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Specific MG Head Upgrade Suggestions

post #1 of 8
Thread Starter 
Hey all,

I'm looking for some specific recommendations on upgrade parts and suppliers. For my MG Head DT, I want to swap out the caps, possibly the resistors, and possibly the pot. Probably the diodes too.

Recommendations on top notch parts (brand names / part numbers) would be greatly appreciated, along with suppliers for all of it.

Thanks and rock on!!

post #2 of 8
Thread Starter 
Anyone? Bueler? Bueler? Bueler? Videoshielded?

post #3 of 8
A while back, Jude posted some pics of a "hot-rodded" MG Head that he used to own. He also explains what was swapped out. I don't know the wheres, whens, or the hows of getting this done. But they are great pics.

The Jude-Meister's ex-hot rodded MG Head
post #4 of 8
Thread Starter 
Oh, yeah, that was in a thread that I started. Cool pics indeed. I'm gonna mod this thing myself but was curious about cap model numbers and suppliers for the parts and that sort of thing.


post #5 of 8
Sorry Dudeness, I was on vacation.

I decided to do my first overhaul without making any modifications to the original circuit design. I am extremely happy with the results. It's a different amp now. More of everything.

Diodes -- I used UF1004's from Digikey (UF1004DITR-ND), with a snubber I added myself, to keep the soft recovery spikes from being sucked back into the tranny. A simpler approach would be to use HEXFREDS, which don't need a snubber, but supposedly aren't as quiet as the UF's with the snubber. You can get HEXFREDs from Digikey; I think their smallest size is 3 amps, but you won't hurt anything going that large.

Power Supply caps -- the Nicihicon's in there are nothing special, so I replaced them with Panasonic EB caps from Digikey (P-5905-ND and P-5931-ND). They're the right physical size to mount easily on the board. I would've used Panasonic TSHB, but they aren't made in the right values. I think the EB's are supposed to be as good as the TSHB's.

Resistors -- I replaced all the signal chain resistors with Riken Ohm resistors from Angela.com. I have experimented with the Riken's in other tube projects and I like them a whole lot. I could have replaced all the power supply resistors with Dale RN types from Mouser, but I'll leave that for next time around.

Coupling Caps -- I had a pair of 2.2uf film and foil Auricaps salvaged from the output cap slots in a preamp. I had a discussion a long time ago with PPL in which he recommended using huge coupling caps to drop the frequency cutoff to the basement and improve bass response. In the MG Head circuit, 1.5 uf would've been big enough to hit is cutoff target, but I used the 2.2's because I had them around. The Auricap is a fast clean cap. If I had tried to go to 1 uf or more with a slower cap like a metalized polypropylene or a paper in oil cap, I'm not sure what would happen -- the bass might get pretty muddy. Also, there's not really room for paper in oils over .47 uf or so. If you try an upsized cap, use a fast cap like an Auricap or a Hovland. As for voltage rating, I'd get it up to at least 200 volts, because I think the caps see high voltage at turn-on before the tubes load. Auricaps are available from Michael Percy Audio, but it will take a LONG time because he's recovering from a move and a broken foot. Welbourne and Antique Electric Supply sell the Hovlands for faster service.

Wiring -- because the filaments in this unit are AC, I ripped out the unshielded filament wiring to the board and between the sockets, and installed twisted pair, shielded wiring it it's place. I used Canare Star Quad in the smaller size, because I had it around and it's very flexible. You can use pretty much whatever fits. In my opinion, you don't need to mess with this, unless you're having hum problems. I wasn't having hum problems, but my next round of circuit upgrades are likely to boost sensitivity and gain a whole bunch, and I was just planning ahead.

I also took out the attached power cord, put the chassis in the drill press, and installed a Neutrik Powercon jack from Mouser. If you were lucky enough to get an IEC version, you're already all set. For the power cord, I used Belden 19364 with a Pass & Seymour 5266-X plug. The Belden is a pain to find, so I'd suggest getting a Volex 17604 cord from Mouser, which is made with 19364, and then cut off the plug and IEC female, to replace with your own choice. The Schurter 4300.0603 IEC female is supposed to be good. If you want to replace the IEC chassis mount, the Schurter 6100.3100 is supposed to be good. I don't use IEC (I use Powercon) so I don't know.

Also, when you're re-installing the board standoffs into the chassis, I'd suggest using Lock-Tite on the screws to the chassis side. It can be a pain to remove the board again later, because the chassis screws loosen and spin when you try to unscrew the board screws on the other end of the standoff. But, it's not necessary if this will be your only round of mods.

You mention swapping the pot -- I think the stock pot is about as well as you'll do with pots. If you want to get a stepped attenuator, that would sound much better, but I'm not sure how you would fit it under the hood. Shunting the pot is probably your best compromise. I'll do that next time around, but I refrained from doing so this time, because it would constitute a circuit mod. Just get a pair of 47k bulk foil precision resistors, like Caddok from Michael Percy or Allied Electronics (or try the Riken Ohm's for a little warmer sound). Put one end of the reisistor into the board where the pot input signal connects to the board (leave the existing input wire between the pot and the board in place, connected the same way it is now). Disconnect the signal wires from the RCA's where they input to the pot, and solder them to the other end of the resistors. Leave the drain wire on the input wire shielding tied to ground, either at the pot or move them to a ground point on the board.

After the mods -- great sound. Really fat, clean, well-separated stuff with awesome pace. Bass is tight and +/- 2 db all the way down to 40 hz, and only about 6 db down at 31.5 hz. Still exhibits a treble hump at 4 to 6.3 KHz, but I've yet to find an amp/headphone combo that doesn't. And the whole deal, with the mods, is still tipping the scale at less that $350! Go for it.
post #6 of 8
Thread Starter 

Welcome home!

Can you take some pics of the modded amp? Was it easy to get 2.2 uF coupling caps to fit in there? The MusiCaps I'm looking at have axial leads. I haven't done the measuring yet, but it seems like it could get tight especially with the new orientation.


post #7 of 8
Sorry, I don't have any way to take pictures of the modded amp.

There was no real problem fitting the 2.2 uf caps in there, since I used film and foil, which is fairly compact. Installation with the radial leads is no problem. The stock caps have radial leads, for all intents and purposes.

As I said above, if you want to try PPL's theory of frequency cutoff, 1.5 uf is all you should need, which will be even smaller and cost less. Of course, if you want to go with more typical cutoff points, a 0.47 uf would be plenty, which would be smaller and cheaper still.
post #8 of 8
Thread Starter 
Hmm, I must have a newer version of the MG Head (mine has detachable cord if that's any indicator) because on my schematic there are no .22uF caps in the parts list. I do have .47uF caps listed, though...


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