Speakers and amps and ohms and magic

[sizzlingbadger]

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

Yes

[GrantB]

I measured the G12M using my amm (for what it's worth) and that tests about 12.5ish. The G12M is rated at 15 ohms (supposed to have special sauce tuning to suit the 1974x...probably just the magnet sticker was changed so it looks more authentic to the OG). The scale graduations are pretty compressed so it's my eye dividing up the space between the grads. Tested the G12H and the needle initially measures about 7 before stabilising just over 9.5.

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

Briefly to test., yes…long term, can’t say. Likely not. I get volume drop when running those mismatched situations.

That 9.5 is weird. It will go up with current…so be more like 11 or 12 even.

[RectifiedAmps]

9.5 ohms resistance is not an issue for an 8 ohm impedance speaker. But I don’t think you should run a mismatch, if that’s what you’re suggesting. Might not damage anything but it won’t sound as good as proper matching.

[RectifiedAmps]

Also, if you measured resistance on the speaker cable-end/plug, rather than the terminals on the speakers itself, it could give a slightly higher reading.

[clubhouse]

Also, if you measured resistance on the speaker cable-end/plug, rather than the terminals on the speakers itself, it could give a slightly higher reading.

Well, sir...were you looking over my shoulder or reading my mind, freaky! You were dead right. I measured across the plug. I measured again, across the speaker terminals, and the needle swung to 2 they slowly climbed and stabilised at dead on 8 ohms, and given the age of the meter, I'll take that

[clubhouse]

Hey cats...you beautiful geeks that make this such a wonderful forum...thanks for all the help and advise. Here's the report.

Flipped the selector to 16 ohms load and the thing roars good enough to be as loud as the original speaker.

I pulled the chassis and had a look for anything obvious. Nothing. Cleaned the selector with DeOxit and switched it a few times for contact cleaning purposes. Tested: Boom...came back to life on 8 ohm output load setting as loud as originally.

Conclusion: oxidised contacts from never being used for a decade +.

Full fanging commenced...much happiness. Thanks to you all, again. I learned a lot

[Dharmajester]

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

I was under the impression that running an 8 ohm out into a 16 ohm load is ok and on the occasions I've done it, sounds fine but quieter than a matched output. Running from 16 into 8 though could potentially be problematic.

The others that have commented above though have much greater technical understanding than me.

[clubhouse]

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

I was under the impression that running an 8 ohm out into a 16 ohm load is ok and on the occasions I've done it, sounds fine but quieter than a matched output. Running from 16 into 8 though could potentially be problematic.

The others that have commented above though have much greater technical understanding than me.

Same here as with you and GrantB. I ran it at 16 in to 8 for testing for about 10 minutes including warm-up as per SB's advise. All was well...no magic smoke released. I didn't run it long enough to note any tone differences...wasn't focused on that. May have caused some work hours/minutes to be lost from the total tranny life but nothing immediately catastrophic.

I don't know where I heard it from but I've always operated under the belief that a valve amp output transformer needs to 'see' a matched or above load to prevent it dumping out excessive current? that causes heat to build up in the tranny faster than can be dissipated and the winding insulation melts leading to open circuit poop. It was an engineering thing rather than a tone thing.

Is this right, or valve amp myth?

[calling card]

Oh there's plenty of threads on this here, seems Fender are quite tolerant. My goldfish like memory forgets exact details of mismatching though.

[RectifiedAmps]

There’s loads of reasoning for both perspectives but it seems the ‘speaker-load >= OT setting’ camp predominates because it intuitively makes sense to most of us. However, there are also strong electrical engineering reasons why a speaker load lower than the transformer setting can be safer. It mainly has to do with load-lines (relationship b/w anode voltage & anode current for specified bias points): higher speaker loads stress the screens, lower loads stress the anode. Screens are weaker and more prone to failure so higher speaker loads are more likely to cause valve shorts.

[Dharmajester]

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

I was under the impression that running an 8 ohm out into a 16 ohm load is ok and on the occasions I've done it, sounds fine but quieter than a matched output. Running from 16 into 8 though could potentially be problematic.

The others that have commented above though have much greater technical understanding than me.

Same here as with you and GrantB. I ran it at 16 in to 8 for testing for about 10 minutes including warm-up as per SB's advise. All was well...no magic smoke released. I didn't run it long enough to note any tone differences...wasn't focused on that. May have caused some work hours/minutes to be lost from the total tranny life but nothing immediately catastrophic.

I don't know where I heard it from but I've always operated under the belief that a valve amp output transformer needs to 'see' a matched or above load to prevent it dumping out excessive current? that causes heat to build up in the tranny faster than can be dissipated and the winding insulation melts leading to open circuit poop. It was an engineering thing rather than a tone thing.

Is this right, or valve amp myth?

That's my understanding too. I've run a jtm45 at 8 ohms into a 16 ohm cab in the past. Sounded great, slightly less volume and a bit more compressed.
I have an Ironman attenuator now though which has a built in Z matcher so I can run any impedance output into any load.

[clubhouse]

There’s loads of reasoning for both perspectives but it seems the ‘speaker-load >= OT setting’ camp predominates because it intuitively makes sense to most of us. However, there are also strong electrical engineering reasons why a speaker load lower than the transformer setting can be safer. It mainly has to do with load-lines (relationship b/w anode voltage & anode current for specified bias points): higher speaker loads stress the screens, lower loads stress the anode. Screens are weaker and more prone to failure so higher speaker loads are more likely to cause valve shorts.

Wow...thanks for taking the time to help me make sense of the issues in play. Is there any commonly held, rule of thumb, tolerance range of speaker load to OT setting that can be assumed? Can I safely plug in the 2 ohm speaker array in a Super Reverb into the 4 ohm speaker output of a Twin Reverb? Can I safely go the other way and plug an 8 ohm cab into a SR or is that too large a jump leading to a valve screen failure?

Probably not as simple as that though, eh

PS: with the meter reading about 1.5 ohms measured across the speaker plug compared to across the speaker terminals...is that food for thought in amp design/electrical engineering or a well accepted loading issue when pairing trannies to valves and speakers?

[RectifiedAmps]

The general rule of thumb that a single-step mismatch is usually ok seems to hold up. So 2 ohm into 4, 4 and 8, 8 and 16 shouldn’t hurt anything in the short-term, whereas a 2ohm load for a 8ohm OT (or vice versa) is ‘bad’. IMHO, the best reasons for NOT mismatching are that the tonal differences aren’t good or at least aren’t beneficial enough to be worth the risk.

But to be honest, I’ve seen lots of fried valves and OTs over the years but I don’t recall ever being able to attribute a failure to a load mismatch (or a lack of a load all together). Certainly it could happen, but I bet it’s very rare.

[RectifiedAmps]

PS: with the meter reading about 1.5 ohms measured across the speaker plug compared to across the speaker terminals...is that food for thought in amp design/electrical engineering or a well accepted loading issue when pairing trannies to valves and speakers?

The resistance of speaker cable is generally the reason why you want a beefy wire gauge and should avoid guitar signal cable. Speakers are relatively low-voltage, high-current so any resistance can result in significant dissipation. However, most standard multimeters (even Fluke) are not very accurate at <2 ohms or thereabouts so values like youre getting are basically not significantly different to zero. Remember that the wire leads of your meter have their own inherent resistance, plus the tips of the probes and the object you’re measuring can have surface dirt, oils or oxidation, which adds more resistance, etc. This stuff is all academic though- interesting perhaps but not worth worrying about. Anyone who tells you they can hear the difference between speaker cables for a guitar amp is probably in business selling speaker cables!

[clubhouse]

The general rule of thumb that a single-step mismatch is usually ok seems to hold up. So 2 ohm into 4, 4 and 8, 8 and 16 shouldn’t hurt anything in the short-term, whereas a 2ohm load for a 8ohm OT (or vice versa) is ‘bad’. IMHO, the best reasons for NOT mismatching are that the tonal differences aren’t good or at least aren’t beneficial enough to be worth the risk.

But to be honest, I’ve seen lots of fried valves and OTs over the years but I don’t recall ever being able to attribute a failure to a load mismatch (or a lack of a load all together). Certainly it could happen, but I bet it’s very rare.

Thanks again, man. I'll make this my new guiding principal. Yeah...there has been the odd time when I've accidentally flipped the switch on a valve amp that wasn't plugged in to a speaker load and had it running for a minute or three before I figured out my goof. After all was made proper there didn't seem to be any complaints from the amp...bad smells or smoke...I just thought I was dead lucky to get away with it.

Along with the 'valve amp must see an equivalent speaker load' belief is the 'SS amps don't mind no load at all (except for hearing the thing) and are electrically stable? seeing no speaker load' idea...BS or is them some element of electrical engineering evidence to support it?