OT relationship between tube impedance and speaker impedance

[5thumbs]

Okay, So my question is: If I have a Fender Twin Reverb and pull 2 of the output tubes - one from each A/B pair. Would the amp perform better with a complimentary change in the speaker impedance? How do I find the winding ratios for Fender OTs? A friend wants to take his Twin and make it lighter and not as loud. He wants to take it down to a single 12" speaker and lower output with the removal of 2 power tubes. Thanks for your help and sharing the knowledge!

Tim

[jazbo8]

I think this was covered not too long ago on another thread... IIRC, if you pull two tubes from each side, then you should move the speaker connection up one level, e.g., from 4 to 8 or from 8 to 16, that way, the reflected impedance on the OPT's primary will be doubled for a lighter load on the tubes.

[5thumbs]

Thanks jazbo8,
"move the speaker connection up one level"? Does that mean increase the impedance of the speaker load from 4 to 8, 8 to 16 or just move the existing speaker to a different jack? I guess I was asking for the math for this. I have found various forum replies and seems they are about half and half as to whether the new speaker impedance load should be doubled or halved, so it is a little confusing and I was hoping if I had the formula I could wrap my head around the concept better.

Tim

[g1]

If you remove a pair of power tubes, then you need to double the load impedance.
If the two speakers are wired in parallel, you can just disconnect or remove one and you should be all good.

[bob p]

To that I would add that when you remove a pair of tubes, check the filament voltages to see if they change. if they rise out of spec, then you'd be wise to add some load resistors.

The previous posts are right -- There is a recent in-depth thread about calculating OT impedance ratios if you want to search for it.

[5thumbs]

Thanks for the info! So if I remove a pair of 6L6s from a Twin (and filament voltage stays within spec), I would plug an 8 ohm speaker into the 4 ohm transformer tap? Have I got it right? Sorry, I used to smoke pot in the 60's. Now I smoke at any temperature ;-) (bad joke)

[pdf64]

It may be beneficial to consider the power rating of the remaining speaker in operation.
The amp's power supply will be stiffer and so the power output is likely to be more than half the previous level.
Plus a single speaker will be 3dB down on available sound pressure level (and is likely to have a lower perceived fullness of tone) than the previous pair, so it may be pushed harder.
The power output of twin reverb models covers a wide range, so more detail of that would be needed before hazarding a guess as to what the minimum rating should be with this arrangement.

Additionally the tone at clipping may be harsher as the ratio of global negative feedback on the power amp will have increased (I think - still trying to get it thought through properly).
Pete

[bob p]

Yes, when you dig into the numbers, you'll see that when you pull a pair of tubes... everything changes a little bit. Although everyone likes to rely on the old rule of thumb that says pulling a pair of tubes will cut the amp's power in half, that rule of thumb is only superficially accurate. There are lots of other considerations that will change along the way: reflected load impedance changes with the speaker/tubes, with less load on the filament supply the filament voltages might rise out of spec requiring the use of a dropping resistor, with less load on the B+ supply the B+ voltages might rise, if the amp is cathode biased, then the bias conditions can change dramatically unless you do some cathode resistor & capacitor switching, if the amp has fixed bias the bias may still change due to the change in B+, and the remaining pair of tubes will likely be worked a lot harder than they were before the number of tubes was reduced.

What's really the point of pulling the pair of tubes? IMO it really depends on what your objective is.

If your objective is to reduce power output, then you're wasting your time -- If you pull 2 of 4 tubes and you compensate for all of the changes that have been discussed, all that you'll realize is a 3 dB drop in power output from the amp. That's an awful lot of jumping through hoops for -3dB, and IMO it's much easier to gain 3dB of SPL reduction by just dialing back on a volume control.

If your objective is to conserve tube life, and reduce the number of tubes that you buy over time, then pulling 2 tubes will help you. IMO that's the only real benefit. The benefits of power reduction are insignificant, so you're still jumping through a lot of hoops to avoid buying a pair of tubes. Not worth it IMO.

Probably the most important consideration when addressing this topic relates to a question that you didn't ask -- what's the benefit of the Boogie-type cathode power switching circuit that "shuts off" one pair of tubes. IMO, that circuit amounts to a marketing ploy to get people to believe that the amp does something really special when it really doesn't. For the cost of a $5 switch, Boogie gets people to think they've got a great dual-power amp. The reality is that with cathode switching, you get about -3dB change in SPL with a lot of other parameters that become mismatched, and you still burn up the filaments on the tubes that are "off". IMO that circuit doesn't provide any real benefit. It certainly doesn't change volume significantly and it certainly doesn't prolong tube life. But it does get people to buy Boogie amps. I guess that's the point.

This business about pulling a pair of tubes? I don't think it's worth the effort. If you want volume reduction, then a volume control and/or an attenuator are better answers. Just my $0.02.

[loudthud]

I think the idea that 3dB is barely noticable came from work done by Fletcher and Munson early in the last century. IIRC it was an isolated test where subjects judged a volume level and then came back the next day and judged it again. Playing in a band is a whole nuther animal. If you have ever mixed an eight piece band, you can go over every mic and adjust the levels up or down to fine tune the mix, then look at the faders and most have moved only 1 or 2 dB. 3dB can be the difference between barely there and out front.

I'm working on an amp based on the Iron Horse Collision (TW) Express. The B+ was a little on the high side and I wanted to try the VVR idea so I hooked up an independent heater supply and used a variac on the B+ and bias. 6dB (half voltage) is a major change in volume. You might not notice 3dB playing solo in a room when you come back the next day, but playing with a drummer, you'll notice right away.

The OP in this thread wanted to make a Twin Reverb lighter and not as loud. Removng two tubes and one speaker would certainly make it not as loud, but I don't think the weight savings would be worth the trouble. Get something like a Pro Reverb if you like the Fender blackface sound.

[bob p]

My recollection is that the limit of perception is 1dB to 2 dB, and that 3dB is what amounts to being "significant" from a perception standpoint.
-6dB is equivalent to doubling the distance from a source. Of course that's very significant.

But is it worth re-designing an amp to get a difference in 3dB? Don't think so. I think the entire tube-pulling thing is much ado about nothing.

[Diablo]

You guys need to try to lift a Twin Reverb, it's a back-breaker. Even worse if it has JBL speakers with the heavy magnets.
Pulling a speaker may help fix that problem, again it depends on the weight of the original speakers.
He could also swap to a Neodymium magnet speaker.
The other part of the weight problem is the 100 watt transformers, and there is no easy/cheap solution for that.

[Zozobra]

I don't think there are any elegant solutions to this problem. All I can suggest is maybe adding VVR/power scaling to the power supply but that isn't trivial with such a high powered amp. Alternatively you could think about replacing the speakers with some which are much less efficient. As for the weight, you don't have many options. Combos are just heavy beasts. Maybe get a dolly truck or something?

[R.G.]

It occurred to me to do an explanation of some of this.

Tube matching to loads is not a thing which has a single right answer, which is what you'd expect from how often a "non-matching" speaker load is glommed onto the output jack.

During the "Golden Age", power tubes were designed, then carefully pored over by the guys back in the lab for what their characteristics were, and how well they would sell. Tubes were to the 40s and 50s what transistors were to the 60s and 70s. Design a new one, and if you sold a lot, you got rich.

The evaluation of a new power tube always involved seeing how much power it could put out. This involved how much current it could pull through the cathode without destroying the cathode and how much voltage it could stand on the plate without arcing over or melting the plate when that maximum cathode current flowed. Once those limits were known, the business of deciding how much AC power could be passed through it started.

They produced graphs of output power versus loading, and also output power versus distortion in most cases. Both curves had broad "sweet spots" where the tube did best. There was a wide range of conditions where output power was maximum, and a similar minimum where distortion was lowest.

It turns out that for almost all tubes, the highest power loading condition was NOT at the same loading as the lowest distortion condition. This accounts for the hifi guys wanting to run 6L6 outputs at 6600 ohms plate to plate, but the guitar guys wanting to use 4000 to 4400. The guitar guys wanted more power, and were perfectly willing to accept more distortion. The hifi guys would accept lower power if they got lower distortion.

As for the tube operation itself, in most cases the incremental output characteristic of a tube in pentode/power beam mode was flat - it looks like a "constant" current source, so the details of what the loading did to the micro-burbles on the tube were largely insignificant. The tubes simply ran between highest voltage and highest current.

Changing the AC load on a tube changes one of the endpoints of the max-plat-voltage to max-cathode-current line. If you put a higher-impedance load on the plates, that means that the voltage changes faster than the current, so you run out of available voltage to swing before you run out of current the tubes can provide. If you put a lower-impedance load on the plates, you run through the available current faster than the available voltage. In both cases, the total power out goes down.

Changing the AC loading at the plates involves changing the AC loading on the secondaries. The loading on the secondaries is reflected to the primary and loads the plates, and this is in parallel with the unchanged primary inductance and series leakage inductances. Changing the reflected load in parallel with the primary inductance and leakage inductances changes the amount of power going into the referred load versus the primary inductances, so *the bass response of the transformer from primary to secondary* changes. This accounts for the tone changes.

What is complicated about pulling tubes and moving load taps is that you're messing with two different transformers in one. One is the normal primary-secondary impedance transformer you think you're using, and then the autotransformer set up by the secondary taps.

An output transformer is designed for a certain set of operating conditions. Normally this is for either one or two pairs of tubes.
The primary impedance will be set by the selected operating conditions for the tubes. For one pair of [for example] 6L6s in a guitar amp, this would be around 4000 to 4400 ohms plate to plate. For two pair, 2000 to 2200 plate to plate. But it could be lower or higher, depending on the preferences of the transformer designer. Small differences will only cause insignificant differences.

This is the impedance the tubes see from the secondary. The primary inductance is then designed to be some number of henries which is at least that many ohms at the lowest frequency of interest. Often, the primary inductance is bigger to much bigger than this minimum inductance, to get better than minimum bass response. Once the core is selected and the primary wound, the primary inductance is fixed.

The load from the secondary is reflected by the transformation ratio. A winding intended to show the tubes 4400 from an 8 ohm load will do that. If you put 16 ohms on it, the tubes see 8800 ohms. This means that the tubes will run out of available voltage (which hasn't changed) before they can swing their max current, and the total power output goes down. The primary inductance didn't change, so it eats relatively more of the signal swing on the plates compared to the higher impedance reflected in parallel with it, so the bass response suffers a bit.

If you put 4 ohms on it, the tubes will see 2200 ohms, and will reach max current before they can swing their max voltage, and total output power goes down. The reflected secondary load eats more of the available plate-to-plate power than the design point, so the bass response gets extended a little.

So much for the first transformer, the one you think you're dealing with. The second transformer is the autotransformer effect of having a tapped secondary. If the secondary has taps at 4, 8 and 16, and you put ONE load on ONE tap (plus common), then the designed load is reflected to the primary. 4 ohms on the 4 ohm tap makes 4000 on the plates, 8 makes 4000 on the plates, 16 makes 4000 on the plates.

If you put a 4 on the 8 ohm tap, it reflects at the "8-ohm rate" but is half as big, so it makes half as much appear on the plates, or 2000. If you put 4 on the 16, the plates see 1000 ohms.

If you put 8 on the 4, it reflects at the '4-ohm rate' and the plates see 8000 ohms. If you put 8 on the 16, the plates see 2000 ohms.

If you have an amp with four tubes designed for 2000 ohms on the plates and with 4, 8 and 16 taps (for instance):

With matched loads on any one tap, 2000 ohms appears on the plates. Pulling one pair of tubes makes this the remaining pair of tubes working with a 2000 ohm load, which is a 2:1 lower load impedance than the one pair can drive to its max-power point. Note that it's BOTH half the available power due to half the tubes driving it, and a lower power as determined by the maximum-power curves for that tube type, which may or may not be as much as half the available power for a single pair of those tubes.

You can correct the loading back to what you'd get from a single pair of that tube type by increasing the plate to plate loading by moving the load on the secondary. To get back to a 4000 ohm plate load for the remaining single pair, you would put twice the load impedance on a secondary tap than it says to put there. Putting an 8 ohm load on the 4 ohm tap, or a 16 ohm load on the 8 ohm tap. In either case, the reflected load the plates see rises to the 4000 ohm nominal that lets it do its best power for a single pair.

Again, this is max power for a single pair, half the power for four tubes.

The bass response will change as noted earlier because of the change in reflected secondary load against the unchanged primary inductance.

[bob p]

You guys need to try to lift a Twin Reverb, it's a back-breaker. Even worse if it has JBL speakers with the heavy magnets.

Backbreaker?!? Gimme a break. A Twin Reverb weighs 69 pounds. With the JBL speakers it's not any heavier than my Super Twin Reverb with a pair of EVM-12L.

Both of those are lightweight combos compared to an SVT. My SVT-2 Pro head in it's rack case weighs about 90 lb. and I dead-lift it to sit it on top of a Mesa Road Ready 2x15 that weighs 165 lb.

If a Twin Reverb is a back-breaker, then maybe this will help:



Just kidding! I hate lifting all of this stuff, and I never look forward to carting any of it up a flight of staris!