[retailer]

The build went reasonably smoothly and was able to do a bench test this afternoon this was when I had a senior's moment that lasted the rest of the day. What should have been 21 odd watts output measured at just 4 watts, all of the valve voltages measured with in spec, I had already checked over the wiring before testing and did find the odd mistake so I was scratching my head and starting to doubt my output transformer, the amp not only maxed out at around 4 watts but also the scope test sinewave did not clip nicely but at around 4 watts it turned suddenly in a horrible jagged line resembling a grimacing mouth.

I tried a known good factory made output transformer - sat it on the bench next to the amp and hooked it up using croc clip flying leads - not quite 20 watts (wrong impedance) but good enough to firmly point the finger and my homebrew output transformer or so I thought. I cursed myself silently mainly because the transformer mounting bolts were under the turret board, as the leads were already unsoldered and before I started on the process of removing the turret board I gave the transformer a check over with the ohm meter - all good - next with the signal gen into the primary just in case I had wired the two primary halves out of phase - then an insulation test at 1000 V (max for my BM10) all tested good, so I clipped it back into the circuit with the croc clip flying leads give it another test, I didn't however use the speaker chassis socket as it was hard to get to but used a spare socket I had lying on the bench - perfect, sinewave started to clip at just under 22 watts, at this point the penny should have dropped, but don't think about it I thought just rewire it back into the circuit, once done I gave it a quick test and sure enough back to 4 odd watts output, how could this be nothing had changed, I was out of ideas, the transformer had to come out. I started to unsolder the secondary leads first and I noticed that I hadn't connected the mains ground lead to the chassis - maybe that was it, a voice inside my head was saying you're grasping at straws here but not listening I clipped it to the chassis with a flying lead and as the secondary leads were unsoldered I quickly clipped them to the spare 1/4inch socket that was lying on the bench and powered up the amp for a quick test, it came up perfect 22 odd watts, this is the second time the penny should have dropped. I was dumfounded and frankly a bit disbelieving, thinking that can't be it, must be something else. While I was poking around I accidently bumped the speaker socket that was hanging in mid air, it touched the chassis and the nice sinewave on the scope dropped to almost zero, finally the penny dropped.

I have a dummy load built into a homemade power meter - a cast Al project box 3 position switch for 4,8 and 16 ohms an AC voltmeter/power meter mounted on the front and a long lead ending in a 1/4 inch jack, I had also fitted a BNC socket on the side so I can connect the dummy load resistor via a BNC lead to one of my scope inputs saves me having to use croc clip leads all over the place. With the 1/4 inch jack unplugged and disconnected from the amp I measured around 3.5 ohms between the 1/4 inch jack tip and the amp chassis, so with the transformer correctly wired into the amp and speaker socket my dummy load presented a near enough short to the amp rather than the 8 ohms I had it set to, clearly I had wired my dummy load incorrectly and had transposed the earth an active wires so the jack tip was connected to the BNC earth which was connect to the scope which was plugged into the mains, the signal gen, also plugged into the mains was connected to the amp input socket. Reversing the connections at the 1/4inch jack, fixed the issue, with everything connected I gave the amp another test it was all good and ready for an actual test by the end user.

The old road signs are cheap if you pick them up from the salvage yard $5 ea nice and thick 2mm and most are good size the only down side is they can be a bit warped around the holes and that paint which is not actually paint but vinyl sticker, the vinyl comes of with a heat gun but the glue sticks like you know what to a blanket so the propane torch is the best thing.

I have a number of speakers available to use and it will depend a bit on how they sound to my son he'll have to make the choice, I have an old mid 70's Celestion Green Back, an old Vintage 30, a pair of Rola C12P speakers and a generic 12inch Chinese music instrument speaker that looks like a copy of the Vintage 30. I also have a old pair of Celestion 10's that I think came out of an old Marshal 4x10 cab.