Impedance question....

[Chad]

I just got an Avatar Hellatone 60 (Celestion Vintage 30) speaker to replace the Celestion G12T-75 speaker that was in my Randall RM50B combo. For those who may not know, this amp is one of Randall's "modular" tube amps that has swappable preamps.

 

Anyway, both speakers are rated to be 16 ohms. But the G12T-75 meters out at about 13.8 and the new Hellatone meters out around 12.8.

 

My amp has a 3-way impedance switch that lets you pick a 4 ohm, 8 ohm, or 16 ohm setting.

 

I've been using the 16 ohm setting, but since I'm actually running a load lower than that, should I switch to the 8 ohm setting?

 

Or does it matter?

 

Thanks!

[tlbonehead]

Originally posted by Chad I just got an Avatar Helltone 60 (Celestion Vintage 30) speaker to replace the G12T-75 speaker that was in my Randall RM50B combo. For those who may not know, this amp is one of Randall's "modular" tube amps that has swappable preamps. Anyway, both speakers are rated to be 16 ohms. But the G12T-75 meters out at about 13.8 and the new Hellatone meters out around 12.8. My amp has a 3-way impedance switch that lets you pick a 4 ohm, 8 ohm, or 16 ohm setting. I've been using the 16 ohm setting, but since I'm actually running a load lower than that, should I switch to the 8 ohm setting? Or does it matter? Thanks!

It usually reads a bit lower than average impedance. You were correct.

[Chad]

Would it hurt to run it in 8 ohm mode? Would it sound any different?

[JamesPeters]

If you're using both 16 ohm speakers in parallel, use the 8 ohm setting on the amp. That is the correct setting.

[Chad]

Originally posted by JamesPeters If you're using both 16 ohm speakers in parallel, use the 8 ohm setting on the amp. That is the correct setting.

 

 

My amp only has one speaker. See original post.

[batotman]

OMG....its the OLDEST ACTIVE POSTER IN HC HISTORY!

[Chad]

Originally posted by batotman OMG....its the OLDEST ACTIVE POSTER IN HC HISTORY!

HA! I've actually been on the HC forum since mid-1997. Some server crash years back reset all the "oldies" to Dec. 1998.

I was thinking my post count was in the 500 to 600 range, but maybe not.

[batotman]

Originally posted by Chad HA! I've actually been on the HC forum since mid-1997. Some server crash years back reset all the "oldies" to Dec. 1998. I was thinking my post count was in the 500 to 600 range, but maybe not.

Oh there was a big crash and people lost tons of posts. Welcome back!

[JamesPeters]

Originally posted by Chad My amp only has one speaker. See original post.

If the speaker is a 16 ohm speaker, use the 16 ohm setting on the amp. There, fixed.

[Chad]

Originally posted by JamesPeters If the speaker is a 16 ohm speaker, use the 16 ohm setting on the amp. There, fixed.

See post #3. We're getting there...

[Chad]

Originally posted by batotman Oh there was a big crash and people lost tons of posts. Welcome back!

Thanks, bro.

[batotman]

I wouldn't run it in 8 ohm mode. There's a chance you could burn the thing up. People mismatch things and they usually work (within reason, like one setting off) but there's always that chance you'll torch your amp.

[JamesPeters]

Originally posted by Chad See post #3. We're getting there...

Use the correct setting.

End of thread.

[Chad]

Originally posted by JamesPeters Use the correct setting. End of thread.

Again, would it hurt to use the 8 ohm setting and would there be any tonal difference?

Also, when does the correct setting become the incorrect setting? By your rationale, are you suggesting that if a 16 ohm rated speaker actually measured 8.1 ohms you would still use the 16 ohm setting?

[batotman]

Originally posted by Chad Again, would it hurt to use the 8 ohm setting and would there be any tonal difference? Also, when does the correct setting become the incorrect setting? By your rationale, are you suggesting that if a 16 ohm rated speaker actually measured 8.1 ohms you would still use the 16 ohm setting?

If it measured out at something like 10 I'd maybe go for the 8.

I don't think there are tonal differences and I've mismatched my ohms before without realizing it. AND i lived to tell about it.

[JamesPeters]

You apparently missed post #13 in this thread.

A "16 ohm" speaker reads around 13 ohms on a meter because you're not measuring its impedance, you're measuring its resistance. An 8 ohm speaker reads about 6 ohms and a 4 ohm speaker reads about 3 ohms.

Use the correct setting--that is my only recommendation.

Here's something to read about it, to make up your own mind:

http://aga.rru.com/FAQs/technical.html#imp-1

Read that, and the answer below it too.

This doesn't explain some things that happen in the electronics in "real life" which make impedance mismatching dicey; suffice to say people who swear up and down it's safe have had their tubes arc and so forth because they thought "it's perfectly safe to mismatch one step either way" and their amp encountered a particular issue which made the mismatch "the straw that broke the camel's back" (at least).

[Chad]

Originally posted by JamesPeters You apparently missed post #13 in this thread. A "16 ohm" speaker reads around 13 ohms on a meter because you're not measuring its impedance, you're measuring its resistance. An 8 ohm speaker reads about 6 ohms and a 4 ohm speaker reads about 3 ohms. Use the correct setting--that is my only recommendation. Here's something to read about it, to make up your own mind: http://aga.rru.com/FAQs/technical.html#imp-1 Read that, and the answer below it too. This doesn't explain some things that happen in the electronics in "real life" which make impedance mismatching dicey; suffice to say people who swear up and down it's safe have had their tubes arc and so forth because they thought "it's perfectly safe to mismatch one step either way" and their amp encountered a particular issue which made the mismatch "the straw that broke the camel's back" (at least).

Thanks for the info and link! That was very helpful. Why didn't you just post that info in the first place instead of getting on a thread closing tirade? lol

[Chad]

This quote from that link is particularly helpful:

"For what it's worth, I've been deliberately "mismatching" load impedances by one tap for years. In other words, either a 4R or a 16R load on an 8R tap, and so on. This small mismatch will limit output power and will change the clipping points of the output tubes, but will not damage anything in a properly designed amplifier. Keep in mind that a higher load impedance in a pentode amp will put additional stress on the screens, so you may want to have at least 1k stoppers installed. A lower load impedance will cause more plate current to flow, and if you're running the tubes at the edge of acceptable quiescent plate dissipation that may push them over the edge into the red zone. If you've got an old vintage amp you'd hate to see get damaged, by all means, play it safe and don't mismatch at all. But if you're wondering about how it sounds, and the amp's got good trannies in it, then mismatch away. Just keep it within ONE TAP please, for safety's sake.

-Ken Gilbert, by way of PMG"

[Critter]

A lot of people confuse 16 ohms of impedence with 16 ohms of resistance. the typical meter with a 9 volt battery in it cannot properly measure speakers ohms of impedence so go with the rating listed on the speaker.
General rule with tube amps. You can set the amp lower than the cab.
for example

a 16ohm speaker with the amp set to 4 ohms will cause a loss in tone but no major damage, it can blow weak tubes or weak screen grid resistors ($50 fix) I've found amps set this way that worked fine for years.

a 4 ohm speaker with amp set to 16 ohms can cause tubes to blow as well as transformers because it increases current flow and adds too much heat as well. ($300 fix) also usually won't hurt the amp but I wouldn't test this too often.

[Chad]

Thanks for the info, Critter!