RD50 BobW mod - I got hummmmmm

Hi from a newbie in the UK and thanks for an excellent forum... 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 really 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.
Here's a link to the schematic http://www.latentlemon.co.uk/LLimage/165 0-RD50-BobWmod.pdf
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

Hi Justin, Maybe try replacing C-8 (.01uf) it should not be leaking that much voltage. How is the 2uf cap after the 1k2watt resistor? Maybe change it to a 10-20uf 450-500 vdc cap. Since you are getting more voltage there now, maybe change the cap to a higher value. I use 20uf/500vdc in my mods there. But my first guess would be C-8, if your getting 5vac. Check to see how many volts DC your getting there. Does the channel switching work ok? Sometimes the 4016 IC gets static and needs to be changed, but if its switching ok, its probably fine. I would first replace C-8. You added a grid resistor to V1A or V1B? I see the 150k at V1B, but that is fine, did you add one at V1A where the 4016 is? How are you powering the Relay? The 8 or 16 vdc power section? Are you getting popping noise when PAB switching? If so you can add a 22meg resistor across each switching contact and you should not hear anything popping. I keep going back to the C-8, give that a shot and good luck,
Ed

Justin, you can email me directly if you need more assistance. That is a great mod by the way, I have done that with some of my own mods, and its is amazing! My email is goforthsound@yahoo.com
Good luck!
Ed

Cheers Ed, I'll try changing C8, check a few voltages and email you, thanks again, Justin

Many thanks Ed. Problem solved! It 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 (I'm ashamed I didn't notice this earlier - I even wrote down the too high voltage on my schematic...oh dear....)
Now on with the fun bit - tweaking.....! I've upped the 2uf cap to 20uf and the 60ufs to 100uf - along with the other mods it's starting to sound grrreat.
Thanks again.

Hi Justin, I'm glad its solved and working good now. We overlook things from time to time, and thats how we can learn the amp better by not giving up and getting it down. Lord knows I have had to recheck things many times. You can change or add a pf cap across the limit volume control (not gain) to smooth out the overdrive harshness with these mods too, anything from the 50pf to 2000pf/.002uf, depending on how much bite you need with you guitar pickups and personal tatse.

Cheers Ed, I'll give those caps a try over the limiter volume. It's so frustrating when you know it's going to be something simple to fix..... but can't see it for looking! Thanks again, you're a star.

Also for those that experience some hum after doing the Bob W. mods, I found that changing the 47k to a 1k that is supplying voltage to the 12ax7, will hum more since the smaller resistor does not isolate hum like the 47k, remember these RD-50's do not have a choke that helps reduce ripple/hum, so using a higher value will help reduce hum. I use the 47k or higher actually with great results, but i use much lower plate/cathode resistor values to makup for it. In practice, using a smaller resistor there, one would believe it will supply more voltage and help the dynamics, but this amp is a different beast with its hybrid design. So on putting a higher value cap instead of 2uf helps tighten up the flabby bass and keeping a bigger resistor like the 47k, helps isolate and reduce the hum.