I’m an amp tech in Memphis and this is the second time this Marshall has come back with an EL34 cracked and gone to air. While testing today, I had the amp at around 65W, running through the power station and two of the screen resistors opened. I replaced them and took some readings in operate and bypass with the same amp settings. Here are my numbers:
You can see in the readings why a few screen resistors opened. Not sure about the amp only pulling 4mA of plate current at 65W going through the powers station. Also, the amp, power station and my dummy load are all at 16ohms. Yes the amp is biased correctly (with no audio). I measured the amp in jack on the power station and it reads 870ohm.
Customer states that his other amps do fine with the power station but they are all master volume amps.
This should eliminate a lot of problems and provide a ground truth as it’s really just a reactive load (of course use flat settings to make it resistive)
Also, yes there is a protection circuit on Speaker 1 so the power station can detect if it has a load.
Thanks for the response. Can you describe the connection diagram for silent mode? It sounds like I need to disconnect the speaker out, leave the amp connected to Amp in, and this would use the internal load in the power station. Correct? I’m seeing an internal load of 914ohms setup in this manner. This sound correct?
Well I finally got the lid off the power station and found this. After resoldering the resistor connections it appears to be operating normally now. Not sure what would have caused this. Any thoughts? Also I found a black disconnected wire and it’s not real clear where it goes or if it’s extra.
Glad you have found the issue. Do you think those problems occurred naturally or might be the result of some hacking? Certainly something bad happened in there.
I don’t have the manufacturing information to be able to help further, please wait for @DavidPhelge to comment. He is on vacation so response might take a week.
( again )
