Trying to nail down the danger level of an impedance mis-match on a tube amp. I've gotten mixed messages on this in the past and now it's become a real question for me.
For example: Impedance switch set at 16 ohms and 4 ohm speaker cab plugged in. Will this cause damage in a tube amp?
Colorcat.
For example: Impedance switch set at 16 ohms and 4 ohm speaker cab plugged in. Will this cause damage in a tube amp?
Colorcat.
Probably not, it just won't perform to peak. generally if you are off one "notch" is won't hurt anything.
4 ohm speakers on the 16 ohm tap will drive the tube current up. Probably not an issue unless the amp is borderline already and it is cranked to 11.
Watch the output tubes when the amp is played hard. If the plates glow red, or the internals of the tube appear brighter on loud playing (glowing screen grid). If so you are in the vacuum tube danger zone!
Watch the output tubes when the amp is played hard. If the plates glow red, or the internals of the tube appear brighter on loud playing (glowing screen grid). If so you are in the vacuum tube danger zone!
Thanks and Here's why I asked
The reason I'm inquiring: I did some work on an amp and thought I'd encourage the customer to plug two different impedance cabinets into the output and set the output ohms accordingly- A 16 and 8 set to 4 ohms, for the approximate 5 ohms load.
The concern is in the event he forgets to put the output back to the correct setting if only one cabinet is used. It should certainly sound not as good but I wanted to be sure there wouldn't be an immediate damage scenario.
Thanks,
colorcat.
The reason I'm inquiring: I did some work on an amp and thought I'd encourage the customer to plug two different impedance cabinets into the output and set the output ohms accordingly- A 16 and 8 set to 4 ohms, for the approximate 5 ohms load.
The concern is in the event he forgets to put the output back to the correct setting if only one cabinet is used. It should certainly sound not as good but I wanted to be sure there wouldn't be an immediate damage scenario.
Thanks,
colorcat.
They have been making multiple impedance output amps for decades, and we don't have an epidemic of blown amps from it.
I made about a dozen or so "Turbo Champ" SE amps about 15 years ago that used a Hammond 125CSE OPT with multiple impedance taps. There was a selector switch to choose any tap, and the user was encouraged to "pick the one you like." Nothing bad ever happened to these amps.
I built several P-P amp heads using Schumaker 6600 ohm OPT's with the usual 0-4-8-16 ohm taps, and 6L6GC or KT88 tubes. Again I told the user to pick the tap he liked. After a year or so I got an amp back because it made a strange sound every once in a while. I could not duplicate this in my shop with my speakers, so I went to his house, and sure enough the amp made a screaming hissing sound that often continued for a second or two after he muted the strings or even unplugged the guitar. This only happened when playing loud, and only with certain notes on the 5th or 6th string.
Autopsy revealed an arc between the plate (pin 3) and the GROUNDED heater (pin 2) at one of the output tube sockets. He had been using a cabinet with 2 X 12 inch speakers wired in series (I assumed 16 ohms total, but didn't test) on the 4 ohm tap because it gave him the distortion he wanted without being too loud. I applied some heat shrink to all the OPT leads over the CERAMIC socket pins, and all was well.
With extreme cases of mismatch on either end of the scale it is possible to get an over voltage, or over current scenario, but neither should be catastrophic in a well designed and built amp.
I measured (with a scope and a voltage divider) over 2500 volt spikes on the plates of a similar KT88 amp that I had built, under similar conditions with a pair of 10 inch EV speakers wired in series on the 4 ohm tap while driving the amp about 10 db beyond clipping near the resonance of the speaker. Nothing was damaged though.
I have seen red plates on a cheap Chinese KT88 when running the same amp into a 4 ohm resistive load on the 16 ohm tap. This was 15 years ago when the Chinese KT88's of the day were pretty bad. The Sovtek's (6550WA's I think) of the day did not complain at all.
I built several P-P amp heads using Schumaker 6600 ohm OPT's with the usual 0-4-8-16 ohm taps, and 6L6GC or KT88 tubes. Again I told the user to pick the tap he liked. After a year or so I got an amp back because it made a strange sound every once in a while. I could not duplicate this in my shop with my speakers, so I went to his house, and sure enough the amp made a screaming hissing sound that often continued for a second or two after he muted the strings or even unplugged the guitar. This only happened when playing loud, and only with certain notes on the 5th or 6th string.
Autopsy revealed an arc between the plate (pin 3) and the GROUNDED heater (pin 2) at one of the output tube sockets. He had been using a cabinet with 2 X 12 inch speakers wired in series (I assumed 16 ohms total, but didn't test) on the 4 ohm tap because it gave him the distortion he wanted without being too loud. I applied some heat shrink to all the OPT leads over the CERAMIC socket pins, and all was well.
With extreme cases of mismatch on either end of the scale it is possible to get an over voltage, or over current scenario, but neither should be catastrophic in a well designed and built amp.
I measured (with a scope and a voltage divider) over 2500 volt spikes on the plates of a similar KT88 amp that I had built, under similar conditions with a pair of 10 inch EV speakers wired in series on the 4 ohm tap while driving the amp about 10 db beyond clipping near the resonance of the speaker. Nothing was damaged though.
I have seen red plates on a cheap Chinese KT88 when running the same amp into a 4 ohm resistive load on the 16 ohm tap. This was 15 years ago when the Chinese KT88's of the day were pretty bad. The Sovtek's (6550WA's I think) of the day did not complain at all.
Really has to do with the quality of your parts, how over designed they are. Are they borderline at the voltage you run at or not. Take a 20W OT and run an amp at 5W and you could probably do anything with the output.
Any tube and transformer amp run with large signal and NO-load is in some danger of internal arcing. Not just Tubelab's big KT88 beasts: there's a story about un-loaded Champs coming back for repair every Monday.
Short of a WAY-high load:
Self-bias amps run maximum-hot at idle. No mis-load will hurt them.
Most fix-bias stage-amps have some leeway for mismatch. "One notch" may be a good guide.
A really souped-up fix-bias amp worked HARD in a low-Z load "could" cook its plates or power supply.
Short of a WAY-high load:
Self-bias amps run maximum-hot at idle. No mis-load will hurt them.
Most fix-bias stage-amps have some leeway for mismatch. "One notch" may be a good guide.
A really souped-up fix-bias amp worked HARD in a low-Z load "could" cook its plates or power supply.
They were relatively tame by my standards, just 35 watts. The recipe was predicated on what I had at hand, about a hundred 90's vintage Chinese KT88's of dubious quality, 200 OPT's rated at "80 VA @ 80 Hz, 6600 ohms to 0-4-8-16" and a box full of power transformers ripped out of HP audio oscillators headed to the metal scrapper. The power transformer and the cheap KT88's were both limiting factors. About 370 volts B+ and tubes that were known to be fragile.
I have seen two tubes break out into internal fireworks hard enough to shatter the glass, One was one of these KT88's in an otherwise working amp, the other was a Russian 6P19 that I miswired.
With that tiny OPT, I think I could blow that thing up by staring at it too hard. My Turbo Champs used Hammond 124 CSE's none of those amps ever blew up, and some ran Sovtek 6550's on 425 volts.
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