TSL blowing 1A HT fuse

[jcm-jmp]

ok got a tsl100 that if you turn volume all the way down and turn on power,.. the HT fuse holds. you can play for hours at high levels no problems. Turn the power off, allow the amp to drain turn standby switch open, and then turn power switch back on allow tubes to heat for a couple minutes and close standby switch, if volume level turned up ht fuse blows, if the volume level is all the way down when standby switch is closed ht fuse holds 
 
 

[HotBluePlates]

ok got a tsl100 ... close standby switch, if volume level turned up ht fuse blows, if the volume level is all the way down when standby switch is closed ht fuse holds 

I'm not confident I'm reading the TSL100 schematic properly.


I think I see the HT fuse not place in series with the B+ voltage (as it would be on older Marshall amps), but between several circuit-ground points, the caps to chassis ground, and the actual earth ground on the power cord. I'm uncertain on this because this makes it look like one side of the (otherwise balanced) heater supply gets connected to earth ground.


I'm guessing the fuse blows only when there's significant ground current, but I don't see an obvious reason why this would happen when your volume is turned up. Maybe someone with more experience on this model or era of Marshall will know more.

[jcm-jmp]

i seen that too. i just assumed that it eventualy made a path to ground, but i still dont understand why volume level causes fuse to blow. it seems like some sort of in-rush current when volume is up with no signal input.

[jcm-jmp]

problem solved! fyi my tube amp friends. DO NOT use GMA style buss fuse in HT fuse on a tsl100. The GMA buss style is a quick blow fuse. The GDA is a time delay style and is what is required in the HT fuse position. The time delay GDA type fuse allows for a momentary "over current condition".
 
Tube amplifiers have a large in-rush current when the standby switch is closed. This "IN-RUSH" current is caused by the capacitors on the B+ power rail charging, which causes the B+ rail to draw more than 1A momentarily until the caps stabalize and are at a charged and ready state. This realy bangs home the need for caps in good working order on th B+ rail.
 
Older caps charge and discharge rates are slower than new caps. This means older caps stay in a "charging condition" longer than new ones and the B+ rail stays in a "in-rush" or "over current" condition longer than a B+ rail with new good condition caps, which causes undue stress on the power rail and the PT and ultimately blows th HT fuse or in worst case, fry's power tubes.
 
Hope this helps   

[HotBluePlates]

Good job sorting that out!


Yes, fast-acting and slo-blo fuses have different response in a tube amp. And most often, slo-blo does seem to be the best choice.