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impedance selection

3/20/1998 6:36 AM
Gary	impedance selection Hi guys! I'd like to start by thanking everyone involved in posting and making accessible all this free information.I've built my first homebrew amp and learned a lot from this site. At this time I'm a little perplexed by impedance selection.Is it harmful to the OT to run the amp on an 8 ohm load while the impedance selectior is set at 4 ohms or visa versa running the amp on a 4 ohm load with the switch set at 8 ohms? I would hate to blow my OT by inadvertantly having the impedance selector switch set to the wrong position. Regards, Gary

3/20/1998 6:36 AM

Gary

impedance selection

Hi guys!

I'd like to start by thanking everyone involved in posting and making accessible all this free information.I've built my first homebrew amp and learned a lot from this site. At this time I'm a little perplexed by impedance selection.Is it harmful to the OT to run the amp on an 8 ohm load while the impedance selectior is set at 4 ohms or visa versa running the amp on a 4 ohm load with the switch set at 8 ohms?
I would hate to blow my OT by inadvertantly having the impedance selector switch set to the wrong position.

Regards,

Gary

3/20/1998 8:07 AM
liam	As far as I know it goes like this. If you run the impedance selector on 4 Ohms into an 8 Ohm load you won't damage the OT. This does lower the power a little, but sometimes this isn't a bad thing. However, if you run 8 ohms on the selector into 4 ohms load the current supplied will go up initially. This risks damage to output tubes and the OT. Avoid at all costs. Hopefully someone who knows what they're doing can comment on the above, but for the moment it works as a rule of thumb.

3/20/1998 8:07 AM

liam

As far as I know it goes like this. If you run the impedance selector on 4 Ohms into an 8 Ohm load you won't damage the OT. This does lower the power a little, but sometimes this isn't a bad thing. However, if you run 8 ohms on the selector into 4 ohms load the current supplied will go up initially. This risks damage to output tubes and the OT. Avoid at all costs.

Hopefully someone who knows what they're doing can comment on the above, but for the moment it works as a rule of thumb.

3/20/1998 11:30 AM
JR	Liam , I think it goes otherway , I'll put it this way , 8 ohm load is more near to open secondary circuit than 4 ohm load when impedance tap is set to 4 ohm . (every tube amp manual says "Do not run without speaker") so ,there's a chance one can ruin OT or tubes with higher impedance load (not necessarily in this case ,but say ,if we're using a 16 ohm load on 4 ohms setting for example) I'm not saying that, what you stated about 8 ohm on tap and running into 4 ohms load ,wouldn't be true ,but think it is not as risky as above However ,transistor amplifiers ,are another story . JR

3/20/1998 11:30 AM

Liam ,

I think it goes otherway ,
I'll put it this way , 8 ohm load is more
near to open secondary circuit than 4 ohm load
when impedance tap is set to 4 ohm . (every tube amp manual says "Do not run without speaker")
so ,there's a chance one can ruin OT or tubes with higher impedance load (not necessarily in this case ,but say ,if we're using a 16 ohm load on 4 ohms setting for example)
I'm not saying that, what you stated about 8 ohm on tap and running into 4 ohms load ,wouldn't be true ,but think it is not as risky as above

However ,transistor amplifiers ,are another story .

JR

3/21/1998 12:44 AM
GVB	Yep, that's right. I've seen countless posts that give the wrong information. Many people assume that tube amps and solid state amps respond the same to a speaker load. This is not the case. Most solid state amps have the load attached directly to the transistors/mosfets. A lower impedance load will draw more current and destroy the output devices. You can run most solid state amps without a load till the cows come home and not have any problems, but I wouldn't necessarily recommend this procedure to anyone. Most tube amps on the other hand, have the tubes coupled to the load through a transformer. Going to a lower impedance load causes an impedance mismatch and therefore the power output will go down. The tubes themselves will run a little hotter, and will not last as long as when run properly, but it probably won't be enough to do any damage. Using a higher than rated load will also cause a reduction in power output. The problem with this is that speakers exhibit back EMF and can induce flyback in the primary of the output transformer. The voltage shoots up incredibly high and arcing occurs across the tube sockets. If you're lucky, all that will happen is fried sockets. At worst this can damage the tubes themselves and/or puncture that insulation of the output transformer primary windings. In other words, do the exact opposite of solid state amps, and you should be OK, but you will ware out tubes faster. GVB

3/21/1998 12:44 AM

GVB

Yep, that's right. I've seen countless posts that give the wrong information. Many people assume that tube amps and solid state amps respond the same to a speaker load. This is not the case. Most solid state amps have the load attached directly to the transistors/mosfets. A lower impedance load will draw more current and destroy the output devices. You can run most solid state amps without a load till the cows come home and not have any problems, but I wouldn't necessarily recommend this procedure to anyone.

Most tube amps on the other hand, have the tubes coupled to the load through a transformer. Going to a lower impedance load causes an impedance mismatch and therefore the power output will go down. The tubes themselves will run a little hotter, and will not last as long as when run properly, but it probably won't be enough to do any damage. Using a higher than rated load will also cause a reduction in power output. The problem with this is that speakers exhibit back EMF and can induce flyback in the primary of the output transformer. The voltage shoots up incredibly high and arcing occurs across the tube sockets. If you're lucky, all that will happen is fried sockets. At worst this can damage the tubes themselves and/or puncture that insulation of the output transformer primary windings.

In other words, do the exact opposite of solid state amps, and you should be OK, but you will ware out tubes faster.

GVB

3/21/1998 6:26 AM
liam	Yes, but running 16 ohms on an 8 ohm impedance select doesn't cause the same voltage rise as running no load. If you're worried about over voltage on the OT the trick is to run a chain of power diodes in parallel with the secondary windings. That way it can't actually happen. But yes, it does seem to give the valves a harder time, and so you get power amp distortion at a far lower output.

3/23/1998 4:17 AM
D.C.H.	I'm confused here (please excuse me for dropping in on this thread)... I thought Liam's reply sounded sensible and correct to me, and I'm no authority myself, but everything I've ever read from others who are seems to say it's basically safe and okay to run your amp into a higher-rated impedence cab, but NOT SAFE AT ALL to run it into a lower impedence cab. 1) Basically - and what I thought Liam was correctly saying - was that it is okay, for example, to run your 4ohm Bassman head into an 8ohm cab. You'll experience some power loss, but this might have pleasing effects. 2) BUT you don't want to run your 8ohm Deluxe Reverb into a 2ohm 4x10 cab because you could fry it. Am I wrong here? Have I misread the replies? I've been doing things this way for some time - as have many others - and know I amps that have survived for years and years this way... and sounded pretty damn good along the way (though I won't argue with correctly matching loads for the optimum).] Please, someone, let us know. This is a real puzzle. Cheers, Dave

3/23/1998 4:17 AM

D.C.H.

I'm confused here (please excuse me for dropping in on this thread)...

I thought Liam's reply sounded sensible and correct to me, and I'm no authority myself, but everything I've ever read from others who are seems to say it's basically safe and okay to run your amp into a higher-rated impedence cab, but NOT SAFE AT ALL to run it into a lower impedence cab.

1) Basically - and what I thought Liam was correctly saying - was that it is okay, for example, to run your 4ohm Bassman head into an 8ohm cab. You'll experience some power loss, but this might have pleasing effects.

2) BUT you don't want to run your 8ohm Deluxe Reverb into a 2ohm 4x10 cab because you could fry it.

Am I wrong here? Have I misread the replies? I've been doing things this way for some time - as have many others - and know I amps that have survived for years and years this way... and sounded pretty damn good along the way (though I won't argue with correctly matching loads for the optimum).]

Please, someone, let us know. This is a real puzzle.

Cheers,

Dave

3/23/1998 8:30 AM
Paul Gratzer	As far as I know, it's OK to run an amp rated at 8 ohms into a 16 ohm cab. You'll just get a lower output, but not much- about 10-15% reduction in volume. You CAN'T run an 8 ohm amp into a 4 ohm cab, you'll fry the output transformer since it see's a much lower load. A Fender repair tech. put it to me this way- it's easy to put a pint into a quart, but you can't fit a quart into a pint. OR for metric minded people- you can fit a mL into a L but you can't fit a L into a mL! Hope this helps- that's the way I remember it.

3/23/1998 8:30 AM

Paul Gratzer

As far as I know, it's OK to run an amp rated at 8 ohms into a 16 ohm cab. You'll just get a lower output, but not much- about 10-15% reduction in volume. You CAN'T run an 8 ohm amp into a 4 ohm cab, you'll fry the output transformer since it see's a much lower load.

A Fender repair tech. put it to me this way- it's easy to put a pint into a quart, but you can't fit a quart into a pint. OR for metric minded people- you can fit a mL into a L but you can't fit a L into a mL!

Hope this helps- that's the way I remember it.

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