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Tuesday, January 08, 2002 9:09:56 AM From: dave////irongater ([email protected]) Date: 1/5/2002 7:56 PM Subject: SE touch sensitivity Would anyone have a recommendation for a PT that would provide maximum current requirements for a single rca 6l6gc in a champ 12? Tube rectumfrier preferred! "The whole reason I'm again considering building this amp is MBSetzer pointing out that unlike class AB1, the power keeps increasing even as distortion goes up, making this design inherently touch sensitive. I have to try it." Me too! From: MBSetzer ([email protected]) Date: 1/8/2002 4:24 PM Subject: Re: SE touch sensitivity Well maybe I can help but I can't just give a part number recommendation since I am always changing everything on the bench. (since I started the reply, now I have actually simulated a supply using Duncan's PSUD2, if there is interest I could post the sim file, it is my first time and could probably use some correction, look below for my p.s.) I like a fairly stiff (low impedance) power supply since it is a bench reference and I can always add sag when I want, plus I alternate between lots of octal tubes and the KT88 was something I wanted to concentrate on, so when I was working my way up through the 6L6's I was already using enough juice for a really hot KT88, KT100 or 6550. I wasn't really running a 6L6 as hot as a KT88 (almost ;)) what I mean is I had enough power supply to handle it. I guess that's what made me push the Sylvania STR to the same stress level (35W dissipation) as the 7581A, I just kept cranking the knobs until it was there and it sat there handling it at maximum dissipation continuously. When I finally got (was given :)) an original NOS RCA 6L6GC, I didn't want to start out at maximum stress so I played it conservatively and did get the impression these are quite toneful tubes. Even though it had its own sound, the electrical behavior was very close to the NOS 7581A & NOS JAN Philips 6L6GC which were the ones about the most reliably available NOS. Eventually I did run the RCA at maximum for a number of hours, and it still seemed to handle it as well as the JAN, but I never did try it around-the-clock. Sounded wonderful too. A really good setting is with about 350 volts on the plate, this may not be quite in the 1 of 3 Tuesday, January 08, 2002 9:09:56 AM most linear portion of the class A curve, but its close enough without having to lower voltage & power to the point where it might seem weak. Once you start overdriving to the point where it is louder than PP EL84's you will be getting pretty much the same distortion as from a more targeted class A operating point. You probably want more or less the screen voltage the same as the plate. Use a 470ohm to 1K screen stopper and when you are massively overdriving the grid of the 6L6 you may see the screen glow but it should still be safe. At some point I must have made the transition from staring at the CRT on the scope to gazing into the power tube while I am playing, but they are still both helpful tubes to observe :) This could be around 90ma idle current for the 6L6, plus I think I have been using stiff supplies so I would want to get similar performance away from the bench, so I would probably want a PT that would easily deliver 150ma, even though mine was tested with a supply twice that capacity. I have done most of my experiments with SS diodes, for tube rectification I would probably use one with the least voltage drop, even then this might be another reason for a higher current PT, so the only serious impedance is the rectifier tube. If you hit 350V B+ with a diode module plugged in your rectifier socket, then use progressively lower voltage rectifier tubes your B+ will decline with each, then you could also increase your idle milliamps to draw more current, well then there will be a number of operating points to choose from but with the lower voltage ones you will already be drawing more than 100ma plate current so the 150ma PT doesn't have much headroom (I mean *distortion headroom*). And with distortion you're not really staying within the theoretical lines where full power occurs at the same milliamps as idle. Continue to add more drive after you have reached full clean power, and you'll need more milliamps if its going to be louder than PP EL84's. So you might want more than 150ma capacity. This is a SE amp, things are supposed to be big ;) You'd need a higher ma OPT also, might as well do the KT88. Realistically, I would compromise and get a PT for 150ma at 350V B+ using SS diodes, but intend to run the amp with a 5V4 instead. I would bias between 30 & 35watts dissipation, and 80 to 110ma idle current. Still would want a 3A rectifier winding so I could use 5U4GB whenever. Probably use a 125ESE OPT with an 8ohm load. I would want to make it easy to upgrade the transformers in the future. Regardless this would be better than a real Champ12 whose PT is too high a voltage and the OPT which is smaller than a 125ESE, they both run too hot and the Fender amp does not sound as good as the lower voltage circuit either. I use a fan blowing on the tube and transformers but its just a small desk fan set behind the amp, I have lots of lab instruments with fans in them but I don't think I will ever want to install a fan in a guitar amp, just put one behind it. Probably change my mind if I tried to assemble a rack unit ;) But I would never go without a fan, that's for sure :) Hope this helps, Mike ps dave, now I do have a preliminary recommendation for a power supply, I have not built it but this is what I would probably do if I had to order new parts: SE 6L6GC 2 of 3 Tuesday, January 08, 2002 9:09:56 AM Hammond 270FX Full wave rectification using 2 SS diodes or one tube 22ohms 100mf 22ohms 82mf - run through OPT to plate 1.5Kohms power resistor 47mf - run through 470ohm to screen 2kohms 2watt 16mf - run to V1B 2kohms 3.3mf film cap - run to V1A Then increase 22ohmers gradually to 100ohms each or higher to get lower B+, difference is more drastic with lower voltage-drop diodes and rectifiers. 3 of 3