Motor Boating

[J Rindt]

Before I just come right out and ask  what, how, why, where.......
Can you guys give me a place to start, some links, point me in the right direction, of trying to trouble shoot an amp that is motor boating.
I took a Crate V18, gutted it, bought a Marshall-18 turret board, and built a Marshall -18, two channel, with tremolo into the Crate chassis.
Can anybody point me in the right direction(s) to rule out some of the most common problems.?
Thank You

[stingray_65]

From Geofex   http://www.geofex.com/

under tube guitar amp tech pages
then tube amp debugging page
click Putt putt like a motor boat


In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)

[eleventeen]

Before you go nuts, have you tried flipping the plate leads of the output transformer?

That's the standard cure for just-built amps.

Basically, your NFB, inverted, is turned into positive feedback = motorboating.

[HotBluePlates]

Further, if this is a rebuild, and you used new filter caps, then they might not have a solid ground.

Good Cap + No Ground = Bad Cap + Good Ground

If your amp has a negative feedback loop, just temporarily disconnect it and see if the problem resolves; if it does, swap OT wires. If it does not, verify good ground for all power supply caps (attempting to solder to the chassis is what trips up most; you need a >100w iron in most cases, and forget soldering on aluminum). If still no good, check grounds for tubes (through cathode resistors), pot harness, input jacks, etc.

[J Rindt]

I appreciate all replys.....will investigate as suggested.
To verify......
This is a new build, with new parts.
I gutted a Crate V18 and installed the Marshall-18 with Trem into that chassis.
I will post some pics after I try some of the above suggestions.
Thank You

[stratele52]

If it is a new built , chance are you make a mistake when built the circuit. Good check up.

[J Rindt]

In one respect or another, that is probably the solution. I am going over the amp now....comparing it to the schem, and using a magnifying glass to look at all the connections.
In the mean time, if anybody want to look at pitures.......Some leads are a bit long....until I am sure everything is where it needs to be. I have done a lot of sticking of the lead dress.....but so far, there is nothing I have been able to move, lift, or push that effects the motor boating in any way.
Thanks Again

http://i111.photobucket.com/albums/n143/zzmoore/Marshall18010.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18009.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18008.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18007.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18006.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18005.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18004.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18003.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18002.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Marshall18001.jpg

If it is a new built , chance are you make a mistake when built the circuit. Good check up.

[sluckey]

Can you post a schematic? Your 18W is quite different from the original. I see a couple full wave bridges and a pot that's not on the original.

Filter caps is a high suspect for motorboating. Divide and conquer. Pull all tubes except PI and output tubes. Still motorboat?

[J Rindt]

Layout
http://www.ceriatone.com/images/layoutPic/marshallLayout/18Wceriatone.jpg

Schematic
http://www.el34world.com/charts/Schematics/files/marshall/marshall_18watt_schem.pdf

Can you post a schematic? Your 18W is quite different from the original. I see a couple full wave bridges and a pot that's not on the original.

Filter caps is a high suspect for motorboating. Divide and conquer. Pull all tubes except PI and output tubes. Still motorboat?

[sluckey]

I'm familiar with Pedal Monkey and Ceriatone. However... "Your 18W is quite different from the original. I see a couple full wave bridges and a pot that's not on the original." What's that about? Could be a factor.

[J Rindt]

Are you kidding me.?
ANYTHING I did could be a factor....
Ayway.....Yeah.
I believe the "standard" 18 was a tube rectified amp.?
The Crate chassis I used was not, so I use solid state diodes for my rectifier. That accounts for one bridge.
Also, I did not make the amp cat biased. My PT had a bias tap, so I made it fixed bias. That accounts for the other bridge you see and that blue pot.
So that is what those things are about. But, just because I know the lingo, and how to build this stuff.....it does NOT mean I know what I am doing. Unfortunately.....my mechanical skills far out-weigh my theoretical skills with electricity/electronics. I read....read what you guys print....read books about tube amps.....but there is still so much I do not know, it is frustrating......almost embarrassing.
My hands have much more ability than my brain at this point. What can I say.? I will try anything you guys think might help.
Will pull the tubes like you said.....get back with that info ASAP.
Thank You