JCM 900 Output Valve Fuse

[dpm309]

Working on a JCM 900 that had blown both output fuses. Replaced both of them and fired it up using the light bulb limiter and it indicated no shorts. My question is what do these fuses protect, the OT or the output valves? I am wondering if the owner inadvertently powered up the amp without a speaker load. Will fire up the amp later without the light bulb limiter. All of the output valves light up and checked them for shorts on my B&K tube tester. All 4 EL34 showed no shorts. Also, someone had replaced the original OT.

[sluckey]

Those fuses will blow if a pair of tubes cathode current exceeds 500mA. This protects the tube and the OT. Most likely cause for blown fuses is bias too hot, or failing tubes drawing too much current, or playing with everything dimed. Playing power chords without a speaker is very dangerous too. The fuses will likely not blow while on the LBL.

[dpm309]

Thanks, Sluckey. I'll pull the chassis and check the bias. Can I get accurate readings while on the LBL?

[sluckey]

Thanks, Sluckey. I'll pull the chassis and check the bias. Can I get accurate readings while on the LBL?

NO

[dpm309]

With the output tubes removed and not on the LBL, I am getting 507VDC on the plate (Pin 3)  and -37VDC on pin 5. It appears the bias circuit is good. I'll put the tubes in and fire it up and hope for the best.

[tubeswell]

507VDC on the plate (Pin 3)  and -37VDC on pin 5.

Might not be quite enough bias voltage for that Plate voltage. Does output tube current still creep up with the bias voltage maxed out?

[dpm309]

With the tubes in and warmed, I am getting 460 VDC on the plates and set the bias to 37mv. The bias seems to be holding steady after I let it run for about 15  minutes so I am thinking the bias circuit is OK. I'll let it run for a few more minutes to make sure. Must have been a bad speaker cable or the amp was run without a speaker load.Dan

[pdf64]

Idle is one thing, moderate signal power output another, but overdriven into an inductive load is the real acid test.