Monoprice/Stage Right 15w combo chassis removal

Wound_Up

Wound_Up

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I pulled the chassis from my amp last night. Finally got around to doing it and it was extremely simple. 6 screws and it slides out the back.

Once I got it out, I decided to compare my pcb's to those in the Laney Cub 12R firsthand so I know for sure whether they're the same. Facts are facts and these are the same circuit. They even share the same pcb layout. R25 on one is in the same exact location as R25 on the other. R10 is R10. C31 is C31. TR1 is TR1. Etc...

They even use the same font for the component labels on each pcb. See for yourself. My Pic isnt lined up exactly with the Cub Pic but it's close enough to see that they're the same.

Cub 12R

Screenshot_20240821-003153_Chrome.jpg

Stage Right 15w combo - the 2 black wires on the bottom right are the red & black wires

20240821_004433_copy_2016x980.jpg

So once I got it out, I shot some Deoxit into the gain pot to stop the scratchiness and that was it for last night. Today, since I had it open and could adjust the bias trim pot, I installed 2 tubes out of the matched quad of Telam/Polam El84's I have.

On the schematic, it appears as though there's a test point on the amp for testing bias voltage. So I looked and looked and looked for it. Nope. There's isnt one. So you have to find R37 & R38 and test between those 2 resistors. It should read -12.5v.

Well, I've read countless posts across multiple other forums from people saying theirs were as high as -7.1x volts. I measured mine with the stock JJ's and it was -12.4x volts. Close enough. With the new tubes installed, it was still -12.4x volts. I'm certain that's correct because I checked in multiple places around those resistors in case I wasn't testing at the correct point and every point read -12.4v.

Works for me. So I slid it back into the cab and that's where it currently sits. I'm also in the middle of adding a couple of speaker jacks so I can connect the speaker to the amp with a patch cable in the back.

This way, it's easier to run the Monoprice amp through a different speaker or run a different amp through the Seventy/80 in the Monoprice amp. It has an external jack already but it doesn't cut the stock speaker when used, AFAIK. So I'm doing this so I can run the stock speaker by itself with other amps.

I think that's it for now. Thanks for letting me share! 🤣😁
 
Awesome. I had a Laney Cub 12R. Kinda wish I’d have kept it. I might go this way next time and save some money.
 
The MP 15/1 is a great amp…I’ve had two and passed them to family.

My MP 5/1 shared a similar fate…also a great amp.
 
I almost killed it yesterday. D'oh!

I forgot I had hooked up the Hot Rod Deluxe to the speaker in the MP15 earlier in the day. We'll, I didn't plug the stock speaker back in when I unplugged the HRDlx and I turned the amp on.

Thank GOD I realized what I did the moment the switch flipped so I IMMEDIATELY flipped it back. It was on for a split second. No problems! Wooohoo!

This right here is why I'm trying to wire in either a 12a Switchcraft switched jack or a 13a Switchcraft "circuit transfer" jack. People say you can use a 12a, which has a normally closed switch with no cable plugged in and the switch opens when you plug in the cable.

I know a 13a will work but I'm still debating on whether the 12a will work. Other people say they've used it for the same reason so apparently it does. I just need to draw out a diagram so I understand how it works and don't miswire it.
 
Almost killed it a 2nd time.

Wired in a shunting jack for external speaker(s). When I plugged in, I got no sound. This time, I had to wait until the tubes warmed up and started producing sound but they never did. Tried it twice more. Nothing.

So I removed the jack and tried it again. Still no sound. At this point, I'm convinced I've burned it up. So I pulled the chassis again and looked over everything. Nothing was cooked or exploded, there were no melted wires, and there wasn't any sort of burned electrical component smell.

OK. So I replaced V1 with no change. Next, I swapped the power tubes with 2 other EL84s. Plugged into the "fx loop return" and I had sound! So I plugged into the front and Presto! It works!
 
Well this has gotten interesting. I bought some EL84 bias probes and returned them because they measured cathode current not plate current. Or it included screen current, not just plate current.

So today, I did some measurements & calculations to figure out how my amp is biased. My calculations showed plate dissipation at around 19mA while I calculated 70% dissipation as something like 35mA. Not sure if I did that right, though.

When you calculate expected dissipation, do you use the stated wattage of the amp or do you use the actual wattage of the power tubes combined? I used 15w in my calculations to figure out how much 70% plate dissipation was. Should I have used 25w since it has 2 EL84s and they're rated at like 12.5w each?

Anyway. I ordered a Eurotubes EL84 bias probe that'll be here next week so I'll be able to confirm whether my calculations were correct. Im pretty certain the actual dissipation is correct. All the specs I used were measurements I took.

Estimated specs:

Plate voltage-300.2v
Center tap voltage-301.6v
Center tap to plate resistance-107.43 ohms
Don't remember if there was anything else

I have a 6l6 bias probe, also, but I haven't used it yet since I don't currently have an amp that it'll work with. Another 24 hours and that'll change. Finally. Friday is my bday so I finally get my new Hot Rod Deluxe! Can't wait.

I also thought I killed the Monoprice 15w combo last week while installing another speaker jack. Thankfully I didn't but Laney sells both transformers for this amp so I can get replacements if I damage something. I am worried I did something because there's a ringing I've been dealing with that started after I almost killed it. Not sure how soon after it started or of it's even related to almost killing it.

One thing I've noticed about both of those amps is that the filter caps dump their voltage pretty quickly once the amp is turned off. Within 60 seconds, the Monoprice is under 40v. Same goes for the Hot Rod Deluxe. 15 seconds after it's turned off, voltage has already dropped enough that it won't kill you. I don't take it for granted and I still treat it as if it will kill me. Best practices and all that.

Like I said...things have been pretty interesting lately lol. I've learned a lot about tube amps and biasing fixed bias amps.
 
The best way to learn about something if often to just do it. Always avoid killing yourself in the process though. It's so very final. ;)
 
soulman said:
The best way to learn about something if often to just do it. Always avoid killing yourself in the process though. It's so very final. ;)
Click to expand...

Lol you got that right. That is the main reason I ordered bias probes to take direct plate current measurements. It's much safer vs me trying to probe points inside an amp that's powered on. This way, all I have to do is use the probes for current and then take plate voltage to calculate plate dissipation.

I already have a probe for 6L6's in the Hot Rod. Currently waiting on the El84 probe I ordered from Eurotubes yesterday. I calculated again and 70% dissipation for an el84 is around 28mA and mine was set at 19 so it was closer than I thought it was.

I'm really curious if I got that right so I'm ready for the Eurotubes probe to get here already lol. I might check the 6L6's in the Hot Rod just to do it and see where theyre set. I need another of each probe so I can check both power tubes at once instead of having to switch 1 probe between tubes.

It sucks because the 6L6 probe from Douk is around $15 while the Eurotubes el84 probe is literally 4x the price.
 
I love messing with guitars but other than to replace a speaker or tubes you will not find me messing inside an amp. My one tube amp is cathode biased and the specs provide enough info for me to know what tubes will tend to work best with it so I can even avoid what you're dealing with.
 
soulman said:
I love messing with guitars but other than to replace a speaker or tubes you will not find me messing inside an amp. My one tube amp is cathode biased and the specs provide enough info for me to know what tubes will tend to work best with it so I can even avoid what you're dealing with.
Click to expand...

Yea I bought bias probes just for that very reason. It makes me super nervous working inside this amp. The Eurotubes EL84 probe I ordered for this amp came in the other day. So I checked plate current with that vs what I calculated and there isn't much difference. 4 or 5 mA

I calculated around 19mA. The Eurotubes probe showed 14 or 15mA, IIRC. It takes a direct current reading from the plate voltage pin so that's all I need is the number. That's around half what it should be. 70% dissipation is around 28 or 29mA.

When I checked the matched quad of Telam EL84s I bought, they drew even less current. They all measured between 10 & 12 mA. I'm not sure what that means exactly. Are they better because they draw less current? Are they dying because they're drawing less current? 🤷🤷🤷 Guess I need to do some reading.

I've got some probes for the 6L6's in my Hot Rod Deluxe, also, but I haven't tested those, yet. It was more difficult than I expected to get them out of the bear traps that hold them in so I gave up ATM. I only had 1 hand available at the time so it was virtually impossible to pull the tubes.

I just ordered some new hooks for the DMM so I plan on rechecking all of the readings I took from inside the amp when they come in.
 
I'm not certain who among us here is the most experienced with amps and the impact of various current and voltage specs. My knowledge is limited to what I know about my Tweaker 15 and even then very little other than knowing the plate voltage and what tubes I've tried are closest to liking it. I'm running a pair of NOS US Tung-Sol 5881s.

One guy I do know of whose a real pro on this stuff is called Aged Horse but he posts more on the TalkBass site being a developer of Mesa bass gear and also of the older Genz Benz product line. There are also guys on the Strat and Tele forums that are knowledgeable amp guys too. You may get some answers posting there as well. At any rate good luck and keep one hand in your pocket.
 
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