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Tuesday, January 08, 2002 9:09:56 AM
dave////irongater ([email protected])
Date: 1/5/2002 7:56 PM
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!
MBSetzer ([email protected])
Date: 1/8/2002 4:24 PM
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
A really good setting is with about 350 volts on the plate, this may not be quite in the
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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,
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:
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Tuesday, January 08, 2002 9:09:56 AM
Hammond 270FX
Full wave rectification using 2 SS diodes or one tube
82mf - run through OPT to plate
1.5Kohms power resistor
47mf - run through 470ohm to screen
2kohms 2watt
16mf - run to V1B
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