These are terrific amps and thankfully quite under rated. They can be had cheap. Similar to a Deluxe Reverb but more of my flavor: EL-84 output tubes for that more "British" sound, better tremolo and more of that wild man reverb than the more refined Fender amps have. So wild that they can run away. A couple simple mods can tame that. In this case I just added a .002 cap from the reverb return to ground and bypassed the 1 meg pot with a 1 meg resistor. This maintains the more crazy and trashy reverb without making it too tame.
Other than that the amp had a crackle and hum problem. Solution was easy: at some point one of the output tube sockets was replaced. The tech put an old socket in but failed to remove the tarnish off the solder points. It had 4 cold joints! So I removed the wires, hit the points with some sandpaper and re-soldered. Problem solved.
Customer loaded this with some fresh "Mullard" EL-84 tubes. They seem to work in this amp just fine. These amps tend to run hot. Most EL-84 amps have about 300 volts max on the output tubes, this one has 398 with each tube running at about 30ma. Nothing is red-plating and it sounds great so I let it be.
Here's a shot of the inside. Built very much like an old Fender amp! Very sturdy, very high quality....
I also changed the electrolytic caps on the circuit board. Someone replaced the ones in the bias circuit but did a sloppy job so I took care of that. The big filter cans can be changed at this point but I left them. Amp runs quite, hum free. Changing the ones on the board cleaned up some crackly stuff.
Notice it's loaded with those lovely Mustard caps. Mmmmm. Yeah, I'm a big fan of the old Traynor amps.....
If this were my amp I would probably change the tone stack myself, maybe set it up like an old Marshall by adding a cathode follower circuit. I recall doing that a long time ago for a client and being really happy with the result. But, I can get used to this. And hearing my client play it, he sounds better than I do naturally. This is his amp and he knows how to work it!
JB
I’d post this comment on a post about a vox amp, but I didn’t see one.
ReplyDeleteCan we discuss EL84’s for a minute? Particularly in the AC30. I picked up a working AC30C2 chassis for a hundred bucks. The amp had been in a flood and the MDF cabinet disintegrated, but the chassis and tubes survived just fine. I figured if there was ever a low stakes opportunity to experiment on an AC30 circuit, this was my chance. I’ve looked at enough Vox schematics over the years to know that the meat and potatoes of the old AC30 circuit is still at the heart of this amp. Yeah it’s on a PCB now, but… you know… fun challenge. I decided to get in there and replace the crappy Chinese coupling caps and resistors with good stuff. A guy named Lyle Caldwell started doing this with Mallory 150’s and Sozos, but I took it a bit farther. I had a nice pile of Mustard caps, tropical fish caps and carbon comps on hand, so in they went. Put a an old Millard in V1. The amp sounds better!
I decided the next step is to replace the Chinese EL84’s. On the advice of a fellow ampaholic, I bought 3 matched quads of Russian 6p14p tubes. The first set red plated, the second and 3rd worked ok, but were far darker sounding than the stock chinese tubes and just sounded sorta meh. Now, mind you, Dr. Z swears by these tubes. A quick internet search finds discussions of the Russian tubes not liking all the current the AC30 throws at it. I suspect that has something to do with it. So here’s my question… I have no idea what these stock tubes are. They literally just say CHINA on them. So, Jef, will you talk a little about modern production EL84’s? Do you have any favorites? Particularly wondering which modern EL84 sounds killer can handle the edge-of-destruction operating conditions of the AC30? Thanks.
Really it's all very subjective.
ReplyDeleteFirst a quick note about current production EL84 tubes. I've had little experience with the Chinese made ones. What I recall is like other Chinese tubes they sound good to my ears. I actually like the regular old Sino 12AX7 myself in a pinch. But the power tubes I've come across are firecrackers. Pop!
Next would be the Russian glass. I haven't had any of them red plate on a Vox, and in an old AC30 they were always my choice due to their ruggedness. I can see why Dr Z likes them. If they are run conservatively they do tend to last. Sonically speaking though, they aren't my favorite. Flat and glassy, definitely darker. The plus side is they tend to have a good punchiness though on the low end.
When I worked in Los Angeles I met Dave Friedman and did some work for him. He was a strong proponent of the Sovtek EL34 in old AC30 amps.
I do find the Sovtek glass to need a good long "burning in" period. Of course this could be purely psychological! My brain will hear something different if I want it to be different, but then again, maybe my experience is true: let them burn for a few days and see what your ears tell you. Still too dark or meh? Move on.
Now, the AC30 is cathode bias, class A. Those little bottles are running hot. Over the years I saw many with a resistor added in series to the 50 ohm bias resistor to run the amps cooler. Russian glass does tend to draw a lot of current so I would start there: Add a 10 ohm to 50 ohm resistor and see what you get. COuldn't hurt. Just make sure your cathode bias cap isn't being overloaded with voltage.
My personal favorite sonically is the JJ EL84. They have that sweet thing I expect from an EL84. Closer to the old tubes. Very musical. However with those you want to make sure they're not biased too hot. They won't last in an old AC30.
Now notice I'm focusing on old AC30s here. I actually have very little experience with the new models. The bit I've seen, I don't think much of them. I don't like anything about the ones I've played though admittedly we're talking about 10 amps or less.
I'm a big fan of the 90's-2000's Korg produced amps, made in England. Those aren't as rich sounding as a true one from the glory days, but as a workhorse they are actually better for a working musician like myself. Good and reliable. They're no longer cheap unfortunately, but can be had for less than $2k. So if I had a choice I'd spend that extra money as it's well worth the investment. Just my opinion.
What don't I like about the new ones specifically? I think they generally sound dull. Clouded... caps can help but then I'd say replace the transformers with better iron too, and maybe the speakers. But the time one does all that you could have bought a better amp.
Thank you for your question! J
Lol you beat me to it… I swapped the output transformer for the Mercury Magnetics Haddon AC30 clone. The speakers are Scumbacks. A scumnico and a S75-pvc (Killer speakers. Jim makes good stuff). I needed a cabinet since the factory one was long gone… bought the north coast music Baltic birch cab. Yeah, I absolutely spent too much, but the AC30 is on my amp bucket list and projects are fun!
ReplyDeletePeople hate on JJ sometimes, but I’ve heard good things about their EL84. That’s the tube I most wanted to try in this thing, but I hear AC30s eat them alive. My plate voltage is right at 315v with the Chinese tubes. Will increasing the cathode resistor to 60 ohms increase the plate voltage? What else could I do to make this amp a little more hospitable to the JJ EL84 without sacrificing the amp’s character? Run it on 110v mains voltage?
I like some of the JJ tubes some of the time..... but not all of them and not all of the time.
DeleteKeep in mind, the amp is cathode biased so yes if you raise the bias resistor value then you also raise the plate voltage but..... you need to subtract the voltage on that cathode resistor from the plate voltage.
For instance, if your plate voltage is 315, and there is 12 volts on the cathode resistor, then the tube is actually seeing 303 volts on the plate.
Say you raise it and the plate voltage goes up to 320, and that cathode voltage goes up to 20vdc. The tube will be seeing 300v. (These are hypothetical numbers of course.)
Try that first and see what you get. There are other things one can do to lower the plate voltage like use a 5U4 as a rectifier. Just make sure the 5V winding is rated high enough current wise to handle that tube.
J
I’ve been pondering this for a week or so while I was waiting for my pile of assorted 10w resistors to show up. The AC30C2 has a solid state rectifier, so can’t swap out a rectifier tube. But what about the Zener diode trick on the HT center tap?
ReplyDeletehttps://robrobinette.com/images/Guitar/5E3P_Build/Zener_Diode_Stack.pdf
My plate voltage currently sits at 315v. If I put in a Zener string to drop the B+ 20-25v or so, would that be a better solution than fooling around with the bias resistor?
That will do it. I've only done this once in 28 years but it did the job.
ReplyDelete