New kit build problems

[tierneyguitars]

Ok, so I put together a hilbishg gt120 kit, having major issues with sound.

Amp turns on no problem, no blown fuses or any weirdness, voltages are correct across power tubes and two 12ax7 preamp tubes are receiving a little over 6.3vac.
As soon as the amp gets signal from my guitar any harder than a very light strum with my thumb it goes bananas, very loud zipper/brown tone farts out of my cab, even if volume on amp is set at 1, the unpleasent sound BOOOMS out of the speakers. Any ideas as to what might cause this to happen. Also all cables and input jacks annnd speakers are in tip top working condition and tested.

[g1]

Please post schematic of actual hilbish.

[Richard]

Does rolling back the guitar volume help at all? can you post a schematic? If you strum very lightly, does it sound normal and then above a certain level make this awful noise? Have you tried a different speaker?

[big_teee]

I had a similar problem several years ago on my first 2204 build.
It turned out to be a wiring error on I think V2, don't remember now.
Check all you preamp wiring.
T

[tierneyguitars]

Rolling the volume back does not help. When lightly strummed it does sound clear and normal. It is as soon as my pick attack is medium strength it goes. Im gonna check the pre amp section again. Thanks. GT120 Schematic .pdf

[Tom Phillips]

Strum lightly then thump on the cabinet or chassis. If that makes it "go bananas" then look for bad solder joints or loos mechanical grounds.

[Enzo]

My first guess is the amp is unstable, and the first bit of signal ti gets sets it to oscillating. I will wager you have the output transformer primary wires reversed.

If the transformer uses American wire colors, the primary has red, blue, and brown wires. The red goes to B+, and I suspect your blue and brown wires need to swap places.

A quick test of my theory is to disconnect the negative feedback wire from the output jack. if that stops the noise, I am correct. If the noise continues, I am wrong.