Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sandoz6G16 on July 05, 2013, 11:38:28 pm
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I have just finished my 6g16 vibroverb and I am slightly disappointed with the trem. It just seems a little too fast for me. Also I didn't have any 6l6 around so I just threw in some kt66 and biased around 50ma, my b+ is 372v, im not sure if this is the problem or what, Please excuse my inexperience any help would be appreciated, I tried searching the forum for bias vary trem tweaks but nothing on the speed came up Thanks.
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Change one of the .01 uF caps in the phase shift oscillator to a .02 uF. The one in the loop from plate to grid after the .02 is probably a good choice to change. That should slow it down nicely for you.
The B+ on the schematic shows 440v...with today's wall voltages that is probably more like 450v. 372v seems a bit low to me....keep in mind that as you adjust the bias hotter, the plate voltage falls....so 50mA may be a little too hot on the bias. The vintage KT66 had a 30 watt max dissipation, but the usual quoted was a 25 watt max. The new tubes are probably in the 25 watt dissipation camp, so 60% to 70% of that is 15 to 17.5 watts. At 372v times 50mA, you are at 18.6 watts, so that is a bit too hot. Another thing to consider with the KT66 is that it requires more heater current than a 6L6GC, and if the power transformer doesn't like supplying the extra, it will run hot and sag with its voltage. I would suggest to back off on the KT66 bias and shoot for maybe 65% to 70% of the max of 26 watts, but even better would be to get a good 6L6GC pair and bias those up to around 18-21 watts dissipation. You will find that the B+ will go up as you adjust bias colder too.
Greg
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http://www.el34world.com/Forum/index.php?topic=604.0 (http://www.el34world.com/Forum/index.php?topic=604.0)
Did you look in Archives?
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Thanks a lot I really appreciate it. I have jj 6l6gc's on the way, I just couldn't wait. This thing sat on the bench for three months with a single missing wire. Absolutely couldn't believe it LOL. THANKS AGAIN!
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Hey thanks for the link, I actually searched and seen that post but was thrown off by the AB763 when scanning. Just thought that was someone talkin blackface opto till further inspection.