Build Your Own Clone Message Board

I powered up my newly built Marshall 50 watt yesterday. It was working for 2-3 minutes when it suddenly shut down. I was making g a video of myself as I played the amp for the first time.
I blew the 500ma chassis mounted fuse about 2 minutes after playing the amp.
It seems like it happened when I jumpered the two channels together. I used the amp that way earlier with no trouble.
I'm waiting for replacement fuses to arrive tomorrow.
Help would be greatly appreciated. Thank you!

You say powering up did you go through the complete testing process? And all went well including all voltage readings in range ? If so, I would start by measuring all your grounds... Not just continuity but read the resistance.

Sent from my Pixel 3 using Tapatalk

When I get the fuse I'll go through the start up process again and write down the measurements. I remember that the measurements were higher than expected but not too far out of range.
Thanks for responding.

Voltage are going to be higher than expected without the tubes in their sockets.

If the internal fuse is blowing, it means that something is drawing too much current. And this is almost always caused by a short to ground.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

Here are my voltage readings from the filter caps just power tubes installed.
Starting from cap #2:
Expected V, actual V
Cap #2
C 355. 480
B. 480. 485

Cap # 1
E 280. 484
D 305. 484

I'm not really concerned with the voltages being higher than "expected" if you have the tubes out, which you do at all points after test point B. What is concerning is that D and E are reading higher than C. Are you certain that these voltages are correct?

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

I am testing with the two power tubes in. The readings here are now from the correct caps.

The large filter caps not on the turret board all read 490 on the (2) red and (2) yellow terminal.
The cap on the turret board has a voltage of 485 on the right and left side.


By the way which are you calling the left and right? It depends on how you are looking at it. I want to be on the same page as you.

The picture that I posted has the 500 ma fuse removed. It had already blown. I have it in for all of the readings that I've listed in this post.

We need you to take the readings with the pre amp tubes installed too. All tubes installed. Otherwise, the circuit will not be loaded down properly and the voltages will be off.

_________________
MasterDelayer/Reverbrador/Ampaholic/TopJacker

Just like the wiring diagram. "Left" is the side with the power and standby switches. "Right" is the side with the input jacks. Test points A through E run fairly linearly from left to right. B would be the red terminal on the second filter cap. C would be the yellow terminal. D and E are out of order, though. E would be on the left side of the filter cap on the board. D would be on the right.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

I reflowed the solder connections on the 4 inputs jacks. I checked the the ground connections for continuity and the resistance readings. All the connections had continuity to ground but they didn't read 0 ohms. There was a .002k -.003k reading on most of the ground connections.
I'm not sure if that is something to worry about.
I also cleaned the sockets and all connections to get rid of rosin residue. (The sockets were clean already).

Here are my readings with all of the tubes installed and the amp biased slightly cold:
Large filter cap#2
Yellow 361 V Red 487 V

Large Filter Cap #1
Yellow 489 V Red 489 V

Turret board cap ( looking at it from the tube sockets toward the pots )
Left 290 V. Right 310 V


I've played the amp for about 30 minutes and so far it's working perfectly. The amp made a " frying bacon " sound when turned on. That lasted about 20 seconds and faded away. I blew the fuse the first time when I jumped the channels but it hasn't happened this time.
I'm not sure what the noise level of these amps normally are but the noise level is higher than I'm used to.

Huh? Sometimes fuses just blow. Hopefully it was just a one time thing.

Do you have a cabinet with shielding? The noise floor should go down a little when the opening of the chassis is covered.

_________________
*patience is a virtue*

Please do not PM me. email is prefered. keith@buildyourownclone.com

Shielding the cabinet seems like a good idea. The input and output leads don't have shielded cable .That would another way to lower the noise level.
This is a very loud amp! I can't raise it past 2-3.