Marshall kit 100WSL - trouble shooting

[angelodp]

Hi, Just finished up a 100W SL build, my second one. I based the layout on the Metro version and it came out very nice and sounded great.... until.

A bone head move, while checking bias at the 1 ohm resistors I shorted V4 - pin 3 to pin 4 - spark and puff of smoke.

I checked the fuses and had to replace the 1A fuse, I also discovered that V1 and V2 pre-amp tubes took a hit.

I know have replaced the tubes and also put in a fresh standby quad of power tubes. I brought up the amp with a 2A fuse , which was what I had on hand and the amp played great, no issues. The 1A fuse arrived and I popped that in and the fuse blew, when I go to play on the standby. It does not blow with the on off switch.

I have my variac modded with Sluckey's current metter and I can see with the 2A fuse back in that the amp does climb up to 1.2 Amp draw.

I cleaned up the bit of carbon on the V4 pins and its cleaned up. The 1K 5w and 5.1k R's read correctly.

So It seems that I have an issue now that I need to track down.

With the limiter and 1 amp fuse it does not blow the fuse, but that is to be expected, I gather from RG's post.

When I play the amp with the 2A fuse the current draw is steady at right around 1A even at high volume.

Could use a bit of a road map to trouble shooting this.

[Stevem]

Your your own worst enemy it seem's.

Shorting output tube pins 3 to 4 should have done nothing since there both at a positive potential, and with certency not caused any harm to preamp tubes.

[pdf64]

Hi, Just finished up a 100W SL build, my second one. I based the layout on the Metro version and it came out very nice and sounded great.... until.

A bone head move, while checking bias at the 1 ohm resistors I shorted V4 - pin 3 to pin 4 - spark and puff of smoke.

I checked the fuses and had to replace the 1A fuse, I also discovered that V1 and V2 pre-amp tubes took a hit. ...

Might the short have been between lugs 3 and 2?

What fault did the valves in V1&2 exhibit, eg just not passing signal, or something else?

[angelodp]

Yes, sorry for the error. It was pins 3 & 2 of V4, the area between those two pins is where the carbon was deposited, now cleaned up.

SteveM - yes - what can I say.... no coffee that morning!!

To make matters worse, I realized that I mis-wired the IEC socket with regard to N & L - now re-wired. So when all this happened the N & L were reversed. Horrible mistake I know. I am including the amp pic to show that it's not all bad here.

The V1 & V2 tubes were very low power after the event so I checked them on my tube checker and they are on replace.
Both sides of V1 and one side of V2.

The power tubes were fine.

At the moment with a 2amp Fuse it plays and on a bulb limiter when I throw the on/off the bulb dims, when I throw the standby it glows then diminishes then comes up slightly but not at all the full 100watts of the bulb.

I have RG's method read up and I suppose I shall embark on that trail.

As I mentioned with the Sluckey style current meter I see and consistent 1.13A when in play mode.

I am preparing to take my medicine and will do what I can to rectify my errors.

Any additional insights are gratefully accepted.

[xtian]

So, HT shorted to ground through the heater winding. Do you have 100R heater-to-ground reference resistors (if yes, check them), or heater CT?

I can't imagine how the preamp tubes got damaged, because the HT would take the shortest path to ground...

[angelodp]

Hi Aaron, thanks for looking at this. I do not have reference r's for the heaters. I do have the heater CT grounded at the pre-amp grounds as in the Larry scheme. I show normal readings on all the heaters and normal readings at the entire amp? As for the EH pre-amp tubes, Mojo is sending replacements and they are for sure not operating normally after the incident. Any areas you would check before tearing things apart.

Best A

[xtian]

I would pull all tubes and make sure amp is only drawing ~10w from wall. Confirm bias voltage and unloaded B+ voltage. Put tubes back in and check bias.

Forget about the damn standby switch. Leave it in "run" position always and never touch it. If HT fuse blows, WHEN does it go? Immediately on power up, or after 15sec when tubes start conducting?

[angelodp]

Ok, HT fuse has not blown ever. It's the 1A fuse that going. I'll try the rest now.

[xtian]

Ok, HT fuse has not blown ever. It's the 1A fuse that going. I'll try the rest now.

I asked about this earlier. The HT fuse should be 1A, and the mains fuse 2A. If you're using a 1A fuse for MAINS, that's the problem. A 100w amp is going to draw more than 1A during normal operation.

[angelodp]

I believe the schematic calls for a 4A fuse for HT and 1A for mains, but 1A fuse keeps blowing.

Here are the results. Once the tubes were back in the amp pulls 114w from wall with 1.09A

Bias was as it was set previous to pulling tubes, with tubes now in 30 -31 range on all tubes.

Unloaded it pulls 5.6w at .07A 120v

Do these readings all seem ok?

BTW - I know it's not a great idea but I still have a 2A fuse in the amp in order to do all this.

[xtian]

I believe the schematic calls for a 4A fuse for HT and 1A for mains

This is backwards! MAINS needs the larger fuse, 4A. HT draws much less current than the entire amp, less than 1A.

[angelodp]

Aaron, egads, correct as always. My review work needs serious sharpening. It's all together now holding the fuses, no blowout and sounds great. My damn dyslexia gets me in trouble from time to time.

This goes for my Grandson who has a great little band going.

Onward

A

[Stevem]

Besides what’s posted above do this so your in a good starting place for putting the new tubes in.

With the right fuses in , the amp off the bulb limiter and all the output tubes removed from the amp set the negative bias voltage on pin 5 of any output tubes to the max negative voltage you can get.

Hopefully you will see more then - 40 volts and then it will be safe to pop in the output tubes.

[angelodp]

Stevem, yes I did just that and all is well. Thanks for chiming in.

[angelodp]

On a side note, the Mojotone fuse holders are gone way flimsy. I do not see Valvestorm anymore. Do you guys have a great source for Marshall style fuse holders that are robust?