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Andyroid

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  1. THIS IS A REPLY POSTED TO THE BC30 MOD THREAD, I THOUGHT IT WOULD ALSO BE USEFUL HERE 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
  2. THIS IS A REPLY POSTED TO THE BC30 MOD THREAD, I THOUGHT IT WOULD ALSO BE USEFUL HERE 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
  3. 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
  4. 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!
  5. 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!
  6. 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!
  7. You say you turned something to get to the main fuse, that sounds like the HT fuse. There is also a 'master' fuse drawer built into the IEC(power) socket. I use a small flat screw driver to lever out the fuse drawer a bit like in this picture, only the BC30 one is much better, it also has a slot for a spare fuse and Epiphone will have included one! There are also seven different fuses on the actual circuit board inside the amp, I think about three of them are directly in line with the power transformer and could be to blame. BTW even with a silicone diode rectifier you only need standby to help protect badly designed DC coupled gain stages and the BC30 doesn't have any, badly designed or not. Standby switches in most guitar amps are really nothing but a mistake copied from Leo Fenders 'mute' switch, and now that guitarists expect them, manufacturers can't really sell amps without them!
  8. I've just turned my spare BC30 into a 50w bass amp with fixed bias, as the BC30 and SoCal use the same power transformer I thought I'd cheat and use the SoCal schematic to derive the bias supply. As this is my first fixed bias amp I just blindly followed the schematic and subsequently turned a capacitor into a firecracker! I was just about to lean over it to measure the bias voltage so I'm lucky it didn't blind me! I'm just posting this here in the hope I may save others from trouble when they recap their SoCals or whatever. Basically, the capacitors are the wrong way round in the schematic, the anode is placed at a lower potential than the cathode. I should have realised as I was building it, but as I said, this is my first fixed bias amp. The capacitors I'm talking about are C30 and C31.
  9. You must have used the standby switch. This amp is notorious for blowing fuses and rectifier valves because of the flyback voltage from the choke when the standby switch is flicked. Just replace the mains fuse (located in the power socket drawer) and check your rectifier, then leave the standby switch in 'Play' mode permanently. Don't worry, this kind of amp (it's a guitar amp, not radio transmitter) doesn't need a standby switch. Andy
  10. I've been recording for about 17 years now and building/selling gear in a music shop for about 6 years, most of my gear only gets used to record full live bands but the principles are kinda the same for bedroom stuff (only no need to worry about venue ambiance and mic phasing). Bedroom/home studio stuff that's one track at a time is easy to set up. All you need is a good dynamic mic for guitars (Shure SM57 is the industry standard) and a good condenser mic for vocals. Finding one that suits your voice can be a costly adventure but a great neutral-ish one I'd recommend is the MXL 2003A (NOT the original 2003, the circuitry in that doesn't match the diaphragm. The 2003A on the other hand is a great mic). A simple USB/Firewire audio interface with a good mic pre is obviously essential, companies like Focusrite and M-Audio will do one for about £100-£200 that will be fine (avoid cheap valve/tube mic pre's at all cost, they are a con, and sound dreadful. Real valve mic pres are EXPENSIVE). And then you need to hear what you are actually recording/mixing so you should spend at least £250-£350 on monitors. KRK seem to be very popular, and they may be the most detailed monitors in that price range I've heard in years. MOST IMPORTANTLY The ROOM you record in MUST sound good acoustically. Microphones pick up the sound of the room more than you can imagine. Right down to early reflections slapping back into the mic and phasing out low frequencies before you can even be aware there is any ambiance, if you see what I mean. Acoustic treatment is what makes a studio. You need lots and lots of thick foam (at 'least' 4 inches thick) and diffusers all over the place to make an average house room sound reasonable. Don't forget foam/diffusers on the ceiling, and a nice fluffy rug on the floor. Oh, and play good music. Have fun! --edit-- 'Acoustic Foam' is often more expensive than the stuff we use to make flight cases but is is virtually the same. 'Open cell' is what is important. So I got enough proper acoustic foam to fill an appropriate amount of wall space and thickened it up with a couple of layers of standard 'eggbox' foam. Do that and you can spend the money you saved on beer!
  11. 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.
  12. 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
  13. 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.
  14. This short article on amp design explains why standby switches are not necessary on guitar amps. P90's education is obviously focused on transmitter valves, a different beast entirely. Power Supplies and Standby
  15. Hi, I've just checked the schematics for your amps and neither of them will really benefit from the use of a standby switch. In fact leaving an amp in standby mode for too long is quite bad for the valves, google 'cathode poisoning'. NOS valves are often less susceptible to this due to purer cathode material, but if there is a chance you could forget that the amp is on then don't risk it. Several hours of standby will reduce the gain and lifespan of your valves. Standby switches are only useful if your amp has a DC coupled gain stage such as a cathode follower. Even then, there is a much better alternative to a standby switch - a reverse biased diode between the grid and cathode perfectly protects the valve. BTW your Univox has an AC coupled cathode follower, so no risk there. Happy Rocking, Andy
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