Blues Jr. Screen Resistor Size

[glebert]

I've got a yellow board Blues Jr. on the bench with a blown screen resistor. Schematic calls out 100 ohm 1/2 watt fireproof. I don't have any of that size on hand, so before I order something up wanted to see if others stick to the original or use something else. I've read theories that the 1/2 watt size was intended as a sort of protection mechanism to burn open with a shorted tube. If so I wish they had made them solderable from the top of the board without having to remove almost everything to change it out.

TIA
Greg

Schem https://www.thetubestore.com/lib/the...-Schematic.pdf

[Jon Snell]

Changing the EL84s on a regular basis will avoid the screen resistor failing.
It takes less than 10 minutes to remove the board from the chassis, once removed, replace the screen risistors with 100R 1/2W FP (Flame Proof) type resistors and then the input jack, (for reliability). If you don't the owner will be in for a nasty shock at the cost of replacing the board when non FP resistors set fire to and destroy the board!
Check the mains inut tappings are correct; 240v for UK not 230!
Mount the new FP type resistors off the board for safety.

To remove the board;
Unplug the anode and CT connectors of the output transformer.
Remove the knobs and plastic guides.
Pull out the pilot light from the housing, gently.
Unscrew the output and external speaker jack sockets.
Remove the fixing screws, around the perimmiter of the PCB and slide the board towards the valve bases gently easing the control side out of the chassis. This may take some care in alligning the tight wire loom.
The reverse puts it all back.

If it's worth doing a job, for goodness sake, do it properly.

[mozz]

I would change the bias resistor too to cool it down a bit, these do run the tubes hot.

[glebert]

Sounds like folks use the values recommended by the schematic, so I ordered those up. Couldn't get fireproof 1/2 watts through my normal supplier.