Bogen Overhaul

Okay, so I was playing around with the Bogen and I finally faced the fact that the preamp was distorting too early and not making enough voltage gain to drive the power tubes to full (clean) output. I've been tossing around the thought of boring out the 7-pin socket hole and putting in a noval socket so I could use a 12AU7 instead of the 6C4 - leave the second the same (as the cathodyne PI) and use the first triode as a recovery gain stage after the cathode follower.

Adding another tube wasn't sitting well with me, and after some consideration I pulled the cathode follower and I'm setting up the preamp as three gain stages:

This is going to have to be tweaked... a lot. I have all the controls in though, so that's something. This preamp should have considerably more headroom, and the overdriven tone should be a lot better, which is good because there's going to be a decent increase in that too. Fitting all that into this chassis is going to be... trying. From here on out, it's going to be turret boards. Turret boards, all the way down.

It's probably going to need more interstage attenuation. Also, now I'm going to reference the heater center tap to the cathodes of the power tubes for a little elevation to help with any noise, as I don't need to worry about a huge Vhk anymore.

It's disappointing that I have to move on from the previous version, but those are the breaks. I'm still planning on doing something with the previous design - maybe a gain stage, then the paralleled stage, then a DC cathode follower, then a recovery stage, then a cathodyne? That's only 3 preamp tubes, might as well add a 4th for reverb. Though I do want to do more work with pentode-based preamps... and that Crate amp that's begging to be turned into a vintage Marshall - I wonder what a Plexi preamp would sound like with a floating paraphase phase inverter? Blues from hell I'm guessing. Not to mention that I haven't even dipped my toe in ultralinear output stages yet.

So many ideas, so few parts.

Yet Another Bogen Update

Okay, so I've finally picked bass and treble controls. They'll likely need to be tweaked, of course. Most amps have tone controls which attenuate the whole signal so you can add in some "boost." Here everything is just cut, so a flat response will be with all the tone controls all the way up, if you really want a sort of Tweed sound.

I might switch the bass control to the one you find in the James stack but I've found even with open-back cabinets you really don't need any bass boost. The response from this bass control will be more like the Orange FAC control - which I've found fantastic - but a little easier to dial in instead of just having a choice of 6 settings.

Bogen Update

Spent the past couple days mostly working on the Bogen. I decided to skip the modified Fender tone stack and instead I'm going to put discrete independent tone controls throughout the amp.

All that's currently built so far.

I've done the usual tweaks - add something then take it out, take something out and then put it back. My thought for a treble control was adding a Vox "Cut" control - a cross-line low pass filter tied across the grids of the output tubes. You can find it on the Vox AC30 schematic. Pretty clever! If you'd like to try it out, this post explains the math simply. I get the feeling that Thévenin equivalent circuits would be handy for stuff like this, but I'm trying to fight it for now. 

Anyway, the cut control didn't work too well for me - the treble roll off was nice, but I think it was loading down the output too much to be useful. I might be back to this eventually, but there are a few other places to hang a treble pot, so I'll try those first.

 I went through the (admittedly simplified) math to design a bridged-t notch filter for a mid control. Not entirely my idea; check out the Gibson GA-30RVT schematic. Man, old Gibson amps were so creative for guitar amps, at least in hindsight. Fender amps were straight from the RCA playbook, apart from that tone stack. Anyway. 

So I went through the math and came up with a great bridged-t, and then I checked Merlin's preamp book and he pointed out that the taper is kind of awkward on the control as Gibson had it, and presented an example of a slightly different bridged-t with a great gradual cut which didn't shift the notch frequency. So I took that. It's fantastic, really changes the character of the amp.

The heater voltage was over 7V, so the antiparallel diodes on one leg brought that down to a more reasonable 6.6V. Still haven't figured out a good place to put the heater elevation.

The overdrive sounds a little nicer without the cathode bypass resistor on the output tubes, but I prefer the transient response and higher output of having it in. Definitely something to remember if I ever build a high-gain cathode-biased amp at some point.

The bootstrapped cathode follower sounds much better with a hefty grid stopper. That 100k is dragging the high frequency response down to ~7 kHz, thanks to the massive gain, so I expected I'd need a bright cap with the gain pot set low, but it seems fine without it. Probably doesn't help that I play Telecaster and  Jazzmaster clones. 

Current plans

So, that's what I'm planning on doing next. Might tweak the cathode follower's load; the compression is nice, but now that everything's built I've found that maintaining headroom is quite a balancing act. The cathodyne phase inverter's main downside (w.r.t. guitar amps) is that it doesn't generate any gain, so all your voltage swing has to come from the preamp. It's a good match for the 7591s here (they're biased at around 18V - almost as easy to drive as EL84s with twice the output power and a decidedly 6V6/6L6 tone) but I wish that 6C4 were its dual triode equivalent (12AU7) instead. Actually, given that the heater voltage is running so high I might have been able to drill the chassis before I started and swapped in a noval socket. Ah well. Or I could undo the paralleled input stage, but it sounds so good I really don't want to. 

The issue right now is that the lion's share of the voltage gain is coming from that one bootstrapped stage before the cathode follower. Surprisingly the overdriven tone is really good too, and I think the cathode follower smoothing it out is a big part of that, but it also helps that the power tubes are breaking up so soon. Higher values of load resistor on the cathode follower should help the headroom a little bit.

I could probably talk about this amp all day, but depending on how it sounds after I get the bass and treble controls wired in I think I'm going to knock the input stage's gain back down to where I wanted it originally. It's hard to explain why, in words; there's just something about how the headroom feels. 

And yet I'm already looking forward to the next build. I'm thinkin a badass Antek toroidal power transformer, maybe wire the Crate amp up for a pair of EL34s instead of the quad of 6V6s it has now. It wants so badly to be a Marshall, might as well let it...

Bogen Update

I sat down to wire up the phase inverter and man, I was just on a tear - got the phase inverter, the cathode follower and the bootstrapped gain stage preceding it, and the gain, master volume, and presence controls all wired up and working beautifully. Oh, and I tweaked the power tubes. All that's left is:

• the first (paralleled) gain stage,
• heater elevation,
• tone stack,
• tweak the power supply,
• tweak the NFB,
• tweak everything else.

Yeah, that's all!

Dear god, never again.

Yeah, that's a real mess. It's working and it's stable but man is that ugly. I really wish they'd mounted all the tubes in line so I had room for a turret board or something. This chassis has been a huge pain every step of the way, and "local star" grounding isn't helping appearances any. With the gain and master volume all the way up there's a faiiiint hint of hiss with your head right next to the speaker, but I still have to wire up the first gain stage and the tone stacks, which will likely both add noise, but I'm also going to elevate the heaters, which should reduce (some) noise. Well, it'll reduce hum, but there isn't any hum (yet.) I'm using 1W metal film resistors throughout and that's probably keeping the hiss at bay. Carbon comps would be a nightmare.

The noise performance should still come out better than average, but there are a couple grounds that I got lazy on (presence pot right to main star ground, OT grounded to main star instead of local ground where NFB returns) so I could wire those differently if need be.

Here's the latest schematic:

Not a whole lot has changed:

• I had to flip the OT primary leads because (surprise!) the negative feedback turned out to be positive feedback with the leads the way they were. I swear to god, output transformers are the USB connectors of the tube amp world. No matter how sure you are you're putting it in the right way, you've always got to flip it (often twice.)
• I did change the 7591s' grid stoppers to 220K. The datasheet value for the max grid circuit resistance for cathode biased 7591s is an optimistic 1M. Ordinarily people make the grid leak resistors larger, but thanks to the low output impedance of the cathodyne there's no real reason to do that. 220K seems large for a grid stopper, and historically it is, because a grid stopper forms a low-pass filter with the input capacitance of the tube it's connected to. In a triode stage you might start losing some highs, but the beautiful thing about beam tetrodes (and pentodes) is the alarmingly low input capacitance thanks to the screen grid. The ordinary values of 1-5K roll off everything above ~300 kHz. With 220K I'm rolling off everything above ~72 kHz, which is nearly two octaves above the limit of human hearing. Considering the average guitar speaker rolls off everything above ~6kHz, we're not in any danger of losing any "airiness" or "sparkle." Hell, I might take the grid stoppers up to 470K, giving me a roll off above 30kHz and reducing the blocking distortion even more. The blocking isn't bad right now, but I'm curious. Plus, NFB stability, right? Yeah, like I need more excuses to tinker...
•  The 7591 cathode bypass cap is 1000 uF because it's the closest value I had handy. 
• The NFB resistor is too large.
• The presence cap is too large.
• The cathode follower's cathode is sitting at 211V; definitely have to elevate those heaters.

Like the overeager child I am, I wired up an input jack across the gain control and took it for a test drive. Understandably, a guitar straight in isn't getting me anything close to full output, but using the line out on a practice amp as a crude preamp worked.

How it sounds right now:

• So much high treble and low bass. I was testing this with a jazzmaster clone, so that wasn't helping, but I won't be adding a tone stack lift any time soon. Pretty sure that bright cap will never get engaged so I might not even bother wiring it up. But what else would I use that switch for? Hmmmm....
• Not a whole lot of sag - the power supply seems pretty stiff, even though it's a voltage doubler with a modest reservoir cap, and the power tubes are cathode biased. Then again, I did add that whopping cathode bypass cap, and there's still another pair of triodes to install. Not that a couple milliamps of constant class A current draw will make a huge difference, but we'll see.
• The clean tone is full, lush, sparkly, you name it. I do miss built-in reverb, but I've got pedals and I probably overuse reverb anyway. A great clean sound was the primary goal here, and that's absolutely a home run.
• The overdriven tone just barks, with a slight bit of beating from the blocking distortion. The transient response is great. Right now I'm pretty much only hearing power amp overdrive and it's fantastic, but I've got to get the preamp finished before I know the whole picture. The blocking distortion adds a splash of ugly and I think it might be better with a little less of that. Maybe not, and maybe I'm just stuck with it anyway. 

Been a good day in the basement! 

Dynaco Mk IV Recap

 

 Okay, these amps are straight up beautiful. If you're unfamiliar with them, they're basically power amps (with phase inverters) - a pair of EL34s pushed by a 7199 pentode/triode. The pentode is a gain stage while the triode is wired as a cathodyne phase inverter. As these were designed to be hi-fi amps the cathodyne is a great choice - fantastic balance and linearity well beyond the audio spectrum. While the long-tailed pair is de rigeur for guitar amps, pretty much every guitar amp manufacturer has used this in at least one notable build: Vox AC100, Fender Tweed Deluxe & Princeton Reverb, the classic Orange amps, and the early Sunn amps.

Dynaco sold them both assembled and as a kit the users could build themselves. The instructions are quite thorough, and you can check them out at this link. These amps are so quiet and reliable that they still have a strong following of devotees. I don't want to say fanatics, I mean, these are tubes after all so that's a little redundant. (How do you know if someone's a tube addict? Don't worry, they'll tell you.)

These units had sat for a few years and the customer was interested in using them both for Hi-Fi as well as guitar amps, and wondered if any mods would be necessary for guitar use. Nope! In fact, the very first Sunn amplifiers were just Dynaco preamps and power amps built into a new chassis and retubed. It's kind of a fascinating story, how The Kingsmen's cover of "Louie, Louie" changed the amp world forever. I'm not even remotely joking.

  

Moving on! The customer had built these himself back in the day and modified the (somewhat silly now) output sockets to standard wiring posts, as well as a convenient post for checking the bias. How clever is this company - a lot of amp builders and owners these days will put 1 ohm resistors between the power tube cathodes and ground as an easy way to measure idle plate (cough and screen cough cough) current. Dynaco ran both cathodes through a 13.5 ohm resistor to ground. Why such an odd value? Because when using the specified 6CA7/EL34 output tubes the bias measurement across this resistor is 1.56V, or as they say in the instruction manual, exactly what the voltage of a fresh D cell battery would be, so even if you weren't too solid on your multimeter skills, you had a standard you could compare to. Different tube types require a little bit of math but considering there are a couple Dynaco-devoted forums out there I'm sure someone else has done the math already.

  

The customer wanted to keep these original so I got a pair of CE Manufacturing cap cans. This company actually bought Mallory's factory and cranks out cap cans to their original specs. Can't get much more original than that! Dynaco calls these "quad caps" - there are four sections, so fair enough. One thing to note is that the capacitors that were in these were only rated for 500V, while the schematic calls for 525V rated caps. With today's higher line voltage, 525V is a necessary minimum, and many replacement cap boards use even higher rated caps. The reservoir capacitor here is 30 uF, which is a little low in the modern Hi-Fi world - with the GZ34 rectifier you could inch up to 60 uF - but these amps are incredibly quiet, so it's clear that more here really isn't necessary. If you're going to use them for guitar power amps, people report negative effects from excessive filter capacity.

There's actually a study some professor and/or grad students did where they "fixed" everything that was "wrong" with a Fender Tweed 5F6 Bassman - I'll update this when I track down the link - and they found that greatly increasing the filter caps they ruined the overdriven tone of the amp. You can design around massive filter caps if necessary - and for many modern high-gain metal-type amps it's a good idea to - but for classic guitar amps it's usually advisable to keep the filter caps close to the same size as they were originally. For Hi-Fi, of course, go nuts, just don't blow up your transformer if you're using a tube rectifier. Data sheets are your friends!

  

Here are the insides as they were originally. I only took pictures of one of these babies, but they were identical inside. There are a couple things to note: a 22 nF cap on the input and 100 nF caps in parallel with each filter cap. The 22n cap was installed to cut some bass as these had been used alongside a subwoofer, and the 100 nF caps were added to decrease the ESR of each filter stage. If you're having problems with excessive ripple, adding a film cap in parallel with your electrolytics can definitely help. Excessively large poly caps in parallel can lead to motorboating though - I've seen both "rule of 100" and "rule of 1000" for deciding how small the parallel film cap should be. This is another trick that can be handy in high-gain designs - we're talking amps with input sensitivities around ~20mV - but if you're building something like that then elevating and/or rectifying your heater supply is probably a more prudent place to start.You can't go wrong with an elevated humdinger, and if you can run your heaters at 12.6V instead of 6.3? Sheeeeeeeeeeeit. Rectifying might be easier if you're starting from scratch, but elevation is definitely necessary if you've got directly-coupled cathode followers.

Too far down the rabbit hole? Yeah, this never ends. You always have to say "good enough" inevitably, unless you've got bottomless pockets, endless time, and don't mind going insane. These Dynacos? Yeah, with the new filter caps you can't even tell that they're on and there's a small-signal pentode in there. That's a testament to how well Dynaco designed these - no crazy layout or handfuls of decoupling caps necessary, and it's straightforward enough that you could build it yourself. If only today's amp companies had a similarly high "good enough" threshold...

  

Okay, back to the plot. Here are the guts with the new quad caps, new electrolytics in the bias supply, and the input coupling cap removed (by request). It's kind of a pain to strip but again I have to say how much I like this 630V teflon-insulated silver-plated 22AWG from Apex Jr.

The bias supply had a surprise in it, dead center in the "before" guts picture. That little green box? Booyah, selenium rectifier that'd gone bad! Fortunately it had just gone leaky instead of bursting into a ball of what I'm told is the most horrific smelling halogen gases imaginable. I've smelled some horrible organohalogens so I'm morbidly curious, but they're not mine so I didn't blow them up. Oh god, I can still taste 1-heptyne. Not an organohalogen, but if burning tires were living things, 1-heptyne is what they would excrete out after binging at Taco Bell. Still, morbidly curious about the selenium rectifiers. That's a chemistry student for ya.

And a shot of the finished exterior. 

I don't know how many more good things I can say about these amps. They're easy to work on, well designed, well laid out... just great amps, both for Hi-Fi as well as for use with guitar. 

One important thing to mention about using a monoblock as a guitar amp is versatility. Easily 80% of a particular amp's tone comes from the preamp. When you compare a lot of schematics, the vast majority of tube guitar amps have cookie-cutter power amps - usually with a long-tailed pair, sure, but cathodyne designs have tons of fans too. You can make this amp sound like just about any guitar amp you want to just by switching preamps. Imagine that, going from a Plexi or an Engl to a Twin or an Ampeg with just one stomp on an AB box. Heck, you can even use modeling preamps, or hook a computer up and use something like Guitar Rig. Even the line out/effect send on an existing amp will work, but make sure that amp still has a speaker load connected.

If you were looking solely for a guitar amp - say you found a monoblock cheap (good luck, btw) - there are a couple tweaks you might like to make. If your distortion ideal is something like an Orange, well, you're golden here. If you're hoping for something more like a Marshall/Mesa, you'd probably want to add a sizeable grid stopper to the cathodyne and lower the coupling cap values to get a nice even grind instead of the crackly farty "Hey Hey My My" tone. Of course, that's assuming you're running at full volume. If you're not planning on deafening everyone at the venue, you'd probably be fine with these stock. The dynamics when playing with separate pre- and poweramps will be a little different because they won't share a power supply, so when you're hitting the power amp hard and the supply voltage is sagging on the output tubes, the preamp won't be affected. Neither bad nor good, just y'know, a little different... possibly not noticeably.

Two Neil Young references back to back. Now I really want to post a Sunn O))) link.

Anyway, to wrap up the Dynacos: phenomenal all around monoblock amps no matter what you do with them, and a real treat to work on.