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View Full Version : Bassman head output is 4 ohm plugged into an 8 ohm cab. What happens?


PRS_CU22
03-26-2007, 09:58 AM
So I have a Blackface Bassman head and a different 8 ohm cab.
Can I use them together?
What could happen to the speakers?
What could happen to the amp?
What might happen to the sound?

I have heard a lot of rumors and I would like to figure this out for sure.

Thanks!
:munch

Phil M
03-26-2007, 10:43 AM
I used to run a 4 ohm Bandmaster head into an 8 ohm 2x12. You lose some efficiency, but I was assured by techs and folks here on this board that it is OK to do so without hurting anything. The problem would be if you had an 8ohm head into a 4 ohm cab; that would do some damage.

Standby for the technical answer...

Jujo
03-26-2007, 11:43 AM
Yea it should be fine. I think you loose some headroom because the amp has to work harder to push the speakers. If your ohms is greater on the speaker side than on on the amps output it should be ok.

alaskagrown
03-26-2007, 01:57 PM
As already mentioned, you'll be fine. You won't lose half the power, but it will be noticeable.

What are the speakers in the cab and what is it - 4x12, 2x12, 4x10? You might be able to rewire it to 4 ohms to get max power!

HandOfTheHost
03-26-2007, 01:57 PM
I was just involved in this debate here:
http://acapella.harmony-central.com/forums/showthread.php?t=1565561

There are directly opposing viewpoints regarding this subject. I personally will only run a matched cabinet, just one less thing to worry about. Check the links and decide for yourself. I defer to others in those links who know more than I (i.e. Michael Soldano, Lord Valve,) and if I HAD to run a mismatch, it would be a smaller speaker load (8ohm head into 4ohm cab.)

ross
03-26-2007, 03:15 PM
I was just involved in this debate here:
http://acapella.harmony-central.com/forums/showthread.php?t=1565561

There are directly opposing viewpoints regarding this subject. I personally will only run a matched cabinet, just one less thing to worry about. Check the links and decide for yourself. I defer to others in those links who know more than I (i.e. Michael Soldano, Lord Valve,) and if I HAD to run a mismatch, it would be a smaller speaker load (8ohm head into 4ohm cab.)
Your link does not work.

HandOfTheHost
03-26-2007, 03:56 PM
Your link does not work.

Hmm yeah, it seems HC is on the blink for the moment. Try it in a little while, or you can just google something like 'tube amp speaker impedance mismatch.'

HandOfTheHost
03-26-2007, 04:10 PM
Here's the Soldano link:
http://www.musicianshotline.com/archive/monthly/qa_soldano/04_03.htm

PRS_CU22
03-27-2007, 12:28 PM
Hmm so far I have votes each way.
I am still not sure what to do...
:confused:

fr8_trane
03-27-2007, 12:40 PM
Speaking from experience as opposed to theory, I have used my Fender Bassman head into an 8 ohm Marshall 4x10 for years with no ill effects. I am not alone. I did some research on this awhile ago and found alot of the conflicting info you did. However, the prevailing wisdom seems to be that the old fender iron (especially the bassman iron) can handle the mismatch easily. You will lose some output power and headroom and the tone and feel of the amp will be slightly different than running at the matched impedance.

billyguitar
03-27-2007, 01:28 PM
The guy that says the 8 ohm load blew his transformer is wrong. The transformer probably blew for other reasons, like old age. I know lots of guitar players and bass players that used Fender Bassmans and Showmans with 8 or 16 ohm loads and I never heard of a one blowing the OT.

John Phillips
03-27-2007, 01:29 PM
I used to run a 4 ohm Bandmaster head into an 8 ohm 2x12. You lose some efficiency, but I was assured by techs and folks here on this board that it is OK to do so without hurting anything. The problem would be if you had an 8ohm head into a 4 ohm cab; that would do some damage.
No it wouldn't - it would be safer than a 4-ohm amp into an 8-ohm cab (assuming we're talking about tube amps). The worst it would be likely to do is wear the power tubes slightly faster.

Tube amps are safer into too low an impedance than too high. (The risk comes from excessive flyback voltages generated by running too lightly loaded, ie too little current draw, not too much.) Most are safe between half and double the correct impedance though, and there will likely be no problem running a Bassman into an 8-ohm cab. You will lose some power and bottom-end, and the sound will be 'flatter' and less responsive.

There's no risk to the speakers if they'll take the power, which is about 70% of the normal power into 4 ohms.

The guy that says the 8 ohm load blew his transformer is wrong. The transformer probably blew for other reasons, like old age. I know lots of guitar players and bass players that used Fender Bassmans and Showmans with 8 or 16 ohm loads and I never heard of a one blowing the OT.8 ohms is OK, 16 ohms is not. I've personally changed a few Fender 4-ohm OTs that were blown by running into 16-ohm Marshall cabs (the most common cabs in the UK for many years). 16 ohms is well outside the safe range for a 4-ohm amp, it's getting too close to an open circuit.

hasserl
03-27-2007, 01:31 PM
I did not read the linked discussion, no need to. There really should be no controversy, it is safe to run your Bassman into an 8 ohm cab, and it is usually safe to run an 8 ohm amp into a 4 ohm cab. In fact the 8 ohm amp into the 4 ohm cab is safer, but both are usually safe. When you start mismatching to a greater degree things get less safe. In these cases it is always safer to go with a lower impedance speaker than the amp is designed for, rather than a higher impedance speaker.

The exact opposite is true when dealing with solid state amps, in that case it is safer, when using a mismatched load, to use a higher impedance speaker than the amp is designed for.

But we're talking about tube amps here, right? In that case the lower impedance speaker is safer.

Phil M
03-27-2007, 01:36 PM
Ha! I'll take your word for it. I'd love to find the thread where I asked this same question about 3 years ago! I got some different responses than I'm seeing here.

ChickenLover
03-27-2007, 02:02 PM
My understanding is what has been mostly said here; to put it very simply and generally:

Lower speaker impedance than the amp expects (4 ohm speaker with 8 ohm amp): harder on the tubes.

Higher speaker impedance than the amp expects (8 ohm speaker with 4 ohm amp): harder on the output transformer.

I have a BF Bassman and have run it into both 4 ohm and 8 ohm cabs many times and A/B tested it (not blind)...with my amp (which has the stock iron) you lose a noticable amount of high end (mainly sparkle), lose depth in the bass, and lose a little punch and volume. I think the correct terms here are that you lose a little bandwidth and power. I really do prefer the tone into the 'correct' 4 ohm load but it's not like a night and day difference...I'm just spoiled after A/B testing it.

ross
03-27-2007, 02:11 PM
I never understood why fender amps were built with extension speaker jacks in parallel with the main speaker jack. If you were to use a second fender cab with your bassman then the amp will be running on a 2 ohm load. I assume that this would have been engineered into the spec of the OT.

hasserl
03-27-2007, 02:17 PM
I never understood why fender amps were built with extension speaker jacks in parallel with the main speaker jack. If you were to use a second fender cab with your bassman then the amp will be running on a 2 ohm load. I assume that this would have been engineered into the spec of the OT.

Give this man a cigar! That is exactly correct. If this was dangerous to do Fender never would have designed the amp this way. They did it because it is not dangerous to the amp.

John Phillips
03-27-2007, 02:20 PM
I never understood why fender amps were built with extension speaker jacks in parallel with the main speaker jack. If you were to use a second fender cab with your bassman then the amp will be running on a 2 ohm load. I assume that this would have been engineered into the spec of the OT.Exactly, and that's precisely why it was done like that... because it's foolproof - more or less. Any combination of cabs (excapt a single 16, which are rare in the US) will result in a load between 2 and 8 ohms, which is safe for the amp. Leo was no fool when it came to making stuff reliable.

billyguitar
03-27-2007, 09:14 PM
I agree. A proper match always sounds better to my ears too. This thread is the first time I've ever read someone say that a lower impedance is safer than a higher impedance for a tube amp. I guess I can learn something new. I'll have to do a little research on that point just to satisfy myself. Nevertheless, I have seen guys run their amps anywhere from 4 ohms to 16 ohms with no ill effects. BUT, this was 30 years ago when the amps were still fairly new. I don't know anybody execpt one bass player that has used one in recent years.

cameron
03-27-2007, 10:06 PM
This thread is the first time I've ever read someone say that a lower impedance is safer than a higher impedance for a tube amp.


The story about lower impedances being unsafe is an urban myth in gear circles that comes up here every couple of weeks, if not every week. The myth is based on the behavior of solid state amps. Tube amps have an optimum impedance, and they are tolerant of mismatches in either direction, but low loads are generally safer than overloads, as John mentioned in his post above.

Solid state amps have a minimum impedance. You get maximum efficiency at the minimum impedance. But if you go below the minimum impedance, bad things happen. (There's no output transformer between the main power transistors and the speakers in a solid state amp.)

The myth is probably due to the extreme urgency of never going below the minimum with solid state amps. It got generalized to tube amps by people with no understanding of the difference between tube and solid state amps.

billyguitar
03-28-2007, 01:36 PM
I know now what I've heard all these years, lower is harder on the tubes, as you've guys said. Weber, Funk and the others don't discuss as much about the OT in their books. Now the light is on! Thanks!!

drbob1
03-28-2007, 04:10 PM
The transformer is designed to balance the internal resistance of the tubes with the resistance of the speakers (and it's an approximate match-can be out of spec by a factor of 2 from the factory without real danger to the tubes). If you have a lower impedance from the speakers, then the "flyback voltage" generated by the speaker coil returning to center thru the speaker magnets field and the magnetic field of the output transformer collapsing on it, will be quite weak when it hits the amp and can't do any damage. The resistance to flow of electrons thru the tubes in the active phase will be lower, resulting in potentially higher output from the tubes, which could burn them out quicker. Still, they're replaceable components (unless we're talking GEC KT66s or KT88s ;)). If the speaker impedance is too high, the flyback voltage is amplified to the point that it can fry components or arc between components in the amp-destroying anything from tube sockets to the OT-very expensive to replace.

When you forget to plug a speaker into a Fender head, the output of the amp is shorted thru the jack (eg speaker impedance of 0 ohms) which is much safer than an open circuit. Also why there's no sound if you plug into just the extension jack (all the amp output is being shorted thru the main jack).