Repair and Rebuild: Marshall JMP model 1987 50W - Part 4: Testing

Testing! Testing the amp at different settings with side, by side comparison of Celestion Creamback M65, Vintage 30 and G12T75 speakers. Comparison of Pentode/Triode mode, Calculating output power with everything on "10."

[@giulioluzzardi7632]

Well done for taking those readings. I had to use a 100watt (8 ohm) celestion10" paired to a "15" watt 2 x El84 amp to get the best frim the amp, full volume...peaked at 60watts at 3/4 bias! Valve amps don't appear to obey any set laws on paper so be prepared and use a 4x12 or at-least 2x12 (70watts each minimum) so you can avoid melting that lovely New transformer or melting voice coils with too much distortion. I reckon after all this you could start thinking about the Vox AC-80 head as a possible build, now "That " would be good but don't rush. Great stuff!!BTW if you just plug an outside source in to amp (CD, phone etc) it could help get clearer readings and free-up your hands.

[@enginerdamps2562]

I may have to take some readings on the other side of the transformer to see if I can get AC output next time.

[@qua7771]

You've gradualy replaced the whole thing. It came out nice though.
I built my amp as well, after decades of never really finding what I wanted retail. I couldn't be happier, and I'm sure you are as well. If you don't like something, you can always change it. That's not always easy with PCB amps.

[@jjsant3250]

Instantaneous DC measurements are not the power output from the tube to the speaker. Nothing DC gets to a speaker. You can measure the ac voltage on the speaker while playing a single note to get somewhat close to a reading, then do the math. Evan that has huge error potential since a speaker does not have flat impedance across frequencies. Use a high-power resistor and a frequency generator (e.g., phone app if needed) to get real figures.

[@jensk4140]

Celestjohn