6505 Plus impedance question

[WingerManiac]

Hi folks, I got a question about the impedance on my 6505 Plus.

My setup is like this: 6505 Plus head running into an EVH 4x12 (16 Ohms) and an EVH 2x12 (16 Ohms) cabinet.
The amp is set at 16 Ohms impedance and sounds great through the two cabinets.

Now to the question: I read somewhere that I should adjust the impedance of the amp to 8 Ohms when I´m using two 16 Ohms cabinets.
I tried that and to my ears it sounded much worse. So is there any harm by keeping the amp at 16 Ohms all the time? Thanks a lot!

[Jacew]

Re: 6505 Plus impedance question

Using higher impedance may burn your OT, but many sources, including this very informative page about tube amps claims using too low impedance should not be that big issue... I wouldn't recommend it, as the amp is designed to be run with 16 ohm load at 16 ohm set-up.

However, the worst I thing I think could happen here is that you burn your tubes out faster...

[WingerManiac]

Re: 6505 Plus impedance question

Thanks for the info. I´ve been using this setup for years now and never had any issue whatsoever...

[Empty Pockets]

Re: 6505 Plus impedance question

When your amp wants a 16ohm load, and you're plugging it into an 8ohm load (like your current rig) the cabs are pushing back (resisting) more of the energy than your amp wants. Eventually your transformer could explode and those things cost about as much as an entire 6505+.

If it sounds good enough to risk your amp going up in flames, rock on man.

[Jacew]

Re: 6505 Plus impedance question

When your amp wants a 16ohm load, and you're plugging it into an 8ohm load (like your current rig) the cabs are pushing back (resisting) more of the energy than your amp wants.

What does that even mean? If you plug higher impedance speaker to amp that wants lower load, too high impedance of speaker circuit will build up current into transformer and destroy it. In opposite way (like in OP's situation) that's not happening.

SS amps will burn, if running lower impedance speakers, because transistors will run above their rated levels. Tubes won't push current beyond their rated levels even if you short circuit the speaker. So at worst you'll just lose the tube(s).

With SS amps you can always go higher impedance, while with Tube amps lower impedance is safer. It's pretty clearly explained in the link I posted above.

[Empty Pockets]

Re: 6505 Plus impedance question

I've always heard it was the opposite, from people who've repaired tube amps for decades. You want the number on the head to be the same or lower than the number on the cab. An 8ohm speaker jack into a 16ohm load is okay, but a 16ohm jack into an 8ohm load is potential fireworks.

FWIW I've run my 69 Bassman which is fixed at 4ohms into various 8 and 16ohm cabs for about 15 years without any transformers exploding.

[Jacew]

Re: 6505 Plus impedance question

I've always heard it was the opposite, from people who've repaired tube amps for decades. You want the number on the head to be the same or lower than the number on the cab. An 8ohm speaker jack into a 16ohm load is okay, but a 16ohm jack into an 8ohm load is potential fireworks.

FWIW I've run my 69 Bassman which is fixed at 4ohms into various 8 and 16ohm cabs for about 15 years without any transformers exploding.

I think you've got that all mixed up with (SS/tube guitar amps/HI-FI gear). Higher impedance speaker will resist the current more than lower impedance speaker, which is opposite what you said in earlier post. I'm not an engineer, but all i've read or heard about the topic, only a handful of random people on internet forums have claimed higher impedance to be better for tube amps, while every reliable source states the opposite.

But I'm not going to argue this further, in any case it's better to just use matched impedance.

[WingerManiac]

Re: 6505 Plus impedance question

I think you've got that all mixed up with (SS/tube guitar amps/HI-FI gear). Higher impedance speaker will resist the current more than lower impedance speaker, which is opposite what you said in earlier post. I'm not an engineer, but all i've read or heard about the topic, only a handful of random people on internet forums have claimed higher impedance to be better for tube amps, while every reliable source states the opposite.

But I'm not going to argue this further, in any case it's better to just use matched impedance.

That´s what I decided to do. I adjusted the impedance to 8 Ohms and tweaked the EQ of the amp to make up for the tonal difference. I think I never ran into issues with my setup because I was using the amp at quite moderate volumes only. Anyway, thanks for your help guys!

[zzmoore]

Re: 6505 Plus impedance question

"I tried that and to my ears it sounded much worse."
That certainly is possible, but normally....... Setting your amp to the "proper" Impedance "should not" make the amp sound bad.
Are you sure everything is connected the right way.?
Is it possible you are listening with your eyes and the numbers on a knob.?
Then again, you very well may prefer the dynamics of the "mismatch".
good luck