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!

More Progress on the Voxy Build

Just a quick update; making some tweaks to the Rockette. The Trainwreck Rocket completely ignores one triode of V1, which seems wasteful, and the AC30 dumps that output straight to the phase inverter, which seems silly. So, I got a little Plexi goin', and ran the "normal channel" into the next gain stage. Basically permanently jumpered channels that you can blend with the two volumes between bright and regular. Maybe I'll split those V1 cathodes if there isn't enough difference between the two volumes...

Moved some other things around, nothing too serious. Might add zeners on the cathodes? Maybe just on one pair? Maybe not, come to think of it.

Oh yes, and the parts are ordered, so the build will start shortly!

Latest Bodie Schematic

Okay, so I've learned to stop romanticizing vintage design yet again. I significantly increased the power supply filtering and oh my goodness is Bodie better now. Punchier with better bass response and less excess treble. I'm going to take this up another couple notches when I have more parts.

I also tweaked the bass half of a James tone stack for my bass control instead of the dual-coupling cap idea. The mid and high attenuation is very helpful, and now the possibility of bass "boost" can kick up a heavier butt for solo playing.

This is getting close to as good as it's going to get. I'm still tempted to tear apart the front end so that the second and third triode stages will be in parallel instead of the first and second. And I'm going to increase the power supply filtering a little more too, when I get some more power resistors. Knock these crazy voltages down a bit more.

Seriously, AES?

A COMPARISON OF CURRENT PRODUCTION 6 L 6 GC TUBES

CE Distribution / Antique Electronic Supply / Amplified Parts just put out this PDF where they plotted the frequency response of a single tube of each of the 6L6 family tubes they sell. 

 Okay, so this looks like a really useful document initially. They put a bunch of tubes on a tube tester, picked the average one, plugged it into an amp, plotted its frequency response, and let a guitarist subjectively describe the tone.

That last bit is a little like asking a mechanic what oil you should put in your car and then asking a guy who likes to drive how the different oils taste on bread, so I'm going to try to avoid commenting on "sizzling leads" versus "balanced and tight." If all you want is lurid prose about how many orgasms each tube brought the guitarist, there's no need to bring a tube tester or frequency plotter into this.

So ignoring the poet, we have the manufacturer's rated specs on the left. These are pretty much crap, as modern manufacturers largely just copy whatever was on the original data sheets. The maximum plate voltage is just a suggestion, let's be honest. Dissipation kills tubes, not voltage. And do we expect guitarists to know what their screen voltage is? Considering how common screen grid failure is, it would've been nice to include maximum screen dissipation buuut it doesn't matter anyway because the manufacturer's numbers are junk.

Back In The Day(tm), for a tube to be sold as a "6L6GC" it had to meet certain specs for plate dissipation, transconductance, envelope size, etc. No one cares about this any more, so it's understandable that there's some variation in rating and size from one manufacturer to another. Not to mention some of these tubes aren't even 6L6GCs, but Russian military tubes which were designed to be clones of 6L6GCs. That's another whole bag of worms, and it's fairly irrelevant, but it is interesting that some New Sensor 6L6GCs can take up to 40W on the plate comfortably.

What would really be interesting would be a comparison of transconductances, but considering a lot of these tubes are "close enough" to a 6L6GC I suspect there wouldn't be too many surprises. Specs on those Russian tubes (6P3S-E) are hard to track down, so it'd be nice to finally get some solid data. Anyway.

There are handy bar graphs of "lows," "mids," and "highs." This was kind of a mistake, or rather, how they define "low" and "high" is problematic. The open low E is 82 Hz, but this chart defines "lows" as 50 Hz. Even a baritone B string is 62 Hz, so the measurement of "lows" on these bar graphs is useless.

"Mids" are defined as 700 Hz, which is pretty reasonable.

"Highs" are defined as 6 kHz, which is not pretty reasonable. Most guitar speakers have already started rolling off around 5 kHz. 3kHz is around the "icepick" region and probably would've been a better choice.

To generate the frequency response graphs (and the guitarist gibberish,) they built a little single-ended guitar amp to put the tubes in. I take issue with a couple things here. First off, they didn't make a neutral Hi-Fi amp, they made a guitar amp. They describe the preamp as being a Blackface Fender design, and that's good for frequency plotting because there's certainly no limit to the low end, but unfortunately there's also a freaking tone stack! Yes, you can get a Blackface tonestack to be almost linear by turning up the mids to 10 and turning the bass and treble to zero, but did they do this for the frequency response plotting? Did they instead take the tone stack out of the circuit? These details aren't provided.

Anyway, these are graphs of just individual tubes so buying tube X and actually getting frequency response X is probably a crapshoot. Those little bumps are the result of manufacturing variances; if they averaged 10 of the same tube the responses would look much more similar brand-to-brand. Plotting dB on a linear scale instead of a logarithmic one is a little disingenuous too; even speaker manufacturers don't try that. Anyway, the biggest difference they show is roughly 2dB which is just under the threshold of what the human ear can discern as a difference in volume.

There is also the sizeable mid-hump which all the tested tubes exhibit. In the guitar community, 6L6s are widely considered to be somewhat "mid scooped" and the swap to EL34s will return these missing mids. At the very least, we can now clearly show everyone that the mid scoop comes from the circuit around the tubes, not the power tubes themselves. I suspect this bump in the frequencies is more related to the circuit, than a characteristic response of a 6L6.

In this case, the circuit these power tubes are plugged in is quite simple. Like I mentioned above, we have to assume they took the tone stack out of the circuit before measuring the frequency response of each tube. The circuit itself is single-ended, with no negative feedback. This is potentially a huge oversight - most guitar amps use a global NFB loop around the power tubes and phase inverter. I honestly can't think of a single 6L6-based amp that doesn't. I'm sure they're out there, of course.

The point being that NFB reduces distortion - the output is fed back to the input out of phase, so if the tube generates a boost at the output, that gets fed back to an earlier stage as a notch, and the result is a flat response. So, it makes sense that you would remove NFB if you wanted to plot a theoretical response of each tube to see if there really are significant differences from brand to brand.

But what happens when you put these tubes in your amp?

Even without NFB, a push-pull amp will cancel a noticeable amount of (even ordered) distortion. As the fluctuations in frequency response are largely sporadic, there would be significant cancellation of some deviations from linearity. Some would reinforce each other, so I suspect the overall response would look less choppy, though approximately the same.

With NFB though, all of that choppiness is going to cancel, so the only difference you're going to see is the overall y-axis offset - also known as the difference in emission & transconductance from tube to tube.

So to wrap this whole thing up:

• All these tubes are basically identical.
• There are some apparent deviations in frequency response at around +/- 2dB. 
• These differences are inaudible.
• Even if they were audible, most amps have NFB, so these deviations would cancel out.
• Only one tube of each brand was tested, so we have no idea if there are any trends among brands, or if CE Distribution just happened to select tubes that were basically identical. 
• This document is just a piece of marketing material, and shouldn't be viewed as anything but an advertisement. 

Bodie with zener-assisted cathode bias

I finally got around to trying the zener trick I mentioned a few months ago. Bodie's idle bias voltage was around 21V, and that surged up to 31V under heavy overdrive. A pair of 12V 5W zener diodes in series now clamp the cathode voltage at 24V. Maximum clean output power has risen from 19W to 23W. On the 'scope, crossover distortion is now only barely present during overdrive and the overdriven tone has increased rather dramatically. I think next time I'm going to have to try just straight fixed bias; 7591s lose so much in cathode bias. Not that I really need another 10-15W of output, but the tone is much improved. I can probably lower the plate voltages to something more sane than, say, 480V to keep the output level reasonable.

I'm going to be tweaking this amp for years, I just feel it.

For now, I still need to come up with a decent bass control. While I really like the "independent tone controls scattered throughout the preamp" approach, there aren't many very good one-knob bass controls. Sure, you can take the baxandall stack apart and just use the bass control from that, but it didn't work too well. It may be time to try a flat tilt control in place of just a dedicated bass control. That would give the user the option of cutting treble early and/or late, which would open up the preamp distortion characteristics a bit.

I'm tempted to start looking at FFTs of the frequency response to see if the NFB loop is causing any high-frequency strangeness. Given the (deliberately) limited bandwidth of the amp, square wave analysis is somewhat problematic, but it seems to indicate there's some unwanted phase shifting at high frequencies resulting in excess treble.

Then again, I might not notice that if I weren't using these vintage EV SRO speakers. It might even be good with a particularly dark speaker, but I feel a well-designed amp should work well with any speaker, though that may be something of an impossible goal.

Anyway.

It seems I lucked out picking a voltage for the zeners to latch the cathodes to; the plates are happy at the dissipation they're subjected to under heavy overdrive. I wonder if I could go a little colder though, get a little more squish out of the output section.

And I'm still thinking about the power supply. It's working fine, but could it be better...?

Yep, years of tweaking ahead.

New Crate Plans

Alright, so, I've decided to take the Crate rebuild in a different direction - Vox land. I was wondering if anyone would be interested in a single channel AC30 with no reverb or tremolo and it turns out yes, they call it a Trainwreck Rocket!

Now, of course, I'm going to have to deal with heat. I suppose I would have anyway (seeing how I don't work for Crate and thus am not comfortable hanging tubes below a completely sealed chassis) but cathode bias ups the heat load I'm going to have to dissipate. Also I'm going to put in (switchable) sag resistors to emulate tube rectification. Needs moar recs!

Here's what I've come up with so far. Still a lot of tweaking to be done. It's based on what people assume is in a Trainwreck Rocket, as well as the original AC30 top boost channel. I've added a handful of tweaks throughout; we'll see how they work.

• Parallel first triode. Why the hell leave it unused. Same gain, lower noise. 
• Arc protection on the cathode follower to help the tube survive switch-on.
• Grid stopper on the cathode follower. Just something I feel like trying. Figured I'd split the difference between 10k and 1M for starters. 
• Reduced grid leaks on the LTP to reduce noise. Reduced cap across the second input to help recovery from blocking distortion.
• Lar/Mar PPIMV
• Sizeable grid stoppers on the power tubes for better distortion.
• Power tubes biased in pairs, partially to spread the heat around, partially so one pair can be pulled. I might do individuals, we'll see.
• Zener-assisted bias for the power tubes. Cathode-biased 6V6s seem to distort nicely anyway, but this might be worth looking into.
• Switchable sag resistor for a "tube rectified" feel. Might make these bigger.
• Heaters referenced to power tube cathodes for free elevation. Humdinger to get the last of the noise out.
• 1k screen grid stoppers. 
• Grid stoppers everywhere. 10k for starters. 

Layout is going to be a big pain. All the tubes are in the center of the chassis, so I'm going to have to get clever. I'm tempted to start drawing it up in DIY Layout Creator or sort of CAD program. I may just use paper. It's going to have to be all point-to-point, but since this isn't going to be as much of a ground-up as Bodie was I'll be able to plan better.

Welp, I've been spending even less time in front of the computer lately, but I'll post again when I've got more to go on.