motorboating?

[titanicslim]

Well, my new Liverpool build is all together, but not yet making sweet music, I'm afraid.

The problems are: 1- a complete lack of signal output; and: b. with volume OFF and other controls in any position, a loud HUMM and, as soon as I TOUCH the volume pot, a very loud staccato that I take to be the dreaded MOTORBOATING I've heard so much about but never experienced until now.

When I first applied power, yesterday, I got a very loud buzzing sound, which I HAVE heard before, so I switched the OT blue and brown wires and that took care of it- but no signal then either. Then in my most recent round of checks I discovered the power resistors were not grounded, so I remedied that. And now I am in the above stated predicament,

I have gone over and over the circuit and can't see anything amiss. Does any of this ring any bells for you?

Dave

[skyboltone]

Photos Dave. Pre-amp grounds.

[titanicslim]

[IMG768]http://i175.photobucket.com/albums/w157 ... CF0814.jpg[/img][IMG768]http://i175.photobucket.com/albums/w157 ... CF0811.jpg[/img]

I didn't know exactly where to star the preamp grounds, so I'm using the clips. Doesn't make much difference.

[LOUDthud]

Done any voltage checks? I know sometimes digital photography can play tricks on you but those carbon comp resistors that should be 100K look like 100 ohms. Check the speaker output with a DVM set to AC volts. An ultrasonic oscillation will show up as a big voltage like 10 or 20V.

[titanicslim]

The CCs are 100k, they're just old and a little faded.

I think the noises I'm getting must be, at least partially, because of the close proximity of the output tubes to the filter caps, no? I was trying to follow the layout...

Say, where do you get 25w resistors that are the size of the ones in the layout? By the time I sized that ancillary board to fit the xicons, there was not room for those big caps between the boards. Also, I couldn't even find a grommet big enough for that huge hank of wires coming from the PT.

There's got to be a supplier for some specialized parts to fit this thing. What gives? I guess I'll do the Fender thing and put those caps on the other side of the chassis. Shield them with a stomp box enclosure or something.

Anyway, I'm a little afraid to leave the thing on long enough to do voltage checks (let alone unplug the speaker to check that voltage) with it emitting that deafening hum. I guess it's time to tear it all apart again and try to figure out some reasonable arrangement.

Dave