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Voltage measurements from a blown 5AR4 vs. good 5Y3
Voltage measurements from a blown 5AR4 vs. good 5Y3
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Old 12th April 2007, 01:37 AM   #1
w5jag is offline w5jag  United States
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Default Voltage measurements from a blown 5AR4 vs. good 5Y3

Playing around this evening with my fried Valve Art 5AR4 I made kind of a surprising observation - losing one of the filaments doesn't seem to have affected it much, at least from a voltage standpoint.

I plugged it into a small homebrew power suppy laid out in the usual fashion: Stancor PN 8403 250-0-250 tranny; rectifier tube, 40 uF, big, but unknown inductance, junkbox choke, 40 uf, 110K bleeder, and measured the voltage output:

Fried Valve Art 5AR4: 383 volts DC, 135 mV AC; RCA 5Y3: 371 volts DC, 45 mV AC.

I wanted to put a scope on the output to look at the AC ripple but in the limited amount of time I had to play I couldn't get a good display, so I just measured it with the DMM. I wonder if the increased AC the DMM is seeing is an artifact of running off one filament.

I did observe a somewhat, not much really, slower voltage ramp up with the 5AR4, but it was only running on one filament after all. I'm not really set up right now to load it to measure current output.

I don't really know what this is useful for or proves, other than that some fried tubes may still have a bit of utility in non critcal bench supplies and the like. It hasn't gone in the trash yet.

Win W5JAG
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Old 12th April 2007, 01:15 PM   #2
Tom Bavis is offline Tom Bavis
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It is operating half-wave. The voltage drop is lower (closer plate-cathode spacing than 5y3), but the ripple has tripled... it will get worse with load. Even worse, the transformer now has a DC component flowing in the secondary (it cancels with full-wave...). The core will saturate at some critical current - its inductance decreases, and if you're lucky, the fuse will blow. If not, I hope you enjoy the scent of roasted transformer...
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Old 12th April 2007, 02:46 PM   #3
w5jag is offline w5jag  United States
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Tom,

I'm certainly open to correction, but to me the available evidence does not suggest that it is operating as a half wave rectifier. Given that E DC = .45 E AC for a half wave rectifier, and E DC = .9 AC for a full wave rectifier, the measured voltages look wrong.

I've let the smoke out of transformers before, and am willing to do so again, in the name of science. If there are any suggestions as to how measure a suspected DC imbalance in the tranny core with the test gear I have on hand, I'll give it a go!

I may try another 5AR4 or 5V4 in it tonight, and see what the ripple looks like on them. Indirectly heated rectifiers may just have higher ripple than a directly heated rectifier. I've never really paid much attention to them.

Win W5JAG
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Old 12th April 2007, 05:48 PM   #4
Tom Bavis is offline Tom Bavis
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Two plates, two heaters. If one is open, it'll be half-wave. Half or full wave will charge to the peak voltage at light load. As the load is increased, the difference will be more apparent. Right now you only have 3.5 mA load from bleeder.
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Old 12th April 2007, 07:52 PM   #5
w5jag is offline w5jag  United States
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Tom,

When I hooked it up, I expected to see exactly the characteristics of a half wave rectifier, and was surprised that I did not.

I left out a fact that didn't seem all that pertinent at first, but the longer I let the fried 5AR4 run, the less ripple I saw on the output with the DMM.

I hadn't considered that a half wave rectifier could charge to the value of a full wave, even under no load. I was considering that enough heat was conducted within the cathode connections from the heated side to the non heated side to still cause some emission on the non heated side, and it is still acting as a full wave rectifier and would until the current draw exceeds the cathode emission.

I think I'll put it on the TV-7 and see if anything is coming off the plate on the cold side.

Win W5JAG
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Old 12th April 2007, 11:16 PM   #6
w5jag is offline w5jag  United States
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I put it on the TV-7/D and it's definitely dead as a doorknob on the cold side - not a hint of transconductance.

Just for fun, I ran the good side all the way up to 12.6 volts and held it there for ten minutes and never popped the filament in it , but at 10 and 12 volts the good side was lit up like a little light bulb. It even handled twenty volts for about fifteen seconds. This was without any apparent degradation to the good side once I took it back down to 5 volts.

Another interesting phenomenon that I didn't expect: at 7.5 volts and above, emission actually dropped off, rapidly at 10 volts and above. That's the opposite of what I would have expected.

Win W5JAG
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