JTM45 Voltage Readings - HELP!

[mutts]

I started testing my JTM45 build and I am concerned about some voltage readings that I'm getting (see attached file). My readings are shown bolded in the table along with the JTM spec values. The readings highlighted in red are obviously concerning especially on pins 1 and 6 of V1, V2, and V3. The higher than normal voltages make me wonder if the 8.2K resistor on my board might be faulty - I haven't checked it yet. For what it's worth the readings were similarily high without the output tubes installed; the readings in my chart are with the output tubes installed.

Other than this the amp does sound great but I think it requires some further investigation before any catastrophe strikes. Any insight from the guys on the forum would really be appreciated. Thanks in advance!

[julkke]

By all means check that dropping resistor.

[mutts]

Thanks Julkke, I think there could also be a problem with the V3 tube. It's glowing but I have to look hard to see it (not like V1 and V2). My thought is if V3 isn't putting out I'm not dropping enough voltage across the 8.2K resistor and thus elevating voltages on V1 and V2. Just a thought.

[mutts]

Just to close this thread I offer my final conclusions to the perceived issue with preamp voltages. It so happens that this was a similar concern experienced by BentFork (see posting below). As our good friend Roe pointed out the preamp voltages in question although high compared to the values in George's instructions aren't really out of wack compared to spec values on an original JTM45. I was not able to find a Marshall schematic for the JTM45 with voltage values but did find one for the Fender Bassman - attached. Roe is right ... no surprise here. So in this case Larry's quote applies "The problem is usually sitting in front of the amp". Indeed!

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