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. :)

Ultralinear Twin - Triode Mode Update

Just one quick update - there's one more recommended mod going around for these ultralinear Twins: wire two of the 6L6GCs in triode mode and leave the other two screens hooked up to the ultralinear taps. I just tried it and my goodness, what an improvement. Certainly warms the amp up nicely, and the amp breaks up even a little earlier, which makes sense as the output power has been reduced.

The bright switch is useable now though; even with a Strat it's no longer a death sentence via icepick.

If the rumors are correct, I have Kevin O'Connor to thank for this trick. Brilliant stuff. 

The Twins!

...Yes, plural.

Okay, so about a month ago I managed to pick up these two, erm, beauties:

Someone order 235W?

On top we have a Fender PA100 from ~1975, and below a Twin Reverb from 1981.

Yes, that's right, the dreaded 135W ultralinear Twin! I was really happy to get this one, even though it's in somewhat rough shape cosmetically, because it's a vintage amp that doesn't have much value (certainly not compared to the Twins even 5 years older) so without worrying about ruining a museum-grade amp I can tweak it into being a great player. On the whole I'm not thrilled with calling an amp that was made the same year that I was born "vintage" but that's how it goes.

The PA100 is also exciting - these were sold as PA heads, of course, but in reality they're Twin Reverbs, minus the vibrato, plus four preamps. All four channels are bridgeable, and while the input impedance and tone stacks aren't too great for guitar currently, those are both easy fixes. I haven't done too much with this one yet as I've been spending my time on the UL Twin.

So, without further ado, here's what I've been working with:

Yikes.

Yeah, it's a real mess in there. At least one other person has been in this amp sometime in the past 33 years - there's the occasional carbon film resistor, and an odd red wire used for the master volume ground connection. The caps, of course, were original:

emphasis on "were"

So, here's where the fun begins, and I haven't even gotten to the schematic yet. The plate voltage is hovering around 510V-520V depending on the day. That's the plate voltage loaded. We're over 530V unloaded. That's a big stupid problem with this doghouse layout! I don't have quite enough room to put caps in series at each node (though I'm going to be looking for small 350V or 300V/47uF caps to try this in the future) so I'm stuck using the only 600V rated electrolytics around. I got these ones from Weber, though I'm sure they're the same as the ones you can get from RadioDaze or other vendors, just with Weber's wrapping on them. Pretty sure the manufacturer is MIEC as they're the only company I can find putting out caps with that voltage rating.

Now, I don't care the slightest bit about mojo, and everything I can find about these caps indicates that they're reliable (I haven't had any problems so far) but down the line I'm going to either look into making a new board with totem-pole caps for each node, or knocking the unloaded plate voltage down under 500. Probably the latter.

Alright, so what's in this beast?

Nice try, CBS.

Okay, so in the interest of brevity I'm not going to go through this part-by-part and list what is designed "poorly" or even "not Blackface-acceptable," and instead I'm just going to post the current as-modded schematic:

Not that much red for a huge improvement in tone.

Now.... now, we bust out the list. Bullets!

• Horrible pull boost and all associated wiring removed. As far as the components for the pull boost, I removed the 12k resistor and left the other components on the board. 
• Screen grid stoppers raised to 2K/5W in hopes of extending tube life. Also introduces distortion a little earlier. 
• Bridged the 2 channels, so now reverb and trem are on both. 
• Removed the bright cap across the master volume. Seriously? How could that have ever been a good idea?
• Removed the death cap. 
• Lowered the output coupling caps to 22n. With the 68k grid leak, this raises the -3dB point to a hypothetically uncomfortable 70 Hz (105 Hz with the 47k) but the bass response is still quite bone-crushing, which is a refreshing change from the overwhelming mud it was with the 100n couplers. 
• Changed the pull-boost MV to a global NFB disable switch. Well, it's normally open, so when you pull the switch it engages the global NFB loop, which maintains the "pull to sound worse" functionality of the original. And sure, a 1M feedback resistor isn't "completely" open-loop, but it's awfully close and this way the switch doesn't pop.

It's pretty close to done, I gotta say. There are some other general maintenance things I've done - replaced the fiber shoulder and flat washers on the 8 ohm jack, and temporarily installed a pair of Peavey Scorpions because the Utahs it came with sound significantly worse.

Oh hey, I should mention the output jacks. This is weird.

• Plugged into the main jack, and nothing in the extension jack, we get a 4 ohm out.
• You can then plug an additional 4 ohm cab into the extension jack. The switching jack puts the external cab and the internal speakers in series, and connects them to the 8 ohm tap on the OT. 
• You can instead pull the main jack, and plug an 8 ohm cab into the extension jack. 

Me, I think they overthought it, I probably would've just gone with a 4 ohm jack and an 8 ohm jack. Decently clever I suppose.

I should mention the poor, abused screen grids in this amp. "Ultralinear" is a configuration for beam tetrodes and pentodes where the screens are connected to taps on the output transformer instead of having their own DC supply. From a sonic perspective, this is a form of local NFB, and now the screen dissipation can also contribute to the output of the amp. You combine this with the much-higher plate voltage, and that's where CBS came up with the 135W claim. (Really ~100W before any clipping, at least according to my scope. Still, this is more than the ~80W clean you really get from a typical "100W" 4x6L6GC amp.)

There are rumors that this amp was designed around a special 6L6GC that CBS got Sylvania to make with significantly more rugged screen grids, but I haven't been able to confirm that yet.

The problem with an ultralinear output section in this amp is the ridiculously high screen grid voltage - roughly 1-2V over the plate voltage, so roughly 60-70V over the 6L6GC's spec for a maximum screen grid voltage. This by itself isn't a huge problem - dissipation kills tubes (rather, electrodes inside them), not voltage. The problem is that the screen grid dissipation in the original schematic gets nuts pretty fast. One person on MEF measured the screen dissipation at roughly 20W per tube (I'll link when I find the post again) which is just a weee bit over the 5W the screen grid is rated at.

In a typical (pentode-connected) output section, the trick is to use a larger screen grid resistor - increased current draw drops the screen grid voltage enough that the resulting dissipation is safe. This is known as "sliding screen" operation, and is a pretty awesome trick. Typically 1K is large enough to accomplish this. The poster at MEF indicated that he needed to use 4K7 screen stoppers to get the dissipation under safe limits, and with stoppers that large the output and tone suffered.

So, we're trying 2K for now. Tubes seem happy enough, the output is plenty, and the added distortion (rather, distortion occurring at lower volume) is welcome.

With regards to "blackfacing" and the future mods I have planned:

The "bias balance" system is poor, and I'm going to change it to a "bias balance and adjust" two-pot system. Considering how beat up these tubes get, I'm not particularly interested in running them any hotter right now. I forget the numbers, but they're on the 50-55% range of plate dissipation right now, and the output looks fine on the scope so this is kind of a low priority.

The phase inverter typically gets the brunt of the "blackfacing" parts substitution. Honestly I see nothing wrong with the phase inverter as it is - it's better designed for a 12AT7, delivers some massive output voltage swing, and is well balanced. I may check the biasing, but those lower plate resistors (47k vs the 82k/100k "traditional" values) are really helpful for driving those 68k grid leaks - which are helpful for keeping the tubes from going into thermal runaway, and minimizing blocking distortion. I could write an essay here, but this is one section where CBS actually got something right.

The tremolo is ticking, so I'll try some of the typical fixes for that.

The reverb driver is biased somewhat uncomfortably warm, so I may try the blackface spec for that.

And someday, someday, I'll look into getting the B+ down. The amp sounds great now, so I have no interest in converting it to a typical pentode-connected output, and I haven't been eating tubes so this really isn't a high priority.

There are some cosmetic issues to address (Scrubbing Bubbles works great on Tolex!) but a full restoration would be ridiculously cost-prohibitive, so for the forseeable future this monster is going to be a little rough around the edges but louder and more reliable than the average apocalypse.

So for now, this one's good. I'll be starting in on the PA100 soon, and I've got to get going on that Voxy build. And updating more frequently. Bah!