Well new tubes arrived and installed. Amp sits at 2kv at idle. Drops about 300v when loaded. Took about 19V of bias. With a pair being driven, seeing about 1750pep into dummy load. Pulling 600mA at full output. ( I had to temporarily install a plate current meter to see what was going on). I’m happy with. If it had 2800v at idle, it would be even happier, with less drive of coarse. As the Grid dissipation on GI46B is 20W.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
GI46b linear, persistent AC hum on modulation.
- Thread starter nomadradio
- Start date
That's good for 700 watts worth of anode dissipation. I don't think I'd want to take it past 1,400 PEP for fear of the notorious internal tube arc. Turns out the data sheet does provide a graph with the curve for bias and indicates this should be showing someplace around 40ma. of idling current with 2kv on the plate and -19 volts of bias.
You would be surprised if you connected a grid meter to see what they draw for current. You might think you were looking at a plate current meter when it comes to these Russian tubes. For the brief period of time I kept three GS35B's on an RF deck, they typically managed to draw in the neighborhood of 1 amp of grid current! Grid over current protection is a good idea for these Russian surplus tubes and the Ameritron GOP-100 accessory board, makes this an easy addition to many triode amps.
It’s not mine or it would have a grid and plate current meter. I had to temporarily install a plate current meter outside the deck so I could get an idea of what going on. I did install a fuse inline with bias circuit. It now has a glitch too on B+. The one thing I never see on Russian datasheet is input impedance. Sure would make it easier.
As a side note, the fuse should have a "cutoff" value resistor in parallel with it, so that if it opens under load, this does not create a large spike in the voltage drop that would be on the cathode. The indirectly heated cathode does not have the best insulation between it and the filament. Since the cathode is effectively in series with the anode, an open in the cathode circuit while under load, causes the plate voltage to briefly want to drop across that path of least resistance. Placing some resistance across what would be an open, that can "hold" the tube in cutoff, prevents this voltage spike from causing internal tube arcs. Without that, if the fuse blows, you can tell by looking at it, that more than the rated 250 volts were placed across it!
The addition of the glitch resistor is perhaps the best, least expensive and easiest to install, protection that you can place inside a tube amp. By itself, it can save the tube from destruction under most fault conditions that would result in an internal tube arc. I managed to learn about both the fuse and glitch resistor in one of my first builds. When the gamma shorted at the antenna, it only cost me one of the two 8877 tubes in the amp... About $1,500 later, they got replaced with a matched pair of ruggedized, Eimac VX-1500 tubes, including glitch resistor, cathode fuse and second cutoff resistor, paralleled with that fuse. Almost thirty years later, it's still running on those tubes.
Needless to say, the parts required to prevent the $1,500 failure from happening in the first place, were less than $10.00.
Last edited:
This would probably be a good time to add some info regarding glitch resistors. It's not enough to just properly size the wattage and ohm value here. You have to go one step farther and consider what is going to happen to that resistor if you ever have to depend on it working during a tube arc.
For all practical purposes, an arc is a short between the two points the arc is occurring at. That arc is looking for ground and when it finds it, guess how many volts it's going to try and drop across that glitch resistor for a fraction of a second? How about ALL of the plate volts! While you'll never be able to practically size the wattage of the resistor to handle this, that is not the goal.
The goal is that the resistor has to be long enough to sustain that voltage spike without arcing from end to end. Remember, an arc is a short. So, if the resistor cannot handle that brief voltage spike, it will arc over and turn your carefully placed 50 ohm glitch protection, into a short right at moment you need it most.
While not the most technical method, I like to see about 2 inches of resistor, per 1,000 volts of B+. Less than this and you run the risk of adjacent windings on the resistor, wanting to arc. Once this starts, it instantly flashes across the entire length of the resistor. Actual high voltage resistors rated for glitch use, are long, expensive "Glow Bar" types. Feel free to circumvent that expense in most cases, by stringing the appropriate value wire wound resistors in series to reach the needed length and breakdown voltage.
For all practical purposes, an arc is a short between the two points the arc is occurring at. That arc is looking for ground and when it finds it, guess how many volts it's going to try and drop across that glitch resistor for a fraction of a second? How about ALL of the plate volts! While you'll never be able to practically size the wattage of the resistor to handle this, that is not the goal.
The goal is that the resistor has to be long enough to sustain that voltage spike without arcing from end to end. Remember, an arc is a short. So, if the resistor cannot handle that brief voltage spike, it will arc over and turn your carefully placed 50 ohm glitch protection, into a short right at moment you need it most.
While not the most technical method, I like to see about 2 inches of resistor, per 1,000 volts of B+. Less than this and you run the risk of adjacent windings on the resistor, wanting to arc. Once this starts, it instantly flashes across the entire length of the resistor. Actual high voltage resistors rated for glitch use, are long, expensive "Glow Bar" types. Feel free to circumvent that expense in most cases, by stringing the appropriate value wire wound resistors in series to reach the needed length and breakdown voltage.
Yes, I have a high resistance resistor in parallel with fuse holder. Can’t remember exact value. I believe it was about 15K ohm
Sounds spot on to me. I used 10K on the larger 8877 amp. Now when the fuse blows, it doesn't cover the inside of the glass, with high voltage carbon flash marks.