Motorboating issue on new build. Extension of the E88CC thread.

[gtrplayr1976]

Well. An issue that appeared overnight. I was going test the amp out. Plugged in a guitar, and when I hit the standby switch I got a motorboating sound. I tried every input ,and no input. So I started poking around ,and could find nothing obvious to me. I then started pulling tubes ,and this is what I noticed.

V1 = No change (left it in place)
V2 = Noise stopped
V3 = Noise stopped

I swapped tubes around ,but nothing changed. So, the noise stopped with either V2 ,or V3 out. I'm attaching a link to the sound. Maybe it will help.

18 watt Marshall clone motorboating - YouTube

[gtrplayr1976]

Could someone give me pointers on lead dress please ? I really need to know what to separate from what.

Thanks folks.

[Gtr_tech]

Tried swapping the output xfmr primary leads around yet?

If the amp uses NFB, connecting the primary leads backwards will result in *positive* feedback shoved into the PI where negative should be. Result? The amp becomes a Theramin. Waving yer hands at it won't influence the freq/amplitude tho

[Gtr_tech]

Hmm....just noticed you said pulling the V2 tube will stop the racket. Primary leads are likely OK as they are. But with lead dress issues in the front end you will normally end up with a higher freq whistle/whine rather than a low freq motorboating. Just moving leads around will point out where the problem is. Generally you want to keep grid leads away from plate leads.

[gtrplayr1976]

Grid leads ,and plate leads. OK I will have to figure these out. This is my first build ,and I'm not close to knowing what all these things do ,or even where they are. I can figure the plate/grid leads by pin out.
Thanks Gtr_tech

[Randall]

If it happened overnight, I would probe around with a chopstick and see if anything makes it change. Push on all connections, tap on the board, move leads around.

[gtrplayr1976]

Thanks Randall. That's what I was doing ,and I noticed when I moved the wires around V3 the noise got quieter. I then took a shot at moving the .047 cap that was attached to pin 6 V3 to the alternate location specified on the layout. Noise was gone after that. I have channel 1 working ,but I still can't get 2 to work. I was sent 3 new preamp tubes ,and all 3 were bad out of the box ,so I'm back to the old ones. I have some voltages to post also. Maybe they will tell someone something. I thought I had found the issue with some voltage that wasn't showing up ,but I corrected that ,and still no channel 2. Maybe I have the input jacks wired wrong.
The V3 on the bottom of the notes is after I found the issue with voltage.

[gtrplayr1976]

Everything working now. Found a bad cap (.0047) off of V3 pin 1 ,and the footswitch jack was wired wrong. Everything seems to work pretty well. Preamp tubes are good. So it's all peachy