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Topic: Which is the worst tube amp impedence mismatch? |
Tom Wolverton
From:
Carpinteria, CA
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Posted 11 Jun 2012 9:05 pm |
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What is worse and possibly damaging to my tube amps?
1. A Showman head with an 8 ohm OT connected to a 4 ohm speaker
or
2. A Dual Showman head with a 4 ohm OT connected to an 8 ohm speaker.
I've always thought that #1 was bad and #2 was ok
or is it all good?
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To write with a broken pencil is pointless. |
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Brett Lanier
From:
Hermitage, TN
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Posted 12 Jun 2012 4:32 am
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| You were right about # 1 being worse. However, with a Fender amp, you can mismatch 100% in either direction without anything bad happening. I wouldn't recommend it, but it's pretty tough to burn up an OT in a Fender. Obviously it's much better to just run the correct speakers. Also, you should be very careful with other amps. A vintage Marshall for instance won't take that kind of abuse. |
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Gordon Hartin
From:
Durham, NC
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Posted 12 Jun 2012 5:14 am
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I use a shoe analogy.
The Foot(Amp) plugs into the Shoe(Speaker Cab)
The amp impedence is the size of your foot.
The speaker load is the size of the shoe.
You will hurt your foot by trying wear shoes that are too small.
But you can easily put your foot into a shoe that is too big, it might flop around a bit 
So you don't wanna plug a size 16 foot(amp) into a size 8 shoe(cab).
Some amps can handle a mismatch, but i wouldn't recommend wearing shoes too small.
Gordon |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 12 Jun 2012 6:34 am
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The answer has not changed since this thread - http://bb.steelguitarforum.com/viewtopic.php?t=170054
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Short story: Using too low a load impedance (a higher than nominal load) is generally less efficient and can lead to more distortion and shortened tube life because the load is drawing more current with less voltage drop to generate the power. But using too high an impedance (a lower than nominal load) is also less efficient, and can damage the output transformer and output tube section due to increased flyback voltage: power is generated in the speaker with a lower current and higher voltage - and higher than normal voltage is reflected back to the transformer, and thus even yet higher than normal voltage is transformed back to the output tubes. The webervst site has a good explanation of what's going on:
http://www.webervst.com/sptalk.html |
Tube amps (which are coupled to the speakers via an output transformer) are different than most solid-state amps (which are generally not). Using too high an impedance is fine for solid-state (although it does reduce the potential output), but not for tube amps. If you must mismatch, better to have a somewhat lower impedance than too high. |
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Gordon Hartin
From:
Durham, NC
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Posted 12 Jun 2012 7:06 am
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Dave,
I think
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| If you must mismatch, better to have a somewhat lower impedance than too high. |
Might not be clear to some of the readers. It is not clear whether your are talking about the amp or the speaker cab.
Just to clarify you are saying that
Using the 4ohm out of an AMP, with a 8 ohm speaker load. Should usually be ok.
Gordon |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 12 Jun 2012 7:35 am
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Gordon - you generally match the speaker (load) to the output transformer, not vice-versa. You have misinterpreted me, and I disagree with your conclusion. What I am saying is this:
Less bad: amp rated for 8 Ohms, use 4 Ohm load.
More bad: amp rated for 4 Ohms, use 8 Ohm load.
Stated another way: for an output transformer coupled tube amp, it is worse to use a load with impedance that is too high than too low. This is opposite from a direct-coupled solid-state amp. There is no danger in running a typical solid-state amp even with no load (i.e., open circuit). If you run a tube amp with no load, you will likely blow the OT pretty quickly due to extreme flyback voltages mentioned earlier.
As Brad Sarno said on the previous thread I referenced, it is probably not a good idea to run anything but the correct load at all. But Fender OTs are pretty robust, and were designed to handle 100% mismatch in the downward-impedance direction. Note that if you have, let's say, an amp looking for an 8-Ohm nominal load impedance and use both the main 8-Ohm speaker and an extension 8-Ohm speaker, you get 4 Ohms. That is safer than running a single 16-Ohm speaker. |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 12 Jun 2012 12:44 pm
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Holy cow! I can't believe I've asked this question twice now. Yes, I never run a different impedence from what the OT is matched to, with my tube amps. I'll just stick to this rule.
Sorry about that. I feel like I just farted in a submarine. Sorry to trouble you all. : )
_________________
To write with a broken pencil is pointless. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 12 Jun 2012 2:21 pm
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| Tom - it's really no problem. This point is often misunderstood, it's good to beat the drum occasionally to remind people that you should deal with OT-coupled tube amps differently than direct-coupled solid-state amps. |
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Tim Marcus
From:
San Francisco, CA
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Posted 12 Jun 2012 3:54 pm
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to make matters even more complicated - its not really the transformer that affects the output impedance at all, its the tubes connected to the transformer's primary side.
transformers are wound to couple an 8ohm (or whatever) speaker directly to the output of a tube (or a pair etc)
If you change the impedance on either side, the other is affected. This does not affect the transformer so much as it cooks the tubes, because they are doing more work on the other side to compensate.
You can get away with it, but its not great for more than a short while - ideally you want things to be in balance.
_________________
Milkmansound.com |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 12 Jun 2012 4:35 pm
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I hear of guys lowering the power of a Twin Reverb by pulling 2 power tubes and running just one speaker (8 ohms instead of 4). I suspect it has something to do with what Tim is refering to.
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To write with a broken pencil is pointless. |
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Gordon Hartin
From:
Durham, NC
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Posted 13 Jun 2012 5:24 am
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Tim,
On the back of many amps the speaker out will say somthing like 4ohm Min.
I have never seen one that says anything about a Max.
David is saying that it is safer to use a cab that is smaller load than is recommended, which is the exact opposite of what i've heard before.
| Quote: |
Less bad: amp rated for 8 Ohms, use 4 Ohm load.
More bad: amp rated for 4 Ohms, use 8 Ohm load. |
Are you in agreement with David on this?
Gordon |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 13 Jun 2012 7:02 am
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Seems like the "oversize shoe" analogy works for SS amps, but for tube amps, no. Best to wear only the correct shoe, but in a pinch, with a Fender, you might get by with a tighter shoe (like an 8 ohm OT connected to a 4 ohm load). Right now is an interested thread about Paul Franklin's speaker loads with his Little Walter amps. Sounds like Paul is mis-matched a bit, unless the LW has a switchable multitap OT.
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To write with a broken pencil is pointless. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 13 Jun 2012 8:05 am
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The issue is that tubes are, naturally, high-impedance devices, while transistors are basically low-impedance devices. The output transformer is there to match impedances - maximum power transfer occurs when impedances between source and load are matched (actually, complex-conjugate matched). Practical speakers are basically low-impedance devices. So you generally need an output transformer to match the high tube output-impedance to the low input-impedance speaker(s). Since transistor output sections are generally low output-impedance, the matching speaker load is also low, and a transformer is not generally necessary. But there are some transformer-coupled solid-state amps (some of the old solid-state Vox amps come to mind).
So, of course, the whole output section is involved in all of this. But the output transformer is in the line of fire and vulnerable to damage from excessive flyback voltages. I sure as hell don't want to have to replace the output transformers on my vintage Fenders with very hard-to-find period-correct replacements. So I want my output section impedances matched as well as possible.
Another factor is the fact that the impedance is complex - there are resistance, capacitance, and inductance involved. So you never get things perfectly matched, and changing impedance relationships also changes the sound. Some players (in my experience, mostly guitar players) don't want maximum power transfer, and make other impedance choices. You can change the sound or lower the efficiency (so tubes can be pushed harder at lower volume) by mismatching - when you run off-optimum impedance-matching, you're cooking the tubes more than necessary. My take is that if the OT is sufficiently robust and you don't mind pushing the tubes harder than necessary, it's no biggie. But again, I don't wanna mess around with my old Fenders.
So - feel free to ignore any impedance-matching advice. However, I think one should do that with one's eyes open. |
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Tim Marcus
From:
San Francisco, CA
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Posted 13 Jun 2012 8:18 am
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well said Dave
another thing to note between tube and SS outputs - there is no output transformer on solid state amps. The transistors generally (sometimes there is a relay or other protection) couple directly to the speaker - this is why lowering the impedance of the speaker can get into dangerous territory. The lower it gets, the closer it appears to the output of the amp to be a short which is death for solid state amps.
Tube amps on the other hand can operate with a short on the speaker output. Fender amps have a shorting jack on the output to protect the tubes in case there is no speaker plugged in, as a matter of fact. Its an open load that will burn up the output of a tube amp.
The best thing to do is keep everything matched as close as you can. That will produce the best tone, the best performance, and the best operating lifetime
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Milkmansound.com |
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Steve Lipsey
From:
Portland, Oregon, USA
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Posted 13 Jun 2012 11:48 am
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Some while ago I even called Fender about those parallel jacks for "Speaker" and "External Speaker" on many Fender amps, where plugging in another speaker would reduce the total speaker impedance to half of what it was supposed to be, and they said it is designed that way because it isn't a problem to have a load of half the recommended impedance....(although he did say that he might not want to run the amp full bore continuously at the mismatched impedance)
And on my Matchless HC-30, there was a "Half Power" switch that cut out 2 of the 4 EL-84's but did not change the output transformer tap to compensate for the impedance change...
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https://www.lostsailorspdx.com
Williams S10s, Milkman Pedal Steel Mini & "The Amp"
Ben Bonham Resos, 1954 Oahu Diana, 1936 Oahu Parlor |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 16 Jun 2012 12:40 pm
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Experience tells me that either is okay with modern Fender amps, and that a lower imprdance speaker will usually work better (give more output) than a higher one. On older, or cheaper-designed amps, there's usually not enough heat sinking capability in the transformers for big mismatches. Also, we must consider that most steelers use only a small fraction of the amps available power, so overheating really isn't the big issue that it is with rock players (who crank everything to "11", and then switch in the fuzz and overdrive pedals).
Most steelers with 200-250 watt amps seldom (if ever) use more than 30-50 watts of that power - kinda like driving a Corvette on a flat road at a steady 35-40 mph. |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 16 Jun 2012 2:03 pm
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For you guys that have Twin Reverb amps in separate heads. If you run one speaker cabinet, it is a 4 ohm cabinet plugged into the main speaker out jack on the back panel, right? But when you run two cabinets (like James Morehead and others), if you plug each cabinet into the speaker out jacks, (based on what we've discussed) it sounds like each speaker cabinet should be an 8 ohm cabinet, to maintain the 4 ohm load the OT wants to see. This would be ideal, right?
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To write with a broken pencil is pointless. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 16 Jun 2012 4:08 pm
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If you are using the internal 4-ohm load, then the smallest additional load you can connect is 4ohms, for a 2ohm total, to stay within the x2 factor mismatch allowed old Fenders.
Best is to run 4ohms for a perfect match, so yes, you're right. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 17 Jun 2012 5:17 am
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As Stephen says, you're correct.
1 cab - 4 Ohms, run from the main speaker output jack
2 cabs - 8 Ohms each (run in parallel) - one in the main speaker output, another in the external cab output.
I had an old Deluxe Reverb output transformer go up in smoke in the middle of a fairly low-volume pedal steel gig - literally, smoke started coming out of the unused input jack - the bass player tapped me on the shoulder and pointed to the amp. I was running the amp quite clean - volume around 5-6, but modulated by the volume pedal, into an 8-Ohm (correct load) external JBL D-130 speaker. My take is that pedal steel can present a pretty taxing signal - wide dynamic and frequency range, lots of sustain. A lot of the old OTs aren't really all that robust, as Donny says.
But the other thing is that badly mismatched flyback voltages can get pretty high and wind up arcing at the OT. I worry this might do long-term damage even with a robust transformer. |
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James Morehead
From:
Prague, Oklahoma, USA - R.I.P.
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Posted 17 Jun 2012 9:18 am
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| Tom Wolverton wrote: |
| For you guys that have Twin Reverb amps in separate heads. If you run one speaker cabinet, it is a 4 ohm cabinet plugged into the main speaker out jack on the back panel, right? But when you run two cabinets (like James Morehead and others), if you plug each cabinet into the speaker out jacks, (based on what we've discussed) it sounds like each speaker cabinet should be an 8 ohm cabinet, to maintain the 4 ohm load the OT wants to see. This would be ideal, right? |
This is the way I rig my speakers with my Twin Reverb in a head cabinet(because this electronic stuff confuses me, too), so my OT "sees" 4 ohms. I duplicate what Fender does--two 8 ohm speakers wired in parrallel, and one plug for BOTH speakers into the main speaker jack in the back of my twin's chassis. I accomplish this with a pair of 1/4" jacks wired in parrallel mounted right in the back of each of my speaker cabinets---I can now plug one 8 ohm cabinet into the back of the other 8 ohm cabinet--doesn't matter which speaker jack(of the extension cabs, that is), then plug either(just one cabinet) cabinet into the back of my Twin for a 4 ohm load. Redundant? Maybe so, but Fender did not wire the extension jack the same all the time, depending on what year your twin was made. THIS way of setting up my cabs, I can be certain I feed the load the amp wants to see for optimum performance. I do not trust my limited electronic skills to sort out what Fender was doing at a particular era. Cut through the chase--safely. See the pics.
Now to do this with a Fender Vibrosonic, you could do the same setup, but with 16 ohm speakers. However, when I take my Vibro on a gig, I just grab one cab and use it alone, as it's an 8 ohm K-130 JBL--and 8 ohms is what the Vibrosonic is designed to use, so I stay optimum. If I need more volume, I take the twin with TWO speaker cabs.
OR if you want to push more air with your Twin, four 16 ohm speakers connected in parrallel like the above pics show, and the twin still "sees" 4 ohms. But now you are much louder, because you are pushing 4X the air with the same 100 watt twin(mine is the 100 watt version--circa '71)
By the way, those speaker cabs were built by Rick Johnson. I sold them awhile back, as my cabs are now all green with gold grill clothe--those are my favorite colors. I give honor to forumite Rick Johnson for such awesome craftsmanship.
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"Good judgement comes from experience, and a lot of that comes from bad judgement"~old cowboy proverb.
shobud@windstream.net |
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Douglas Schuch
From:
Valencia, Philippines
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Posted 1 Oct 2012 9:07 pm Pulling Two Tubes To Reduce Power?
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I have a question related to this topic. It no doubt has been asked before, but I could not find it without reading hundreds of posts from a search.
Tom:
| Quote: |
| I hear of guys lowering the power of a Twin Reverb by pulling 2 power tubes and running just one speaker (8 ohms instead of 4). I suspect it has something to do with what Tim is refering to. |
I have read this several places as well. I am looking at a 64 Dual Showman at a good price next weekend. Does removing two of the power tubes actually change the impedance from 4 to 8 ohms on these amps?
Thanks!
Doug
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Bringing steel guitar to the bukid of Negros Oriental! |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 1 Oct 2012 9:31 pm
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Doug - yes, that works. Pull two of the power tubes (either the outside pair or the inside pair), and run into an 8-ohm speaker if you want to run at half-power. If it was matched with 4 tubes - 2 push-pull pairs - into 4 Ohms, then it will be matched with 2 tubes running push-pull into 8 Ohms.
Myself, if I'm using a 100-watt 4-6L6 amp, it's because I need the extra juice. Otherwise, I'll use a Vibrolux, Super, Pro, or something like that. But if it's all you have with you and you need to cut the power back, it does work. |
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Douglas Schuch
From:
Valencia, Philippines
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Posted 1 Oct 2012 9:56 pm
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Dave, thanks for the reply. I have a "practice" amp I bought when I got my first steel, but the bass response is basically zilch on the the low strings now that I have a U-12...might as well thump the top of the guitar as play those strings through that amp! If I buy the Showman, I am thinking I will build two 12" cabinets to hook up to it (keeping packages small and light) and will try it as stated for practice and see if it sounds good without pissing off the neighbors!
Again, Thanks!
Doug
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Bringing steel guitar to the bukid of Negros Oriental! |
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Tommy Boswell
From:
Virginia, USA
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Posted 2 Oct 2012 5:37 pm
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| Tom Wolverton wrote: |
| Sorry to trouble you all. : ) |
Tom, no trouble! You might have helped me big time.
I just happened to read this post while I was trying out my new vintage 8 ohm speaker with my Bassman 70 head, --that some fool sold to me --. Played for no more than 20 minutes before I saw this post, and unplugged it real quick. The strange thing was that when I then plugged it into a solid-state amp it buzzed real loud. Tried different cables, and even a different speaker to isolate the problem. I thought I had ruined my vintage speaker. But after a few tries the buzzing went away.
So, since we've got all these electronics gurus reading this post, can anyone tell me what was going on? My vintage Altec speaker has been doing great ever since, works great with the Peavey 112 solid state amp, and with my Blues Jr. which requires an 8 ohm speaker. Did it built up some kind of nasty charge while I had it plugged into the Bassman? |
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