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Another BC30 design flaw...

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I've just found another design flaw that may be the sole reason the BC30 self destructs. There is not enough limiting resistance before the rectifier. Limiting resistance helps keep ripple current within limits and is stated on the datasheet.




It is worth noting that the choke will slightly lower the required limiting resistance, but not by much. The above datasheet chart provides the calculation necessary to determine the limiting resistance provided by the transformer:


Rs = Rsec + N2Rpri


I have measured Rpri to be 5.5R and Rsec to be 37.5R (75R for the entire winding) so...


Transformer ratio N = 350 / 240 = 1.45 so...


37.5 + (1.45 * 1.45 * 5.5) = 49R


The chart shows that for a 350V tap, the limiting resistance needs to be around 105R for EACH PLATE of the rectifier, and this is for a fresh valve manufactured to 1959 standards. So lets assume 115R will be safer for modern 5AR4s,


115R - 49R = 66R


68R is the nearest standard resistor, two of these should be chassis mounted inside the amp, and the transformer secondary taps (350V) should be wired directly to these and then the other ends of the resistors wired to the PCB. I recommend 25W types, as the voltage rating should be in the region of 550V. The working voltage in practice will be much much lower than this but at the moment of power on, it will be higher and it's nice to know that nothing can go wrong! I over spec almost everything in my amps!


BTW, using the standby switch makes the problem worse because the rectifier is fully ready to conduct. If you are unlucky enough to flip the switch at the moment the mains AC voltage is at it's peak then destruction is almost guaranteed. Starting the amp from cold (without the standby) will slightly reduce the likelihood of failure. So, for a happy BC30, don't use the standby and install limiting resistors!

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Hi, my previous post may be a little difficult to read, I've never been good at writing. So, I'll show you two schematics, the original, then the modded one. The first one shows the original power supply. The transformer secondaries (350-0-350) go to the 'Standby' switch and then to the main circuit board. These are relatively thin red wires that come from the power transformer, through a rubber grommet in the chassis and then go directly to the 'Standby' switch.




The next schematic includes the two new 68R resistors, the green arrows point to them.




There is plenty of free space on the end of the chassis to bolt on the new resistors. This is probably the trickiest part of the mod if you want it to look tidy, as you obviously have to drill from the outside, but the resistors go in the inside. The important thing is that the mounting holes are the correct distance apart and that it is electrically safe. I sort of eyeballed it and used the resistors as a guide to get the holes the right distance from each other. If you see what I mean...


On to the wiring. All soldered connections should be covered with heat shrink, simply slide an appropriate length of heat shrink down the wire before soldering, then you can slide it back over the finished solder joint when you are done.


I recommend cutting these secondary (350-0-350) wires and inch or so from the standby switch (to allow easy joining of new wire). You now have two long (6 or 7 inches) of thin red wire coming from the power transformer. These will be tied to other wires to keep things tidy (you may find it easiest to cut the plastic cable ties to free things up). Strip some of the sleeve from the ends of these two and solder them to the new resistors. You then need two additional pieces of wire to go from the other ends of the resistors to the inch or so of wires hanging from the standby switch. Attach them accordingly and don't forget the heat shrink!


I find the best way to attach two lengths of wire is to twist and tin each of them, then bend them into hooks. Hook them together and squeeze them tight using pliers. Then solder them together properly, cover with heat shrink, and you're done!




Too the guys that say you can't run KT-66 tubes The guy who made the BC30 said it could handle the 66's plus

the ot is 22k ohms and the KT-66 are rated at ahhhh 22k ohms. Sounds like it might work...



In an amp like the BC30, KT66's actually want to see something like an 8k load. The BC30 transformer presents something like 4.4k. So simply plug you speakers into the 'next socket down'. On the back of the amp there are sockets for 4, 8 and 16 ohms. Relabel them to 8, 16 and 32 ohms and you wont go far wrong.

BTW I ripped the guts out of one of my BC30 amps and gave it a Selmer Treble and Bass preamp, Fender-ish reverb and a sort of SoCal power amp using KT66s. Sounds fecking awesome. Turn the Presence and Reverb down and it's a killer bass amp too! (Still building the head cabinet though, no bass combo should ever have valves in it!!)


Hope this lot helps!


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