MusicMan RD50 Output Modification

[hyderr]

Hope someone can help me out here. I have an old MusicMan RD-50 that I want to modify a little bit. These amps did not come with a Hi--Lo power switch. Looking at the prints and measuring voltage off the transformer, I am not sure it can be done. If someone knows how, I would appreciate some instructions please. I also would like to add a switch to disable one of the 6L6GC output tubes without removing it from the socket. Can you simply add a switch to interrupt the voltage going to the heater? Or would it be better to switch the screen or control grid...or something else completely? Thanks for any help you can provide.

[PRR]

Welcome.

No, disabling one tube would be like cutting-off one of Ursain Blot's legs to make him run half-speed.... basically lame.

Please get a smaller amp. I understand the attraction of the early MusicMans but too many other classic amps have been cut-up and ruined within my lifetime (some at my hands). And the early MusicMans are very hard to understand and service.

Not to mention how MANY variations of plan (-A? -B?) were produced.

[pdf64]

  … some at my hands …

[pdf64]

… I also would like to add a switch to disable one of the 6L6GC output tubes without removing it from the socket…

Do you like the sound of the amp with only one output valve fitted?

[hyderr]

Thanks even though your reply was not what I was hoping to hear. I do like the amp and don't want to damage it. I was hoping for a less expensive solution than buying a new amp when all I need to do is reduce the output a bit. I do not want to "beat a dead horse" with more questions pertaining to the same. But are there any other modification suggestions that would reduce the output? Such as replacing the 470 ohm screen grid resistors (R55 & R56 on schematics) with 1k ohm? I do not mind experimenting a bit with different values, but not at the expense of damaging the amp. That is of course why I am asking those who would know.

The other part of the question was about adding a "HI-STBY-LO" power switch from the power transformer to the output transformer. Many tube amps come with this switch from the factory, mine did not.
I did not think to add a quick link to the schematics for the amp (sorry). https://el34world.com/charts/Schematics/files/Musicman/Musicman_rd50a.pdf

A brief explanation of my situation is this. I bought this amp new. I used it as my main amp for club gigs. Then, 35+ years ago, I got a job working night shift. Finding fellow musicians who worked similar hours to play with was impossible (for me). I lost interest playing alone. So I have not played in roughly 30 years. I recently retired, and hope to play again. A fixed income makes me reluctant to spend much money on a new amp when I do not know if these old fingers will find their way around the guitar again. Or be more like your Usain Bolt analogy...just lame.

I did manage to go to tech school and have a degree in industrial electronics. However, tubes are not taught any more. I know little about them other than they are very different from solid state.

[hyderr]

… I also would like to add a switch to disable one of the 6L6GC output tubes without removing it from the socket…

Do you like the sound of the amp with only one output valve fitted?

Honestly, at low volumes, I can't tell much difference in volume or tone with only one, or both tubes installed. That said, in this scenario, I am down sizing everything I can. I am not using the 12" JBL speaker that I have in the RD-50 cabinet, I am using a small monitor cabinet. Again, attempting to reduce physical size and volume. I would likely mount the amp in a rack (with fans) with my effects unit to keep out of sight. Having only a cable for the guitar, speaker, and switch pedal connected to the effects unit (Ibanez UE405) from the rack to where I am playing.

[sluckey]

You probably won't hear any difference by changing 470Ω screen resistors to 1K.

Some amp manufacturers are calling the pentode/triode switch a full/half power switch. I think this is about the only option you have for this hybrid amp. If the tube sockets are on a circuit board I suggest you let it be.

[pdf64]

… I also would like to add a switch to disable one of the 6L6GC output tubes without removing it from the socket…

Do you like the sound of the amp with only one output valve fitted?

Honestly, at low volumes, I can't tell much difference in volume or tone with only one, or both tubes installed…

What’s the benefit of switching one of the output valves off then?

On a theoretical point, I think it may be feasible to switch the amp’s HT rectification arrangement between the existing voltage doubler and a full wave bridge. Thereby significantly reducing the HT voltage to the output valves, and hence power output.
Alternatively, the feed to the OT CT could be switched to the screen grid supply.

[passaloutre]

You’d have a better volume reduction using an attenuator or even a lower efficiency speaker.

I made this one and I’m very happy with it: https://guitar.com/guides/diy-workshop/diy-workshop-build-your-own-attenuator/

[PRR]

....calling the pentode/triode switch a full/half power switch. ....

These MusicMans run the plate and screen at VERY different voltage. Like 550v and 275V? You can't just switch the screen grids to the plate leads, tubes will melt quickly.

From 20 feet away, it "looks like" you could feed the PT CT from the 270V screen feed. This would be more like quarter power, which is surely a good goal. ("Half power" is not much softer.) But that leaves the too-clever power supply rectifier working as half-wave, big buzz and DC flux in the iron.

You could sell that treasure and buy house-size amplifiers all day long, at least to re-find your chops. Most rock shops take trade-ins just to move the good-commission goods, have a room heaped with $30-$50 transistor amps, used with 30-day bring-back-and take-another warranty.

[acheld]

Check out the M2 attenuator over at the Marshall Forum.  Works great, nice little project.

[hyderr]

Thanks for all the input. I will likely use an attenuator as mentioned and leave the amp stock. Since I plan to use a smaller speaker, my point in reducing the output was two fold. Yes, to reduce volume, but also to help NOT have the ability in an over-zealous moment to send more power to a small speaker than it could handle. Removing the amps ability to do that by 50% by disabling/removing one tube. Along with other things like Hi-Lo transformer switch was what I thought made most sense to accomplish both.
Seems that each method has its own caveat, and I am just not comfortable with the likelihood I will damage the amp. I like it too much to sell it for a cheap transistor amp (what is the point of that? I won't know if it is my playing that sounds bad or just the amp). If all goes well and I get my feel for things back...I'll miss her!!! And hold you all personally responsible...LOL
So, I will likely remove one tube and leave it out (the only change with the amp I think would not be harmful). Add attenuation after the output (just picked up an old Altair unit). And use a small directional speaker. Anyone see an issue with this?
If that isn't enough...I will hand out earplugs!!!
Again, thanks for the input.