Marshall 18watt, el34's

[Dave H]

Dave H,,What is the current rating of your PT?. I have a Hammond 270FX 275-0-275 rated 150 ma that I want to try with two EL34's cathode biased. may be a little on the light side but then the hammonds are under rated. What you think? (sorry to derail)

My transformer is 275V, 300mA, 82.5VA (bridge rectifier). The 270FX is 550V, 173mA, 95VA so it should be fine.

[Dave H]

Just realized something kinda embarrassing. I'm actually getting 550V to plates. I have bad short term voltage memory i guess... lol so its supposed to be biased around 32ma according to weber vst. Does it make sense I could get so much wattage now? especially since its at 46ma now... well I changed it a teeny bit without a meter now, so it should be slightly less, whatever. So what do you guys think?

550V on the plates could be because of "...it had weird voltages on the amps normal rectifier, so i put in a bridge rectifier". The PT has a centre tap. If you have the bridge rectifier across the ends of the PT with the centre tap open it will try to generate double the voltage of a two diode rectifier with the centre tap grounded. 75W output sound possible now as you are also measuring it distorted "...always a nasty looking wave". It shouldn't be nasty looking. It should be a sine wave at the onset of clipping (a tiny flat on the peaks).

[isaac]

I'm not sure if the settings on my scope are weird, because it used to just look like jagged mountains, now even with the same settings it goes crazy, so I'm guessing that the scope is set kinda wrong. I mean it is only nasty after it starts clipping. up until about 11 oclock on the volume its a nice wave then it starts clipping and whatever, I need to finish a couple solder joints and I will have basically a complete trainwreck. Except in the process of doing that i made up my own crazy preamp, (which didnt work) but in that I converted to a cathodyne PI so now i havea trainwreck and I still have the extra preamp tube socket, (not used currently) which i wanna do either a reverb/Fxloop, or drive channel with, adn since im sing cathodyne PI, I have a couple extra eyelets back on my eyelet board. ;-) I'll report how it goes in a minute once IM done with it.

[isaac]

Hey, I got a new Power transformer, although I'm not sure if I needed it or not but anyway, it doesn't have a bias winding, just a standard high voltage 290-0-290. So I used a standard kind of bias thing by putting a resistor off of the main HV winding right before the normal rectifier likeunto Almost ALL vintage examples, ;-) but its not working right for some reason. I drew up a schematic of what I have, and then I drew up a schematic of what I'm thinking of. Number 1 is what I have, and number 2 is what I'm thinking of. I wrote the voltages by the wires. I have no idea whether it would work or not, but I measured the voltage there and it looks like it might work. I just don't know if the way that these things work would allow for this. so #1 is what I have that won't work for some reason... and #2 is what I think might work but need help with.

[Dave H]

Neither bias supply will work with a bridge rectifier. If you use a bridge rectifier like that with a 290-0-290 transformer the B+ will be far too high. Use the standard two diode full wave rectifier circuit which has the centre tap of the transformer grounded then you can use bias circuit 1.

[isaac]

I had started another thread after this one, since before you hadn't gotten any response on this particular one, so I got that, but THANKS. You guys have been helpful. ;-)

[isaac]

Hey guys, I tried your suggestions and ideas, and the amp is AWESOME now!!! I had some preamp issues to work on, but I'm pretty sure I got them in my latest change, as I didn't get to test it when I was done, BUT nonetheless, I am pretty sure I got it. Before, the old PT for some reason generated a TON of noise, now this one is quiet, has enough voltage to produce the correct wattage (which I calculated out to 55W ;-)) and is all around great. Except for the fact that I'm not sure of whether it is completely correct or not right now, because, as I said, I had a preamp problem, which I'm pretty sure was just the preamp driving the phase inverter too hard, so I put in a voltage divider and that helped, but then the low frequencies were still too much, so they would mess with it, but as I said, I Think I got that worked out, but not sure as I didn't have the time to test it when I was done. But thanks a LOT guys. ;-)