| Author | Message | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
I am sure that most people on this forum can understand it when I say it was exciting to have a free Saturday with 1/ a sorta broken Music Man amp, 2/ parts to fix said amp with. This RD-65 came to me with the label 'needs some work' and a description of 'when the standby switch is set to on, the fuse blows'. Ok, first I forwarded the the suspect looking tubes to the nearest trash can (one was an old Sylvania 6CA7, the other a newer EL-34 with masking tape on it). I then looked over the rest of the amp. Sure enough, one tube socket had a nice black spot between the plate and ground pins (more on this later). What convinced me that there would be absolutely no power applied to this amp right then, however, was the leaking filter cap in the 'pan'. Instead I spent some time cleaning/vacuuming and removed a bit of rust and corrosion. This was a couple of weeks ago. Today I had fresh tubes (plus a retired gig set that work fine but have gone microphonic), fresh caps and an assortment of other replacement components to work with. First I replaced the 100uF electrolytics and the 150k bleeder resistors that had stretched substantially. Next I hooked up the Variac and tested without tubes in place just to see if there were any shorts I could have missed. It turned out that if I brought the B+ to the OT past 175V or so, I would get a cartoon-noise 'bzzzz' (think Dr. Frankenstein's lab done by Hannah-Barbera). At the same time a nasty 'fried electronics' smell would fill my workshop. At first I thought one of the flyback diodes had cooked, but they both tested ok. Puzzled I fired up the Variac again and this time I saw where the noise came from: The tube socket with the black spot was arcing! Thankfully I had a replacement tube socket, so that was a relatively easy fix. Next was testing with a tone generator. No sound, bummer! After poking around for a bit I figured I should hook up the foot switch and see if it made a difference which of the two volume/gain control pairs I was using. To my relief the 'hot' pair of controls did pass sound so after some more poking around I traced the problem to be a bad connection around the 'clean' gain control pot. With my microphonic tube set still in place (and no bias adjustment done) I quickly put the amp together enough to test it with a guitar. I'm happy to say that it sounds great! Tomorrow I'll get going on putting tolex on the relacement upper back panel and putting in the new tube set. I'll post again when I'm done and if I can make it work, I'll post some pictures too. Thanks for reading. /Lars Verholt www.captain-foldback.com | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
The amp is now ready for real live testing. Here are some pictures (cell phone cam, so a bit fuzzy). Amp: http://hamtech.dk/MMAMPS/Image0094.jpg Bad tube socket: http://hamtech.dk/MMAMPS/Image0097.jpg New back panel in the works: http://hamtech.dk/MMAMPS/Image0096.jpg The only problem left is that I personally think the Bright filter is waaay too hot and actually seems to overload the distortion stage in a very unmusical way. It may of course be that the filter is simply designed that way and I am not liking it. Others with more experience with the GD-2/GD-2A version of the RD series are welcome to chime in! Cheers, Lars Verholt | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Damn, Lars. The socket photo looks like my bassman gig amp after a POS set of Groove tubes went Chernobyl on me. I've put in a lot of sets of GT's and they ALL seem to be junk. If it says GT on it now, I pretty much junk them if they want ME to work on it. I ate too many that came back that they said I did something wrong. Mike. | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
So Thursday night was the first time testing the RD-65 in a live situation. Unfortunately I still have a few things to take care of. It turns out that the speaker plug has the dreaded age rot and starts to lose contact when the amp warms up. This problem may or may not be the cause of the amp being too quiet in general. In particular the clean 'channel' lacks punch. I'll do the usual Switchcraft replacement of the speaker plug and then test again. Also, I think I will upgrade the bright filter to GD-2A specs as the stock GD-2 is a tad too aggressive. The deep filter is a bit overwhelming (and unfortunately placed before the distortion circuit :| ) but it's not a pressing issue as yet. On the upside I found the distortion effect to be surprisingly useful with my Strat. I will report back when I make some headway. PS - this amp will be for sale once I get it back in working order. Cheers, Lars Verholt | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Well, so much for thinking that a 2 dollar plug was my problem (honestly, I was only hoping it would be that as I would like to make a couple bucks on this thing). Some more testing revealed that the output stage was only putting out a fraction of the power it was supposed to, and a pure sine wave looked quite funny on a scope especially if I pushed the amp a bit. After hooking up a spare OT that used to be in a 65R combo, it was clear that I have a bad transformer. Drat - it seems that whatever profit (= payment for the time I have spent on it) on this amp is disappearing. Add to that, I am having trouble with my login on the mojotone website. Oh well... On the upside, I am convinced that this puppy will sound like a million bux when I'm done with it. Stay tuned! -Lars Verholt | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Oops, a month passes by so fast. Though I'd just post an update. The new OT arrived from Mojotone last week and I installed it and tested it with a scope on the bench. So far so good. Last night I finally had time to put the amp back together and take it to the weekly jam session to check it out. Wow! This amp sounds amazing now - and it's darn loud too. I'm starting to think it's even louder than my usual 112-65 combo. The RD-65 112 is now 99% done. The only thing I want to do now is to change the bright switch components to GP-2A specs as it's just too 'hot' to use. It sure feels good to have rescued a sick puppy like this - it was a complete basket case when i got it. Now to find someone to sell it to. Cheers, Lars Verholt |
Tue, 03/25/2014 - 09:35
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