Andy Le Blanc wrote:...
The amp I have set up at the moment also uses the heaters of the first to
preamp tubes to bias a pair of 6l6 or el34, which helps to regulate the diode
heaters some.
I just caught this. Are you running class A, or lower than usual B+ on the outputs? Unless I'm missing something, that's 75 mA per tube
Its set up hot. I've 6ca7/EL34 at nearly "Design Center" maximum, but its
cathode bias. The tube type has a 25w plate rating and 8w screen rating,
so that's 33w total. Today wall ac gives a 382V plate and the reading across
a 1 ohm resistor in the cathode circuit was .149 A. So.... 56.918 W or......
28.46 W per tube. That's correct, your not missing anything.
Class A?..... 149ma plate current at zero signal
Testing for a maximum output gives me 17.4 v across a 8 ohm load.
So... P = E2/R ... 37.84w = 302.76/8 , and a max plate current of 181ma.
So... CA = 2*cos-1(-149/181) = 290.81 deg.
% (AB) = 180/CA = (180/290.81) = 62% of the way between A and B
Another way to look at it is the conduction angle or CA for Class A is 360deg.
You get this when you put "1" into the equation. -149ma/181 = -0.82
.82 where 1 is "Class A", it's not technically class A but it's in the neighborhood.
Cathode bias is just about the safest choice, Ive seen that repeatedly, many places.
I went thru an old alamo amp a couple years back, never seen 6v6's run so hot.
Ive seen a few others like that too, decades old tubes run hot, still running fine.
The trannies are off the the shelf generic fender replacements, no problems.
I have an order to fill which will be constructed in much the same way.
50w trannies, power tubes biased thru the heaters of the pre. Same diode bias,
but with a VOX style inverter and a dirt channel. These design choices really
simplify the build, the pre's are just one resistor and a cap on the parts board,
and one wire for the heater dress, saves time.
The power side of the amp has been trouble free for most of a year now.
Andy Le Blanc wrote:...
The amp I have set up at the moment also uses the heaters of the first to
preamp tubes to bias a pair of 6l6 or el34, which helps to regulate the diode
heaters some.
I just caught this. Are you running class A, or lower than usual B+ on the outputs? Unless I'm missing something, that's 75 mA per tube
W
Andy, can you explain this a little more? The heaters of the first two pre-amp stages are playing the role of cathode resistor for the power tubes, I guess... how are they wired?
And Re 6L6 vs. EL34, the bias points are different, how is that accommodated?
Finally, how was Wayne able to estimate the output tube idle current so closely?
two series dressed double triode in series. 12ax7, 12au7, 12at7, 12ay7 etc...
anything with a 12.6 V 0.15 A heater, series, the numbers just happen. I'd like to think
that someone back in the day designed it that way. Danelecto manufactured
amps like this, two 12ax7 biasing either a pair of 6l6 or a quad of 6v6.
And there are two draw backs.....
ONE: You get what you get, there is no accommodating, 2 6l6 and el34 just
happen to be close enough. Its nice to worry about bias, this design is very
general, even the diode bias in the pre is a generalization.
TWO: The heaters are also rated for a controlled time warm up, they take a bit to get going.
Now this might be a good thing, everything cycles gradually,
as things heat there are no hard switches. But its a wait to go through standby.
martin manning wrote:
Finally, how was Wayne able to estimate the output tube idle current so closely?
MPM
I had make a couple of assumptions. I assumed that the tubes were heated in series (12.6V) rather than parallel, because the other option would have meant 300 mA shared between two power tubes. I also assumed that Andy would have made sure the heater voltage for the pre tubes was at least nominally near rated voltage so they would operate normally.
I guess until I design a cathode-biased amp and push it to it's limits I'll always underestimate how much you cathode bias lets you away with!
Andy Le Blanc wrote:two series dressed double triode in series. 12ax7, 12au7, 12at7, 12ay7 etc...
anything with a 12.6 V 0.15 A heater, series, the numbers just happen. I'd like to think
that someone back in the day designed it that way. Danelecto manufactured
amps like this, two 12ax7 biasing either a pair of 6l6 or a quad of 6v6.
And there are two draw backs.....
ONE: You get what you get, there is no accommodating, 2 6l6 and el34 just
happen to be close enough. Its nice to worry about bias, this design is very
general, even the diode bias in the pre is a generalization.
TWO: The heaters are also rated for a controlled time warm up, they take a bit to get going.
Now this might be a good thing, everything cycles gradually,
as things heat there are no hard switches. But its a wait to go through standby.
Thanks.
I had thought through the start-up... The power tube filaments have to get hot before the pre-amp tubes can even begin coming up to temperature, and going on standby shuts down the pre-amp filaments.
If the if the two pre-amp heaters in series are operating on design that would give 25.2V above ground and 150mA. The current is just about where you are. Happy coincidence that it all works out, I guess. Did you happen to measure the cathode voltage?
Andy Le Blanc wrote:...TWO: The heaters are also rated for a controlled time warm up, they take a bit to get going.
Now this might be a good thing, everything cycles gradually,
as things heat there are no hard switches. But its a wait to go through standby.
I know you've been using this arrangement for a while with no trouble and I don't mean to poke it to death, Andy, but please bear with me for a second for the sake of my further self-education.
Does anything strange happen on start up? I'm thinking - the power tubes are already heated, they'll conduct whatever the bias allows. The preamp tubes are dead cold, and will look awfully "short" when you first apply B+ to the output section. Fully heated + near-short in the cathode + full voltage on the 6L6 should = ... do you see where I'm going with this?
Hopefully, the 12AX7s come up to temp fast enough to keep it in check.
The cathode voltage is around 24.4, jumps to around 18-20 then stabilizes.
a dead cold start must have a transient, but theres the DC resistance of the
opt and the resistance of the tube, and an apparent short very quickly becomes
filament heat, thats the soft start, there's zero signal until the stages of the
pre heat. The place where we might be concerned is the the plate voltage
of the pre amp tubes jump as the tube heats before the diode forms a bias, it deosn't
exceed the 300v design max, and it only happens on the first cold start,
But this is not reflected as significant voltage on the cathode/diode of the pre
stage, it bumps up to 5-6v before leveling out to 1.5 - 1.7v. All in all it seems a softer
switch over voltage condition than in a typical old fender standby,.
If it does exceed the heater to cathode rating of the power tube, it appears
only briefly just before the heaters convert the current into heat.
The cold resistance of two 12ax7's 12v heater in series is around 27 ohms.
