Glad you solved it  :dbtu:And welcome to the Foum  :cheesy:

And for not-that-large power amps, think 15W up to 30W, maybe using 1 or 2 TDA2030/50 or LM1875, search for extruded aluminum such as used fo kitchen windows or patio/garden doors, some are reasonable thick (at least 1.5mm, 2.5 or 3 mm even better) and generally being complex shapes that increases surface and acts as fins.

Ask at a local junkyard or neighbourhood aluminum window maker, any scrap they have for peanuts or free will do for quite a few projects at nil or no cost


worst case, you´ll have to use the old

Post schematic and if possible a gut picture, showing internal layout and wiring.

Motorboating is often caused (specially if "just appeared" ) by dry capacitors, because they allow low frequency signals to "travel backwards"  from current hungry power stage into sensitive preamp sections, but it may also be caused by poor grounding, signal travels back through imperfect ground connections or layout.

Your amp may also be oscillating at inaudible high frequencies, you won´t hear *them* but it works like a so called "blocking oscillator": oscillation is strong, blocks next tube, stops, starts again, you won´t hear the ultrasonic oscillation but the on-off "thump" , repeating regularly.

*Really*  , the best tool here is a scope, because we are dealing with too high or too low frequencies to be heard, we are dealing with their "side effects", but hey, we´ll try to do our best with what´s available .

 :dbtu: :dbtu: :dbtu: :dbtu: :dbtu: :dbtu:

Oh, "shorted" I expected, what I didn´t get is the simultaneous "open" bit 
It´s either one or the other

But now that you cleared the point, fine with me  :tu:

Quotethey had their tech put a 4 ohm 25W resistor in series with the + speaker lead and otherwise just hook it directly into headphones. the headphones they are using are 38 ohms that can handle 1.5W.

Madness.
They need to add a couple 220 or 470 ohm resistors, one in series with each earphone, at *each* headphone out jack.

As is, they will blow earphones, as in catching fire, and kill or gravely damage users.

You must feed *milli* watts to earphones, not full watts (not even 1 W).

"shorted open" ??

Yes, please repost pictures using a less greedy site, such as imgur.
Avoid sites like google cloud, dropbox, etc. which *require*  you to register or become member to see images.

Since it´s the so called "Class A driver" and it´s passing some 100mA , I guess original was a TIP30, letter B or higher,rated 1A.
Of course, 3A TIP32 or 6A TIP42 will also work.
Get letter B or higher, although lately letter C is "preferred" so it might be easier to find, I mention this just because maybe you have some B letter ones in a dusty old drawer.

Only precaution is that 22V x 100mA means over 2W dissipation, and transistor will heat up a lot unless prooperly attached to some heatsing, don´t forget insulating mica, grease and plastic nipples if needed.
Unless it´s mounted on a floating own small heatsink on the board itself, not touching ground or chassis.
You´ll still need some grease.

Post the amp schematic.

Ok, ok, it´s older than Santa Claus, I get it 

Plan B : post some sharp well illuminated gut and board closeups (which in general means no cellphone pictures but any old crummy real camera you have lying around) so we have an idea of what you have.

Gaving only 5 transistors I guess it´s s real old job, with a driver transformer, and in that case you most certainly have some crossover distortion.

If so, you´ll have to get used to it, no practical way to solve that.

For one I do not yet what kind of noise we are talking about, there´s many different types with different origins.

And in any case primary connection would have little or nothing to do with "noise"  so don´t mess with it.

I)f possible, post an MP3 showing the noise, and move controls to see whether any of them has some effect.

Don´t connect the headphone jack to the XLR connector.
In fact, remove that XLR to avoid confusion (as in: "WTF? ... I plug this into the mixer and it does not sound")

Make/buy a headphone plug to regular guitar plug cable and send headphone out to a mixer "Line Input" or if more than 20/25 meters away, ask Soundman for a "Direct box" .

That said, I guess you are inserting too much and worse: contradicting elements in your sound, I guess I know what sound are you trying to achieve and you will not.

Want a *real*  lo-fi sound? : plug your vocal microphone into a small cheesy amplifier, even the Honey Tone might do, put amp inside a closed case or if recording at home,in another room, a bathroom is perfect because of the natural reverb, and pick it with a microphone.

The *acoustic*  element is a large part of that sound.

Or use just the tube preamp, drive it hard so it becomes gritty, send it straigt to mixer, and there cut all bass, all high treble and increase some nasty mid-high frequency, think 2000 or 2500Hz .... instant Lo Fi 

If yu want a boxy sound increase around 300/400 Hz.

I see 5 transistors , please read what they have printed on the case and label them in the pictures.

You also have 3  2 leg thingies touching the heat sink, might be 3 diodes or 2 diodes and 1 resistor, please label them in the pictures.

There´s a couple details to take care of:

1) that amplifier is a single supply (+46V)  type, any chipamp you use will also need to be used in a single supply mode, some datasheets suggest one.
Most are dual supply.

2) being single supply, it needs capacitor coupled input *and*  output.

3)an LM3886 would be happy with a +46V supply and will last forever; a TDA2050 is just on the edge, will work properly but heat sink it well just in case.

*Forget*  the Yamaha distortion, don´t waste time on it and even better don´t risk hurting the amp, just use the meanest pedal you want in front of it.
Let the pedal do the drive and the Yamaha provide muscle ... which it has in spades.