ehedwr Posted September 25, 2009 Share Posted September 25, 2009 I've recently done a few mods to my BC30 like fitting a bright switch and converting to EL34 output tubes. Whilst playing the other day the mains fuse (F9) blew on me. Thinking it was because I had changed tubes I put the 6L6 tubes back in. I tried a few more fuses and they kept on blowing when I switched from standby to play. I then pulled all the internal fuses and tried it by putting em back in different stages to try and isolate the fault. Guess what? From then on it worked fine. Today I had the chassis out taking notes of different voltages at various points. Everything was hunky dorey until I plugged in the guitar and started giving it a bashing. Pop! The fuse blew again! I replaced the fuse and connected my multimeter inline with the power supply to see what was happening. Don't compare my readings here in the UK to yours elsewhere. US currents would double what I have. On switching on the power switch the current quickly settled at about 300 mA. When I switched on the standby switch it rose to 500mA. The original fuse was rated at 1.6 amps. I tried this a few times when I noticed the rectifier tube would sometimes arc whilst switching on the standby switch. The reading on my meter spiked and a crack could heard coming from my mains transformer. I had a Harma GZ34 fitted and only a few weeks old too! I put the original Chinese rectifier tube back in. Now there is no spiking, arcing or cracking and with Channel 2 cranked right up, full volume, full gain, full sag, it's drawing 650 mA. I'm hoping that this is a just a case of an iffy valve and not some other problem causing the rectifier to go bad. And I hope I caught it in time to save my power supply TX from blowing. As for the EL34's? They'll be going back in soon as soon as I'm absolutely positive they didn't cause this. Though I don't see how they could have. Anybody have any insights on this? Link to comment Share on other sites More sharing options...
ehedwr Posted September 25, 2009 Author Share Posted September 25, 2009 Ok. I just tried this again to see what current readings I would get with the EL34's fitted and the good rectifier tube installed. At first all was well. The plate voltage for some reason had risen from 420 VDC (with 6L6's) to 430 VDC. I turned off to measure mains current, started it back up and POP! Another fuse gone, the power TX farted and a flash from the rectifier tube. What next? I know the heater filaments draw more current with EL34's but the 6.3V winding is capable of 5 amps and only feeds the output tube heaters. Maybe it's time to look for a new power transformer and fit a solid state rectifier. Oh and finally I must add that I uprated the filter caps from a pair of 100mF/400V electrolytics to a pair of 220mF/400V electrolytics. I did this to reduce sag. Do you think the larger caps are sucking too much current on startup? -------------------------------------------------------------------------------- Link to comment Share on other sites More sharing options...
MikeRom Posted September 26, 2009 Share Posted September 26, 2009 The F9 fuse on my amp is blowing too. Power up is OK but when I hit the standby switch it blows almost immediatly. I'm thinking it's the power transformer cuz it seems to me that since everything on the other side of the transformer is fused with lower amp rated fuses that if something on that side was bad it would blow 1st. Except for componets along the PT13 line ... Having to replace the rectifier tube would be much better than having to replace the power transformer tho, I think you're on to something. Keep us posted. Link to comment Share on other sites More sharing options...
ehedwr Posted September 26, 2009 Author Share Posted September 26, 2009 I've ordered a Sovtek Solid State rectifier from Hotroxuk.com which plugs straight into the rectifier octal base. I'm expecting the voltages to be a little higher so I'll be watching out for that but at least it'll take more current than a tube. I will also fit a second temporary standby switch at a different point in the circuit to control the amount of inrush current on startup. I'll probably put the original filter caps back in too. If that works then fine but if not I'll be testing the transformer with a megger to see if it's breaking down. I'll keep you posted. Finding a replacement TX could be difficult because of the unusual 12V winding. Link to comment Share on other sites More sharing options...
ehedwr Posted September 26, 2009 Author Share Posted September 26, 2009 Well a fine gentleman from another forum informed me that the filter cap maximum for a GZ34/5AR4 rectifier tube is 64uF. C33 and C34 in series work out as 50uF. I had replaced mine with with a pair of 220uF/400V caps which gave me 100uF. This was drawing too much current through the rectifier on startup. I put the stock caps back in and then modified the way the standby switch operates. At present when you switch on your main on/off switch power goes straight to the heater filaments for all tubes including the rectifier. When you switch your standby switch on current rushes through your warmed up rectifier to fill up your filter caps and provide power to the rest of your amp circuit. I believe this current spike and the arcing in the rectifier is what's causing the main fuse to blow for alot of people. Options? Fit a solid state rectifier which can take more current or alter the position of the standby switch in the circuit. I'm still awaiting my solid state rectifier so I dropped the standby switch out of it's current position in the circuit, instead the rectifier is now connected directly to the transformer. I have used one of the standby contacts and wired it in series with the fuse F8 on the HT line. It now switches the 420VDC output from the rectifier to the choke T4. Now when I switch the amp on the rectifier comes online with the main switch and starts to trickle charge the filter caps as it warms up. A little more gently than before. On switching the standby switch the filter caps are already full which aids the rectifier in charging the rest of the circuit. Well thats my understanding of it and it's working at the moment! Only thing is I'm not entirely happy with switching 420 volts with the cheaply made switch that comes with the amp. I'll keep you posted of any developments. Link to comment Share on other sites More sharing options...
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