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I've posted here before about a tone cabinet conversion to guitar amp that went very very well, and now I'm giving it a go with a from scratch build of a Hoffman AC-30 using mostly new parts, with some recycled transformers/sockets/etc.
At first test I was using the field coils of 2 old electrodynamic speakers as a filter coil. The results were thumping, loud hum on warm-up, no signal I could hear, and it all happened regardless of the volume. I swapped in an actual filter coil for the second test, and that stopped the thumping, but there is still a loud hum (except in standby it's quiet) and also I noticed I could hear a faint signal through the speaker that is almost drowned-out by the hum. I tried pulling tubes and there was no change by pulling preamp tubes or individual power tubes, but it was silent when I pulled the all power tubes. I was checking grounds and found the negative side of the speaker was open to ground! I hope I didn't kill the amp with an open speaker ground. I grounded it and checked again and it's still the same. I can't get voltages because of the massive hum.
To sum it up:
1. Loud hum comes on as the tubes warm up regardless of volume setting or if there is a guitar connected or not. So bad I have to shut it off within a few seconds.
2. I can hear a faint guitar sound with the volume up, which may be slightly distorted, but its hard to tell because the hum is so loud.
And now my soldering iron died too
Help
At first test I was using the field coils of 2 old electrodynamic speakers as a filter coil. The results were thumping, loud hum on warm-up, no signal I could hear, and it all happened regardless of the volume. I swapped in an actual filter coil for the second test, and that stopped the thumping, but there is still a loud hum (except in standby it's quiet) and also I noticed I could hear a faint signal through the speaker that is almost drowned-out by the hum. I tried pulling tubes and there was no change by pulling preamp tubes or individual power tubes, but it was silent when I pulled the all power tubes. I was checking grounds and found the negative side of the speaker was open to ground! I hope I didn't kill the amp with an open speaker ground. I grounded it and checked again and it's still the same. I can't get voltages because of the massive hum.
To sum it up:
1. Loud hum comes on as the tubes warm up regardless of volume setting or if there is a guitar connected or not. So bad I have to shut it off within a few seconds.
2. I can hear a faint guitar sound with the volume up, which may be slightly distorted, but its hard to tell because the hum is so loud.
And now my soldering iron died too
Help
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