Push/Pull Pair Removal Theory & Questions

[bob p]

just in case the OP is still wondering why it's taken 4 pages for him to get the answer he wants, the problem may be that the answer he wants isn't forthcoming. as you can see, there are many various design considerations that need to be discussed, rather than just settling on a simple answer.


there are lots of reasons that 50W amps are designed they way they are, using the components they use. and there are lots of reasons that 100W amps are designed the way they are, using entirely different components. i think that in 4 pages we've covered every reasonable thing that would need to be reconsidered when changing from a 4 tube to a 2 tube setup, with the only item that hasn't been mentioned yet being grid loading. Fender's designs from the SF era forward took parallel loading into consdieration, while Marshall's didn't. Personally I think it makes a difference that matters, though others (such as the BF fans) may disagree.

What all of this goes to show is that every feature of a 100W amp is changed when scaling up from a 50W amp, and similarly, every feature of a 100W amp would need to be changed if you're scaling it down to a 50W amp, if your intent is to do everything "right". But there's a lot of leeway, which is why you can get away with a 50/100 switch for occasional use. But if I weren't going back and forth, and I wanted a dedicated 50W amp, there's just no way I'd use a 100W platform as a starting point. and if I was intent on building my own amp to do both things, then there would be a lot more compensation involved than just pulling a pair of tubes for flipping a cathode interruption switch. pulling 2 tubes works. it's often good enough, but there are better ways to accomplish the same goal which involve more design considerations.

re-designing an amp involves a lot of thought. just about EVERY design consideration that went into designing that 100W amp needs to be re-evaluated if you're going to convert it into a dedicated 50W amp. It's a great learning exercise if that's what you're after, but if you're an old salt you'll probably conclude is that the easiest way to make a 100W amp sound "smaller" is to add a little bit of attenuation on the output. taking the output down a few dB doesn't change tone appreciably, and plugging in an attenuator is so easy that nobody in his right mind would consider modding a Twin Reverb downward... that is, unless they just want to do it as a learning experience.

My personal opinion on this: if you really want to quiet down a Twin Reverb by only 3 dB then the shortest distance between two points is attenuation. YMMV.

[uneumann]

... 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).

Ok - 4 pages and no one has mentioned the option of lowering screen voltage with a zener chain.
This approach leaves all your tubes in place but runs them at whatever power reduction you wish so they last much much longer. Since your stated issue is tube replacement costs - this is an option to consider.

Since the tubes remain in place, your filament voltages don't change. You can even keep the bias currents about the same (bias voltage will need to be lowered) so the tube/OT idle currents don't change. There will be a difference in sag since you won't pull as much B+ current at max output, but you could just increase the B+ filter chain resistors (or add them) to increase sag.

The lower screen levels will change your optimal load (increasing it) so you would still consider putting your 8 ohm spkr on the 4 ohm tap (or similar change). Impedance matching is less critical with lower screens since the tubes dissipation is reduced regardless of impedance mismatches.

So, I should probably point out that I have not done this on a 4 bottle amp myself, so I'm not claiming a history of experience and successes, but I have used the screen drop method on a 2 bottle amp - with good success. I suggest using a chain of low voltage (~10v) 5w zeners between the screen supply cap and the screen resistors. Using many smaller voltage diodes means you spread the heat dissipation over many diodes so heat sinks aren't required. I'd even consider two chains, one for each tube pair to spread the heat even more. OK - so you don't normally see 10 zeners in most screen supplies, but they would lower your screens by 100 volts - which is significant and zeners are cheap BTW. Stepping the voltage down/up (adding or removing diodes) also gives you the ability to tune a higher impedance load line to the tube curves. Not sure how much you care about that, but it will impact your OD tone and a better load line fit to the tubes will generally be less stressful to the tubes.

Just throwing the idea out there since no one has mentioned it.

[Helmholtz]

Never heard of grid (parallel) loading. Which grid? Could you please explain?
Does OP mean original poster? I hate abbreviations.

[bob p]

OP is original poster.

by parallel grid load I was referring to the reduction in value of grid load resistors when running power tubes in parallel. maybe my choice of words was poor.

[bob p]

... 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).

we can yak on for pages and pages, and fill your head with all sorts of ideas. IMO the best way for you to really gain from this discussion is to grab your soldering iron, impose some of the recommended changes, and take measurements while you listen to the results.

there is no replacement for actually doing the mods and hearing them. book reading is supported by laboratory exercise.

i think you've gotten a lot of good advice already. time to start testing some of it out and draw your own conclusions.

[Enzo]

Agree^^^

SOmetimes it is a lot faster to take five minutes to solder in a couple parts and find out how it sounds rather than bat it back and forth for a week here. It is hard to harm a tube amp, and changes are easy to undo.


As to abbreviations, once you know them you won't forget. I'd hate to have to spell out AC, DC, LCR meter, and microfarad.

[Gregg]

Ok - 4 pages and no one has mentioned the option of lowering screen voltage with a zener chain.

The screen variety of power scaling first came to mind but since the discussion headed to a minimalist approach I considered this suggestion out of question. However a switch, couple of zeners and one more bias resistor setup maybe falls into this category.

[Helmholtz]

OP is original poster.

by parallel grid load I was referring to the reduction in value of grid load resistors when running power tubes in parallel. maybe my choice of words was poor.

Thanks Bob.

So you were referring to the grid (leak) resistor Rg1-k, which in most amps is higher in value than the tube manufacturers' recommendation for fixed bias, thus increasing the risk of thermal runaway.

(Regarding abbs, my attitude towards them often changes once I understood the meaning. But to non-native english speaking people the meaning may not be as obvious. Sometimes I wished to find a glossary. No problems with technical abbs though, as a great deal of my physics and technical literature has been in english since my university days.)

[Mike Sulzer]

Thanks Bob.

So you were referring to the grid (leak) resistor Rg1-k, which in most amps is higher in value than the tube manufacturers' recommendation for fixed bias, thus increasing the risk of thermal runaway.

(Regarding abbs, my attitude towards them often changes once I understood the meaning. But to non-native english speaking people the meaning may not be as obvious. Sometimes I wished to find a glossary. No problems with technical abbs though, as a great deal of my physics and technical literature has been in english since my university days.)

There do seem to be a few to choose from.

• Abbr. or Abbr
• Abbrev. or Abbrev
• Abb
• ABB
• Abbrvtn
• A
• Abbrn

[Helmholtz]

/\ I see you got my point. Abbs can be quite ambiguous to non-insiders.

[the fatch]

After a brief glimpse of the rant you linked - where the author claims 'that's bogus', '[they're] wrong', 'dead wrong', etc. - I'd say that you're better off taking the argument with a grain of salt.

There are circuit-dependent factors that can affect the usefulness of any thumbrule. Any determination aught to be done with a specific amp as an example, only then can any general case be extrapolated. Some amps can operate in the envelope with two tubes pulled, others may be over-dissipating. A cathode biased amp will change the bias operating point (not sure if there are any examples of a cathode-biased 4-tube amp?) and need compensation.

As a general rule (after advising you to be suspicious of general rules) pulling two tubes and moving the load to the next lowest impedance (thereby doubling what the reflected primary Z sees) should keep the amp inside the designer's operating envelope.

If your objective is to optimize the amp's behavior with 2 tubes versus 4, then all the technical suggestions come into play. Bias, check plate and screen dissipation, check output signal, etc. But you'd likely take some of those steps with any simple tube change.. if it's the tone you're chasing.

I agree with your 'general rule' as long as it's a fixed biased amp. Trying it with a cathode biased amp (e.g. AC30) is a recipe for disaster.

[nickb]

I agree with your 'general rule' as long as it's a fixed biased amp. Trying it with a cathode biased amp (e.g. AC30) is a recipe for disaster.

I'm pretty sure that's not a general rule either

If you have individual cathode resistors, it's not going to be a problem.

[pdf64]

I agree with your 'general rule' as long as it's a fixed biased amp. Trying it with a cathode biased amp (e.g. AC30) is a recipe for disaster.

I think that the key issue with pulling power tubes on an AC30 is not due to it being cathode bias, but because it's a shared cathode bias arrangement.
Hence it depends on all 4 tube being in place and operational for the intended bias voltage to develop.
If an individual cathode bias resistor per tube is used, then it's much less of a problem; if a pair were pulled, stress on the remaining tubes would still increase a bit, due to the reduced HT current draw causing reduced sag at the HT nodes, hence voltages (and so dissipation) would increase.

[the fatch]

So I found this majorly-conflicting-with-everything-i-know post

[ATTACH=CONFIG]47107[/ATTACH]

Can someone give me a basic run down of the "what happens" when you pull 2 tubes and half the impedance switch on a typical push/pull w/ 4 power tubes? Like.. the theory of what happens?

Voltage and current draw on the 2 tube plates left running go up, the overall impedance doubles, the overall resistance goes down (I think?). Things like that. My understanding is fuzzy but would love a quick professional and fast to-the-point of what exactly happens here and what if anything additional needs to be done. (Parts that get more stress/less stress?)

I was under the impression of puling two, half the impedance, and you're good. You might have to adjust bias but as bias is mostly a preference within acceptable range thing you can get away without having to adjust after pulling two given that there is no red-plating happening. Is this inaccurate? Not should you still adjust the bias (we know we "should") but do you have to? Does anything negative happen to the amp with the change, specifically, will it "actually" cause any problems by consistently running 100 watt heads w/ only 1 pair?

I was told at an expo event that this will always damage an amp because of the change in current and stress on some parts. I have also been told the opposite, that it's fine, you don't need to adjust bias (but you should) and that it will not fail because of running at half power. Then, I see a post as above, which also conflicts.

I'm hoping to get a solid understanding of what happens when you pull a pair in a push/pull and what changes if any are necessary after the fact (and to avoid misinformation).

Thanks guys!

Pardon me if I'm teaching Grandmothers to suck eggs, but don’t try this with a cathode biased amp such as the AC30. If you do, just hope that a fuse blows before the two remaining tubes shuffle off this mortal coil and join the choir invisible.