Next on the Bench: Carlsbro 60PA

Realised I was pretty sick of Sound City guts now. Nee to sort a better cleaning plan for the previous one, it is still mostly black on the chassis, I picke up some Fairy Power Spray for stripping some Warhammer models (you wouldn’t be reading this blog if you weren’t a dweeb. Also, cheers to Doug Dreadful for the recommendation, seems to work). So I went back to an amp I started fiddling with about a year or so ago, maybe more, I stopped when it started working.


The Nichicons on the left are the previous work. They fixed the crazy mains hum, and I called it good to go. Check out that burn under the resistor, before my ownership. Basically, it is still noisy, and I swear it has got more noisy since I last used it, all the rest of the electrolytics need to go, and obviously none are same values as the stock I have for the Sound Citys. Apparently the burgundy resistors are bad news too. I’ll change those if the electros don’t fix it. Still trying to find out the real reason why they are bad. Other thing to note is the star grounding, by the fuses an input voltage selector, all the stuff grounds to the same point, so no gournd loops. This is what the Sound Citys lack, and all the noise mods on the net for them drop the gain to drop the noise. Might eventually have a go at star grounding one, butĀ  lower gain valves achieve the same thing without mods.


Rewired cab

Don’t have sleeve, used heatshrink to attach the cable together, I assembled the look then put it in, could have been a bit neater, actually quite hard soldering with it dangling, will masking tape it in place next time. Should be stapled down, but there isn’t much slack going on, ugliest thing is the different length wires from the input, figured chopping that back was a no.

Took apart my Carlsbro 60w PA, as it has become wicked noisy since I last turned it on, fair bit more confident since I replaced some electros in that, figure I’ll do the rest now.

Time for a cab rewire

SO, before I plug in a cab to one of my nice amps, I check the DC resistance, make sure I’ve not got a blown or miswired cab. This one comes up 10.6 ohm, which is wrong, should be 14-15 for a 16 ohm cab. So it is probably 3seriesĀ  speakers in parallel with one.

Messy wiring

All cable tied tightly, and all the same colour, so hard to follow them, the meter says wrong so not much point anyway, cable tying to the center post is a good idea, as it stop loose wire penduluming and straining the joints. Better option is to not have a ton of excess wire.

Sign something is wrong when there is a taped joint in the wire.

This is the proper style, no joints needed. I’m aiming to follow this, Rooted through my wire bundle (recovered from a scrap washing machine), looks like the input wire is the original, so here is the nearest match:


Starting number 2

The first one is now working, noisy, but that is by all accounts part of the original circuit, and I’m probably gonna hand it to a tech with more test equipment to make some mods to help that. For number three, I picked out the nastiest looking one, rather than the easiest. Grubby at the front and all the knobs destroyed. Just a grubby sound city on the outside. Inside we have this:



Not really sure how an agricultural amount of filth can be in an amp, possibly fog machines don’t help, but this is chunky and thick.

Inside the chassis isn’t loads better:

OK at first glance, standard Mark 4 scruffiness.

The bias cap has exploded which has covered everything in the furry stuff, I think it is paper fibers, but it looks a bit like cat puke. I’m not sure what the glistening gob of stuff at the bottom is. Other mods are some replaced diodes in the rectifier, and a cap bridging some resistors, I’m not sure what it does, hoping it is a noise reducing mod I can copy, but bearing in mind the new bias cap is 10uf instead of 100uf, I’m not optimistic.

The little cap right above the yellow box on the left is the addition.

First casualty

Mate came round today to borrow a wadpunch, so he could open out his straps for straplocks (they work best if the hole is big enough for the thread to pass through, so the washer can clamp down tight). Post doing that, was time to plug in amps, top of the pile was the Carlsbro with a clapped out valve, John Chambers kindly repaired the duff power transformer for me a while back. It was sounding great, then it died. Will have to investigate, but fairly sure the duff valve is now a dead valve, and I need a new HT fuse. Should be something to learn from that. Like maybe be arsed to pick up several amps to get to the properly working ones.