Musicians Roadhouse

Slo-Blow fuses are used when there is a situation where inrush current will fry a fast blow fuse repeatedly. Remember as well as supplying power to the circuit the caps are charging and this creates a momentary high current draw which quickly levels off. A slo-blow will with stand this inrush current.

ClassicRock88 wrote:Well I pulled the fuses out of my two Grangers to feed them to the Super Bass and I know they were slow blo. The new ones I just bought from a local amp tech. I would assume that they are slow blo. What dire consequences will arise if they aren't?

There's a good chance they would blow on switch on. The end cap of the fuses bears the name and will show a 'T' or a 'F' before the '4'.

I believe that they all say T/4A on them.

I would take a look at the underside of the pcba. I have repaired amps where the traces will start lifting away from the board, and connections can become intermittent/open/ or even short to another component. Theres probably a simple solution to the problem, but you really gotta perform some thorough diagnostics/troubleshooting. Dont give up!

Are the rectifier diodes ok?

SteveD wrote:Are the rectifier diodes ok?

No idea. Amp is at a shop now. :(

Just to add a note, the JMP model 1992 schematics indicating 6550 power tubes had a timed (slow blow) 5 Amp main fuse.

Ok I got it back. What was done:

New tubes, bias cap replaced, board electrolytics checked over, fixing the impedance and voltage selectors so they both work correctly, checking tube sockets, fixing bad solder joints and cleaning up some old repairs that were done sloppily.

Now the only weird thing it's doing is when I turn it off. After I turn the amp off the pilot light still glows faintly. If I unplug it it stops. If I switch it off of standby before I unplug it I don't get any sound and the filaments are't glowing, but the pilot light is still glowing 1/3-1/2 brightness. Is this a bad switch and should I be immediately worried about it?

Looking at the schematic the pilot light is running off the heater winding. If the power switch is off that light should be off.
http://www.drtube.com/schematics/marshall/1992u.gif

Is the polarity correct in the amps power wiring and or the outlet? How much voltage is still coming from the heater winding when off?

Anitoli wrote:Looking at the schematic the pilot light is running off the heater winding. If the power switch is off that light should be off.
http://www.drtube.com/schematics/marshall/1992u.gif

Is the polarity correct in the amps power wiring and or the outlet? How much voltage is still coming from the heater winding when off?

Ok it's learning time! What's the best way to do this and not die?

A this this amp rating, most slow blow fuses have a coiled element, fast blow (or standard) fuses have a straight wire element.

On top is a slow blow, below is a standard

ClassicRock88 wrote:

Anitoli wrote:Looking at the schematic the pilot light is running off the heater winding. If the power switch is off that light should be off.
http://www.drtube.com/schematics/marshall/1992u.gif

Is the polarity correct in the amps power wiring and or the outlet? How much voltage is still coming from the heater winding when off?

Ok it's learning time! What's the best way to do this and not die?

An easy way to see how much AC is still present is to just remove one preamp tube and measure the AC voltage at the tube socket of either pin 4 or 5 and pin 9.

http://www.tubebooks.org/tubedata/HB-3/ ... _12AX7.PDF

Will get on this. Other concern is when I got to run it up there was a sort of "warbling" in the low end, if that makes sense. I'm guessing this is old filter caps?

Anything is possible there. Rule out the easy stuff first like try a different cab, can you dial out the offending overtone, and the like but an amp that old would benefit from at least a filter cap change. It wouldn't hurt.