[Videoshielded]

I’m getting ready to upgrade my MG Head DT by putting a pair of constant current sources (CCS) on the plates of the driver tube. As long as I had to disassemble the amp to pull the plate load resistors and put in the connecting leads for the CCS circuit boards, I did a couple other mods I’ve been putting off:

Coupling caps – I pulled out the Auricaps I’ve had in there for over a year and replaced them with Dayton film and foil caps from Parts Express, part number 027-458. They cost $1.38 each; the Auricaps are about $7.75 each. Both are true film-and-foil caps (as opposed to metalized film caps, which don’t sound as good IMHO).

Shunt pot – I shunted the Alps volume pot with a pair of Caddock MK132 resistors, size 47K. The easiest way to do this is solder one resistor so it connects the input of the pot (the one with the wire from the RCA jack on the back of the chassis, duh) to the wiper of the same channel of the pot (center terminal, connects to the “input” terminals on the circuit board); then do the same thing with the other resistor on the other channel of the pot.

Just for giggles, I also redid the negative feedback circuit so I can switch it on or off with the “high-low” toggle on the front panel. Then I put everything back together and tack-soldered the plate load resistors back in, so I could be sure everything was working well before I add the CCS boards this weekend.

The Dayton caps are good – just as good as the Auricaps, I think. I’ve used them in other projects and been similarly impressed.

The shunt pot mod – amazing. I’m on record as hating volume pots. It’s astounding how cruddy they make things sound. But there’s really no room for a resistor-attenuator in this chassis; even the DACT looks too big. The shunt mod helps a whole, whole lot. Best example I can give is one track I like on a Latin jazz CD recorded in Havana; it has a strange background noise in the studio that I could never quite sort out. Some kind of percussion, but I couldn’t tell what. Turns out, it was raining like hell. Now I know.

 

[grinch]

Quote:

Originally posted by Videoshielded The shunt pot mod – amazing. I’m on record as hating volume pots. It’s astounding how cruddy they make things sound. But there’s really no room for a resistor-attenuator in this chassis; even the DACT looks too big. The shunt mod helps a whole, whole lot. Best example I can give is one track I like on a Latin jazz CD recorded in Havana; it has a strange background noise in the studio that I could never quite sort out. Some kind of percussion, but I couldn’t tell what. Turns out, it was raining like hell. Now I know.

awesome set of mods! glad they came out great.. and being able to hear rain in the background is damn smooth.

 

[sneared]

Videoshielded,

Can't wait for the listening tests!

What kind of circuit are you using for the constant current source?

I've been real interested in these since I saw/heard some Cary amps.

Care to post a schematic?

 

[Nick Dangerous]

Camille's C4S mod! Yes! I know we have discussed this via PM so I'm happy to see that you are taking the plunge. Should be a terrific modification. Do keep us informed... this could be a very significant upgrade!

Speaking of mods, I have been reading a lot about the 6N1P tube. It's a $6 tube in current production that is similar to the 6DJ8 but uses a higher filament current. I hear that it outperforms the 12AT7, 12AX7, and 6922 types... even expensive NOS ones! If this is true, it would be a GODSEND!

I have decided to tweak my Paraglow amps and swap out the 12AT7 driver tube for a 6N1P. Additional wiring and component changes will be necessary... but I'm expecting promising results. I'll post an update when I complete the modifications.

 

[braillediver]

Can somoeone post the details of Camille's CS4 Mods?

6N1P interesting. I'm just collecting parts for a Morgan Jones Mini Head Amp that uses 6DJ8's.

 

[eric343]

I may have to try that shunt mod... Sounds interesting!

 

[Videoshielded]

I'm still working out the kinks on the constant current source mod. The circuit consumes current, and it looks like the power tranny in the MG Head doesn't have a lot of current to spare, so I'm having to drop the circuit bias as well as work upstream through the power supply in an effort to balance things out. Unfortunately, I'm out of town for the next several days, so it will be later in the month before I have full information on how to implement it. But, my experience putting the same circuit in other amps has been fantastic -- much better than coupling cap upgrades, etc. Better clarity, drive, and much tighter bass control, not to mention great isolation from any residual nastiness in the power supply.

The C4S circuit is proprietary. You can buy it for $35 at: http://www.bottlehead.com/et/adobespc/C4S/c4s_kit.htm

The kit comes with a great manual, boards, and parts. You need a few resistors in sizes that depend on the application. When I've got it successfully implemented in the MG Head, I'll post the resistor values. For all you compulsive tweakers, the ASL AV8's are next in my sights.

Sorry for the delay, but duties as daddy and boss take precedence. I love audio, but I love my family and my job much more!

 

[Nick Dangerous]

Cool... we'll be here.

I'll be working on that 6N1P Paraglow mod.

 

[mezzman]

wow, I just got the caddock resistors today and soldered them to the Alps pot on My MG Head OTL, and what a difference!
There is much less grain during quiet passages, I can't wait till I get the Raytheon red label windmill getter 5751 tomorrow

!

The Auricap couplers(1.5uf), Solen fastcaps (20uf 250v) at the OTL jack, Riken resistors in the signal path, UF1004 diodes with snubber, pot shunts, and better power supply caps are really shaping up the quality of the MG Head (which was really good to start with).
All I have left to get is the Equinox cable for my HD600's and I think I'm all set!
This constant current source is very interesting, can't wait for the results...

 

[Videoshielded]

Very cool! I've done many of those mods myself (though as you know I just took the Auricaps out of coupling duty in favor of Brand X).

I really love the tube you mentioned -- it's my favorite in this application. I think you'll be impressed.

I took another whack at the C4S this morning and didn't make any real progress yet. Pulling out R14 got my B+ on the plates of the 6Bq5's up almost high enough, but I'm not getting sufficient voltage differential off the C4S circuit at the plates of the 5751 yet. Not sure why. I have a couple more tricks left to try, but then I might have to beg assistance from the Bottleheads again. Stay tuned. Sorry.

 

[Videoshielded]

Okay, the Bottlehead Corp. constant current source (C4S) is finally installed on the plates of the driver (center) tube in my MG Head and debugged.

It sounds really, really good. The results are consistent with other amps that I've put the C4S in. Much clearer sound, much better attack and decay, and much better bass control. It seems like it's also giving lower frequency response, but it could just be that the bass is better quality.

First -- don't try this mod unless you know how to modify tube equipment without hurting yourself. Tube amps pack lethal currents. For example, at least one other member of this forum has tried to do previous MG Head mods I posted with the amp plugged in and turned on. Fortunately, it shorted to ground and threw the breaker in his household breaker box. It could've just as easily flat-lined his heart. Also, try this at your own risk of messing up your equipment, yadda yadda.

Second -- this is for an MG Head DT. I've never seen the inside of the new OTL version, so I don't know if the main board is the same. I think it probably is. If somebody wants to send me a picture, I'll look it over.

The way this works is you first measure the plate voltage you're getting on your amp. With the amp off & unplugged, use clip leads to connect the red lead of your DVM to the bottom end of R3 (the end closest to the tube) and the black lead of the DVM to a ground point in the amp. Then stand back & turn the amp on, and read the voltage after about five minutes, once it's settled. It should be in the range of 50 volts to 70 volts, but it may be a little more or less. Power down the amp, let the caps discharge, and do the same thing at R4 (the other plate of the driver tube). Write both values down. Power down & discharge the caps again.

Next, remove the circuit board from the MG Head, and desolder the two plate load resistors from the driver tube. They are 100k resistors located at R3 and R4 on the MG Head board. Then cut four pieces of 22-gauge solid core insulated hookup wire, about three inches each. Put these wires in the four holes vacated by the resistors, with the long ends sticking up on the component side of the board, and solder the wires to the pads on the underside of the board. Also, examine the vacant spaces on the board marked C5 and C6. There are two holes on each of these spots, and one of them is for a ground connection. Cut two more pieces of the hookup wire the same size as the others, and solder one wire in each of the ground connections at C5 and C6, in the same way as the ones that replaced the plate load resistors. Finally, remove R13 (10K resistor) from the board and replace it with a short wire jumper. Reinstall the MG Head circuit board back into the MG Head. [DON'T mess with R14, notwithstanding my prior post above.]

Assemble the two C4S boards as described in the Bottlehead kit manual. Follow the instructions for the current source circuit -- not the current sink circuit! Use the right transistors included for the current source version.

You have to provide your own R1 and R2 for each of the two C4S boards, because these values are specific to each circuit application. For R1, after a lot of tests, I settled on 2.37Kohms, rated 1/4 watt or more. For R2, I settled on 60Kohms, rated 1/2 watt or more. I'd suggest you start with these values.

Connect one C4S board to each plate of the driver tube, using the wires you installed on the MG Head board. For each plate, the high voltage/B+ wire is the one at the top of the plate resistor spot (the end farthest from the tube socket); connect this to the B+/Input terminal on the C4S board. Connect the output of the C4S Board to the plate of the tube using the wire at the other end of the plate resistor spot on the MG Head board (closest to the tube socket). Now, as you can see, you've replaced the plate load resistor with the C4S circuit. Get it? One last thing -- you have to connect the ground terminal of each C4S board to the ground of the MG Head circuit through the wires you installed on the MG Head board at space C5 or C6.

In my case, I trimmed the hookup wires before I soldered them to the C4S boards, so as to install the boards parallel to, and almost touching, the bottom plate of the amp. As a result, I had to insulate the heck out of the inside of the bottom plate. If for some reason one of the boards breaks free, it's carrying almost 130 volts & I don't want that shunting to the external case of the amp, even if it is grounded! I tried some other positions, and one of the boards always seemed to pick up hum from the AC filaments. Anyway, the 22 guage solid core wire is stiff enough to keep the boards from flopping around, so you don't need to worry about mounting them with standoffs or anything (so long as the bottom plate is insulated!).

Fire up the amp again with the tubes installed. If the LED's don't light up, post a message to the Bottlehead forum and they will help you get it straightened out.

If they do light up, remeasure the plate voltage with the C4S installed (positive lead of your DVM to the output of the C4S board; use clip leads & observe appropriate safety precautions, yadda yadda). It should be pretty close to the original measurements (use the same tube, because they vary quite a bit usually). If it's way over, you need a slightly higher value for R1. If it's way under, you need a slightly lower value for R1. Post your original measurements and your C4S measurements to the Bottlehead forum, and somebody will help you recalculate.

I strongly suggest that you do not attempt to do any other mods at the same time as you install C4S boards. If you do, and the amp doesn't work, you'll wear yourself out trying to figure out whether it's the C4S boards or the other mods that went astray.

Once you're done, relax and enjoy!

 

[Nick Dangerous]

Congratulations! I was wondering who would be the first to try this mod. After wiring up my Paraglows with new C4S boards, I realize that I have two left over. Either I'll buy another MG Head and try the mod, or send them to you. I'm CERTAIN you will have a few MG Headders contacting you in the near future...

 

[andrzejpw]

This sounds great! Now, too bad I don't have an MG head...

So Nick, making a comeback tour?

 

[tanfenton]

Videoshielded,

I've offered little resistance so far. CCSs? Why not!

Leave no part stock,
NGF

 

[tanfenton]

Videoshielded,

What material did you use to insulate the bottom plate? Speaking of coverings, were there any positives to using shielded wire in the run between the power transformer and its switch--between the tube sockets? What other wires do you see as having high potential for noise induction?

Turning 180 degrees: What kind of power tubes are you running?

Many thanks,
NGF