Hey guys. I built a Marshall single channel 18watt clone, but I built it with el 84's. Now I found Transformers that should work. The OT works fine, and I'm pretty goldarn sure of the power transformer. Anyway, I built it just like the el84 version otherwise, but I only got 30W out of it, and it didn't even sound as good. But the problem is that I had a Bridge Rectifier (not sure of part # or whatever) but it worked great. The problem with that is that the power tubes would redplate, and that the preamp would distort really bad the the voltages were so high. So I replaced the bridge rectifier with two in4007's and it worked for a while but it was only 30w... So I went to put in a real bias supply, and I have a 100V winding on my PT. I used two in5400 diodes (big guys) and I KNOW I put the components in correctly, but I'm barely getting any negative voltage. All I'm getting is -2, and for some reason, I actually got upwards of +50VAC after the diodes. I can't figure that out. So anyway, the circuit I tried is like this. Winding/both sides connected to reverse diodes/filter cap (also reversed, +to ground)/at the junction of the cap and diodes there is the top lug of a 25kb pot/ then the wiper of that pot goes to the two 220k resistors which connect to the grid stoppers. the bottom of the pot goes to ground. I didn't use a dropping resistor since I drew too much current through the 2W I had used (which blew up) when I tried drawing the voltage from right before the normal rectifier, which is two in4007's running in parallel which works fine. But this amp has always had that problem of drawing WAY too much current through the rectifier. I had THREE 1k 9W resistors in parallel for about 300ohms and 27w capability because the bridge rectifier (i had at the time...) had too much V. But it blow those resistors, and once they got SO hot that the solder holding them in place was as wet as if I had held the soldering iron there. But now it resides with the funny bias supply and two in4007 diodes. as the two rectifiers in the amp. Something that just came to mind is that the bias winding on teh PT does NOT have a Center tap, so I'm wondering if it needs one to function, since I only was getting -2Volts, which I Think were basically "ghost" volts. If I got a center tap would that work? (although you can't really "get" a CT) but would fashioning a "fake" CT with two 100k resistors to ground work? Like many filaments do?
Also, I read that to prevent redplating on the power tubes you can just raise or lower the cathode resistor (when running in cathode bias mode) is that true? Could it be possible for me to run the I believe around 450V through EL34's as long as the cathode resistor is +or-'ed and as long as the screen voltage is lower than the plate voltage of course? Hope these ramblings made enough sense for someone to help me. I need the help. I really want to get this to work.
Also, I read that to prevent redplating on the power tubes you can just raise or lower the cathode resistor (when running in cathode bias mode) is that true? Could it be possible for me to run the I believe around 450V through EL34's as long as the cathode resistor is +or-'ed and as long as the screen voltage is lower than the plate voltage of course? Hope these ramblings made enough sense for someone to help me. I need the help. I really want to get this to work.
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