LS50/GU50 Both Grid Drive

[kenpeter]

The important thing is to have the cameras rolling.

When, ahem.. I mean IF, If it blows up. Yeah if...
thats what I meant to say, at least your story will
be worthy of the telling.

Time to earn that lab coat.

[Wavebourn]

Originally posted by smoking-amp
"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?

I'm not so selfish. Let it be the common name for both grid driving.

[kenpeter]

Originally posted by tubelab.com
I tend to "sniff" my tube circuits with an RF spectrum analyzer while driving them to clipping. My full time job is transmitter design, so it is just a habit.

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...

[Wavebourn]

GU-50 tubes were working fine today, driven by variable G1 and very stable G2. Though, the festival was called Burning Amp, there were neither flame, no clipping...



(Schematics on top: I was tired to explain to everyone what is inside)

[Mazuki]

How many watts can this amp output?

[tubelab.com]

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?

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.