smoking-amp said:"vague chicken scratchings on the back of a pizza coupon"
You mean like this?
I couldn't find a crayon handy though.
Don
OOPs, I just noticed that the GU50 g1 drive signals need to be crossed over between tubes due to the Mosfet's inversion at its drain. Hmmm, so the non-linear screen current only effects the "OFF" tube's g1 drive. Hmmmm, but then the Mosfet driving the "ON" tube's g1 has no current thru it's drain resistor. I think some fixup is called for here.
Thanks!
Yes, control grid signals have to be swapped, also 100K grid leak resistors are needed connected to 100K pots to set idle currents, and -60V source is needed for pots.
After presenting Zobsky with the schematic, he seemed enthusiastic
to actually built it. There will be some minor changes to accommodate
a few things I didn't realize (for example: his Karlson K15 is loaded
with a vintage 16ohm coaxial, not 8 ohm as I had assumed.)
And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit.
I am still intending to abuse only positive drive(s), no caps, and stay
entirely in A2 at all times.
to actually built it. There will be some minor changes to accommodate
a few things I didn't realize (for example: his Karlson K15 is loaded
with a vintage 16ohm coaxial, not 8 ohm as I had assumed.)
And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit.
I am still intending to abuse only positive drive(s), no caps, and stay
entirely in A2 at all times.
smoking-amp said:"vague chicken scratchings on the back of a pizza coupon"
You mean like this?
I couldn't find a crayon handy though.
Don
OOPs, I just noticed that the GU50 g1 drive signals need to be crossed over between tubes due to the Mosfet's inversion at its drain. Hmmm, so the non-linear screen current only effects the "OFF" tube's g1 drive. Hmmmm, but then the Mosfet driving the "ON" tube's g1 has no current thru it's drain resistor. I think some fixup is called for here.
By the way, I like your drawing: it has the name of the topology stamped on it already: "Free Cookies"!
"it has the name of the topology stamped on it already: "Free Cookies"! "
Yes, I like the name too! The Free Cookie! (seems there is a Free Lunch around also.) Can we use it to refer to gm scaled g1/g2 drives in general, or do you want to reserve it for your specific implementation?
"When it comes to legible renderings, Don is the one true master!"
Good thing I had some junk mail around to write on, I would have had to use TP otherwise. Some of those sale tubes came wrapped in it.
Don
"And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit."
If you scale the drives by 1/gm, you can just use the standard curves with the gm doubled (effectively halving the drive voltage from g2 or g1 only drive).
Yes, I like the name too! The Free Cookie! (seems there is a Free Lunch around also.) Can we use it to refer to gm scaled g1/g2 drives in general, or do you want to reserve it for your specific implementation?
"When it comes to legible renderings, Don is the one true master!"
Good thing I had some junk mail around to write on, I would have had to use TP otherwise. Some of those sale tubes came wrapped in it.
Don
"And I will probably try to find a way to drop G1 down to about half of
G2, even if that means going off the known LS50 G1=G2 curve a bit."
If you scale the drives by 1/gm, you can just use the standard curves with the gm doubled (effectively halving the drive voltage from g2 or g1 only drive).
Since it's my particular GU-50 in question and I am pea-brained, I had to ask:
"is it safe to run the tube and draw grid current in A2 CONTINUOUSLY ?" I understand the driver limits the current that the GU-50 grids see but I always considered A2 as something that tubes only operate in intermittently.
Thanks
"is it safe to run the tube and draw grid current in A2 CONTINUOUSLY ?" I understand the driver limits the current that the GU-50 grids see but I always considered A2 as something that tubes only operate in intermittently.
Thanks
Watts is Watts. Voltage times current is Watts. Voltage is low
here, and those grids look pretty darn heavy under the XRay.
I wasn't around when they wrote the spec sheet for G1=G2.
Who knows what they had in mind?
I'm not sure its the smartest thing I've ever suggested, to have
a positive G1 focus the flow of electrons into its shadow (where
G2 normally hides)... G2 either melts down in a huge industrial
accident when it gets too hot, or it doesn't.
The drive circuit was specifically designed to limit the wattage
that could be sucked from the grids. 1 combined watt or less.
Forget how I computed that figure now??? And anything extra
that strikes, only serves to lower the voltage. Its a theory...
------------------------------------------------------------------------------
If you don't care for positive drives, Telefunken's G1=G2 curve,
or something closer to g1=0.5*g2. You are going to need a lot
higher B+ with conventional negative g1 drive. Maybe around
800V would do it, not counting AC added by the plate swing.
I thought you wanted this to be safe, cheap, and simple?
At least safe for you in the same room, if not for parts...
I got a lamp dimmer and some incandescent bulbs to fake a
poor idiot's variac. We could bring up B+ slowly, in the dark,
while watching for any unusual fireworks... Would two fuses
in series be a ridiculous suggestion?
And as its entirely DC coupled, might add a subsonic filter.
here, and those grids look pretty darn heavy under the XRay.
I wasn't around when they wrote the spec sheet for G1=G2.
Who knows what they had in mind?
I'm not sure its the smartest thing I've ever suggested, to have
a positive G1 focus the flow of electrons into its shadow (where
G2 normally hides)... G2 either melts down in a huge industrial
accident when it gets too hot, or it doesn't.
The drive circuit was specifically designed to limit the wattage
that could be sucked from the grids. 1 combined watt or less.
Forget how I computed that figure now??? And anything extra
that strikes, only serves to lower the voltage. Its a theory...
------------------------------------------------------------------------------
If you don't care for positive drives, Telefunken's G1=G2 curve,
or something closer to g1=0.5*g2. You are going to need a lot
higher B+ with conventional negative g1 drive. Maybe around
800V would do it, not counting AC added by the plate swing.
I thought you wanted this to be safe, cheap, and simple?
At least safe for you in the same room, if not for parts...
I got a lamp dimmer and some incandescent bulbs to fake a
poor idiot's variac. We could bring up B+ slowly, in the dark,
while watching for any unusual fireworks... Would two fuses
in series be a ridiculous suggestion?
And as its entirely DC coupled, might add a subsonic filter.
tubelab.com said:
I got a pair of WA5VJB log periodics that cover 400-2600Mhz.
2Gig tek scope at work can FFT a plausible spectrum over at
least the low end of that range. At home, I'd probably have
to use a slotted line and try to measure a standing wave.
Probably all that matters is knowing the detected level.
I also have an LNA. Does pass/fail RF oscillation test typically
require sensitive front end amplification at such close range?.
I mean, if a tube is gonna oscillate, gonna oscillate BIG, right?
Also have plenty of attenuators on hand if needed...
Sorry, I missed this question when it was asked.
Some circuits may rudely oscillate at an RF frequency while still doing a reasonable job of passing audio. The oscillation is more or less continuous, and may cause distortion, or a vague foginess in the sound. Sometimes there are no audible artifacts. These can often be detected with an AM radio or TV set in the same room. Often one or two TV channels are totally wiped out even though the TV is connected to cable and recieving a strong signal. Often moving parts around will change the affected channel, and help lead to the offending circuit. These are the easy ones.
More often a circuit may breifly emit a burst of RF energy only under certain conditions, often as one tube in a P-P amp cuts off or returns to conduction, or at the onset of clipping. Sometimes this can be seen on a scope. I use an old HP 141T spectrum analyzer with a 0 to 1250 MHz RF unit. I "sniff" with a piece of coax that is stripped back exposing about 2 inches of the center conductor. The entire end is covered with heat shrink to prevent accidental contact with a live circuit. I do not use an additional LNA since the 2 inch wire will pick up local TV, radio and cellular towers.
Operate the amplifier with an oscillator and a load resisitor and with a real speaker and music. Drive the amp to clipping, while "sniffing" around. The best places to find anything is right next to each tube. I usually crank the amp hard with some bass heavy music and look for anything at all on the spectrum analyzer that pulsates in time to the bass. I set the analyzer to scan zero to 50 or 100 MHz at maximum RF sensitivity. Even with the small "antenna" you will find all sorts of signals that vary as you probe around inside the amp. Welcome to our RF polluted environment. In my case TV channels 2 and 4 (analog) are very strong. So is the local high school radio station at 88.5 MHz. Find anything suspicious? Without moving the probe, shut the amp off and see if it goes away.
I now have a new driver board running, and I am working on a multi - PowerDrive board. Sooner or later I will do some testing with fully adjustable dual grid drive. I can even flip the phase of individual drive signals, so G1 can be driven in phase with G2 or out of phase. Maybe I will even try seperate drive for all 3 grids. Why? Because I can, and I am curious as to what will happen.
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