Mesa/Boogie Fifty/Fifty question. Was mine modded?

[SocialInsect]

Hello, I'm new to this forum. I just bought a Mesa/Boogie Fifty/Fifty power amp, and I opened it up and checked out the main board, and found something that I'm not sure is stock. I've taken pictures I'm going to post them below. I would really appreciate it if someone could tell me what this mod actually is, if it is in fact a mod. The first and third picture I made a note in the photo of what I think might be modded. The power tubes that are in there are Sovtek 5881's.

 

[IronSean]

I don't think that's a mod, just looks like a repair job.

From the schematic, R423 and R424 are the grid resistors on V4 and V5. That resistor used to go from where it's attached on the left to the pad on the right, which connected directly to the end of the red wire, which leads up to the tube socket. Looks like the joint cracked or the resistor fried at some point, and instead of trying to reattach to the base of the red wire, they just wired the new resistor directly to the connection on the tube socket, bypassing the red wire.

Should work identically, and if it was a fried resistor it probably just happened when a power tube failed at some point so I wouldn't worry too much about it having needed a repair either.

 

[SocialInsect]

Awesome, thanks for answering my question.

I was also wondering if I should change the tubes to 6L6GC's, or leave the 5881s in there. What do you recommend? Also, I have one of the 4ohm speaker outputs going to a 4ohm speaker. Is that alright? Also, it is safe to plug a guitar directly into one of the A or B inputs, and play it like that without a preamp? Is it also safe to have a distortion pedal in the chain going into the input of the amp?

 

[JMMP]

You should only plug an input signal (guitar or preamp) into a channel that you have connected to a load. Using only one 4 ohm output means you should only use channel A OR channel B. The unused channel should have its volume at zero.

Plugging a guitar directly into the input of the 50/50 is ok, as well as guitar -> distortion pedal -> power amp. Just make sure whichever input you use has a load (speaker) connected to that channel.
I'm not sure what the bias requirements of a 5881 are versus a 6L6, but I seem to recall Mesa selling 5881s for use in their amps, and since there is no 5881 bias switch, I'd imagine they use the 6l6 bias, so you should be fine to keep them in the amp.