G12-65H: 16 Ohm or 8 Ohm, any difference?

[Fischerman]

Thanks for posting that heisthl...as I was reading that last portion I was thinking; "Damn this sounds vaguely familiar."...then I noticed kg wrote it and I used to always try to listen hard when kg was talking.

For those of you with triode output tubes, or a triode switch, you're better off running into a HIGHER load impedance.

I did not know that it was just the opposite for triodes.

A Super Reverb wants a 2-ohm load; running it at 8 ohms pretty well guarantees you'll be making a substantial contribution to the Lord Valve Home for Lord Valve.

Hey...I contribute to that charity too.

[thyx]

Pay attention to the current. Current is what kills. Same power (roughly), or energy, with a lower impedence equals higher current...times the impedence of the OT gives you power which is dissipated in the form of heat.

P=VI=(RI)I

Therefor, too small a load and the OT will fry.

Too high an impedence and you risk arcing within the OT due to flyback voltages from the power tubes.

Bottom line...there are no hard and fast rules. It depends more on how well the OT is constructed than anything else. With an old BF or SF Fender OT from the 60's or 70's, you could go up or down a step and not worry about it. Try mixing it up with an old Marshall and your OT will poop out real fast.

Either situation can be ultimately unhealthy for the tubes...but that greatly depends on the tubes being used. EVH used a circuit to get the "brown sound" in later years that was absolutely murderous on tubes, but because he used an extremely robust tube he got away with it.

[scotto]

FWIW the owners manual for my 100 watt Seymour Duncan Convertible (EL34 based from the 80s) suggests that you experiment with mismatches and go with what sounds best. Says you can't hurt it so long as there is a speaker of some sort connected. I've been using this amp for many years with various matched and mismatched speaker loads. No problems. This was a high end amp back then so the iron is likely quality stuff.

[heisthl]

Therefor, too small a load and the OT will fry.

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

[thyx]

Therefor, too small a load and the OT will fry.

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Your thinking that I meant a smaller value of impedence for speakers is correct...and, yes, greater load as far as the OT is concerned.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

I think that that's a requirement, based on how the OT is wired, in order to give you the alternate output impedence at the non-grounded jack(s). But be warned...you'll still fry the OT if you play it without a speaker connected for very long. An open circuit is actually a better protector of the OT when using tubes...closed circuit for solid-state.

[scotto]

Back to the original question, A properly set up Overtone with an 8 ohm 12-65 speaker will sound fine. Mine does, but it didn't until I sorted out the numerous amp issues including specific components/values, tubes, trimmer settings and lead dress that were all gleaned from posts in this forum. I would focus on that and just go with the 8 ohm speaker. Much more to gain by getting that other stuff right vs. 8 or 16 ohm speaker. Speaker will take time to fully break in. Just my .02 cents.

[heisthl]

I think that that's a requirement, based on how the OT is wired, in order to give you the alternate output impedence at the non-grounded jack(s). But be warned...you'll still fry the OT if you play it without a speaker connected for very long. An open circuit is actually a better protector of the OT when using tubes...closed circuit for solid-state.

Careful - you want me to hand you a shovel so you can keep digging or do you want to retract that statement? If you put a short (closed circuit) on a solidstate output section you have a really nice space heater until it either erupts in a cloud of smoke or engages some form of protection circuit. As to Fender needing a ground for the other speakers impedance you'd better relook at an AB763 schematic - there is no purpose for shorting the jack other than OT protection, in fact the other jack won't even work by itself. I agree, even on a Fender wired this way you don't want to leave it with the speaker disconnected for a long period of time.

[thyx]

I think that that's a requirement, based on how the OT is wired, in order to give you the alternate output impedence at the non-grounded jack(s). But be warned...you'll still fry the OT if you play it without a speaker connected for very long. An open circuit is actually a better protector of the OT when using tubes...closed circuit for solid-state.

Careful - you want me to hand you a shovel so you can keep digging or do you want to retract that statement? If you put a short (closed circuit) on a solidstate output section you have a really nice space heater until it either erupts in a cloud of smoke or engages some form of protection circuit. As to Fender needing a ground for the other speakers impedance you'd better relook at an AB763 schematic - there is no purpose for shorting the jack other than OT protection, in fact the other jack won't even work by itself. I agree, even on a Fender wired this way you don't want to leave it with no speaker disconnected for a long period of time.

LOL...nah...you can keep your shovel. It really depends on the OT in question. For some Fenders (like the BF and early SF), you're right about the other jack. But for other later Fenders (and my old Boogie, for example), you'd be wrong. Check out the wiring for most Hammond OTs and you'll see even more examples of what I'm referring to. You really have to look at the circuit to be certain which, if you're not familiar with the amp. As for soild-state, I was thinking about a transformerless output, direct couple you find in a lot of stereos...not guitar amps. Solid-state guitar amps should be sacrelige around these parts!

Well...maybe except a Jazz-Chorus.

[thyx]

Back to the original question, A properly set up Overtone with an 8 ohm 12-65 speaker will sound fine. Mine does, but it didn't until I sorted out the numerous amp issues including specific components/values, tubes, trimmer settings and lead dress that were all gleaned from posts in this forum. I would focus on that and just go with the 8 ohm speaker. Much more to gain by getting that other stuff right vs. 8 or 16 ohm speaker. Speaker will take time to fully break in. Just my .02 cents.

I think it depends what you want. You're going to be cleaner, all other things being equal, with an 8-ohm speaker. Current is like a drill sgt making you stay in line. Lower impedence and same power equals more current to the speaker. The biggest difference will be at higher and lower frequencies. Not too much difference in the mids.

[heisthl]

As for soild-state, I was thinking about a transformerless output, direct couple you find in a lot of stereos...not guitar amps.

So all those output transistors my shop used to replace in DC amps made by Crown, QSC, BGW and even Peavey power amps that were caused by shorted speaker cables were just a coincidence?

[thyx]

As for soild-state, I was thinking about a transformerless output, direct couple you find in a lot of stereos...not guitar amps.

So all those output transistors my shop used to replace in DC amps made by Crown, QSC, BGW and even Peavey power amps that were caused by shorted speaker cables were just a coincidence?

Study the circuitry, then you tell me. What would happen voltage/current/power-wise in the event of an open circuit in those amps? What would happen voltage/current/power-wise with a short?

Personally, I wouldn't recommend either scenario...but solid-state is much more tolerant of a short than tube. But if you run the amp flat-out it's not going to matter much which scenario you've got (short or open) as you're going to fry the amp. You try running a tube-amp with a shorted output at any volume and it'll just be a matter of minutes before you smell the smoke.

[heisthl]

Study the circuitry, then you tell me. What would happen voltage/current/power-wise in the event of an open circuit in those amps? What would happen voltage/current/power-wise with a short?

Ok I'll bite - here's a picture of a DC amp I got off the web - with no speaker connected it will run all day and not even get hot - Short the output and you'd better put liquid nitrogen on the heatsink.

[Max]

In any case, I've heard it too many times from folks that really know their stuff (the likes of Aiken...but I can't say it was specifically Aiken) that the one to avoid is higher load on a lower tap. That much I'm almost certain about.

Hi Fisherman,

if you are that certain about this, how do you explain, that HAD (Or do you think he isn't someone "that really knows his stuff"?) delivered most (if not all) of his "silverface" ODS Combos (50W and 100W) with 4 Ohms OT (Bassman, Dual Showman) and 8 Ohms speaker (Altec, EVM).

I have never heard of someone using these early amps without a switchable (8 Ohms/4Ohms) version of OT to complain about blowing transformers or blowing tubes because of a speaker mismatch.

Obviously the opinion of Mr. Dumble in this matter was: If you prefer to use it as a Combo with just the built in 8 Ohms speaker (and this is the reason why there are combo-amps), feel fine and play some good tunes; if you prefer to use it with an extension cab (to get a bit more loudness or a different sound or whatsoever), feel fine too.

Why do you think HAD did wrong in using 4 Ohms OT in combination with 8 Ohms speaker? And what is from your point of view wrong with the arguments of the Electrovoice tech I asked about this (see my last post in this thread)? Could you (or someone else) explain this to me?

Cheers

Max

[drz400]

Therefor, too small a load and the OT will fry.

I'm thinking when you say "too small a load" you mean a lower impedance value of the speaker (i.e. 2 ohm) but the reality is low ohms is a greater load.

Fender amps like the AB763 or even the modern amps like the Hot Rod Deluxe, actually ground the speaker jacks (the greatest load of all) when no speaker is connected just to protect the output transformer.

This will protect it since the amp will blow a fuse first, Flyback spikes of no load is what kills a transformer. Quickest way to blow a transformer is with no load, not to be confused with a higher load impedance. At least that is my experience. I have replaced many OT for old Marshalls and the most common cause was , there was no load on the amp.

[butwhatif]

My advice, make sure you like g12-65s, while they might be able to tame an otherwise trebly amp, they are quite midrangey and dark to my ears in a dclone. Audition speakers first if u r able--try as many as u can. The cabinet can also have a considerable effect on the sound.