Musicman RD50 mods - done!

[Jooshtin]

27th August 2007:
I finally sorted the hum (B+ filtering and UK mains...) and just couldn't help tweaking some more. The amp is played in and has been used in the studio on a few sessions and sounds fantastic! A very versatile little amp. I've now sold my Boogie MK3 - it sounded so dull and boring next to the MM. I've attached a couple of schematics for info.

- I mounted the OD channel on it's own board on the side of the chassis.
- Added a PAB which gives a nice fat "vintage" tone boost
- Added an op amp boost on the input, this retains the tone of the amp and can be set for a bonkers boost if you're like that...solid state clipping isn't pretty though...
- mid shift added, with all the boosts on gives a full on motorhead/sabbath od - rocktastic
- a 50pf cap over the limiter volume smoothed out the tone at high volume without changing the amp character
- upped the filter caps from 60uF to 100uF and the B+ cap initially to 20uF , then a bit more...
- the input pad on the od section is set for the recommended Dogears value of 235K with the pad in. This mellows the tone nicely, with the pad out the amp's got a sharper "Marshall/Fender" tone for when I'm in that sort of mood..
- a bright cap added to the overdrive pot
- dc raised the heater supply with a couple of 100R resistors off the 30v rail
- footswitch with leds for PAB and input boost relays
- replaced the speaker with an eminence Swamp Thang, it took a while to play in but sounds fab, big fat well defined tone
- I've used a couple of TAD short bottle power tubes and a Watford Valves selected TAD pre tube. Took a little while to "burn in" and lose harshness, but now sound very sweet.
- raised the B+ voltage to get as close as I could to 200v-ish on the plates

LeeMo - I'd certainly give the BobW mods a go as they sound great, I just went for a more vintage vibe, you can pretty much drop in the D*mble version of your choice in the OD section! And the input op amp boost is easy to do (don't have to remove the PCB if you're crafty...). These are great amps, have fun!

Cheers and huge thanks,

Justin



Hi all, greetings from a newbie in the UK, now it's my turn to ask for help...

I've been given a 1981 Musicman rd50 combo, the amp was dead when I got it - one of the power tubes had been put in wrong....no inserts in the valve bases to lineup the tubes! Thankfully the transformers all checked out ok and I recapped it and replaced the blown components - and as I've never been a fan of the limiter (distortion) channel on these amps I have given BobW's mod a go that was posted over on the music electronics forum. The clean channel works and sounds great, no noise or hum at all. The limiter channel is sounding good but has a loud hum - it's there whether or not the guitar lead is plugged in, the hum increases with the master volume for the channel. About halfway through the travel of the gain pot the hum decreases slightly, then rises again as the pot is rotated. I've changed R61 to a 1k 2w to get more voltage to the plates, and added a PAB relay and a grid resistor to the input of V1 - but it was humming before and the extra mods have made no difference. It's a 250volt model running on 240v uk mains.

On the second stage output of V1 I'm getting +5v AC between output capacitor C8 and output load resistor R16. Everywhere else it's about 100mA ie at C7 on the first half of V1. I've tried everything I can think of - using known good tubes (from my boogie), checked the PCB and cabling over and over, moving earth points etc. and re-wiring the filamets with a couple of 100ohm resitors to ground. It's got me beat and is making my brain ache! I'd be sooo sooo grateful for any thoughts the group have, many thanks, Justin

[ampdoc1]

If you get hum with the master up, but the gain control doesn't affect the hum, then the problem is most likely a leaky cap C8, or the switching electronics, IC2.

ampdoc

[Jooshtin]

Cheers Ampdoc, I've changed C8 and the other caps in the V1 circuit and it has reduced the hum a tiny bit. I'll check out IC2 some more as it was playing up when I got the amp, I changed both the IC and socket which wasn't making good contact with IC2. On Ed Goforth's recommendation over on the MM forum I've ordered a new 20uf filter cap to replace the 2uf now I've got higher voltage on the plates. I just know it'll be something simple but it's driving me nuts! Thanks again,
Justin

[ampdoc1]

Check your cathode cap and the ground connections at the cathode also.

ampdoc

[LeeMo]

Sorry to go OT but I have a RD-50 that I bought new in 1980. I've never been in love with the limiter channel's sound. It comes off as muddy.
Do you like the mod that you performed? I looked also at heistal's more severe mods. I might have to go that way. However, I preferr to keep my reverb.


LeeMo

[Jooshtin]

LeeMo,

Even with the hum (waiting on a few parts, fingers crossed...) the mod sounds great, much better than the original stodgy limiter channel. I'd say it has more of a D*mble/plexi-ish feel to it now and less of a change in character between the clean and limiter channels. I'd use the plate/cathode values as a guide and change them to suit so you get a decent plate/cathode voltage - I found, just as Dogears recommeds for the D*mble od, it comes to life with plate voltages around 200-210v. I've been surprised by just how much difference even the smallest changes make.

If you try it and it's not for you, you can always go the Heistal route - which looks amazing, but I'm a coward........! I'll let you know how I get on with the mods. Cheers

J

[LeeMo]

Thanks!
I'm a bit yellow for the full jump, also.
If I can help getting that hum in line by measuring anything on my stock amp, please let me know. I can follow directions pretty well. I'm just a not as advanced as some of the others here.

LeeMo

[Jooshtin]

Cheers - problem cured! Turned out to be various reasons - a dry solder joint that I'd missed (I've re-soldered the whole board now), replaced cathode caps with good quality ones, replaced old cable to V1, and some muppet in the past had replaced the zener diode in the 30v supply with a regular diode.

Now on with the fun bit - tweaking.....and a nice cup of tea....

LeeMo - I'll let you know how the amp ends up & sounds.

Many thanks.

[LeeMo]

Cool! Thanks!

LeeMo

[BobW]

Jooshtin,

Sorry for the late reply. Am glad you found the noise source. Have you tried lowering the V1 plate voltage down to 200V yet?

I hope you like the mod. BobW

[Jooshtin]

Cheers BobW,

The amp is sounding REALLY good, I prefer it to my MkIII Boogie. Thanks for sharing your mods. I've still got more hum than there should be, which increases as the limiter gain is raised. I'd be grateful for any thoughts on this. I need to get the scope on it and track it down... It's getting better as it improves....!

After a lot of experimenting (I had most of the V1 circuit on breadboard for a while) I kept coming back to the 1977 OD (Fender) values of 100k/1k5 plate/cathode resistors. I found the 180k/150k etc plate values sounded very good and smooth, but the 1977 values have a more "open" fender/plexi-ish blues/rock tone. Maybe it's down to the plate power supply too. The B+ and plate voltage supply isn't as high as on the schematic so I've ended up with about 190v on the plates of V1 (I put it down to the age and huge number of gigs it's done, and the 250v transformer running on 240v uk mains?). I played around with all sorts of plate/cathode values, and the closer to around 200v a nice tone/breakup develops - but the D*mble ratios sound best to my ears....

I've also added a PAB relay, a midboost relay (both footswitchable) and an "OD entrance" pad that switches between the 1977 (very plexi) and the Dogears 235K total resistance (smoother, warmer). I've replaced the MM speaker with an Emminence swamp thang, it's got a nice big full sound. I've also added a 220pf pull-bright cap accros the gain ... the Marshall factory isn't far away from me so it has to be done!

LeeMo - I haven't forgotten, once I get this last bit of hum sorted I'll post a schematic....

All the very best

Justin

[BobW]

I need to get the scope on it and track it down... It's getting better as it improves....!

A scope is my best approach to identifying noise sources too. I believe UK uses 50Hz? Verify the noise frequency. If it's 50 Hz, it's before the filters and probably heater related, if it's 100 Hz, it's a filtering and/or grounding issue. You may also want to add a DC offset on the heaters. A pic of your layout is worth a 1000 words, and may help identify the noise source to us.

[Jooshtin]

Cheers BobW,

Yes it's 50Hz hum that appears to be coming from V1. With the gain control on about 2 there is no hum. As the gain is increased the hum also increases together with the master volume, so at higher volumes it's quite noticable. I've recorded old Marshalls/Voxs etc. that are far worse, but I'd like to get this one as quiet as I can for studio use. The clean channel is beautifully quiet at all volumes.

I've attached some photos - the birdsnest of wiring was almost neat for a while! Since the pics were taken I've raised the heaters on the +30v dc supply and the od section is grounded via the 20uf filter cap at the original V1 ground point on the pcb bus near the clean channel treble pot. I've also isolated the input jack and run the ground from the PT secondary centre tap to the original heater ground lug. This has helped but there's still that bit of hum.... Any ideas gratefully received!

The OD circuit is basically the 1977 rev A1 minus the snubbers. On the od entrance I've used a switchable 150k resistor with the total resistance between treble wiper and grid of 235k with the 150k in circuit. On the od exit I've added the 150K resistor as on the schematic which really improved the tone. When I can I'll put up a schematic.

Many thanks,

Justin

[Jooshtin]

And a couple of pics of the od pcb squeezed into the chassis.... J

[BobW]

Jooshtin,
I noticed the heaters, (twisted Green wires) are laying on top of the board, right next to the F/S and Reverb jacks. Try moving those wires off and away from the pc board and your OD circuit. hth