Easy Questions for tube amp dummies

[ColdSweat]

Ok, I have a Crate Blue Voodoo 120 and two 4 x 12 cabs. One cab is a BV cab and can be set at 16 ohms or 4 ohms. It also shows that it can run 8 ohms if used in stereo. My other cab is a 16 ohm Peavey XXL cab that I use for practice etc. The amp has a selector switch that lets me choose 16 ohms or 8 ohms. I would rather run 8 ohms but am not sure how to get an 8 ohm load from my cabs.

I've been searching online and can't find anything about it. The amp has two speaker outs, one labeled main(use first) and the other labeled ext. speaker. Can I run a cable from each of those outs to the stereo inputs of my cab to create an 8 ohm load?

I've been running everything on 16 ohms just to be safe, and would really like to try an 8 ohm load. I really don't want to use both cabs at the same tie, but might like to have that option as well someday.
Any ideas?

[tboy]

I would rather run 8 ohms

Why?

[Gtr_tech]

There is a school of thought where if you run the amp at 16z, you use the entire secondary winding of the output transformer, leading to better efficiency and tone. I'm sure the difference is subtle. It really doesn't matter with tube amps *what* load you use, so long as its matched to the amp's impedance setting.

Solid state amps are a different story.....

[Ronsonic]

Set the cabs at 16. Plug them both in. Set the amp for 8. Done.

Use the BV cab, set it on either 4 or 16, set the amp to match. You may feel a bit of a difference between the two settings.

Running a cable from both outs into the two 8 ohm inputs gets you 4 ohms. Set the amp for that.

There is nothing gooder about 8 ohms compared to the alternatives.

[kg]

i use secondary impedance taps as tone controls.

fwiw
ken

[Gtr_tech]

Not a wise idea IMO. You go 2 steps too high (4z amp to 16z load ) on the load end and yer askin' for arc'd tube sockets and/or output transformer. At your own risk....and I'd surely not advise this as normal operation....

And if you think it doesn't happen, tell it to the Twin I have in right now with the damaged output transformer due to series wired 8z speakers.....

[kg]

Not a wise idea IMO. You go 2 steps too high (4z amp to 16z load ) on the load end and yer askin' for arc'd tube sockets and/or output transformer. At your own risk....and I'd surely not advise this as normal operation....

And if you think it doesn't happen, tell it to the Twin I have in right now with the damaged output transformer due to series wired 8z speakers.....

to each their own.

perhaps fender opts are crap compared to the hammonds i always use.

incidentally, how do you know the damage was due to the series 8r speakers?

[pdf64]

Yes, the guy holding the smoking gun isn't always the murderer, though Occum's razor would indicate that a strong burden of proof is required to assign the blame elsewhere!
It's a fair point though; check all the speaker circuit, connections and wiring etc, for potential or intermittant open circuit. Pete.

[Gtr_tech]

to each their own.

perhaps fender opts are crap compared to the hammonds i always use.

incidentally, how do you know the damage was due to the series 8r speakers?

The Schumacher transformers used by Fender are known for their durability....so its not the transformers fault. It was simply operator error by running it too hard into a mismatched load. Primaries don't just go bad for no reason......something has to cause it unless its a mechanical short to the core.

[kg]

The Schumacher transformers used by Fender are known for their durability....so its not the transformers fault. It was simply operator error by running it too hard into a mismatched load. Primaries don't just go bad for no reason......something has to cause it unless its a mechanical short to the core.

something, you can't rule out of course. nor can you rule out a nick in the enamel on the winding, etc.

the reason i am dubious about the "too high primary z caused my opt death" is because tube amps survive NO LOAD situations all the time. obviously infinity is a great deal higher impedance than 16r.

[tubeswell]

... Occum's razor would indicate that a strong burden of proof is required to assign the blame elsewhere!

I've nicked my chin numerous times with that damn Occum's razor! Last time the bleeding went on for 1/2 an hour!, And I was in a hurry to get to work too! Why the hell can't he make better razors?!

[kg]

let's assume gtr_tech is correct and a 4x mismatch caused a primary winding fault.

questions:
1) were the tubes damaged in any way, g2 shorts in particular?
2) were the g2 stoppers overdissipated (as one would assume if there was a g2 short)
3) what KIND of damage to the opt was observed? open primary? shorted turn? shorted to core?
4) is that the FIRST one you've ever seen? if not, what did the other ones look like?

steven conner's experiments of a few years ago regarding OPT protection methods proved that even a completely open secondary did not generate the enormous flyback voltages at the primary that most (including mysef) believed would happen. my theory on why is that the non-infinite plate resistance of the conducting phase output tube greatly damped the spike.

can a few extra volts cause an insulation punch through? yes... but i believe it will only happen at a weak point. perhaps it had arced at that spot once before, after being abused in some other way.

all i can tell you is that in ALL my 20+ years of faffing about with tube guitar amps, using, abusing, and repairing them quite regularly, i have never destroyed an OPT by a secondary Z mismatch.

[Gtr_tech]

I assume you meant secondary....

[kg]

I assume you meant secondary....

now i'm confused.

was the bad fender opt you had in for repair a primary or secondary fault?

where do you think i meant secondary?

[Gtr_tech]

The failure mode is a damaged primary *caused by* mismatched load on the secondary. Very common when you run the amp hard into a larger load than it is set/intended for. The reflected Z on the primary becomes incorrect and mismatched as well. The energy/spikes created by the output tubes needs to go somewhere, and if it can't be absorbed by the intended load, it *will* go somewhere. So either it arcs a tube socket or it shorts adjacent turns on the primary out the output transformer. In extreme cases the primary burns open....

<<<"too high primary z caused my opt death" is what you said.