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.