8 ohm speaker on a 4 ohm output

[zentman]

I have a peavey VTX MX amp I like to use the power section on for my Axe FX. The internal speaker is 4ohm and the back says 4 ohm minimum load. The manual says you can hook 2 four ohm load up for a total of 2 ohms just fine and I do by adding a 4 ohm cab.


I want to change the internal speaker to an 8 ohm though. 4 ohms are just too hard to come by. Will it fry the tranny?

 

[squank]

I'm confused - the back of the amp says minimum 4 ohm load, but the manual says you can go down to 2 ohms?

I don't know of any modern guitar amp with a tube output section that recommends ever going down to 2 ohms. Be careful here. Better yet, you should check with Peavey. And do it via email, so you have it in writing and they will have to replace your output transformer if (when) it goes up in flames.

 

[glip22]

You can go lower not higher. 16 ohms into 8 or 4 would be fine. So 8 into a 4ohm impedance is fine. 4 into 8 would probably open your OT windings and short it.

 

[squank]

You can go lower not higher. 16 ohms into 8 or 4 would be fine. So 8 into a 4ohm impedance is fine. 4 into 8 would probably open your OT windings and short it.

16 into 4 is NOT fine if you are using distortion. It's the square wave that causes the dangerous flyback voltage, and it doesn't really matter which way your impedance mismatch goes. 4x is dangerous.

I think Randall Aiken did the definitive work on this subject. He was looking a the specific problems that attenuators can create when running amps full on and attenuated hard (which often causes significant impedance mismatch), but the conclusions are applicable here.

I was initially skeptical of a purely resistive load causing any problems at all, so I decided to do some bench tests myself.

Running a sine wave input with the output not clipped, there is no danger of either flyback voltages or screen overdissipation. The problems occur when the output stage is heavily overdriven into a square wave. But who would do that with a guitar amp, anyway???

Into a purely resistive load, the flyback voltages aren't bad at 2x mismatch, but at 4x mismatch they can get rather high, primarily dependent upon the leakage inductance of the transformer used. Inductive loads can really get out of hand.

Here are some pics of the plate of one tube with the output stage being driven really hard into a resistive load at different impedance mismatches. Note the voltage scale is not correct, as I had to divide it down to avoid killing my Tek scope:

http://www.aikenamps.com/resistive_load_1x_match.jpg
http://www.aikenamps.com/resistive_load_2x_mismatch.jpg
http://www.aikenamps.com/resistive_load_4x_mismatch.jpg

The real surprise was screen current. It increased dramatically at higher impedance mismatches, again, only to dangerous levels when driving the output stage into a hard clip, particularly when there was duty-cycle modulation of the waveform as you would get in a non-master Marshall cranked up. I don't remember the numbers I measured, but it was way over the max screen dissipation limits of the EL34 tubes. I suspect screen overdissipation is the cause for most of the reported tube failures and fuse-blowing incidents when using these devices.

If you don't drive your amp hard, you should never see any problem with a purely resistive load, even with a 2x mismatch. I'd still be leery of running a 4x or greater mismatch. There's no such thing as a "universal" or "one-size-fits-all" attenuator. It is best to match the impedances unless you like to live dangerously.

Randall Aiken

 

[zentman]

I'm confused - the back of the amp says minimum 4 ohm load, but the manual says you can go down to 2 ohms?

I don't know of any modern guitar amp with a tube output section that recommends ever going down to 2 ohms. Be careful here. Better yet, you should check with Peavey. And do it via email, so you have it in writing and they will have to replace your output transformer if (when) it goes up in flames.

http://www.peavey.com/assets/literature ... 343009.pdf

Second to last page.

 

[squank]

I'm confused - the back of the amp says minimum 4 ohm load, but the manual says you can go down to 2 ohms?

I don't know of any modern guitar amp with a tube output section that recommends ever going down to 2 ohms. Be careful here. Better yet, you should check with Peavey. And do it via email, so you have it in writing and they will have to replace your output transformer if (when) it goes up in flames.

http://www.peavey.com/assets/literature ... 343009.pdf

Second to last page.

Ok, the Peavey manual you posted clearly states that by plugging in a second 4 ohm speaker, it activates the output transformer's 2 ohm tap. You should be good to go using the stock 4 ohm internal speaker along with a second 4 ohm extension cab.

 

[zentman]

I'm confused - the back of the amp says minimum 4 ohm load, but the manual says you can go down to 2 ohms?

I don't know of any modern guitar amp with a tube output section that recommends ever going down to 2 ohms. Be careful here. Better yet, you should check with Peavey. And do it via email, so you have it in writing and they will have to replace your output transformer if (when) it goes up in flames.

http://www.peavey.com/assets/literature ... 343009.pdf

Second to last page.

Ok, the Peavey manual you posted clearly states that by plugging in a second 4 ohm speaker, it activates the output transformer's 2 ohm tap. You should be good to go using the stock 4 ohm internal speaker along with a second 4 ohm extension cab.

That I do but I wanna just put an 8 ohm speaker in it and bring no cab. Can I?

 

[squank]

I'm confused - the back of the amp says minimum 4 ohm load, but the manual says you can go down to 2 ohms?

I don't know of any modern guitar amp with a tube output section that recommends ever going down to 2 ohms. Be careful here. Better yet, you should check with Peavey. And do it via email, so you have it in writing and they will have to replace your output transformer if (when) it goes up in flames.

http://www.peavey.com/assets/literature ... 343009.pdf

Second to last page.

Ok, the Peavey manual you posted clearly states that by plugging in a second 4 ohm speaker, it activates the output transformer's 2 ohm tap. You should be good to go using the stock 4 ohm internal speaker along with a second 4 ohm extension cab.

That I do but I wanna just put an 8 ohm speaker in it and bring no cab. Can I?

Are you playing clean or distorted? Believe it or not, that is the MOST important question. If you're playing clean, there's virtually no worry about flyback voltage on a 2x mismatch.

However, you WILL put extra strain your output transformer. Will it be too much strain? That depends on how you play it. Volume hi and/or distorted = lots of strain. Lower volumes and cleaner sounds = less strain.

Impedance mismatch is always a gamble. There are many variables, and therefore, no set "yes it's safe" answers. Would I do it on my amp? No. But I play loudly and I use high gain. I try to never mismatch impedance because of that.