Rectifier question for Bob

Bob, I will soon continue the assembly of an ST70 I pieced together from parts around the internet (sorry, I collected the parts about 4-5 years ago and didn't know about you at the time). I somehow found in my parts box a 1N2389 rectifier. I am curious to try it. Are you familliar with this device? It seems to be a pair of diodes in a metal can, the size of a small metal tube and wired to replace the usual types like 5U4, 5AR4, etc., and similar to a Weber Copper Cap without the time delay feature. Looking on google came up with specs of 600mA and 1600PIV. So I'd guess it could handle being in a stock ST70?

Would it be appropriate to use it instead of a new production 5AR4? I have a couple of new 5AR4's, but I keep reading of short life spans and fireworks when they go.


Thank you,

Tony

I agree that "cathode stripping" is overblown. I have other tube equipment and it seems to me a bigger issue is coupling capacitors that become leaky over time will destroy an output tube much faster. I have replaced coupling caps and output tubes (6BM8 and 6BQ5) that were nearly dead on my B&K747b. And since I have an 80,40,30,20 quad cap rated at 525Vdc I will use the chinese 5AR4 I have and report what happens.

Thank you,

Tony

I used solid state rectifiers (with no delay) for a few weeks in my ST-70. I found 7199 triode plate voltage to exceed 400 volts immediately following power on during the few seconds before plate current started flowing. I realize that in many tube amps that are factory equipped with solid state rectifiers this situation has been going on for decades with little trouble. But when one of my 7199s shorted plate to grid in the triode section I went back to a GZ34 to take advantage of the sequencing it offers. It is worth noting that 7199's are rated for 330 max plate volts.

Also I will offer my .02 on "hot switching" or "short cycling". Yes this will eat a GZ34 on occasion. But what to do about those brief power line interruptions over which we have no control? This may not be for everyone but it has worked fine for me for several months. I changed out the recommended 3 amp fuse with a 1.5 amp fast blow fuse. No problems for several months now and this is a daily driver. Here is why this works: The amp draws about 1.3 amps most of the time so the 1.5 amp fuse is happy. If you crank it loud the current through the fuse will get up to 1.8 amps on the peaks, however a typical 1.5 amp fuse needs at least 3 amps flowing for about 5 seconds to blow it. I have had absolutely no nuisance fuse pops. I did however get up the nerve to test this idea (on a low cost GZ34) by deliberately "hot switching". The result? A blown fuse. I replaced the fuse and the GZ34 was fine. At the greatly elevated rectifier current flow following a "hot switch" the lower rated fuse will open in as little as 10 milliseconds per the fuse charts and it will be able to save the tube. A 3 amp fuse will not be able to open in time, or at all, to save the tube. The 3 amp fuse will save your iron, never your tubes. In addition, the 1.5 amp fuse will protect the EL34s. I have learned these lessons the expensive way. I encourage anyone reading this to check out a fuse time/current chart and take a close look at a complete GZ34 data sheet. It will be an eye opener!