Andyroid

The problem with the rectifier tube is caused by the power transformer not being specced correctly, its DC resistance is too low. But don't worry, simply stop using the Standby switch. It is perfectly safe to leave it in Play. On amps like this, the Standby switch is only there because guitarists tend to think that all tube amps need them, they don't. There is nothing about the BC30 that demands a Standby switch. I would suggest only using 12AX7 in every position in this amp, I've tried all sorts and 12AX7 always came out best. 1.2ohms for the reverb TX secondary sounds right. Maybe the tank is plugged in backwards?

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! 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! Andy

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!

I used modern Tung Sol 6V6GT. So they were operating way out of their league. I've heard the modern JJ/Tesla 6V6 would probably handle it but would sound like a cross between a 6V6 and a 6L6. As for the EL84, I know someone round here has had great luck with the adapters you can get, Tone Bones or something like that. Don't have any inside pics, sorry. Just noticed you mentioned the standby switch, you can safely hard wire it in the on/play position. There is no need for a standby switch on this amp, in fact it's affectionately called the 'self destruct switch' by many, as it can and will destroy your rectifier valve and blow fuses.

I tried a 5Y3GT and a pair of 6V6GTs a few years ago and had hum and a little red plating. I fixed the red plating by increasing the cathode resistor, but lost the amazing tone in doing so. If I didn't need to look after my 6V6s (I use them in my main amp) then I would try again and add bigger screen grid stoppers instead. Just an idea I thought would be worth mentioning! I think the hum might be coming from the 5Y3, you could try the 5Y3 with your 5881s to check that. Another thing, my FAVORITE mod is a pair of recessed side handles. Makes the amp so much easier to lift, especially when you make it a two man job! Have fun! Andy

For an effects loop you could try something like this. I haven't tried it yet but I'd expect it to reduce the level sent to the reverb tank a little, also the FX return is quite low impedance so line level FX modules will probably work better than typical stomp box type FX. I'd use shielded cable to link to the pot and sockets on the back of the amp. To increase return level and impedance you could probably use a 2k2 resistor instead of the 1k. -------------------------------------------------------------- Schematic above is updated to fix a bug in the FX return.

When removing bright caps you don't just cut a leg, you carefully desolder and remove the capacitor completely because otherwise vibration in the amp may cause it to short back into the circuit and crackle badly. Also if you dont like the change you can simply put it back (assuming you havent cooked it with slow work!). Any cheap solder sucker (desolder pump) will really help. If you want to try it, then remove them completely. Also you must realise the the big filter caps can and will hold enough charge to kill you, so before starting work simply use a wire with insulated crockodile clips to short the anode/plate resistor of valve1 (the one nearest the instrument input) to the chasis. Within a couple of minutes the resistors in the amp will have drained the filter caps. BTW I've got a fair bit experience with modding this amp and the bright caps are good as they are. If yours has ice-pick syndrome it's likely to be because of the valves you're using, not the bright caps. Cutting the caps really only ruins the overdrive, sucks the life out of it.

As I'm sure you're all aware, Channel 2 has always had less bass than Channel 1. This is largely because the extra 'Midrange' control can never be fully turned off, it always filters some bass out. You can fix this by removing C14 thus disabling the 'Midrange' control. You could alternatively replace VR3 with a pot that breaks the circuit when fully turned. Note there is still a little more chime on channel two, probably because of the extra valve stages with cathode bypass etc... ;-) Andy

'Would it be wise to discharge the electrolytic caps. before doing any soldering? ' YES! I do it by unplugging the amp from the mains but leaving it 'on' and in 'play' mode overnight, I hope this is a safe way, I'm still alive... Thanks for the C15 advice Dave, I'll print that out for future reference, cheers, though having done the other mods im actually pretty happy with where it is now, and also I havent figured out why my reverb keeps cutting out. If its an invisible crack in the PCB then messing with it may only make it worse. :P So far I've noticed that Channel2 is still brighter than Channel1, wouldn't reducing C15 from 22uF to 2.2uF take more bass out and make this problem even worse? It seems like a big (and possibly risky) job which might not be worth it. What I might be willing to do is put a cap in parallel with the plate resistor to tame some of the highs on Channel2, thats where the schematic gets too complicated for me, is R20 or R48 the plate resistor? (or both!? or neither?) anyone? Actually if C30 and C31 are trebble roll of caps then I could change both their values to brighten the clean channel AND tame the overdrive, I really wish I knew what I was talking about! LOL Electronics course here I come!

Hi guys, just about caught up with these mods. Before doing any more I decided to read up a little about electronics and I found this page on carbon resistors http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#CarbonComps , so there may still be some more sweetness to inject yet! (this site has LOTS of mod ideas!!!) Also this page helped me get to grips with what the bright cap mods actually do http://www.diystompboxes.com/pedals/tubedummy.html I havent changed C15 yet 'cause I'm pretty sure I'll have to remove the PCB to get to the solder, Papa, you've done this already, is there anything I should be aware of before attempting this? or should I just get stuck in and be carefull? Actually, does anyone know how to remove a cap like this (cylinder damn close to the PCB) WITHOUT removing the PCB? Cheers and happy modding! Andy

Thanks PapaMidnight, thats put me straight. It says 471 on one line, 1kV on the next and sec on the third. Dont know what sec means. Though I have read that the 1 in 471 is a multiplier, that confused me a bit, originaly i was looking for a 471pf cap!! LOL I would have simply moved C3 to C10, but it looked far too fiddly. Ive found some on rapidonline.com anyway and they're based in England, £0.06 each and £5.00 postage!! better order a few bits n bobs to get my moneys worth i think. BTW what is some-timers disease?

Hi, I'm urgently in need of some advice please! I've been looking round for a replacement C10 when I suddenly realised im not sure what value its supposed to be (I'm certainly no electronics technician!), the schematic says 471/1kV, is that 0.471 microfarads / 1000V? also what kind of capacitor is it? a decoupling ceramic disc capacitor or metallised ceramic plate capacitor or what? They all look the same to me and I dont want to mess this up!! Also my mid pot is damaged from transit, all the schematic says is A250k. No mention of maximum wattage or voltage. Here in Hull noone sells 250k pots. The nearest I can find is 220k 0.2W 500Vdc logarithmic, would that be safe? I dont want to kill my amp or myself! Cheers