Strange 5AR4 "Problem"

[Zer09]

I have a 5e8a twin that I built with the ability to use 6l6's or el34's. When I use el34's I use a single gz34 rectifier. I've had no issies with this setup. I prefer the sound of 5881's (real 5881's, not the sovtek wxt) and I was switching back the other day. Just for the heck of it I decided to test the tubes that I pulled. The rectifier is a fat bottle Sylvania 5ar4. When I tested it the heaters lit up but under test only one plate was allowing current to flow. The other tested completely dead. This was tested on a Weston 981 tester. I have 2 of these. It tested dead on the second one as well. The result was confirmed again on an EICO 666 emissions tester that I have. I put the tubes back in the amp and they work fine.

Is it possible that one half of a 5ar4 is supporting 2 el34's @ 420v @41ma and doing so in a stable manner? I find this hard to believe. Or is something just preventing this tube from being tested correctly?

[Steve Conner]

Yes it is possible. The remaining good half of the tube will function as a half wave rectifier, so you'll still get power. But, if you tested the amp's output power you'd find it was low because the B+ would sag too much under load. Also the PT may well run hotter and make a different, rough buzzing noise than it would with a good rectifier.

[greekie]

Lucky it didn't develop an internal short! I'd really recommend that you install a couple of diodes in series with the tube rectifier. That trick from RG has saved my ass a couple of times over the years.

[Zer09]

Thanks for both of your posts. Chucked the 5ar4. I'm going back to back to 5881's anyway. As for diodes in series with the rectifier plates, I'm not really worried about that. I have HT fuses to protect my transformers but in 30+ years of playing I've never had one fail. Adding diodes would just push my B+ too high for 5881's. There are times though when I'm gigging frequently that I'll stick a weber copper cap in an amp just to avoid the physical abuse to a $100 Mullard. But the advice is always appreciated.

[pdf64]

'Adding diodes would just push my B+ too high for 5881's'
The diode greekie is referring to are in series with the tube rectifier.
They would only ever raise the voltage if the tube rectifier develops a short.
Pete.

[Greg-M]

Like this....

Added diodes to my 200S amp (same as a Dynaco). It's a happy camper.

[Tom Phillips]

Nice illustration drawing. Reminds me of the old Heathkit construction illustrations.

[Greg-M]

I wish I could take credit for the drawing. I got it off of a Dynaco forum

[Zer09]

OK. What am I missing? The drawing above shows the PT secondaries connecting to the diodes using unused rectifier tube socket pins. The other end of the diodes connect to the plates. So the ac is rectified by the diodes before it ever gets to the plates. How does this not increase the b+ over just a tube rectifier? I understand that when the tube fails you lose the voltage drop across it. But aren't you still starting with a higher b+ before it fails?

[jmaf]

OK. What am I missing? The drawing above shows the PT secondaries connecting to the diodes using unused rectifier tube socket pins. The other end of the diodes connect to the plates. So the ac is rectified by the diodes before it ever gets to the plates. How does this not increase the b+ over just a tube rectifier? I understand that when the tube fails you lose the voltage drop across it. But aren't you still starting with a higher b+ before it fails?

The maximum +B is the peak of the AC wave right before the silicon diode anodes. There is no way that adding a passive component there would increase +B from 1/2 p-p AC. So adding a silicon diode does not increase the voltage, it in fact decreases it a diode drop.

Edit: And when a rectifier fails, it often arcs from anode to cathode, it's a short, you don't lose only the voltage drop across the tube, you lose all voltage in fact, blown fuse, magic smoke.

[pdf64]

The 1N4007 1kV rating is likely to be marginal or insufficient for many instances of this application.

[Greg-M]

I put two in parallel at each point, just to make sure

[jmaf]

I put two in parallel at each point, just to make sure

Add them in series. You don't need the current there, but the peak reverse voltages.

[Zer09]

The maximum +B is the peak of the AC wave right before the silicon diode anodes. There is no way that adding a passive component there would increase +B from 1/2 p-p AC. So adding a silicon diode does not increase the voltage, it in fact decreases it a diode drop.

Edit: And when a rectifier fails, it often arcs from anode to cathode, it's a short, you don't lose only the voltage drop across the tube, you lose all voltage in fact, blown fuse, magic smoke.

The last part was in reference to what happens when a rectifier tube fails with diode backups.

When I refer to b+ I am referring to the dc side of the rectifier, and the subsequent plate voltage on the 5881 tubes. So you are telling me that putting diodes in series with a tube rectifier is not going to increase the dc voltage coming out of the rectifier tube? I mean according to this calculator PT Current Draw there is a 40-50v increase in rectified voltage going from 5u4 to diodes. What becomes of that difference when you put a tube after the diodes? This is what I'm missing and that's what concerns me. I don't want to end up with 500v on the plates os my power tubes.

[pdf64]

The tube diode drops 40V (or whatever) and the Si diode drops ~0.6V. The 2 are in series, with the same arrangement each leg of the B+ winding. Therefore total voltage drop is 40.6V.
The characteristics of the tube diode massively dominate those of the Si diode, in terms of volt drop, internal resistance, switching time; such that for normal operation, the Si diode may as well not be there.
However if the tube diode shorts then the total volt drop reduces to 0.6V, the Si diode takes over and prevents a massive Vac being fed to the reservoir cap etc, and keeps the show on the road, allbeit with hotter operating conditions.
Pete.