I pulled 2 power tubes...its a little quieter

[JFrankParnell]

'73 bassman100. I pulled the two inner power tubes after a lot of reading on how that will make your amp quieter. I dont know fer sure if that cuts the watts by half, but it def makes it a bit quieter.

fyi, this amp has 2 channels (only one that I use, cuz the other one is broked). So I use the 'bass' channel. And it has a master volume. What was bugging me was the tone on lower volumes was not good. And I dont mean that I couldnt get distortion or whatever. It was just that there was a tipping point at about 2.5-3 on both channel vol and master vol, below which the sound was very anemic. Above that point, sound was good. Now, of course, 3 on this amp is loud enough to be heard throughout the house. That's ok most of the time, but who wants to hear me practicing some lick, working on my speed, going deedly-deedly 500 times in a row?

So, yeah, I pulled 2/4 power tubes. There still seems to be that tipping point, but the actual db's are much more acceptable above the tipping point. I can keep the MV at 3.5 and have a much nicer range on the channel volume. And, I think there's plenty of power still to play with a band. I cranked it up, and it still roars. If not, I can just plug the tubes back in.

cool, right?

[gfxbss]

Pulling have of the tubes works well for a class A/B amp, but be aware that if you have a class A tube amp this will not work.

Also, the reason it only seems a bit quieter, is cause the human ear senses dB on a logarithmic scale. This means that you can half the wattage(power) of your amp, and only hear a minimal difference.

Tyler

[thetragichero]

make sure you set the impedence selector when you remove 2 tubes... if it's going into a 16 Ohm load, set it to 8 Ohms (set the amp to half of the Ohms of the speaker load)

[gfxbss]

Tragic,

Not by any means saying that you are incorrect, but I haven't heard that one. Just was curious about the origins of it.

Tyler

[thetragichero]

www.marshallforum.com/marshall-amps/3992-took-out-2-power-tubes.html?highlight=Pulling+tubes

tl;dr - so you don't fry your OT

[gfxbss]

I'm looking through my books to find this.... I know I read about it somewhere....

I am trying to decide the math on it... would this be R=V²P?

Obviously P would be divided by 2 since two tubes have been pulled. But I'm not sure what the operating voltage would be...

Tyler

[JohnH]

I have also heard about changing the output impedance setting. It makes sense to me. It means that the two tubes that are left are seeing the same share of load that they had when the other tubes were there. Experts on the Marshall forum think it is important, and also to check bias on the remaining tubes when done.

[gfxbss]

It just seems to me that your impedance hasn't changed... if you where running an 8Ohm cab before, you still are running an 8Ohm cab....

Tyler

[thetragichero]

it's not the cab that's changed, it's what the OT needs to "see" to operate correctly

[ashcatlt]

I don't know from nothing (though I no sumbody who does), but I'm pretty sure that a tube amp won't blow up if you plug a 16Ω cabinet in where it expects 8Ω. Huge mismatches in either direction are best avoided, but I don't think this'll hurt anything except (maybe) tone.

[gfxbss]

I would agree w/ the biasing....

The part that is throwing me off is....

When you set the impedance on the head, that is telling the amplifier how much impedance to expect...

Just by pulling two tubes doesn't change how much impedance the amp should be expecting, so I don't understand why you would change the impedance....

Tyler

[JohnH]

It’s hard to explain it properly without a full understanding of it myself. But imagine this:

Your 100W amp had 4 tubes driving say 8 Ohms, and the transformer tap was set accordingly to the correct turns ratio between primary and secondary.

You can think of this as two amps in parallel, and to an extent it is actually true (eg the tubes are in two parallel pairs). So you can think of this 100W as two 50W amps, each with two power tubes, each driving 16 Ohms. Each wants the same tranny turns ratio as the full amp so that each tube is working the same as in the full amp. So to actually split (or halve) this amp, the transformer is same in each, and it is connected the same. But what was correctly set to drive an 8 Ohm load is now correctly driving a 16 Ohm load.

So if you pull two tubes, check bias, and then use the tap labelled for 8 Ohms to drive a 16 Ohm cab, or that labelled 4 ohms to drive 8 ohms.

[JFrankParnell]

wul, on this amp, there are 2 speaker outs, says "4ohms total load" by them, and I still have just the one cable going into one 4 ohm cab

[thetragichero]

to be honest, a 100w amp should be run with all of the power tubes it's designed to run at... i would look to low efficiency speakers before taking out tubes willy nilly, as the volume difference is next to negligible

[sumgai]

Whoa, whoa there, Bobbalouie, I say WHOA!

Let's all siddown and pass the bowl around for a bit, eh?

OK, first things first.

Biasing is nearly always seen as the Mecca of obtaining tone. Nope. It's just one of many, many components in the signal path. To be sure, a poorly biased amp can harm one's tone pretty quickly, but getting from 90% to 95% of the maximum Mojo possible, that can take a helluva lot of effort and time. Achieving 100% Mojo means constantly playing with stuff, whether it needs playing with or not. Plan on short visits to your family, 'cause you're gonna spend the majority of your waking hours playing Mad Scientist, down in the dungeon you call your Electronics room.

Biasing is not easy, done correctly, and the converse is true too - biasing is easy, when done incorrectly. In a nutshell, don't do it just for bragging rights. Either do it correctly, or pay someone else to do it for you (and make sure they know the drill too, or else you're just wasting your geld). Long story short, I set an amp's bias when asked to by the customer, or else when I've replaced any parts in either the power supply or the output stages. Otherwise I leave it alone. Why? Because the tubes have a lot of leeway, and will sound 90% of "good" no matter what the bias is set to. It's that last 5 or 10% that causes a customer's wallet to shrink back and stammer about costs and things like that.

'Nuff said.

Now, as to impedance. For you relative newcomers, ChrisK and I have often posted about this topic, but IIRC, there really was only one thread where we explained it in some pretty deep and technical terms. I prefer to play the KISS card, so I'm not gonna link to that thread just now, instead I'll do the Cliff Notes version.

As you remove tubes from an amp's output stage, you decrease the overall current taken from the power supply. The voltage remains the same (or it should, all things considered), so Ohm's Law says, the impedance changed. Which way? Easy.... use that law - E = I x R. In order for the voltage to remain the same, if the current went down, then the impedance must have gone up, right? (Hint: say "yes".)

Since the transformer remains unchanged (you didin't take it apart and rewind it to get a new turns ratio (I hope!)), then the higher impedance on the primary is reflected on the secondary side as well. If it doubled on the primary, then it doubled on the secondary. Hence, your 4 Ohm tap is now presenting 8 Ohms for a speaker to see. As well, if you have a 2 Ohm tap, that's now giving off 4 Ohm's.

This substantiates tragichero's statements above, and I believe some of the rest of you have also gotten it right. But don't pass over newey's statements either, he's right - you can go half or double the expected impedance on a speaker, and still sound pretty good. As well, the life expectency of your amp's components will still be unaffected. However, I don't think I'd recommend going outside of that range. IOW, it's a shady proposition at best to round up 4 cabs each with a 4 Ohm rating, hooking them all up in parallel, and then shoving that into a 4 Ohm jack. The amp's gonna be looking at 1 Ohm (!) of output impedance, and that's gonna make the power supply work like a sunnuvagun, trying to supply all that current! The tubes and the output tranny ain't gonna thank you for it either, trust me on that.

We can discuss ways around that, should the need arise, but for now, I think I've answered the original question sufficiently. How about you, Tyler, does this get your squeal of approval?

HTH





sumgai

[JFrankParnell]

haha, well the original question was "cool, right?"

[gfxbss]

SG,

Sorry for hijacking the thread there for a bit.

I didn't realize that the current dropped when you pulled the tubes.

Thanks, I understand now.

Tyler

[sumgai]

Dec 1, 2010 15:05:45 GMT -5 JFrankParnell said:

haha, well the original question was "cool, right?"

Spoken in a straightforward manner, the answer would be "Yes", it's cool to remove one pair of output tubes, from a setup that has 4, 6 or even 8 tubes in all. In higher power amps, you can figure that you're cutting the current draw by 1/2, 1/3, 2/3, etc.

Those of you who spotted the shortcut, go to the head of the class. If I take the 1/2 figure (by removing two out of four tubes) and invert it, I "double" the speaker jack's impedance rating. If I take 4 tubes away from a 6 tube power section (think Fender Super Twin here), I cut the current draw by 2/3 - I now have 1/3 the overall power draw, compared to the full measure. Take that 1/3, invert it, and multiply the impedance rating at the jack. For instance, a 4 Ohm jack would become 12 Ohms. Ditto for removing 3 pairs from a 4 pair'ed monster (Marshall Major springs to mind, but a few Ampegs also fit the bill) - I'll quadruple my output jack's rating.

Just remember that you're inverting the number, not tossing out the numerator. If you remove only two of the six tubes, you still have 2/3 of the total current draw, don't you. Multiply the output jack by 3, and divide that by 2 - 4 Ohms becomes 6 Ohms.

Simple, yes? ;D

HTH



sumgai

[newey]

But don't pass over newey's statements either, he's right - you can go half or double the expected impedance on a speaker, and still sound pretty good.

Thanks for the shoutout, but it wasn't me who said it . . .

[sumgai]

ash is gonna think I don't like him anymore!

Dec 2, 2010 6:42:56 GMT -5 newey said:

But don't pass over newey's statements either, he's right - you can go half or double the expected impedance on a speaker, and still sound pretty good.

Thanks for the shoutout, but it wasn't me who said it . . .

Sorry ash!!

ash said it in reply #9 - newey has only now spoken up. And I've called the nurse to see about upping my meds.....



sumgai

[JFrankParnell]

with a new set of tubes and the fun meter pegged, I took a measurement :

Those two tracks are recorded and played identically (as possible) with the only difference being 2 or 4 power tubes. I'm not sure what that equates to in terms of spl.

[JFrankParnell]

and, btw, holy CRAP that muther is LOUDDDD!!! with only 2 tubes!

[sumgai]

jfrank,

In terms of your measurements, it looks to me like you've got approximately 3 more dB of power going out to the speakers. This should translate into about double the available power at the speaker terminals, which should in turn give you about twice the SPL... provided that your speaker(s) and amp are matched, Ohms-wise, and provided further that you haven't exceeded the speaker's headroom.

I forgot to actually look at the label on the back of your speaker cab - what is the rated Ohms on that thing? And then, just for grins, take a meter reading off it, will ya? I'd expect the DC resistance to equal about 65 to 75% of the Ohms listed on that label.

I'll have more for ya when I've got those numbers in hand.





sumgai

[JFrankParnell]

it says 4 ohms on the cab. the amp says 4 ohms total, and has two jacks. i measured the cab by testing the end of the speaker cable, it was 3.5-3.6 ohms (digital meter)