New Marshall JTM-45 clone build makes harsh loud noise.

[PfeifferElectronics]

I just finished my JTM-45 clone and it makes an awful loud noise when I switch on from standby. This is not a background hum, it is very loud even with all controls at zero and nothing plugged into the inputs. Here's what I know so far:

With just the rectifier tube (JJ GZ34 S) installed I get 505 VDC on pin 8 (seems high). On the other side of the standby switch, I get 8V.

With the 3 preamp tubes installed (12AX7s) and standby off (energized plates):

All filaments are at 3.5 VAC

V1: 257V on pin 1, 2.1V on pin 3, 250V on pin 6, 2.1V on pin 8.
V2: 214V on pin 1, 1.4V on pin 3, 352V on pin 6, 213V on pin 7, 214V on pin 8
V3: 261V on pin 1, 47V on pin3, 255V on pin 6, 47V on pin 8
V4: 494V on pin 3, 493V on pin4, -45V on pin 5, 494V on pin 6
V5: same as V4
V6: 492V on pin 2, 360V on pins 4 & 6

No noise until I install the power tubes, then everything gets loud and noisy and I'm afraid to leave the amp on for very long so I don't burn something up.

If I remove the 3 preamp tubes and install the power tubes (KT66). The amp sounds like a machine gun. (with all controls at zero).

I have triple checked the OT wiring to make sure I have it correct. I've installed a bias pot to adjust the bias with no better results. Also checked the grounds on the power tubes. I've tried swapping out the KT66s with 6L6s. None of this helps.

Any ideas?

[Ronsonic]

I have triple checked the OT wiring to make sure I have it correct.

Have you tried swapping the primary leads, anyway?

Unless it is from a kit, there's a real chance the color codes are off. Even then ...

[tubeswell]

Yes - first suspect is positive feedback (if not a microphonic pre-amp tube - but the clue is in the 'machine gun' oscillation you report with the pre-amp tubes removed). So try disconnecting the NFB resistor (where it goes to the speaker jacks). If that stops the noise, then do what Ronsonic said.

[PfeifferElectronics]

That was it! Thanks guys!