Ultralinear vs Distributed Load in the Late 70s through Early 80s Fender Twin Reverb

Okay, I kind of played it fast and loose on the technical side with my last two updates concerning the "Ultralinear" Twin.

If you'd like a primer written by someone much more well versed in the effects of connecting the screens to taps on the OT, read this: Oestex - Ultra-Linear

Say we're designing an amp, and we want to connect the screens to taps on the primary side of the OT. If we connect the screen taps to the OT center tap (0% of the primary turns), we get pentode operation, albeit with no ripple filtering. if we instead connect the screen taps to the plate taps (100% of the primary turns) we have triode operation. Anywhere in between we get what is known as "distributed load" operation, which has performance somewhere in between pentode mode and triode mode.

Pentode mode has higher output, but also higher distortion. This is how nearly all of guitar amps are typically connected. Certainly all amps you can buy in the store. Many guitar amps have a "half power" switch, sometimes properly labeled as a "triode/pentode" switch which allows the user to pick which mode the output tubes are connected in. These switches aren't very popular with users (though they have some fans) partly because the loudness doesn't seem to drop to half (pesky logarithms!) and partly because the tone is much duller in triode mode.So, I'm sure the average guitarist has some familiarity with amps with this switch. To summarize for guitar purposes:

Pentode:

• Higher output
• Brighter tone 
• Crunchier distortion
• More distortion

Triode:

• Lower output
• Darker tone
• Muddier distortion
• Lower distortion 

Well, friends, there's a whole spectrum in between 0% and 100%. Someone back in the 50s (or possibly the 30s, there are disagreements about who/when) figured out the optimum ratio for balancing high output and low distortion and they decided to call it ultralinear. It turns out (hah, "turns") that the optimum screen tap turns ratio for hi-fi is in the ballpark of 40%, though the optimal tapping point is different for every power tube. At this magical percentage, output actually increases (slightly) over typical pentode connection, the amount of distortion decreases to something similar to triode connection, and the resulting tone is somewhere in between.

Brilliant stuff if you're building a stereo. We're building guitar amps; who cares? Cut to CBS/Fender in the late 70s.

Some clever engineer realized that they could slightly increase the output of their flagship Twin Reverb by using an ultralinear OT. Fender, even in Leo's day, was always about maximum clean power, so this makes a lot of sense. CBS (and Leo, admit it) also really liked pinching pennies, so the prospect of saving the cost of a choke and a filter cap must have played into the decision. To polish it off, they upped the plate voltages (and by extension the screen voltages) to 500V and blam: the Twin is now putting out 135W. At least by guitar math. Remember that the 100W Twin was identical to the previous 85W Twin. It's pretty easy to move the goal posts when measuring output power, especially if you're selling amps to musicians.

Anyway.

So it's been largely assumed that CBS/Fender used the typical 40% taps for the balance of high power and low distortion. But they didn't!

I measured the turns ratios in my Twin's OT and here's what I found:
Ra-a = 2.26k
Screen tap at 12.5% of the winding 

Wild, eh? Pretty far from 40% of the turns. Here's a graph of the power, distortion, and output impedance of various tapping percentages for a 6L6GC:

Table courtesy of jazbo8 and bob p over at M-E-F

 The striped box indicates "ultralinear" conditions, and the green box indicates where Fender put their taps.
So we're basically still in pentode mode! Huzzah, the "Ultralinear Twin" is really pretty close to just being a Twin!

As we can see, the power is slightly less than true-pentode connection (yeah, yeah, these are "beam tetrodes" but they're wired up like pentodes so go stuff it), the distortion is a hair lower, and the output impedance is lower as well. Those last two, well, just think of them as being effects of the NFB that you get from connecting the screens to taps on the OT. Needless to say, we absolutely do not need an additional global NFB loop, the kind you typically see on guitar amps.

So how is fender making more power if the screen tap they chose (or really any screen tap for the 6L6GC) indicates it should be putting out less power than a typical pentode-connected power amp?

That's where the increase in plate voltage comes in.

Now, Fender has always pushed the voltage ratings of tubes, and it's fair to point out that voltage itself isn't what kills tubes, it's how much current is flowing at that voltage which kills tubes. Literally, W = V*A. The typical solution is to include a screen grid stopper. When the plate voltage dips below the screen voltage, all those electrons that would have loved to keep whizzing by instead are drawn to the screen grid and screen dissipation increases dramatically. Putting a resistor on the screen grid means as the current increases through the screen, the voltage at the screen will decrease. Good ol' Ohms Law n'at. This is a huge concern at overdrive, when we're going to slam the plate voltage as high as it can possibly go and then back to as low as it can possibly go.

In a typical pentode-connected power amp, all the current goes to heating up the screen grid; it's wasted. In this distributed load connection, some of that current goes to the load - it makes more output - so the screen dissipation is actually staying constant. The formula here is P = EI - ei, where uppercase 'E' and 'I' are the DC voltage and current, while lowercase 'e' and 'i' are the AC voltage and current.

source: http://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_2/100_UL_Screen_Grid_Dissipation.pdf

Pretty neat, huh? Basically the rule of thumb for DL operation is "if the screen is fine at idle, it's fine up to full output." The question I have to ask though, is what happens at overdrive? This is how guitar design is still something of an uncharted frontier; overdrive conditions are largely dismissed in the golden-age literature (why the hell would anyone want to make distortion...?!). To meet this "if it's fine at idle..." condition, the screen voltage (and thus the plate voltage; with regards to DC they're going to be nearly the same) needs to be under the maximum listed on the datasheet. While there are some current-production tubes which list Vg2max as 500V, the historical standard is 450V, and that seems like the more reliable number.

Back to the guitar world, the screen dissipation at overdrive for 6L6s in Fender's distributed load experiment has been measured at ~20W. That's just a hair bigger than the 5W that they're rated for. So it seems fair to assume that these amps only don't eat tubes like a Mesa-Boogie would like popcorn because they're not routinely driven to overdrive. Reportedly increasing the screen stoppers to 4k7 can save the screens, but the output and tone suffer heavily.

To me, all this adds up to a poor tube choice for the iron. Sure, we could add a variety of bandaids to try and get the plate and screen voltages into the safe region of <450, but they all involve throwing power away as heat, which just seems stupid.

Personally I think the best solution is to run a pair of KT88s in the outer two sockets and double the speaker load (8R into the 4R tap, or what have you). KT88s will eat this plate and screen voltage for lunch, and at 12.5% they'll actually be closer to the ultralinear region:

KT88 distributed load characteristics. Source: oestex link above

Going off the datasheet values for a pair of KT88s with B+ = 510V and Raa = 4k5, we're looking at 100W of output maybe. Probably a hair less, something in the 85-90W region, which is preferable. The bias voltage will have to be ~10-20 ish volts more negative, so the headroom is going to certainly increase. Then again, since I removed the global NFB loop, the headroom is likely to be less than it was in stock condition. Also the tone is likely to change, but we'll see how much.

So, this is my preferred option because it requires the least adaptation of any tube change I can think of. There are other options, of course. So, assuming we want a 30 year old amp to make it last to 60 and beyond:

• Run the amp with the stock quad of 6L6GCs, preferably the most rugged you can find. This may be the Sovtek 5881/WXT whatever designation they're calling it now. Put in the largest screen stoppers you can tolerate and don't let it get too loud. THIS IS STUPID LOUD IS AWESOME TINNITUS FOR ALL
• Run the amp with a quad of 6L6GCs in pentode connection. This will involve adding a new power supply node, and if you want to stay all Fendery about it a choke and a new filter cap. If you don't want to try and stuff a choke on this crowded chassis a resistor will work fine. Looking at the Ra-a of the OT, you probably want the screen voltage to be somewhere in the 300-350V ballpark. If you connect the choke to the B+ node that's feeding the plates, you're going to want at least 1K resistors on the screens.
  
• Run the amp with a quad of KT88s or 6550s in distributed load connection. If a quad of KT88s will even fit, they're way too close to each other for safe heat dissipation. You'll need to add a separate heater supply for the preamp tubes. KT88 screens are more rugged than 6550s, so for distributed load connection these are preferred.
• Run the amp with a pair of KT88s or 6550s in pentode mode. You'd need to lower the screen voltage considerably for this option. Fortunately there's a nice node right between the stacked reservoir caps (where the PT CT connects). And yes, you can draw current from here without disrupting the voltage balance of the stacked reservoir caps! Check the 'evil twin' schematic; this is where Fender connects the plates for the 25W low power node. 
• Run the amp with a pair of KT88s or 6550s in distributed load connection. Double the speaker load. The heater current draw is slightly less than the quad of 6L6GCs it was designed for, so no problem there. KT88 screens are more rugged than 6550s, so for distributed load connection these are preferred. 

 Any change to KT88 or 6550 will require these mods:

• You'll almost certainly have to modify the bias circuit to safely run these tubes. You'll need a raw bias voltage adjustment and a balance adjustment as a minimum; independent bias pots for each tube are preferable.

•  Pin 1 will have to be rewired and connected to ground.  

•  The 'bear trap' tube retainers will have to be replaced with spring-type retainers.

•  The sockets may have to be rotated - check the Genalex datasheet for details. 

•  Additional cooling (via fan) is a good idea, possibly necessary. 

You know, I haven't looked into EL34s yet, because I certainly don't plan on adding an auxiliary heater transformer. Plus I'm a beam tetrode kinda guy. I'll leave the research into these and KT77s for someone else.

I am accepting donations at this time for funds to get a pair of KT88s. :)