I have heard high frequency like that on those small portable Yamaha digital amps. Perhaps something to step up to high voltage but low current.

I don't think this is the case but I remember playing around with led strips that put out a high pitch noise. To make it worse the person using these led strips was putting them as guides on their guitars fret board. The power source has to be put somewhere where it won't pick up the frequency. But that Yamaha modeling amp put out this constant noise and it sounds similar to the high voltage/low amperage led strips. They use DC batteries to invert back to AC which stepped up to 100v. Maybe that Yamaha modeling amp didn't have a little noise gate to get rid of the high pitch when idle? Perhaps your amp has a gate? I would clean up the pots and switches as well. Justin Case.

Well how about some voltage checks on the transistor legs? Personally I never pull a transistor unless it's dead or very questionable. I do chuck old dried up electrolytics in the garbage. Voltage checks and tracing signals are your best bet. A good transistor may never work if you have a cracked trace in the circuit board. What part of AZ are you? I'm in Scottsdale 👍

One thing is the jacks must have insulating washers. Otherwise they will oscillate. 

Maybe not. Weird. It looks better before uploading it?

I think this one is better.

You don't need there to be crossover distortion to find the sweet spot with the ammeter. The scope is just there for detail. Honestly you just need to monitor current flow with a meter then find the sweet spot and back it of a touch. Try it on some amp one day and you'll see just how easy it can be.

MEF will be back up I am sure of it.

The Enzo method is good enough to keep your job. You dial in crossover distortion of a test signal 1k by making the bias pot set to coolest setting. You will need to measure current with an ammeter as you adjust the bias pot. Now turn up the bias pot while watching the scope wave form and the ammeter. As you increase the bias you will see the wave form become more linear on the scope. There will be a sweet spot where raising the bias pot causes the ammeter to register current flow. You will see the current meter start to rise and when it is in that sweet spot you just back it down a hair. So it will be idle and safe.

There is a lot of information about this on MEF but it's CURRENTLY down.

Acoustic 470 or 450?

I should make a few and put them up on eBay! At the same time I should make myself a more suitable one as well. The LBL's are easy to make but I have to wonder about liabilities selling something like this? We'll compared to the Chinese crap main's wiring I have seen I am sure anything I build would be better.

Please post the forum where they posted about it. I would like to read what they posted.

Oh yeah The Dude said it. Scrape off the green stuff and bridge the crack. I sometimes use a piece of stiff wire to make the connection with solder. If it is a big trace I have even used scraps of solder wick and that works amazingly well.

Remove the speaker from the amp. If there is DC voltage leaking into your speaker it may damage it. Next use a multimeter set to read dc voltage across the speaker leads. What does it read? We don't want to accidentally short them together and definitely remove the speaker. If it is not DC voltage on the output it may be something right in the preamp. But first test the power amp side to be careful.

Q11 & Q12 caught my eye too but understanding each transistors role in a circuit is difficult. I don't know what Q11 & Q12 actually do but it never stops me from guessing. Are they current limiting the circuit in case of overload? Not sure if they are pre drivers for the outputs or what. I'm curious to know how bad of a guesser I am. 

The 30 vac refers to the AC voltage but after rectification to DC voltage it will be higher. 30vac * 1.414 = 42.42vdc.

There are amps out there like Mesa boogie simulclass where they have a pair of each EL34's & 6l6's. But these are pairs of tubes and the amp is designed to allow this arrangement. No amp that I know of uses one of each different tube as a pair. It just doesn't work in a balanced way. People spend all day trying to create a matched pair of tubes for their amp circuits. EL34 tubes require a power transformer that can handle the current demand which is more than a 6L6 demands on a circuit.