Push/Pull Pair Removal Theory & Questions

[bob p]

the forum seems to be having a latency problem with the page loads. it took a long time to commit my last post.

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.

You should always check bias on the amp when you're servicing it. Always. It's just good practice and it's easy to do.

There are a few things that can cause problems with pair-pulling that most people don't think about. Although most people are familiar with the Z-matching problem, most people aren't aware of the voltage problems that can ensue with tube pulling.

Your PT is designed to provide a specific secondary voltage, and that secondary voltage will vary somewhat depending on the load being placed on the transformer. An amp's transformer is designed to provide relatively constant voltage supply with a design range of varying loads. The extent to which that voltage does not vary much within the range of specified loads is determined by the "regulation" specification of the transformer.

Going back to our 6L6 data sheet, the idle current for a pair of 6L6 at 450 VDC is about 122mA, and as I mentioned in the previous post the peak current is about 232mA. For a normal pair of 6L6 you might expect to see load vary by about 110mA between the idle and peak operating points.

Those numbers would be doubled for a quartet of tubes: about 2 x 122 = 244mA at idle and about 2 x 232 = 464 mA at peak. That's a range of 464 - 244 = 220mA.

From a practical standpoint the 2x6L6 PT should be designed to maintain a reasonably constant voltage when loaded with about 110 to 232mA of current, and a 4x6L6 PT should be optimized to deliver twice that. When you pull two tubes, you're asking a PT that's designed to be fairly constant in the range of about 244 - 464mA to work in the range of about 122-232mA. Chances are that your transformers regulation won't be as good at half of it's intended current, so voltages may rise. How much? Hard to say with hand-waving, but it could be significant.

That increased voltage could cause problems. Anything that ends up operating outside of it's proper operating voltage range could experience a shorter lifespan. If you go pulling tubes, you should expect that the B+ voltage would rise, and the bias voltage should change along with it. IMO re-biasing is important if you pull a pair of tubes.

Other things to look at would be the heater voltages. If your heater voltages were to rise by say 10%, that could put them outside of their normal tolerance range and shorten the lifespan on all of your tubes.

There are a lot of things to consider, but most people turn a blind eye to them. If you really wanted to be optimal about pulling a pair of tubes, you might want to consider replacing them with a dummy load so that your PT remains functioning in it's intended load range. I'm probably the only guy who bothers to do this.

[Chuck H]

I don't think that post reads so much like the Crate-peg guy as the "I invented led cathodes" guy (and it could be he). At any rate, it's not so much wrong information as terribly incomplete with all it's caveats to the normal mo of matching the impedance to the tubes. So at face value it should be taken to the top of a freeway overpass and thrown at a passing car. Keep balancing your push/pull sides and keep matching the impedance when pulling tubes.

[Mike Sulzer]

This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*

*That is, the person who wrote the response in the attachment to the first post of this discussion.

[g1]

This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*

Agreed Mike, if you want to stay with the original load impedance. Alternately, you can maintain the original turns ratio and change the load impedance. This is the most common, simple solution when the amp is already built.

[Chuck H]

What the guy in the OP's attachment DIDN'T say is that you can't run tubes out of spec like that at any sort of stressful levels and also can't expect the same level of performance when doing it. The average guitar monkey armed with only that post for advice is going to blow up parts in their amp. (<that's a period)

[Mike Sulzer]

Agreed Mike, if you want to stay with the original load impedance. Alternately, you can maintain the original turns ratio and change the load impedance. This is the most common, simple solution when the amp is already built.

But that solutions side effects:

1. The percentage losses in the transformer approximately double. That is, winding resistance is bigger than it should be because there are too many turns and the wire is too small.
2. The leakage flux becomes more important. This can cause a loss of high frequencies.
3. The magnetizing inductance is larger than it needs to be, and so the small signal bass response has gotten better, but the power handling capability in the bass has not improved.

So if you really want to double your power, you need a bigger transformer wound correctly to specs.

[alexradium]

he said that primary impedance doesn't change,i'd say there is no absolute primary impedance value,only a primary/secondary relationship that can also change depending on the load/ no load condition.
That guy is a total BS and should present himself with his real name to prevent people from disaster.

[Chuck H]

he said that primary impedance doesn't change,i'd say there is no absolute primary impedance value,only a primary/secondary relationship that can also change depending on the load/ no load condition.
That guy is a total BS and should present himself with his real name to prevent people from disaster.

Yes, in that statement he was dead wrong. He probably meant, thinks or believes that because the transformer turns ratio is constant. But a transformer is an coupling device. The primary to secondary are not mutually exclusive. Altering the circumstances on the secondary absolutely does have an analog to the primary and if you don't correct for it then you get inefficiency and losses. And, as already pointed out, you get losses even when you DO correct for impedance, but not usually a big deal for guitar frequencies and not such that you shift into dangerous operating conditions or noteworthy power losses.

[Enzo]

Transformers may be designed for certain impedance ratios, but the transformer itself has no impedance, it only has a ratio. If someone says they have a 6k primary OT, it is not something you can measure with a meter. It ONLY presents 6k ohms impedance to the tubes IF the specified load is connected.

So the primary impedance changes with the load impedance. That is why they wind OTs with multiple output windings or taps.

[bob p]

This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*

...So if you really want to double your power, you need a bigger transformer wound correctly to specs.

All valid points, but wasn't the OP's question directed at pulling a pair of tubes from a quartet type amp, in order to halve the power, not adding a pair to double power?

[Chuck H]

It's just discussion of peripheral aspects about the subject at hand Bob. g1 and Mike drifted into some general considerations. It's all food for the forum.

[bob p]

Oh, I understand the food for the forum thing. But the OP's question was about tube-pulling, not modding a PP pair amp to a PP quartet amp. From a practical standpoint, there are other things in an PP pair amp that will preclude the idea of adding 2 tubes to increase power, like lack of space, PT limitations, OT limitations, etc. I understand the value of theory, but I guess I'm too grounded in practical application to understand the deviation toward the physically impractical/impossible considerations. I hope that hasn't confused the OP.

[Mike Sulzer]

Oh, I understand the food for the forum thing. But the OP's question was about tube-pulling, not modding a PP pair amp to a PP quartet amp. From a practical standpoint, there are other things in an PP pair amp that will preclude the idea of adding 2 tubes to increase power, like lack of space, PT limitations, OT limitations, etc. I understand the value of theory, but I guess I'm too grounded in practical application to understand the deviation toward the physically impractical/impossible considerations. I hope that hasn't confused the OP.

Sorry to be confusing. So let's pull out one tube (of two) on each side. We continue to drive the same transformer. Entirely independent of how the taps to the load are adjusted, or the load changed, there is a potential problem. We have only one tube per side providing the magnetizing current to the primary inductance. Since this current increases with decreasing frequency, it becomes a problem at twice as high a frequency, and we lose bass.

If you include the low frequency specification, transformers are designed for a specific impedance.

[g1]

Agree with the theoretical concerns, differ on the practical ramifications.
I'd think this would be more of an issue with hifi than guitar amps, but point taken. It's still real and I imagine the tx designers have a specific primary z to build around.

[Mike Sulzer]

Agree with the theoretical concerns, differ on the practical ramifications.
I'd think this would be more of an issue with hifi than guitar amps, but point taken. It's still real and I imagine the tx designers have a specific primary z to build around.

I agree that it could be of no practical concern in a guitar amp. Or maybe it could sometimes. Output transformers used in guitar amps can be smaller than those used for equivalent power levels in HiFi, and so the safety margin for 80 Hz might not be as big as it might seem.