Marshall JCM600 heater fuse blowing

[pontiacpete]

This amp starts up and plays great sometimes but other times it blows the 6.3A fuse, F101.
The only thing I've tried so far is to replace the power tubes.
I'm not sure where to go from here, the caps or the bridge rectifier or what else?
Has anyone come across this sort of problem before or have any suggestions. ?

[pdf64]

Is a T rated / slow blow replacement fuse being fitted, ie T6.3A?
On a side note, those pre-amp tubes must be seeing 8Vdc on their heaters; doesn't that cause early failure?
Pete

[g1]

You must use a slow-blow fuse in this location. Otherwise, as you mentioned it's most likely the bridge or leaky caps.

[pontiacpete]

T6.3A fuse was the original one that blew when it came in.
My replacements have been 6A slo blo .
I ordered the 6.3's and the caps/.

PDF do you mean early failure of the ax7's?

Thanks

[pdf64]

Yes, sorry, early failure of the 12AX7s.
The other key thing to check is for carbon tracking on / around the output tube sockets, most likely on the socket face nearest the tube base.
If replacing those 3300uF caps, look to increase the voltage rating, especially if one of them proves to be the root cause.
Pete

[g1]

those pre-amp tubes must be seeing 8Vdc on their heaters; doesn't that cause early failure?
Pete

I'm also always suspicious of higher heater voltages when the AC heaters are rectified like that. But the "theoretical" usually ends up lower when I do the actual voltage readings. Maybe the 100R grounding resistors will load down the DC heater voltage?
Please post the final AC and DC heater values once the repair is complete.

[pontiacpete]

with all tubes pulled out the heaters on the AX7s is about 8.7vdc
Will check the VAC on the el34s

I ordered 25v caps, they will just fit between the chassis and board.
This amp has a really strange board position in the chassis, it's perpendicular to the bottom of chassis.

[pontiacpete]

Here's the outcome of this, so far, I replaced the 4 3300uf caps. I used caps rated a little higher at 25v.
They had to be laid down in order to fit in the space.
The bridge rectifier had created a burn mark underneath which I cleaned/scrapped carbon mark. I cut traces that came closest together
in the burned area and added wire.

The voltages on the heaters 7.7vdc and on power tubes 7.7vac on the power tubes.
Each reading was taken with one tube removed,, it's difficult to probe down into this chassis.
Thanks for the help!

[Lee_ranaldo]

I've had issues with these marshall amps that used rectified heater supply for some reason. Specially the DSL combo's
What i had to is totally replace the bridge rectifier with one that had a higher current capacity. It was causing the preamp tubes to die
literally. Your bridge rectifier shows sings of over-hearing and that "might" have caused the heater fuses to be shorted

Check your heater voltage with all the tubes running, If drops the voltage too much might be a sign that you gotta do this modification too.
Also it improved the sound SO MUCH because the heater voltage was close to its nominal value and made the tube sound better.

[pontiacpete]

This amp has come back for a 3rd visit with the same issue.
I did the up grade of the bridge rectifier on the last visit. The amp got used a bit over the year but the owner tells me not much.
So now it's back with the heater 6.3 fuse blown and not sure where to go from here.
Any suggestions?

[Mick Bailey]

Re-check the rectifier to see if it's blown again.

I had one of these amps doing the very same thing and the CF was occasionally breaking down and spiking HT onto the heater circuit. An indicator of this is one or both of the 100R balance resistors opening.

[pontiacpete]

In circuit they are both measuring 50ohms. The first time here, I raised the wattage of these resistors to 2 watts.

The amp is working with the new fuse in place. Turned it on and off many times trying to see if it will blow.

BTW 'CF'? cathode filament? or follower?

[Mick Bailey]

Cathode follower. A tube in this position can break down due to H-K voltages being too much for certain 12AX7s.

[pontiacpete]

Thanks Mick!
So are you suggesting changing the pre amp tube and would that be V103?

[pontiacpete]

Also, how bad would it be to do away with the fuse and just wire it up for AC heaters?