Push/Pull Pair Removal Theory & Questions

[bob p]

^ load resistor.

[trip]

Thanks everyone!

I originally was writing a response and forgot to post it, before I knew it this thread blew up so thanks for all the good info (and not good info too)

I have a better / fun question.

Given these options ala Mesa and others with a half power switch, which can be implemented a few different ways, what would you recommend as a modification to ensure the best way of running these heads on 2 tubes over 4 permanently? Is there an ideal fix that can be applicable to all and achieve the best results? (Removing from circuit vs grounding, etc?)

[bob p]

just my two cents: if what you want is really a 40-50W amp, then buy a 40-50W amp instead of trying to down-convert an 85-100W amp.

[trip]

just my two cents: if what you want is really a 40-50W amp, then buy a 40-50W amp instead of trying to down-convert an 85-100W amp.

I have a few that are low powered. In reality, the high powered amps are hella cheaper and way easier to come by. I've never paid more than 300 for a head and most of them way less than 200. So if I wind up with a buncha high powered stuff, that's exactly what I want. It has nothing to do with volume or loudness per se, I do live in an apartment now so I can't be blasting any 4x12s, but if it costs, say, and estimated 80 per head / 15 heads, it's 1200 to retube all the power's, whereas running em all at half will be half that. I don't see a need for the extra 3db that 4 tubes gives me over 2. The tone difference is almost non-existant, given I'm not going to be cranking these at stadium volume. I would prefer to run that with 2 for as long as I own them if possible, which is what I've been doing for some, also enabling lower power cabs to be used. I would like to make it permanent / modify it to be ideal if possible because it's fun, I want to, it's useful to learn and understand (to me), and, I can see continuing to do this for quad-powered amps for the conceivable rest of my life when I feel like not using 4 tubes or only have a single matched pair available, so, I totally get what you're saying, but, it's something I want to do and want to understand the best possible way of achieving it with what I have. Looking through the replies, some transformers don't differ between the 2 and 4 models of the same amp, so I'm sure there's a few things that can be adressed to make the match more ideal than switching the ohm rating and removing two tubes. I'd love to throw a switch in there so I don't have to actually pull the tubes and can keep em in there. (the Kruise attenuator is pretty cool too as an alternative for power limiting, but I'm not shipping things out)

In summary, yes I agree, and I do, but I like doing things, and I want to understand the best way to achieve this and why it is so. (Because I'll be doing something eventually regardless of opinion against it, but would prefer solid opinions on best practices and from people who have experience doing the same). Don't buy a high powered amp is the easy answer. I want the complex switch the grid stoppers / soak it in water / do whatever makes sense to alter in order to keep them running with 2 tubes and problem free for a lifetime.

(I do appreciate all of the insight you have lent to the thread!)

[Chuck H]

just my two cents: if what you want is really a 40-50W amp, then buy a 40-50W amp instead of trying to down-convert an 85-100W amp.

There are good reasons to want to do it. One would be that your gigs require a four tube amp but it's too loud @ settings for the practice space. Another would be that you love the four tube amp but things change and now you need a two tube amp but you want to keep your tone. Another you be that you happen to have a four tube amp, need a two tube but don't have the bread to buy one and don't want to sell the four tube amp just to trade down in power because the four tube amp is collectible and going up in value.

Though I'm sure the most common reason for doing it is just GMS (gear modification syndrome).

[Gregg]

4 pages and this thread still going on. Guys, really?!

just my two cents: if what you want is really a 40-50W amp, then buy a 40-50W amp instead of trying to down-convert an 85-100W amp.

+1. An amp with two tubes doesn't sound the same like an amp with 4 tubes. The same applies for a 4 tube amp with two of them removed. Everyone who is not deaf yet knows that. Call it the "feeling is different" or whatever but it's a fact.
Removing two tubes just for the sake of lowering the power in half is just stupid. It can be a temporary fix (like if one of the tubes blows) but not a permanent solution. Your 100W amp is too loud? There's a simple solution to that: don't krank it up to the max but keep all 4 tubes running as intended.

[g1]

Calling it 'stupid' is a bit strong but you are of course entitled to your opinion. There are plenty of manufacturers who put in a switch that drops out a pair of power tubes in an effort to cater to the 'stupid' crowd.

[trip]

There are good reasons to want to do it. One would be that your gigs require a four tube amp but it's too loud @ settings for the practice space. Another would be that you love the four tube amp but things change and now you need a two tube amp but you want to keep your tone. Another you be that you happen to have a four tube amp, need a two tube but don't have the bread to buy one and don't want to sell the four tube amp just to trade down in power because the four tube amp is collectible and going up in value.

Though I'm sure the most common reason for doing it is just GMS (gear modification syndrome).

^This, awesome i'm being understood. It probably is GMS and it is addictive. Guitar amps were meant to be modded. Would they be as fun if it was the opposite?

4 pages and this thread still going on. Guys, really?!



+1. An amp with two tubes doesn't sound the same like an amp with 4 tubes. The same applies for a 4 tube amp with two of them removed. Everyone who is not deaf yet knows that. Call it the "feeling is different" or whatever but it's a fact.
Removing two tubes just for the sake of lowering the power in half is just stupid. It can be a temporary fix (like if one of the tubes blows) but not a permanent solution. Your 100W amp is too loud? There's a simple solution to that: don't krank it up to the max but keep all 4 tubes running as intended.

yeah, 4 pages and most of it was people arguing over what they think is obkectively correct over the other, I'd be happy for it to go on 4 more, also considering there are threads going on for years or less with hundreds of pages, I don't get the surprise, also considering this is a question related to my OP. Dudes, really?!

Yes, there is a difference. Yes, I can hear. Sometimes it is a big difference. Sometimes it is negligible. For Nth time, I am not concerned over perceived loudness or the barely audible difference in volume. It is an economy of cost issue with owning so many quad powered amps, and also, I don't see what the big deal is. Yes I am aware I can buy lower powered amps but that's not what I want? It's not the power I'm really concerned about, it's the tube life and how often I can replace them, as well as learning the correct way, instead of a brushover "get a lower powered amp" answer. Maybe I only have one pair available and I will only ever have one pair available?

Am I really asking for something SO crazy that my answer is continually "buy a lower powered amp"?

I'm looking for the HOW and the WHY.

What needs to be changed (components or otherwise) to make an amp built for 4 tubes be able to function healthy with just 2? (beyond the already stated, or the mesa style switching)

What modification can I do to facilitate the fact I WILL be running them with two tubes for the conceivable future.

They'll last long enough as is. But as pointed out above with "GMS," it's something I really want to understand because it's something I really want to implement. (I'm not even that concerned about the cost to replace 4 over 2, but spending 40-60 a pop is much better than 80-120 minimum, at that price I can score another head, or replace two instead of one with 4.)


Really not trying to sound like an Ahole, but the "get a lower powered amp" is not the answer I am looking for and I'm hard-pressed to believe a forum full of passionate people who modify electronics is going to basically just say "it can't be done" when it apparently has been done for years by companies and their stuff runs fine?

Phew

It's like going to an ice cream shop and them saying "just eat a salad instead"

There are plenty of manufacturers who put in a switch that drops out a pair of power tubes in an effort to cater to the 'stupid' crowd.

^ What else on top of that would be an ideal change to facilitate the healthy running of just two?

[g1]

You might want to consider how the lighter load on the power supply raises the B+ voltage and decreases the supply's sag. (also affects heater voltage)
The fact that you now have PT and OT that are way over-rated for their new job is not really something you can change, only compensate for.

[trip]

You might want to consider how the lighter load on the power supply raises the B+ voltage and decreases the supply's sag. (also affects heater voltage)
The fact that you now have PT and OT that are way over-rated for their new job is not really something you can change, only compensate for.

Well a transformer operated above it's rate will overheat, and I'm under the impression there's a lot out there with severely underrated power transformers for how they're built. Does it not work like a power wart where as long as you under the rated maximum you are ok?

B+ goes up and the heater voltage goes up. Does it really go up so far to blow it mega-out of spec (knowing that tube amps have a decent amount of tolerance, or should) for it to melt down eventually? Or just diminished life of the tubes now that they are being taxed harder? I assume it would also affect some component possibly on the cusp of max rating. What cheap components would manufactures throw in there that couldn't handle this change and need to be replaced?

With that in mind, the simplest answer would, ideally, be that we need to reduce both the B+ supply and the heater supply levels back to the correct level. If we could achieve that in a perfect world. ?

[trip]

Also I was extremely curious about the dummy load statement. Is there a way to wire something "safe" up with some unused tube bases to insert in the open sockets to compensate when running without two tubes? ( I mean my mind says this probably isn't possible but figured I'd ask)

[trip]

Well a transformer operated above it's rate will overheat, and I'm under the impression there's a lot out there with severely underrated power transformers for how they're built. Does it not work like a power wart where as long as you under the rated maximum you are ok?

B+ goes up and the heater voltage goes up. Does it really go up so far to blow it mega-out of spec (knowing that tube amps have a decent amount of tolerance, or should) for it to melt down eventually? Or just diminished life of the tubes now that they are being taxed harder? I assume it would also affect some component possibly on the cusp of max rating. What cheap components would manufactures throw in there that couldn't handle this change and need to be replaced?

With that in mind, the simplest answer would, ideally, be that we need to reduce both the B+ supply and the heater supply levels back to the correct level. If we could achieve that in a perfect world. ?

I realized that's vague as you'll never know the jump unless your measuring that specific amp, I'm not sure what the math would be, wouldn't the current double w/ ohms law? I mean yeah I want to understand the theory but my other questions are more important. Am I toasting my filaments when running on two?

[Mike Sulzer]

Your 100W amp is too loud? There's a simple solution to that: don't krank it up to the max but keep all 4 tubes running as intended.

That's fine if you are happy with preamp distortion, as some people are. But if you like the sound of a distorting output stage, that is not an option.

[bob p]

Calling it 'stupid' is a bit strong but you are of course entitled to your opinion. There are plenty of manufacturers who put in a switch that drops out a pair of power tubes in an effort to cater to the 'stupid' crowd.

Count me in among the Stupid Crowd. I am using a Mesa amp that has the 60/100 switch. Actually, it's a Mk IV so it's more of a 40/90 switch, but that amounts to splitting hairs.

Is it useful? Yes. Does it make the amp do things that it couldn't do otherwise? Yes. Does the amp sound anywhere near as good as a real 40W amp when it's switched down? No. Or anywhere near as good as a real 90W amp when it's switched up? Yes. If I were building from scratch would I bother with the 60/90 switch? No.

In the big scheme of things, the power switching thing makes the amp a little more versatile. But another way to look at that is to say that it makes it a jack of all trades and a master of none because it involves design compromises. Truth be told, a dedicated 40W amp that's properly tweaked will sound better than a 40/90W switcher, and the same could be said for a dedicated 90W amp.

The OP already has multiple amps. So I don't understand the desire to make one amp do everything, unless he's just got an irresistible urge to tweak.

That's fine if you are happy with preamp distortion, as some people are. But if you like the sound of a distorting output stage, that is not an option.

If he's after the sound of a distorting output stage, a well built attenuator will get him A LOT farther than a 60/100 switch.

[bob p]

You might want to consider how the lighter load on the power supply raises the B+ voltage and decreases the supply's sag. (also affects heater voltage).

That is probably the smartest thing that I've heard anyone say in this thread.

(which is exactly why I said it in post # 16)