Chimike

That's Amazing, Great, Thank You. I'll get them ordered up, together with the 4 220uf 200v caps in the power supply section that were stressed. My friend who owns the amp said it was still functioning , hard to believe , but worth a go . C5 and C6 had exploded, and are completely unrecognisable. Best Wishes, Mike

Thanks, Electrolytic's, Yes . Clearly marked on PCB C5 and C6.

Could someone with a Mark Bass Little Mark 250 please look inside their amp, and note down the values, voltage and orientation of the Capacitors C5 and C6 , located close to the output Transistors . ( or does anyone have a circuit diag ? ) PLEASE only do this if you are competent and safe to do so , make sure the amp is unplugged , and do NOT poke inside . The Caps should be clearly visible to a knowledgeable person . Thanks , Mike

Bit of Fun, last week, playing with the youngsters !

My recently built Greenboy 12/6 , and 15/6, although on the 15/6 I had to move the port to the rear of the cab, and add a false floor to get the "Vox" look ! I still LOVE 15's.

I have to Thank the Basschat members that have helped me here . Finally had time to remake the power transformer connections , and carry out a test, all seems fine on 240v. Great to be able to do away with the drop down transformer, on this B15, at least !

Better pic ! Seems these were used on the '61 B15 ,the again on the '64 version, that used Silicon rectification . Maybe my late '64 valve rectifier amp was built using the early '64 boards that had the "thermister type thing ". Later B15's did away with that component, so I think I'll do the same. Also, there were two versions of the PT108 mains transformer, part no 300...- 1 for 117v operation, and part no 300...- 3 for 230v operation. Unfortunately, the circuit diagram I found on the Ampeg website with that info is a bit smudged, but I think I'm sorted now. Will try the 230v connection on the transformer, through my Variac, and keep a good eye on the output voltages, JIC. Many Thanks for the help. Mike

That circuit diagram is similar to mine, at least, on the mains input and primary side. Seems that the symbol is an old way of showing Resistors, so you're probably right as to it's " soft start " function . I'll measure it ! I'll also get a better picture ,will help to confirm. Thank you Both.

Not the best photo in the world. It's the silver thing between the pink condenser and the white resistor. It's shown on the circuit diag under the amp, ( B12N AND B15N ) between the on / off switch and the mains fuse. Any help greatly appreciated .

So, Continued searching, and found a mention of a similar transformer on a '63 B15 on music electronics forum . Guy named Boroman , in Germany, was recently asking the same question as I have re the Blue and blue white taped off wires. Conclusion was that it was indeed a dual voltage transformer. He was advised to remove the death cap ( done that ), And add a resister into the neon pilot light circuit before making connections for 240v. Only problem I have now, before I try it ( on a variac ) is that there is a " thing " that I don't recognise in line with one side of the mains, unsure what it does, or what it's voltage rating is. Will take photo, and show it to my tech friend , when I next see him

Hi, So, taped up leads, blue and blue/white show no connections to other leads from Transformer. Measure 4 ohms across leads. This is similar to the Primary leads , black/white and black , which measure 3.8 ohms . I'd love to think I have a dual primary transformer, but I'm not willing to risk my old amp on a guess. BTW. Have done a cap job, and also fitted the protection diodes to the rectifier ( as you have suggested in the past ). Did the same on my SB12, but lost the bias 'till I realised I had to move that connection as well !

Thanks, I'll do that, and report back.

I'm aiming for correct internal volume inside a visually correct Vox AC30 bass cab, although slightly larger, to get the necessary 30.5 inch dim . So far , I've drawn the cab with a false wall on one side , but I'm having to move the port slot to the rear of the cab , to accommodate the black panel / gold stripe on the front .