Amp has started to howl and squeal: I need advice

[Resonator Guy]

About a year ago I bought this neat little home brew amp. It has a 12AT7 preamp tube, a 6005 power tube and a 6X4 rectifier, along with a 6" 4ohm speaker.
I put in new filter caps along with one other .22uf cap, new(er) tubes, a 3 prong plug and it sounded pretty darn good.
The last time I plugged in though, I was met with a terrible howl if I turned the amp up past half volume, and a terrific squeal if I turned the tone control much into the treble area.
I have checked solder joints, filter caps and am stumped as I have never encountered anything like this before. My reading suggests that this is oscillation, but I can't find much about how to correct it except getting lucky.
While checking the amp out I discovered two odd items. First, a 2200pf @ 400 volts cap between the two leads to the output transformer, quite close to the OT.
Second, or B, a ground attached to one of the output leads from the OT.
Again, I have never encountered either before. Removing them, one at a time, makes the amp even worse re howls and squeals.
One other odd item is a light bulb inside the chassis wired directly to the PT.
It seems to serve no purpose other than to light up the interior of the chassis. Perhaps it was to help the original builder see while he searched for oscillations.
I won't even mention the four banana plug inputs alongside the 1/4" jack.
Oh, there are also two parallel resistors between the volume pot and the tone pot?

I would really appreciate it someone here could point me in the right direction as to solving this. I am at a complete loss as to where to continue.

Thanks. Here are some pics . Well just one pic this time. The downloads have stalled out. I will post more soon.

[Resonator Guy]

Here are the rest of the pics. It is a cool looking little guy and I do want to get it working properly again.
If there is no chance then I might gut it and build a champ type amp out of it, but with the OT over by the inputs there might always be trouble.

[52 Bill]

While checking the amp out I discovered two odd items. First, a 2200pf @ 400 volts cap between the two leads to the output transformer, quite close to the OT.

This limits the high frequency response of the amp.

Second, or B, a ground attached to one of the output leads from the OT.

This is common to most amps guitar.

Have you checked the tubes? Are any of the tubes microphonic to the touch?

[Amp Kat]

^^^ what Bill said and if it's not that try reversing the Secondary leads on the OT.

[Tom Phillips]

I remember you posting the pics when you first got it. Hard to forget the light bulb and the 4 single conductor jacks.
In addition to tips above, keep an open mind about what you might find. There is no evidence that this amp ever worked properly. Right?
Have you traced it out and drawn a schematic?

[Resonator Guy]

I have indeed checked and switched tubes. I have also tried reversing the leads to and from the OT. As I mentioned, the amp was working beauty after I recapped it. This started later.

[Resonator Guy]

Now that I have had a couple of coffees and am more awake -

Tom, I started to do a schematic, however, in the cramped space of this chassis, there was so much going on around the preamp tube and the power tube, I got lost. Both of these sockets have centre posts on the chassis side that are used as grounds and there are too many wires attached to each for me to make sense of. Again, this is a set-up I haven't seen before.
Bill, when you say the ground attached to one of the leads from the OT is common to most guitar amps, I get confused. I have about 30 different amps and none have this feature. It is a wire coming fom the centre post of the preamp tube (ground?) spliced into one of the leads from the OT going to the speaker.
I am going to try again with my schematic, but it isn't making much sense. The inputs (banana plugs) also add a good deal of complexity to the drawing.
Also, when switching tubes with known good ones, I tried different amplification factors with no luck. 12AU7, 12AY7, 12AV7, 12AZ7, 12AX7 and 5751.

[madkatb]

Have you tried "chopstick"ing the wiring? Must be some kind of positive feedback going on. Try moving the 47k(?) input resistor to the preamp tube base instead of directly on the input jack.
Or trace the oscillation with your oscilloscope...

[Resonator Guy]

I have chopsticked the wiring quite a bit, gently though with no success. I will try moving that resistor, and I might just remove all the wiring for the banana jacks as I don't believe I will ever use them.
Today I moved the big orange drop resistor out a bit and then put in a 12AV7. The howling stopped. I played for a bit and then put a different 12AT7 back in. The amp worked beautifully for a while then mid-chord it started howling again. Something getting too hot and acting crazy??
Right now I have a 12AY7 in it and it wants to work so I will go play for a while and see if it acts up when warm.
If it starts howling I will put the scope on it. I was afraid that waveform might hurt the scope.