the factory! It has a 4 ohm load on the 8 ohm tap. The aux jack was its 4 ohm tap!If it is a 135 watt you can easily use the main jack and an open jack in the aux to get at the four ohm tap.
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David Doggett
Member From: Philadelphia, Pennsylvania, USA
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posted 07 April 2005 10:57 AM
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Bob, from what I remember of the old posts on this, increasing the impedance on a tube amp is okay (e.g., a 4 ohm amp with an 8 ohm speaker), you just loose a little volume or headroom. Decreasing the impedance (8 ohm amp into a 4 ohm speaker) actually increases the volume, but as Ken says, it's harder on the tubes. Supposedly Fender tube amps can take about half the recommended impedance okay, so you are probably okay no matter what you do. But running two 4 ohm speakers in parallel would have the amp seeing 2 ohms, which would seem to be bad for an 8 ohm amp. Running those same two speakers in series would of course be 8 ohms. What happens at the extension jack varies from model to model and is very confusing to me. I would take Ken's advice about that. |
Ken Fox
Member From: Ray City, GA USA
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posted 07 April 2005 12:00 PM
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They did the transformer re-tapping on the UL amps via the extension speaker jack. Isee that same arrangement in some of the new amps as well (such as the Hot Rod Series of amps). Blackface and SIlverface were jack parallel jacks for aux and main. A common thing to all the Fender amps is a dead short to ground occurs on the main jack, when nothing is plugged in. Tends to save the output transformer from dying with a no load condition. With nothing plugged in the speaker jack a signal delivered to the primary windings of teh output transformer has no place to go except right across the primary windings! It can tolerate a dead short better as it couples the energy on across to the secondary windings and on to the dead short. Now don't try that on a direct coupled output, transistor amp! A dead short there works just the opposite and takes out the output transistors. Ouch!!! |
Bob Carlucci
Member From: Candor, New York, USA
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posted 07 April 2005 12:12 PM
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David,, I always assumed the same thing as you,that running an 8 ohm load with a 4 ohm amp,would result in somewhat decreased out put but would not hurt the amp... However I just came off a couple of tube amp websites that stated the OPPOSITE! They claim a tube amp would be hurt LESS by running at some what lower impedance[8 ohm head 4 ohm load] and MORE by running higher [8 ohm head 16 ohm load).. also they claim the reverse is true in solid state,, That running lower impedance would burn it up fast,but running higher is NO problem...GRRR... I'm just gonna do what I always do... plug it in and start wailing!!!bob |
John Daugherty
Member From: Rolla, Missouri, USA
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posted 08 April 2005 06:00 AM
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Bob, a simple test that very few people think of is the old "touch test".
After playing through the amp for a few minutes, touch the output transformer. If it is warmer than normal (hot), you have a problem. Same goes for the speaker voice coil.
A lot of people ask me "how can I tell if my power cord or power supply is big enough?"
Answer: Touch and see if it is hot. |
David Doggett
Member From: Philadelphia, Pennsylvania, USA
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posted 08 April 2005 09:13 AM
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Bob, over on the Weber Speaker site in the speaker question and answer section there is a discussion of impedance mismatch between speakers and amps. It gives some technical reasons for the problems caused by mismatch. There are reasons why mismatch is bad in either direction. However, it doesn't discuss how much mismatch is required to create these problems. It seems like our commmon exerience is that a mismatch of about half in either direction is tolerable for practical purposes (e.g., 8 ohm amp with 4 ohm speaker, or 4 ohm amp with 8 ohm speaker). I think it also matters how much you stress the system. Because of the way we use the volume pedal (attack notes with the pedal half off) we steelers don't really stress amps and speakers much. So what causes a problem with a 6-string rocker who plays his amp at 10 all the time might not be a worry for us. |
Dave Grafe
Member From: Portland, Oregon, USA
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posted 08 April 2005 10:42 AM
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quote:
Because of the way we use the volume pedal (attack notes with the pedal half off) we steelers don't really stress amps and speakers much
This is true, unless you are using a stomp-box for distortion effects -- that will heat things up right pronto. [This message was edited by Dave Grafe on 08 April 2005 at 10:43 AM.] |
Donny Hinson
Member From: Balto., Md. U.S.A.
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posted 08 April 2005 01:30 PM
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John Daugherty has hit upon the real crux of the problem, and that problem is heat. How much of a mismatch an amp will handle has to take into consideration both the program material (how hard the amp is being pushed), and the ambient conditions (temperature and ventilation). It's no secret that an "upside-down" chassis configuration causes excess heat on the chassis and all other components. This condition is exacerbated when the amp is tilted back, placed too close to a wall, or played in direct sunlight. All of these conditions will increase operating temperature, and that is what kills all the components, transformers, resistors, electrolytics, you name it. So, how long an amp will run a certain mismatch without damage depends on the amount of mismatch and those above mentioned operating factors. I can say this, I ran a Super-Twin Reverb (normally rated for 4 ohms) into a 2 ohm load for years, and pushed it very hard, without any ill effects. Had I not been conscious of the temperature factor, I might not have been so lucky. The moral of all this is that you can run an amp at a lower-than-recommended load, and get more output, but it had better be well ventilated! A small whisper-fan installed in the enclosure will add years to the life of components. Call it "audio overclocking".  |
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