Metropoulos Forum

Quick question: I have a Peavey 5150 amp and cab. I have pulled two tubes but not sure what ohms to run it at with one cabinet? Thank you for help!

Did you try Google?

I Googled it and found that I should run it at 8ohms, but as someone that does not know much about amps and such, Metro Amp is the only place I trust when someone gives me advice!!!

Put the two tubes back in. There isn't much volume difference and the 5150 power section is (probably) designed to run clean anyway.

However, if you want to use just two power tubes, set the amp to half of the cab's impedance. i.e. - 16 ohm cab, set amp to 8 ohms

Toner thank you for your response. Just curious, why do you suggest to put the two tubes back in? The reason I ask is that I am not savvy to working on amps, but, when I take the two tubes out, it makes a huge difference in the sound. I have googled and asked questions and most people have said that although its not good for the amp, it probably won't hurt it. Is there a reason that the amp or tubes will be hurt by pulling two tubes out?

First of all, I'm not very familiar with 5150's so please take what I say here as general suggestions only and search for more info.

If your goal in removing two tubes is a reduction in volume, it won't make that much difference. If you want more power tube overdrive, it may help a little but that amp has a LOT of preamp overdrive and usually similar amps are designed to run the power stage clean and sound best that way (very different from vintage Marshalls, etc.).

More importantly, when you remove half of the power tubes, the reflected primary impedance of the output transformer (OT) on the remaining tubes has changed. Sorry, I don't recall the technical details right now but it is always safer to operate into the proper load.

Can an impedance mismatch hurt the tubes or amp?
Yes. The power tubes will likely be stressed and can generate more current through the OT than normal, which in turn may damage it over time.

I probably didn't fully answer your questions but hopefully this will help point you in the right direction. However, it is common knowledge to set the amp impedance to half of the speaker impedance when doing this.

Toner, thank you again for your responce. You didn't exactly answer my question but have pointed me in the right dirction so thank you! However, I am not pulling the tubes to lower the volueme, I pulled them because the sound difference is inccredible!!! I don't realy know how to explian it, but the amp just seems to come alive so to speak; its still very loud (!!!), but it sound much much better, which is why I would like to pull the tubes if it is safe. I do not diss-like the normal tone of the 5150, but without the two tubes, its like a 5150 on steriods so to speak; it just sounds better!!! Thank you again for your feedback!