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hi guys,
I've been an amateur amp tech for several years now and can usually diagnose and resolve common issues, but have reached a conclusion about a JCM800 I have that I'd like some feedback on before I start buying parts.
This is a 1988 JCM800 4010 50W combo with EL34 output tubes. It's a Canadian version with two 0.5A HT fuses in the power supply (between the PT and the two sets of rectifier diodes instead of a single one after them). I don't know if that was common to the Canadian version or all models in '88.
Some observations:
1. The amp blows the HF fuses when powered up with a full tube compliment and speaker connected.
2. I can run it without the power tubes (and all other tubes installed) and the HT fuses do not blow. I measure ~483VDC on pin 3 (plate) of both output tube sockets in this case.
3. With my 40W light-bulb current limiter in series with the amp (where the bulb glows dimly), I can power it up with all tubes and without the HF fuses blowing. I can also play through it and get some signal through to the speaker... sounds about like a 2W amp. The plate voltage on the EL34s is ~293VDC in this case, no doubt because of the limited current, but the amp works and all controls seem to function normally... it's just way quieter.
4. Removing the current limiter after trying #3 immediately blew both HF fuses (as in #1 above).
5. Another set of power tubes has the same result.
So... since everything seems fine when the current is limited to the amp, but fuses blow when not, I'm thinking my Power Transformer is dying. But I'm not clear exactly what the failure mode is for the PT here... if it was a short in the PT the mains fuse should blow immediately. So what's going on?
What do you think?
Thanks fellas!
I've been an amateur amp tech for several years now and can usually diagnose and resolve common issues, but have reached a conclusion about a JCM800 I have that I'd like some feedback on before I start buying parts.
This is a 1988 JCM800 4010 50W combo with EL34 output tubes. It's a Canadian version with two 0.5A HT fuses in the power supply (between the PT and the two sets of rectifier diodes instead of a single one after them). I don't know if that was common to the Canadian version or all models in '88.
Some observations:
1. The amp blows the HF fuses when powered up with a full tube compliment and speaker connected.
2. I can run it without the power tubes (and all other tubes installed) and the HT fuses do not blow. I measure ~483VDC on pin 3 (plate) of both output tube sockets in this case.
3. With my 40W light-bulb current limiter in series with the amp (where the bulb glows dimly), I can power it up with all tubes and without the HF fuses blowing. I can also play through it and get some signal through to the speaker... sounds about like a 2W amp. The plate voltage on the EL34s is ~293VDC in this case, no doubt because of the limited current, but the amp works and all controls seem to function normally... it's just way quieter.
4. Removing the current limiter after trying #3 immediately blew both HF fuses (as in #1 above).
5. Another set of power tubes has the same result.
So... since everything seems fine when the current is limited to the amp, but fuses blow when not, I'm thinking my Power Transformer is dying. But I'm not clear exactly what the failure mode is for the PT here... if it was a short in the PT the mains fuse should blow immediately. So what's going on?
What do you think?
Thanks fellas!
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