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Post subject: Yet another Impedance question...
Posted: Tue Feb 17, 2015 11:22 pm
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Hey guys

Time for another 'I don't understand impedance well enough to figure this out' type question.

The amp: 67 Super Reverb.
The issue: weird sympathetic vibration on certain notes, and in the lower bassy register.
The question: is it the tubes or is it the speakers?
The Trouble shoot: hook up the amp to an extension cab and see if the issue persists in a different speaker configuration (if it does, its the tubes, if not its one of the soldered in speakers)

The rub: I don't want to blow up my amp finding out, and this would be a lot faster than unsoldering each speaker (which may happen next)

So for the forum: Can I unplug the amps internal speakers, and hook up an 8 ohm extension cab for a 1 minute to test at bedroom levels, without risking harm to the transformer?

Thanks


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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 12:01 am
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" Can I unplug the amps internal speakers, and hook up an 8 ohm extension cab for a 1 minute to test at bedroom levels, without risking harm to the transformer? "

As you know, the impedance load is nominal 2-ohms. You can run an 8-ohm cab, for 1 minute, no problem. The OPT is heavy duty and can handle momentary mismatches.


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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 12:05 am
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Be sure to check the amp and cabinet itself for physical anomalies. A loose screw holding the baffle board to the enclosure, a speaker to the baffle board, or even a transformer to the chassis can mimic the symptom you've noted.

Arjay

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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 2:01 am
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BMW2002Ti wrote:
" Can I unplug the amps internal speakers, and hook up an 8 ohm extension cab for a 1 minute to test at bedroom levels, without risking harm to the transformer? "

As you know, the impedance load is nominal 2-ohms. You can run an 8-ohm cab, for 1 minute, no problem. The OPT is heavy duty and can handle momentary mismatches.


Would you stop it with the misinformation! It has zero to do with straining the transformer. With an 8 ohm load on a 2 ohm amp, there will be less power output, less current to the transformer, and less strain on the transformer than with a 2 ohm load.

However, using too high an impedance is what can and does harm a tube amp, and when you get above twice the impedance rating of the amp, bad things can happen. 8 ohms is four times the impedance of a 2 ohm amp.

I would not use an 8 ohm speaker load on my 2 ohm rated amp, for any length of time. Someone else's amp, sure, go for it. :mrgreen:

Using a 4 ohm speaker load with a 2 ohm rated amp will be fine for any length of time. Always keep an impedance mismatch, high or low, to within a 2 to 1 ratio and the amp will be fine.

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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 7:17 am
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Shimmy, running an 8-ohm speaker for 1-minute is not going to hurt the Super Reverb's output tranny. A tranny of this size that has an issue with that scenario has a lot more problems than simple impedance mismatches.

Explain to me how this mismatch setting damages the OPT. You talking saturation? In one minute?

How many times have you worked on an amp that had a burnt-out speaker? Maybe two, in a SR. I've only had to replace an output tranny in a couple of amps --- a Vibro-Champ and a Princeton (non-verb). And ppl had used the amp, at volume, for hours, day, months --- not ever realized one speaker is blown.


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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 1:20 pm
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That's exactly what I mean by misinformation, it's as if you don't understand at all about output transformers. You mention saturation, the maximum current that the OT will deliver, which is the exact opposite of the real issue, which is when approaching an open circuit (infinite ohms, worst case of too high an impedance), the lack of flyback can allow the voltage in an OT to build to the point that it arcs from winding to winding internally. This is why many amps use a shorting speaker jack, to protect the OT in case no speaker is plugged in. Note that initially, Fender and others didn't use shorting speaker jacks, which might have lead to blown OTs, so at some point the shorting jacks became standard. A shorted output on a tube amp will protect the OT from the open circuit, and can cause no harm due to too low an imedance (zero ohms)

For a four speaker amp such as the SR, one blown speaker out of four still leaves a load of 2.66 ohms, well within a 2 to 1 ratio for impedance mismatch for a 2 ohm rated amp. Two blown speakers out of four still leaves a 4 ohm load, again, within a 2 to 1 ratio for the impedance mismatch. Blow the single speaker in a VC or Princeton, as you mentioned, or use the amps with no speaker connected and no shorting speaker jack and voila, blown OT, due to too high an impedance (open circuit).

At some point approaching an open circuit, the impedance will be too high, blowing the OT very quickly. There is no real chart for when this will happen, or at what initial point the impedance is too high, thus the recommended safe mismatch ratio of 2 to 1 high or low will definitely cover it.

How long would you use a tube amp with no speaker connected and no shorting jack to protect the OT? Hopefully your answer would be "not at all, unless I am trying to blow the OT". :wink:

And back to the scenario of using the SR with a blown speaker or two without damaging the OT, that is just more confirmation for my contention about a safe impedance mismatch, although it still has zero to do with the OT robustness. As you stated, they use the amp with a blown speaker or two and never know it. With an impedance mismatch, high or low, the amp (tubes) simply do not develop the full rated output power, that is all.

If you have the chance, check out some of the curve traces found in some RCA receiving tubes manuals, and note those that show the output power from a tube across various load impedances. Note the the output power curve is a parabola, with the peak at an optimum or "matched" load impedance, and the power drops off if the impedance is higher or lower than optimum. Since the power output from tubes drops off with a "mismatched" impedance, there is less current through the OT, which means less "stress" on the OT when mismatched impedance, and the most "stress" on the OT when the impedance is matched. That is, up to a certain point, when the impedance can be so high that the OT will arc internally.

I don't know of any greater authority on tubes than the RCA Receiving Tubes manuals. 8)

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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 5:37 pm
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Ok. Got it. I understand that in tube amps you are less likely to cause damage if the impedance load is too high. And in solid-state, too low an impedance load.

I just think that the OP doesn't need to buy another 2-ohm impedance speaker(s) load, to test whether it is his OEM speakers that are causing problems or not. Not for a momentary test. I posted that 2-ohm is the nominal impedance of the amp, for regular use. Well, my 2¢ worth...


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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 6:31 pm
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BMW2002Ti wrote:
...in tube amps you are [more] likely to cause damage if the impedance load is too high. And in solid-state, too low an impedance load...


Fixed. :wink:

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Post subject: Re: Yet another Impedance question...
Posted: Wed Feb 18, 2015 6:57 pm
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Vous remercier. Beaucoup!


:D


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Post subject: Re: Yet another Impedance question...
Posted: Thu Feb 19, 2015 4:10 pm
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Hey guys

Thanks for all the info.

It was in fact the speaker. I pulled it out and the issue was gone. Replaced it and it returned.

Speaker was the same one that I had recently had reconed, so Its going back for another repair (hopefully not on my dime)

I know move on to the next issue Im having with the amp...In my next post


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