Musicman RD-50A

[Platefire]

I'm helping a friend trying to get this amp going he just bought. It turns on but is weak and distorted. Right off the bat he found two burnt resistors a 470/1 Watt screen resistor and a unknown resistor that I can't figure out that has color code with 5 bands of color. Don't recall seeing this before. Please take a look at attached picture and tell me what the value of this resistor is. The color bands appear to me a Blue/purple/Gold/Gold/Yellow? Even better if I could find where this resistor is located on the schematic. Platefire

[acheld]

Hard to be sure, but I'd guess R51, R54. 

[Platefire]

Thanks, I believe your right!

Can anyone explain the bias on this one?

[glass54]

Correct acheld. I used to see a lot of the MM amps about 20-35 years ago, (here in Aussie) but only the odd one these days (very gutsy amps, best with 2xJBL speakers)
Basically IC4a mixes Preamp+Rev+Feedback and drives Q3 (JE1692). IC4b is simply a unity gain phase inverter driving Q4. A voltage to current conversion takes place IC4 to Q3 or Q4 and this controls the Current through Cathodes at V2 and V3. Grids of V2+V3 are hold constant at +30V DC. There is extensive DC housekeeping around Q3+Q4 including DC+AC feedback via R68and R69.
I have experienced failures in Q3 or Q4 but rarely both, R51 or R54, D16 or D17 and some other components in that differential Drive. I always changed both transistors (to be sure, presence of high voltage in the tubes) and any components that was a little "off". Rarely had an Op Amp failure (IC4). Possible cause?? Flashover of tubes??
I do have a later improved schematic for the driver which includes a trimpot to adjust Iq (Quiescent current in tubes) plus extra diodes possible for "Vbe" type tracking of Iq.
I can scan that for you if you are interested.
Trust this helps.
Kind Regards
Mirek

[Platefire]

Thanks Mirek
Looks like your very knowledgeable of this amp. Thanks for sharing it with me even though it's mainly over my head.Have had hardly any experience with solid state or Hybrid amps. I am comparing the schematic to your explanation to see what I can
make out of it.

on "cause" what do you mead by "Flashover of tubes"? It that the same as "Red Plating" or is that something different?
  Bob

[glass54]

Very Occasionally, I have observed a "flash" in a tube, associated with a horrible thump/short "thunder-like" sound in tubes, especially if you have just removed the chassis from cabinet. The cause?? Possibly some residual manufacturing schwarf in tube housing causing temporary short, after being dislodged when chassis is rotated!! Who knows
Catching a flashover is like watching the sky for meteorites 
Please let me know if you want a scanned copy of the later Driver section with adjustable Quiescent current.
Regards
Mirek

[Platefire]

I think my friend (Bear) is going to replace the burnt resistors, install new filter caps and pull a set of JJ 6L6GT's out of another amp to test this amp. I think he's got in mind to do bare minimum in an attempt to get it up and running. So thanks for the offer on the upgrade plan to the driver section but no, I think right now he don't want to do that. Appreciate your help
on the amp. Bob

[Platefire]

For anybody like me looking for the PI and bias in this amp, this thread explains a lot about it. I see even PRR got in on the discussion. Never seen a driver section set up like this! See reply #6

https://www.diyaudio.com/community/threads/music-man-rd-50s-why-so-many-output-section-issues.373188/

[Platefire]

Got a concern regarding these new metal film resistors for this amp. Got replacement 6.8, 1 Watt, 1% resistors that go in the PI network. Also got some 1 watt 470 screen resistors. Attached is a picture of the 6.8 and the 470 side by side.

So the specs seem to match with high tolerance 1% and 1 Watt on the 6.8 R. The concern is the small size of the resistors compared to the CC resistors that came out of the amp or even the new 470's . Makes me wonder if I missed something??? Sure don't want cause problems, but hopefully repair correctly!

[shooter]

in the 'ol days size was a good indicator of W, the modern replacements seem to be "1 size smaller" than their predecessors.
that said, the 470's do look like 1/2w.
I've been known to do some math, gator-clip up a test circuit and smoke test...just to be sure 

[Platefire]

Ok Thanks, I was thinking the smaller one was the 6.8 , but I checked the color code and yes it's the 470. Here is a pix:

[AlNewman]

Those look like 1W, compared to ones that I have.
When they're working, put your finger on them, they can certainly get hot, even within spec. 

[rake]

I just got some 1w carbon film resistors in and thought this must be a mistake! They looked like 1/4w resistors!
So, I got out the calipers and the spec sheet from the manufacturer and HOLY CHIT, they are their 1w's! Yageo is
the brand. If they keep getting smaller, I'm gonna need new glasses! 

[Platefire]

Well I'm glad I'm not the only one having problems with these small resistors

My friend that I'm helping on this checked back with AES where he got them from and said they verified they are Vishay 1 Watt. So I guess he can put them in and still feel OK about it.

Thanks, Platefire

[shooter]

I checked the color code

since the "modern resistor" came out, I never rely on color-code anymore, always ohm them.  I've found other peoples orange is my brown, yellow on a blue background isn't yellow anymore!

[Platefire]

Amen! and believe me I do. Ever resistor installed by me, is checked on MM first. 

[jojokeo]

Hello Platefire, I have one of these amps on my bench currently. How did things turn out for you on the amp overall? I've got several issues with the one I'm working on. I have new power tubes and replacement transistors on the way. BUT - there's a strange low level fuzz-type of distortion whether on the clean channel OR the Distortion/limiter channel. I've replaced the TL072 chips but no change. At the moment while waiting for parts I can't put my finger on why the weird low level buzzing that almost seems like a parasitic oscillation thing? Wondering if anyone's heard/dealt with what I'm describing on a Musicman amp before???

[Platefire]

On this one, I didn't have my hands on it but was trying to help a friend who also does a little tinkering around with amps.His had low power output, distorted and power tube red plating. None of the stuff we tried helped. He ended up taking itto an Amp Tech in his area and  he got it going. This was his report:
Got the Music Man back from the shop yesterday. It needed op amps. Don’t really understand the whole thing but he couldn’t find the exact value, and to take the 6.8 ohm resistors I replaced up to 22 ohms to have them work right. Thing is loud and clean on the “clean” setting. Not sure I like the “limiter” setting that gives you overdrive. I like the overdrive of the 6L6 Champ and 5F2 you built me much better. I don’t exactly know what the 12AX7 tube is doing. Read online that it’s just a phase inverter on one page and then that it’s a preamp tube for the limiter “channel” on another page. So who knows…

[jojokeo]

Geeze Platefire, that guy is an amp tech yet doesn't understand how an overdrive channel works and can't understand the difference between a phase inverter and cascaded gain stages? BTW, the majority of Musicman players LOVE the limiter/OD channel and think it's a thing of beauty. Potato potaughto as they say... In many of these amps the power tube's cathode's need a 30v zener replacement as it's needed to bias the grids properly. Sometimes the cathode transistors can go bad and cause output issues, a service bulletin recommends replacing the JE1692's w/ 2N6488's as a superior (more robust) part. Then there's screen resistors under-rated which burn along with a couple other locations I've seen. Normally the OpAmps never have a problem and rarely need replacing. They do have +/- 8V feeding them (two more zeners) which needs to be checked which is more commonly an issue there rather than the OpAmps themselves. This info may help others down the road?

[Platefire]

Hay jojokeo, think you misunderstood my reply. The reply in italics was the guy I was trying to help via E-mail and was what he reported back to me after he got it back from his amp tech after repair.

What little I learned about this amp when trying to help my friend is they are a pretty popular amp and fetch a pretty hefty price.

So have you got your issues solved on your MM yet? and if so, what to you think about its sound and operation? Plate

[jojokeo]

I found I had to replace the driver transistors among others, those being the first order of business. I used the 2n6488's as per the Service Bulletin #7. They were matched perfectly at 243 hFE under the same identical test voltage and current. This is important to try to get them paired as close as can be. It wasn't the easiest changeout due to difficulty unsoldering these boards while keeping the traces from lifting and then pre-bending the leads, fitting into the heatsink, etc... Normal solder suction doesn't work on these boards at all. The amp works very strongly afterwards BUT THERE's STILL an original issue that I suspected from the beginning. But the next thing I had trouble with was the Reverb. I used a completely different reverb tank and the issues with that went away. However, there's still a low-level static/distortion on the notes - more pronounced in the lower guitar frequencies while testing and playing. It rides the notes for a few seconds while sustaining the note then goes away while the continues. Also, there is noted lack of sustain due to this - both typical parasitic oscillation behavior. I have tested having the chassis on my cradle, the subbed reverb tank sitting separately on the bench, and then the cab isolated away preventing any vibrations. The oscillation is coming from the pre-amp no matter which channel is used. Since it contains a pcb, checking or modifying typical grounds, signal path crossings, shielded wire, etc. techniques aren't really in play? I've replaced all three TL072's, no difference, using new NOS 6L6s, tested all caps, new matched driver transistors, etc. nothing else is out of place (aside from the old reverb tank).
The big question now is how to deal with an amp like this with so little remaining to try and attempt further work-arounds?

[Platefire]

Well your right, you've done the complete work around to get it working right. So the power is up to full potential but the low level static riding on the notes is the problem on both channels? So with the reverb, switching it off or dis-connecting it didn't lose the noise you are hearing? It almost sounds like DC is getting in the grid path but you said you tested all caps.I know this is pcb wiring but could some live chop sticking might be in order? It seems your almost there and that it could be just a bad connection acting up, but just finding it?

If your like me, when I have something like this I finely decide to give it a rest but my mind keeps drifting back trying solve the problem in my mind. So I hoping it will come to you , you'll find it and start enjoying your amp.   

[jojokeo]

Thank you Platefire. I've replaced every opamp, transistor (in signal path), and tube. Lifted and tested all electrolytics, tested for DC voltage on the clean Vol control, checked all jacks including the Ext speaker jack which has built in switching contacts to the main jack, chop sticked, alligator clipped, you name it!
Additional work:

*The reverb tank ended up having one of the small holding springs come loose so the internals were shorting against the inside housing. Works normally after re-attaching.
*The preamp opamps' power supply rails are not decoupled as should be done for stability and preventing oscillations, etc. So in a bi-polar power supply one needs to use 2 - 100nF caps - one for each rail per opamp. After soldering these w/ short leads on the underside of the pcb and tested, found no change!? DAMN I really thought this could help.
*After previously moving/separating all wiring best as could be, the Gain control wiring had 3-wires twisted running along the other pot's wiring all jammed under the pots towards the input jack/preamp controls. I removed & replaced w/ 2-conductor shielded cable. THIS HELPED!
*I swapped various newer opamps into the IC-1 position and gained a little extra positive difference as well.


The parasitic oscillation demon issue is improved and lower in volume. It is livable for a gigging musician and would be hard pressed for anyone to hear it during an event/gig. Recording or playing quietly in your room is another thing. For me, it would drive me crazy. All of my own built amps do not have this very annoying problem.


* have found another possible grounding issue where the pre-amp tube (12ax7) cathode's ground is made at the socket then routed to the speaker ground jacks. This is another grounding No-No! But, if the OD/Limiter channel is NOT activated it shouldn't be causing this issue correct? (this is a question for likes of Merlin or PRR maybe?) The signal is not "in circuit" but it just might be affecting things anyways? The parasitic demons are very much the "ghost in the machine" and require an exorcism. I'm running out of options and things to do...




Here's a link to hear the oscillation (prior to the latest improvements): https://soundcloud.com/jojokeo/musicman-rd50b-preamp?si=a7d812a78eed43ec8b9f350db4aad9b5&utm_source=clipboard&utm_medium=text&utm_campaign=social_sharing

[jojokeo]

Some updated progress photos

[Willabe]

Just a thought, maybe 1 of these guys might know and throw you a bone?

Kevin O'Connor, London Power Amps, he has a web site.

Todd Sharp in Nashville, he has a big shop. He builds/repairs amps along with guitar work. Ed C told me he visited Todd a few times and spent several hours talking with him in his shop. Hes was very impressed. See below.   

Jeff Bakos, Bakos Amp Works, east coast, Tone Quest Mag used him a lot, very experienced.

Larry Craig in Cali, Neil Young's guitar/amp tech for several decades. Now he owns and runs
Vintage Instrument Rental in San Anselmo, CA.

 

Edit: Woops, it was Jeff Bakos Ed visited, not Todd Sharp.

[tubeswell]

Got a concern regarding these new metal film resistors for this amp. Got replacement 6.8, 1 Watt, 1% resistors that go in the PI network. Also got some 1 watt 470 screen resistors. Attached is a picture of the 6.8 and the 470 side by side.

So the specs seem to match with high tolerance 1% and 1 Watt on the 6.8 R. The concern is the small size of the resistors compared to the CC resistors that came out of the amp or even the new 470's . Makes me wonder if I missed something??? Sure don't want cause problems, but hopefully repair correctly!

Resistor miniaturisation tech is following the same trajectory as other electronic components and the temperature tolerances seem to be higher than the old days. But those little guys give me the heebie geebies. I’d be forever fretting about return work. Maybe order modern 2W resistors next time?

[Platefire]

jojokeo

Your probably gonna want to kill me for bringing this up after all you've done on this amp but I have to ask;

Have you tried a different guitar with a different cord with this amp just to eliminate the possibility it may be external?

Have you tried plugging it in and testing in a different room on a different electrical circuit and away from existing lighting?

Now you can whomp me

[jojokeo]

Hello again, it's okay to suggest the obvious but this isn't my first or even 100th rodeo, lol. Yes, I've used a couple of guitars, chords, speaker & cab, and a different reverb tank over the weeks troubleshooting while waiting for parts in my garage, family room, and office. Nothing has gone very smoothly on this amp and process in that I initially ordered TAD red base matched & burned 6L6s which one of them came w/ a milky area internally at the flashing which I had to return for replacement (filament didn't even glow), they should be arriving today finally. The first order of supposed NOS 2n6488 driver transistors came very mismatched and were not manufactured by Motorola as advertised (and paid a bit more for) and had to order another set of 10 by On Semiconductor which were matched perfectly... So, there's not much that has gone easily throughout this process. Even used a new set of reverb tank RCA cables, etc.
These MusicMan pcb's are very difficult to work with, solder doesn't suck out all the way making replacements a pain and the component side of the pcb is not labeled either. The tested results were interesting to see on my transistor tester:

[Platefire]

Well I'll say one thing, when you finally get this thing ironed out==I think you will be one of the foremost MM RD-50A specialist around. I hope those TAD 6L6's do it!

[jojokeo]

Thanks Plate, I'm feeling like I have this entire circuit and layout memorized now! It's so hard for me to come up with a good reason for the oscillation I'm having? It just won't go away after doing so many things.
I'm including a signal path flowchart to the schematic eliminating the unnecessary distortion/limiter channel and reverb circuits. The issue occurs at all times and w/out those being active/turned down completely. I've even removed IC2 and IC-3 to test, unplugged the reverb tank, etc... those have no bearing on it. However, I've wondered if it's possible they could influence/cause harm to nearby HIGH Z wires even turned down? My answer is YES it could be possible and why I shielded the Gain wiring routing under the pcb...


It's such a unique design in the phase inverter/power amp section using the driver transistors inserting the preamp signal into the cathodes and +30vdc biasing the grids... a complete oddball in our world for sure!
Part of the reason for all the documentation is to help others down the line as there's not a lot of info on these amps except for a few places other than here, and they are old and could disappear some day? Maybe it'll help draw more people here to Doug's forum as well?


I'm still surprised PRR, Sluckey, Merlin, etc. hasn't chimed in?


Lastly, I've pondered the 1n916 diodes in two places in the circuit specifically just prior to the Clean Volume control? Guessing that they are there to minimize possible signal clipping over 8v p-p or likely 16v p-p? from hitting the following stages? Same for them prior to the tube limiter channel?

[shooter]

reason for the oscillation I'm having?

haven't been following very close, ya'll use a lot of words n it messes up my dyslexic Brian 


does the oscillation go away when NFB is disconnected?

got a scope shot of the oscillation??

[Platefire]

Looking at your signal path drawing.

Don't know, just asking. Can you determine where oscillation is entering the grid by using a listening device. If you could tell what location in the circuit it's coming from, seems you would know where to start concentrating on????

After you test the new tubes and if it's still present after that, I would take another check out of OT and its connections, Why not try the NFB on the 8 Ohm tap or disconnected as shooter said----of course you probably already tried that!

[jojokeo]

Thanks platefire shooter, normally parasitic oscillations occur from a poor grounding plan in higher gain amp circuits, high impedance wires close or running parallel to others rather than 90º angles on different planes, proximity and alignment of transformers (also near sensitive wiring), and other reasons making a signal tracer fairly useless since it doesn't emanate directly from a single source? If you heard the sound example - it exhibits the classic parasitic phenomenon in sound and behaviors sounding like a mosquito on the note that is somewhat frequency dependent, lasts for several seconds, then goes away as the original note still decays, and it negatively affects sustain - worse on some notes than others. I don't believe that a short temporary anomaly riding the beginning of a note can be located exactly? By definition of the word "parasite" it needs a host/organism/wire/component to ride on and it's only there upon the initial string attack & frequency of the note(s). I don't think I've ever heard anyone on an amp forum saying that they found any one thing that a parasitic precisely came from? Rather it's a set of things which added up that helped it go away (my best way of describing this non-musical demon).
Thank you for your input and I'm going to give your suggestions a try anyway.

[Platefire]

jojokeo
You have a very detailed organized picturesque way of explaining parasitic oscillation. I kinda figured a listening amp wouldn't work but wasn't for sure. I did listen to you sound sample and it was most apparent to me on what I think was a open g string. I first experienced that on the first 5F1 Champ I built back in 2001. I had routed the grid wire from the Vol Control to the second 12AX7 gain stage underneath the board. When I would turn the volume up to about 75% it would start oscillation with a very degraded sound. Once I found it and re-routed that grid wire its been fine ever since.

Well I know your going to find it. Please report back to this thread when you do, so we can start the celebration. When the battle is the toughest the victory is all the sweeter!

[jojokeo]

Just a quick update:
I rolled a bunch of various opamps in the 1st preamp position and some were better than others. I opted for a Burr Brown 2134 and after putting everything back together the oscillation issue became practically unnoticeable. Why? The only thing making sense to me is the Faraday Cage grounding affect since the enclosure has a mesh screen under the top. The crazy thing to me is that the amp had the issue before I took it apart in the beginning and re-installed it a couple times throughout the process. But something I did at some point must've helped or worked?
For other comments and thread continuity, I did disconnect the neg feedback loop and this gave the amp improved raw tone but increased the oscillation buzz proportionally. Without the oscillation, it would be a great option to put this option on a switch.


My rock n roll hall of fame customer (FL & LA) picked up the amp yesterday. It was satisfying seeing the big smiles on his face putting the amp through it's paces. Then received a nice glowing report on the amp's first event last night back in action!


Thank you Platefire, Shooter, & Willabe for your time and comments!

[Platefire]

  Hallelujah! Praise the Lord! I'm Glad----May it give many many more hours of service