Which way should I wire these speakers?

[slidincharlie (Carlo P)]

I'm trying to find out the right way to load this amp.
It's a Peavey Classic VT Series 100 50W combo with four 16 ohm speakers, that I could wire in parallel (4 ohm total) or series-parallel (16 ohm total) configuration.
The amp has got a Main (8 ohm) and an Aux (4 ohm) speaker jack.
HOW do I have to wire the speakers and WHERE do I have to plug them?
It came with the speakers wired in parallel (that is 4 ohm) plugged into the aux jack, leaving the main (8 ohm) jack unloaded.
Is this wiring correct?
The amp schematic is attached here.
TIA

[wizard333]

The stock wiring is correct. You want the 4 16s in parallel for 4 ohms and plug them into the 4 ohm output.

You COULD wire them in two series sets of two speakers in parallel for 16 ohms, and plug into your 4 or 8 ohm output. It wont HURT anything, but it will change the cutoff frequency for bass response and reduce your bass response in the 8 ohm output or even more in the 4 ohm output.

If you wire them to 4 ohms, DONT plug them into the 8 ohm output. That can damage things.

Since you can wire them to 4 ohms and you have a 4 ohm output on the amp, I'd do that.

[slidincharlie (Carlo P)]

Won't it hurt the OT to plug the speakers in the AUX 4 ohm jack leaving the MAIN 8 ohm jack unloaded? I thought that you should never leave the main output jack empty in a tube amp...

[wizard333]

No if its wired the way you posted the un-used tap will be unused. As long as something is plugged in to one of the transformer taps you are ok, as long as you dont underload the tap. If you arent sure you understand that, you should take it to a qualified tech to do the wiring.

[Enzo]

I thought that you should never leave the main output jack empty in a tube amp...

You should never leave a tube amp without a load. That is not the same thing as which jack to use. In a lot of ands like Fender, there is a grounding contact on the main speaker jack, so if nothing is plugged into it, the output is shorted. That is prefrable to no load at all, but if you plug into the AUX SPKR instead, the MAIN jack will still be shorting across the output. SO in those cases, yes, you have to use the main jack. Not because it is a tube amp, but because the output is shorted.

In the old original Classic - NOT the Classic VT - the main output does indeed have a shorting contact. But your VT is more modern. The main out jack is just wired to the AUX SPKR jack. The AUX jack merely switches between impedance taps. This is the same arrangement as the Classic 30 today. The internal speaker on teh C30 is 16 ohms. Plug anything into the extrenal jack and it switches the output over to 8 ohms. This is intended so th einternal speaker will be in parallel with teh external jack for a 16 ohm there making an 8 ohm total. But nothing wrong with disconnecting the internal wiring and plugging an 8 ohm sopeaker into the external jack.

Look at the schematci or at least the wiring in the amp. The Aux jack selects 8 or 4 ohm output from the transformer, the main jack is not grounded.

Plug the 4 ohm load into the 4 ohm jack.

[slidincharlie (Carlo P)]

Thank you wizard,
I know how to do the wiring.
Cheers.

[Amp Kat]

If you wire them to 4 ohms, DONT plug them into the 8 ohm output. That can damage things.

.

Actually it won't harm the amp to do that it would just be mismatched and may not sound as good but no harm done. What kills trannys are to high of an impedance. bad cables causing opens when the amp is cranked or nothing plugged in at all. Shorted tubes and bad filter caps that should have been changed eons ago. They wouldn't put a switch in the back for 4,8,16 ohm selection if it would cause damage under any 4,8,16 load situation.

[wizard333]

Plugging a 4 ohm speaker load into an 8 ohm tap could indeed damage the amp, don't try it.

Plugging a HIGHER impedance speaker load into the tap wont hurt it, it will just change the cutoff point for bass frequencies and may sound a bit thin. UNDERLOADING can DEFINITELY cause damage, dont do it.

[Ray Ivers]

KB,

Actually it won't harm the amp to do that it would just be mismatched and may not sound as good but no harm done. What kills trannys are to high of an impedance. bad cables causing opens when the amp is cranked or nothing plugged in at all.

Each type of mismatch has its own set of failure modes, but I agree that the 16-ohm-load/8-ohm-tap mismatch has a much greater chance of incurring immediate amp damage.

The OT-primary overvoltage situation that exists when using a 16 ohm load on an 8 ohm (or even worse, 4 ohm) tap is far more likely to quickly damage the OT and/or tubes via arcing or insulation punch-through than the 4 ohm load/8 ohm tap connection.

OTOH, the 'hot-bias' folks may also experience tube-overheating-related failures with this (4 ohm load/8 ohm tap) hookup, since the output tubes will try to draw more current around waveform peaks while returning to the same smoking-hot bias point each cycle. This may or may not eventually cause tube failure, but it will surely reduce tube life - and if a tube then shorts a screen grid and smokes a screen resistor, the tube will go open-circuit resulting in even more massive overvoltage spikes than the first type of mismatch - probably on a nice toasty OT, too.

Ray

[Amp Kat]

Ok Ray I can buy the tube wearing part but were talking about RCA blackplates here bud ! The output transformer can see a variety of impedances whether it's lower or higher from 0 to 50 ohms during a typical operation and the math part is much more complex than I want to get in this discussion but the little bit of difference the OT sees from 4,8,16 is so little that it makes hill of beans as far as damage which mismatch you use. As I stated before no harm done whatsoever. What OT's don't like is opens not shorts. You can almost run a dead short on a tranny and it won't harm it at all however negative ohms is not good under any situation but your amp can sound louder with different settings. Menno Van Der Veen states in his book that the average impedance transformers see is 5.3 ohms which is the reference point he uses in his Toroidal OT's from Plitron.

[wizard333]

Where is your information coming from? I must say that this is the first time in my life I've ever heard anyone recommend under loading a transformer, eg. plugging a 4 ohm speaker load into a 16 ohm tap. I wouldnt do it and I've never heard anyone say they would, so I'm awfully curious why you would?

[Amp Kat]

I'm not saying that it will sound good I'm saying that the difference in 4,8,16 ohm taps are so small that it won't harm it. The only thing it will do is effect the dynamics and possibly tone but as far as causing damage it won't. I've done the math and have had discussions with Kevin O'connor on this website many moons ago about this very topic. It may still be in the archives if you want to look it up. But from testing many amps in my shop in for repair I really don't need the math or anyone validity because I've tested it to many times.

[wizard333]

Not wanting to be argumentative, but since that goes against all conventional wisdom and any amp manufacturer's or transformer manufacturers recommendation, what conditions have you tested it under? A short test run at moderate operating conditions in a shop might very well not produce any damage, but an amp run hard regularly during practices and stage work is another matter entirely.

I may be wrong, but I'd be curious if you can point me to any manufacturer of guitar tube amps or transformers that would recommend continued real world use of a 4 ohm load on a 16 ohm tap.

[Amp Kat]

I love arguments Wizard, can't you tell ? Your missing the point though. I'm not saying you should hook your 4 ohm cab up to your 16 ohm tap on your OT. I'm saying that if you do it,it won't hurt the OT. Do you think an Amp Co. would put a 4,8,16 ohm tap right there where Mr. enduser could set it wrong and blow his amp up ? Don't think so and not only that but when the amp is cranking for what it's worth do you think the OT see's 4,8,16 or whatever ? Not really and all of the parameters change and the math involved is quite complex tstl as many things change impedance wise and magnetically. You should get Menno Van Der Veens book Transformers and tubes as it explains all of this quite well. I see club players all of the time using 4 ohm or 5.3 ohms 4x12 cab (8-ohm speakers wired series parallel ) and they don't have a clue where that switch is set and jam night after night ,week after week with no problems at all. Not supposed to but they do it not knowing with no harm done.

[Regis]

Hi Carlo,

This page should help you regarding cab wiring and impedances.

Regards,

Regis


PS: I don't want to pour gasoline on an argument, but I've been running a pair of SF Bassman tops into an 8 ohm load on and off for over twenty years with no ill effects, and I'm talking diming the amps here.