Guitarnut Posted October 4, 2010 Share Posted October 4, 2010 I have been trying to do some mods on my BC30. Understanding the importance of draining the caps first, I uplugged the amp, removed the chasis, set my DVM to 3000VDC and checked for voltage at the anode resistor (R4) of V1...nothing. Checked every resistor coming off of the power tubes (R40 - R46) ...nothing. I can't get to the pins of the caps without removing the PCB. Anyone who's done the C5, C6 mod, MV mod care to share how they drained the caps? Is it possible the circuit has bleed resistors that drain them off before I can get the tubes cooled and the chasis removed? Did anyone ever actually meter any voltage in the circuit? I've tested the DVM on several known DC sources...9V battery, 12VDC/24VDC power supply and it appears to be fine. I've never had any issues with it in the past. Jefrs has been a huge help in explaining the mods but he's 6 hours ahead of me and probably sound asleep as I'm posting this. I was hoping to get this buttoned up tonight. Short of metering the chasis when it's hot, I'm at a loss. BC30 Schematic Peace, Mark Link to comment Share on other sites More sharing options...
Andyroid Posted October 5, 2010 Share Posted October 5, 2010 Hi, I've never found any residual voltage. R50 and R51 are indeed bleeder resistors, it seems like they drain the filter caps in a matter of seconds, maybe minutes. You do not need to remove the PCB to get to cap terminals 'as such'. As other components are mounted on top of the PCB, for example one side of R49 to 'chassis' will give you a measurement of C33/C34. Andy Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 5, 2010 Share Posted October 5, 2010 Wouldn't C35/36 be the first storage caps in the power supply? Is it not possible to measure at these terminals? I have been trying to do some mods on my BC30. Understanding the importance of draining the caps first, I uplugged the amp, removed the chasis, set my DVM to 3000VDC and checked for voltage at the anode resistor (R4) of V1...nothing. Checked every resistor coming off of the power tubes (R40 - R46) ...nothing. I can't get to the pins of the caps without removing the PCB. Anyone who's done the C5, C6 mod, MV mod care to share how they drained the caps? Is it possible the circuit has bleed resistors that drain them off before I can get the tubes cooled and the chasis removed? Did anyone ever actually meter any voltage in the circuit? I've tested the DVM on several known DC sources...9V battery, 12VDC/24VDC power supply and it appears to be fine. I've never had any issues with it in the past. Jefrs has been a huge help in explaining the mods but he's 6 hours ahead of me and probably sound asleep as I'm posting this. I was hoping to get this buttoned up tonight. Short of metering the chasis when it's hot, I'm at a loss. BC30 Schematic Peace, Mark Link to comment Share on other sites More sharing options...
mjlewis22 Posted October 5, 2010 Share Posted October 5, 2010 I think based on the fact that no one can actually tell us theyve ever measured any residual voltage or been shocked by this particular amp that it must have bleed resistors on the filter caps that drain them before you can get the chassis out of the cabinet. Furthermore since no one can say "To properly drain the filter caps on the BC30..." I dont think anyone does it. H0: Work on the BC30 fearlessly H1: BC30 gonna shock the sh*t outta you Since there is not sufficient evidence to prove the alternate hypothesis that the BC30 can kill you, we must accept the null hypothesis that it cannot kill you. Link to comment Share on other sites More sharing options...
Guitarnut Posted October 5, 2010 Author Share Posted October 5, 2010 Wouldn't C35/36 be the first storage caps in the power supply? Is it not possible to measure at these terminals? Not directly because their terminals are under the PCB. And they wouldn't hold a voltage very long anyway since the balance/bleeder resistors, R50 and R51 mentioned above, give them a direct path to ground. Peace, Mark Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 5, 2010 Share Posted October 5, 2010 I think based on the fact that no one can actually tell us theyve ever measured any residual voltage or been shocked by this particular amp that it must have bleed resistors on the filter caps that drain them before you can get the chassis out of the cabinet. Furthermore since no one can say "To properly drain the filter caps on the BC30..." I dont think anyone does it. H0: Work on the BC30 fearlessly H1: BC30 gonna shock the sh*t outta you Since there is not sufficient evidence to prove the alternate hypothesis that the BC30 can kill you, we must accept the null hypothesis that it cannot kill you. "Since there is not sufficient evidence to prove the alternate hypothesis that the BC30 can kill you, we must accept the null hypothesis that it cannot kill you." That there is no proof "They" are out to get you doesn't mean they are not. Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 5, 2010 Share Posted October 5, 2010 Not directly because their terminals are under the PCB. And they wouldn't hold a voltage very long anyway since the balance/bleeder resistors, R50 and R51 mentioned above, give them a direct path to ground. Peace, Mark Thanks for the clarification, I thought this might be the case given the op mentioned not wanting to remove the PCB. Now I'm curious. I'm far more familiar with home audio and have just recently been looking into buying/building an amplifier for my guitars. Modifying seems quite prevalent on this forum when it comes to amplifiers. I assume a large portion of that is due to the current popularity of the Valve Junior with such a simple, budget oriented circuit that it sort of screams to be improved. I understand each circuit is different but, just what can you modify on the average guitar amp's circuitry without removing the board? Is there an answer to such a question? Link to comment Share on other sites More sharing options...
Guitarnut Posted October 5, 2010 Author Share Posted October 5, 2010 Thanks for the clarification, I thought this might be the case given the op mentioned not wanting to remove the PCB. Now I'm curious. I'm far more familiar with home audio and have just recently been looking into buying/building an amplifier for my guitars. Modifying seems quite prevalent on this forum when it comes to amplifiers. I assume a large portion of that is due to the current popularity of the Valve Junior with such a simple, budget oriented circuit that it sort of screams to be improved. I understand each circuit is different but, just what can you modify on the average guitar amp's circuitry without removing the board? Is there an answer to such a question? Modifying amps is more than prevalent, it's epidemic. What can be modded depends on what you want to accomplish. On the BC30, the non-polorized caps and all of the resistors are top mounted on the PCB. The mods I have planned are lifting a couple of bright caps and adding a Master Volume which can be done from the top side. The electrolytic caps are radial packages and both of the leads go thru the board...the can sits on top of them. So, the PCB has to be removed to replace or meter them directly. Peace, Mark Link to comment Share on other sites More sharing options...
Andyroid Posted October 6, 2010 Share Posted October 6, 2010 Not directly because their terminals are under the PCB. And they wouldn't hold a voltage very long anyway since the balance/bleeder resistors, R50 and R51 mentioned above, give them a direct path to ground. Peace, Mark Just thought I should point out, rather pedantically, that R50 and R51 don't really give a path to ground, C36 goes directly to ground. When the power cable is unplugged there is actually no ground but R50 and R51 will still bleed the caps, by dissipating heat. Link to comment Share on other sites More sharing options...
jefrs Posted October 6, 2010 Share Posted October 6, 2010 I've never had any problems with just switching the BC30 off (with the standby switch on) whilst the valves are hot and letting it discharge itself. This is probably not necessary but by the time the valves are cold enough to handle the volts are gone. Note: Some amps do not do this and the caps remain hot but the BC30 has discharge resistors too. Always disconnect and remove the mains lead before pulling the chassis out! It is good practice to check the volts with the meter from an anode to chassis ground, the connections to the valves are easily accessible on the PCB. If there is no voltage on the anodes then there is no voltage on the capacitors and the circuit is safely dead. The voltmeter itself can be used to discharge the circuit since it has insulated probes and a suitable resistance to bleed the caps off without sparks and bangs, when it gets to zeroish the circuit is safe. Earth and ground: the two terms are used interchangeably but in UK parlance "earth" is really the mains earth and "ground" would the the chassis, the local-earth, whether it is currently connected to the mains earth by the power lead or not. Somewhere near the building, the earth will be connected to a long metal rod driven into the ground. They have different symbols, earth is a stripy triangle and chassis ground is a hatched rectangle. The potential difference of the big smoothing capacitors is across the HT (B+) and chassis ground. There is usually, or should be, only one earth connection to an amp chassis, there may be several chassis ground connections. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.