Regarding pulling a pair a tubes for half wattage....

[VBCheeseGrater]

I recently had an At-100 amp repaired and had the tech go ahead and use only two power tubes for 50 watts along with rebiasing. I mentioned about having heard of the necessity to halve the ohm setting on the amp vs the cab homage, so an 8 ohm cab would require the amp set to 4 ohm.

So contrary to everything I've ever heard, he flat out told me this was incorrect, and I should continue to match the cab impedance to the amp . Again, this goes against everything I've heard so I asked him again when I picked up the amp...same answer..."No need to halve the ohms, use 8 for 8, 16 for 16...etc."

So can anyone confirm this that actually knows the math behind it? I've still been running half ohms since then out of caution, but if I could go back to matching it would increase my options on speakers and cabs.


Another question - can I expect less overall stress on the transformer and other parts by running at half wattage? The amp is not the most reliable so it's my backup, but I do love the tone from it and enjoy gigging with it here and there.

 

[sage]

The tech is incorrect. Pulling two tubes doubles the ohms of your amp and you should be using a 4 ohm out to run an 8 ohm cab. Doing otherwise could fry your output transformer. Having a lower ohm output on the head and a greater ohm load from the speakers is ok, but the inverse, which is what you would have using the 8 for the 8 in your case, is damaging.

You cannot really expect less stress on the other parts.

 

[Zehailiu]

As with everything else related to guitar, music, life, love, and the universe, there is no simple straight answer.

If he insists, get him to agree to fix it for you for free if something goes wrong, otherwise, take his word over forum posts, lastly, rely on your own further research and/or instincts.

Or try the reverse order if you're more self confident.

 

[TheDepthsWillRise]

I'm not sure how amps vary case by case. But, I can say I've been running my dual rec with 2 tubes removed, though the 8 ohm out, into an 8 ohm Avatar 212 for about as long as I can remember with no adverse effects thus far.

 

[imaginary]

I use whichever speaker output sounds the best to me.

Technically, if you have an 8 ohm cab, you should now plug it into the 4 ohm output for the same reflected impedance. If you don't, it doesn't necessarily mean your power tubes will exceed their maximum dissipation.

If you really want to know, you would calculate your "loadlines".

How to calculate loadlines:
The Valve Wizard -Push-Pull

50watt mode will draw much less current than 100watt mode, so yes, it should be more reliable... maybe/probably.

 

[VBCheeseGrater]

The tech is incorrect. Pulling two tubes doubles the ohms of your amp and you should be using a 4 ohm out to run an 8 ohm cab. Doing otherwise could fry your output transformer. Having a lower ohm output on the head and a greater ohm load from the speakers is ok, but the inverse, which is what you would have using the 8 for the 8 in your case, is damaging.

You cannot really expect less stress on the other parts.

This is what i've heard on the internet also, but unfortunately does not explain why we must halve the ohms.

I'm not sure how amps vary case by case. But, I can say I've been running my dual rec with 2 tubes removed, though the 8 ohm out, into an 8 ohm Avatar 212 for about as long as I can remember with no adverse effects thus far.

This is some good anecdotal info. Although your amp is a Mesa and may just be handling the stress well!! I'm leaning towards listening to the tech. After all, the internet is the internet.

Still, would love to hear the science behind needing to halve to ohms, other than "a mismatch will fry your amp" - which most guitarists with a tube amp have learned

 

[sage]

The reason that you halve the ohms has to do with what the transformer actually does. The input side of a transformer is expecting to see somewhere in the neighbourhood of 1900 ohms from the four power tubes. Pulling two tubes doubles the impedance to the input of the transformer to around 3800 ohms. I don't know why the impedance doubles. Outside of my area of expertise, but it was explained to me by Vancouver's legendary amp tech, Mr. Ho, long before the advent of the internet (yup, I'm that old). Anyway, the transformer doesn't change at all. It's set up to turn a big number of ohms into a fraction of that, not convert any number of ohms into 4, 8, or 16. So, because you are doubling the input to the transformer, you double the output.

The transformer doesn't switch between being a 4, 8,or 16 ohm transformer when you hit the switch on the amp or plug into a different jack, either. The transformer can have multiple taps into the winding. So, when you activate the 8 ohm setting by either plugging into the 8 ohm jack or hitting the 8 ohm switch, the sound signal goes through all the windings that get it to 16 ohms, then goes through all the windings that halve it to 8 ohms, and then on to the jack, bypassing the windings that would further halve it to four ohms.

I hope this makes some sort of sense. There is anecdotal information abound on the internet about guys who never knew about this and operated their amp safely for 10 years without halving the ohms to their cabs. There is anecdotal info to the contrary, also, of guys who melted their OT's by having their amps set to 4 ohms and running a 16 ohm cab. Personally, after being lectured by Mr. Ho, I followed his advice. halved the ohms, and didn't have a problem running a Marshall with two tubes pulled running KT88s for years. Mayhap I wouldn't have had a problem anyway if I'd have just run it straight.

Funny part of the above story, I didn't run two tubes for any reason other than that I couldn't afford 4 KT88s at the time.

 

[Rook]

If output impedance of the primary circuit is Zp, the impedance seen looking into the secondary, Zs = Zp*(Nprimary/Nsecondary)^2 where Np is the number of windings on the primary side, Ns on the secondary. To tidy that up a bit, the windings on the transformers are assumed not to change with Zp (duh) so let's call (Np/Ns)^2 the transformer factor (a phrase I just coined ) some arbitrary constant Nt.

[EDIT: I can derive this math if anyone cares, but it's very basic, have a go at finding it for yourself if you're really interested ]

If Zp changes by the factor of a constant, so Zs changes by the same constant i.e.

ZpNt = Zs
2ZpNt = 2Zs

So.

Assuming in your amp by removing two tubes you have doubled the power stage's output impedance, the impedance ratings stated on your amp (eg 4, 8, 16) are actually outputting double what they say (8, 16, 32).

So if you have an 8 ohm cab and want to match (you do), your 8 ohm output is now labelled 4 ohms because you've doubled Zp and hence Zs.

If the two push pull pairs are in parallel, removing one will double the total output impedance. One would assume they are parallel for obvious reasons

 

[VBCheeseGrater]

Thanks a bunch to sage and Rook and everyone else's 2 cents on this.

Half ohms it is then! No point in gambling in the face a facts and reason, especially given the reliability record of the B-52.

To be fair, i've had 3 of these amps with only 1 ever having issues, and it's the 2x12 combo. I keep coming back to it, there's something about the distortion i really dig over some much better built amps i've had.

 

[Rook]

It'll sound weird if it's mismatched, mismatching up (applying a bigger load) causes damping, which comes off like a roll off in presence and a more indirect low end, a mismatch down, smaller load than expected, will add a bit of high end and can make the low end roll off a bit.

Neither is always the case, but it'll feel very different.

 

[TheDepthsWillRise]

I think I'm gonna go put my tubes back in, give my amp a back rub, and kindly thank it for not shitting the bed.