Fuse protection for tubes?

[Crusher]

I've just been experimenting with T match circuits. I have done a couple and then decided to built a test jig to figure out from 13 ohms up to about 60ohms. Copper is a lot cheaper than a decent air variable.

On the other hand for a PI input on a single 3, I have used .26uh, .295 or somewhere around .195-.2uh. Works well. Depends on Q u want. The .195 also works well for a pair in parallel.

 

[Mr Fix it]

Check out these sites

Here are a few sites which make the calculations super easy

Online Calculator .:. Matching Network

Jim Hawkins' Java Radio Calculators

Online Calculator .:. Helical Coil

GM3SEK's Amateur Radio Technical Notebook

 

[Crusher]

I been using jim hawkins calculator. I like it.

 

[Crusher]

T match with 19 turns of #14 enamel wire on a 5/16 form for each coil with a 110 pf variable cap will give you a 1.1 swr with a single 3cx3000a7. This tube has a cathode impedance close to 50 ohms.

What band does this work on?

 

[Mr Fix it]

The same band that shockwaves 3-500z amp worked on. I should add that the fuse in series with the cathode return (dc bias lline) is common practice on all henry hf and commercial amplifiers. Its a simple thing to add if not already present on your home brew or even commercially built amplifier.

 

[Mr Fix it]

Crusher you should try the pi and pi l calculator on the gm3sek site.

It allows you to add other parameters. Which will save you time overall.

I use this calculator when I build pi networks for my hf amplifiers.

73

 

[GnG8d]

To my surprise the conditioning recommended for these tubes prior to use is very beneficial in terms of breakdown voltage within the tube.

Yup, they need to sit and burn in just the filament for about 24 hours before plate V is applied.

I've read that the filament and blower should be ran for 24 hours before hitting the plate with HV. But I was recently told to go ahead and apply plate voltage, but with a 47k 5w resistor in series with the anode feed.

Any thoughts?

 

[Crusher]

I know reason to burn in, but the 47k in series with HV. Not sure why. I can see 50 ohm @ 25W to act as a glitch.

 

[GnG8d]

It didn't sound like he was using it as a glitch resistor as much as a current limiting resistor. He also mentioned that he'd gotten some bad gs35b's and wouldn't mess with them anymore. But the ebay sellers selling gs35b's still have good seller ratings, so I don't know.

 

[Crusher]

47k will have a substantial V drop on it. 5W rating aint enough. The last batch of 35B's I bought, I bought 5 in a case lot. 4 were good. Already burned in and tested. My only complaint with these 35b's is the ship them in the styrofaom container but with nothing else. The styrofoam is pretty ridgid and doesn't absorb shock very well. So if mailman is rough or whoever handling, I think this is how they get damaged. When I first get them, I put just heater and fan on, burn for a few hours. Then apply low plate V. Let sit for another hour or so. Then key amp and see what happens. I make sure cathode is fused like recommended by shockwave. Used diagram right off of GS35B.com. Then I add a glitch on HV. Usually a pair of 27 ohm @ 20W. If it shorts, you will know. Just make sure you have a shop vac ready.

 

[Shockwave]

The 47k resistor is a current limiter that will stop an internal arc from causing tube damage during the burn in process. It is better to use a variac and apply the voltage in stages but it you can't isolate the plate transformer or don't have a variac, the current limiting resistor is the next best choice.

Do not skip the filament and plate burn procedure with these Russian military surplus tubes. I've hi-pot tested many from the GI7B to the GS-35B and nearly half will show leakage at rated plate voltage before burn in. The vast majority of these would fail upon power up or application of drive.

They will show very poor emission on a transconductance tube test and excessive filament current as a result of sitting on the shelf for 25 years. This burn in process will burn off any ionized gasses that are trapped within the vacuum.

After the burn in process these same tubes can hold back more than double their rated plate voltage without leakage on the hi-pot tester. The best GS-35B tubes have been able to withstand just over 6 kv DC on the plate at full rated grid and plate current. That's an amazing amount of voltage once you factor in the RF on top of the DC!

Added note about the 47k resistor: This resistor should be long enough that is can withstand the entire voltage of the power supply without arcing across the resistor. If it can arc, you have defeated the purpose of using the resistor to limit the current since the arc will appear as a short across the resistor.

It really does not have to be a very high wattage resistor and several can be strung in series to make the total resistance over a longer resistor that is less prone to arcing.