Kevin Brown Posted August 3, 2008 Share Posted August 3, 2008 New amp, but it is strange. I get like a low power hum on the 15 watt setting, but not on the 30 watt setting. On either setting even with nothing plugged in a get a harsh ( but quiet, not loud ) crackling sound. Arrrrgggghhhhh. I bought it in a shop in Amsterdam and I am in Germany.... Plugs are three pin grounded. Tried different plugs in apartment - same story. Any ideas? Link to comment Share on other sites More sharing options...
ricach Posted August 4, 2008 Share Posted August 4, 2008 Both the crackling and hum can come from dirty pin sockets, or even a tube. Even though yours is a new amp, pull the plugs and use some kind of electronic cleaner in the pin sockets. And while you're at it, go ahead and switch the power tubes placement, and do likewise with the preamp tubes - just make sure you're putting the same type of tube back in the socket. Reason for this is that the crackling or hum caused by a preamp tube might reduce to almost nothing (or possibly get worse) if placed elsewhere in the circuit. And for the power tubes, you would be using a different set of power tubes when only in the 15W mode. Link to comment Share on other sites More sharing options...
Kevin Brown Posted August 4, 2008 Author Share Posted August 4, 2008 OK Sounds like a starting plan. Thanks, I will let you know..... Kevin Link to comment Share on other sites More sharing options...
ricach Posted August 4, 2008 Share Posted August 4, 2008 I'm sure you understood, but I meant 'pull the tubes', not the plugs. :- Link to comment Share on other sites More sharing options...
Kevin Brown Posted August 4, 2008 Author Share Posted August 4, 2008 I'm sure you understood' date=' but I meant 'pull the tubes', not the plugs. :- [/quote'] Yes indeed. In the early 60's my father when he was repairing TV's made me tube tester. How I regret now not paying attention as he tried to teach me basic circuit knowledge....... But yes, I stay away from capacitors...... thanks KB Link to comment Share on other sites More sharing options...
MikeRom Posted August 5, 2008 Share Posted August 5, 2008 And for the power tubes' date=' you would be using a different set of power tubes when only in the 15W mode.[/quote'] What do you mean a different set for the 15W mode? The 15W/30W switch changes the output transformers winding point from the middle of the winding to the end (half or all the output winding). Link to comment Share on other sites More sharing options...
ricach Posted August 6, 2008 Share Posted August 6, 2008 I don't have a schematic to look at , but I assumed in 15W mode you only use half the power tubes. Do you have a link to those schematics? Link to comment Share on other sites More sharing options...
MikeRom Posted August 6, 2008 Share Posted August 6, 2008 Yes I do, but not on this computer. Plus I'm out of town for 2 weeks but I will try to remember when I get home. I think I just googled for it anyway. Link to comment Share on other sites More sharing options...
ebb Posted August 6, 2008 Share Posted August 6, 2008 I'v the schematic but i dont no how to host/link it, so if anyone wants to pm me their email, i'll send it on. I'm not gettin any real hum on 15W, except for when i use shoddy cables, which is sadly too often8-[ Link to comment Share on other sites More sharing options...
cGil Posted August 6, 2008 Share Posted August 6, 2008 I'm looking at the version 1, BC30 schematic and that "half power" switch is really just a triode/pentode switch. It's not quite triode mode since there's a drop across a 500R screen grid resistor for each tube, but it's close enough to lower the output a notch. Whatever. At any rate, the amp has DC filament heaters, so as a last resort, you might consider swapping out C37, which is supposed to be a 25000uF/25v cap. I'm pretty sure Rat Shack has a 48000uF/25v or 50v radial cap that'll do the trick if you can't find a 25000uF. But first thing to do to is prove it's safe to poke around in there. Use a voltmeter to check for DC Voltages on any resistors next to any caps. Especially the big ones. If everything's below 200mV, you're safe. If not, R50 and R51 should bleed the charge off within 20 minutes. Maybe! NEVER ever trust your life to a 20 cent resistor. Always check with a meter and BE SAFE. An important step toward eliminating hum is to clean up the usual factory mess of power transformer wires. Anything that carries AC, can and will induce hum in other wires and components. AC is in the power cord jack's AC wires to the front panel power switch, and from there to the PT's primary and secondary winding wires... . So twist 'em into corresponding pairs and zip tie them as far away from the board as best as possible. BTW, both transformers should have stickers on the top with the color codes. Just flip the amp over so the transformers are on top. You'll see the color code stickers. There are some international primary winding taps that can be cut (and insulated); or just disconnected from their dead-head parking lugs on the pcb, coiled up, and tied in the corner of the chassis if you think you might need 'em in the future. Be sure to insulate the ends really well, cuz those slip boots over the quick connects will slip, and if the quick connect contacts the chassis it'll destroy your amp. So pull 'em back, shrink tube the connector, and then pull the boot over the shrink tube'd connector. Little chance of a short to the chassis if you do that. Oh yeah, and any time you need to remove a wire with a quick connect on it from the pcb, use a poker of some sort to pry the spring clips apart. Then the connector will just slide off the pcb lug, and not destroy the lug's fragile solder connection. They're VERY fragile, so be careful. The power and standby switches are wimpy and exact replacements are almost impossible to find, so no tugging on those connectors, either. Gil... Link to comment Share on other sites More sharing options...
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