5e3 weird voltages and B+

[dango123]

Hi everyone - excited to be posting for the first time!

I have a 5e3 that I put together. Sounds excellent, and seems to perform as expected, volume up to 2 is funky clean, starts to break up nicely as you spin the volume up to 12, channels interactive, etc. etc. Very cool amp. And loud! I have an Eminence Maverick installed so that I can ratchet down the volume level - works well and sounds very nice.

So on to the problem - I started getting occasional arcing in the rectifier tube, together with some audible sputtering. I could see this was happening inside the rectifier tube, so I got a replacement (an NOS GE 5Y3) - worked fine. But I noticed the amp was getting a bit hot (note that I have a 30uf cap in the first filter cap).

So I pulled all the tubes and started taking voltage measurements and saw something weird. The voltages going to the rectifier (pins 4 & 6) via the High Voltage taps on the PT are around 180 volts each (I would have expected closer to 350 at each pin). The filament voltages seem OK at 6.1vac, the rectifier filament voltage seems ok at 4.8vac -- so I'm scratching my head here.

With the rectifier installed, the B+ at the first filter cap is around 180v, which is half what I'd expect it to be - and I see that situation across the power tubes as well (under 200v from plate to ground).

To add to the mystery - the amp sounds great, and is still loud.

Questions I'd love your help/advice on:
1) any idea what could be going on here? Could the PT be defective, or would you suspect a wiring issue on my part?
2) if I continue running the amp this way (with vastly reduced B+) do I risk damaging anything? As mentioned, it sounds great!

THANKS THANKS THANKS for any advice you can throw my way!

Best,
Dan

[Chuck H]

Try disconnecting the mains transformer HV leads from the rectifier socket and test the voltage. If it's still low the mains transformer is wrong, bad or the primary is wired incorrectly (this isn't likely since you seem to have correct-ish filament voltages). This is only a start, and doesn't explain the problems with your first rectifier tube.?. It's possible that when the tube failed it took the transformer with it, or vice versa.

EDIT: Forgot to say... There's still a chance that there is a mistake in the wiring that may have caused this failure. And it occurred to me later that there could also be a problem with your HV wind center tap.

[nashvillebill]

Do you have a good ground?

[Chuck H]

Do you have a good ground?

HA! Simulposting. Love it. My edit was done at the same time you posted.

[nashvillebill]

Great minds think alike, or so they say. Of course, they also say even a blind hog can find an acorn every now and then.

[dango123]

Thanks guys, REALLY appreciate your help on this. I had checked my HV center tap, I have a Zener diode on it to bring the voltage down a bit - the ground connection seemed to be ok when I checked with my multimeter but I will certainly take a closer look per your suggestions.

While I'm sorting this out, could I potentially do any damage playing the amp as it sits currently? As mentioned it sounds great, near zero hum and plays pretty loud.

I'm on a 4th of July gig for the next 2 days so I won't be able to get back to this for a day or two, but I will report back as soon as I have more info.

Thanks again for all your help, and happy 4th to you all!

Best,
Dan

[g1]

Judging by your description of the amp's sound, there would seem to be nothing wrong with it. You would have never suspected a problem with sound quality, correct?
So there is the possibility of measurement error/meter problem. Do you have another meter you could try (and have you checked the meter battery ) ?

[dango123]

Thanks, I had changed the battery in my DMM and still had the same issues with my measurements. I'll find another meter and see if I get the same results.

Best,
Dan

[Chuck H]

And... You mentioned a zener on the HV CT to reduce voltage. If your new measurement with a different meter corresponds with your old reading try bypassing the zener. I'll bet a dollar the problem is there because it seems VERY unlikely for a PT to fail with all windings correct other than the HV being low but remaining balanced.

[loudthud]

The zener between CT and ground will offset the AC voltage (to ground) at the 5Y3 plates possibly confusing your meter. Try connecting the other lead of the DVM to the CT.

[mac dillard]

"So I pulled all the tubes and started taking voltage measurements and saw something weird. The voltages going to the rectifier (pins 4 & 6) via the High Voltage taps on the PT are around 180 volts each (I would have expected closer to 350 at each pin"

So are you taking the readings with all of the tubes out? If so the readings on all of the plate pins should be about the same and high. I would first do as Chuck suggested, pull the rectifier and measure what the transformer HT reading is. If the HT voltage is not around 390 or more (according to which PT you are using) you will have isolated the problem to the PT or its wiring.

[dango123]

Yes, the 180VAC I measured from the HV taps was with the tubes all pulled. To your point, I'll take a look at the center tap wiring and the Zener diode to see if that's the potential cause of the problem.

Was wondering how likely it would be that the Zener would fail in this scenario? Also, if the amp sounds good this way, why bother to run with higher voltages at all? Running the 6v6s at 180 Volts sounds like the tubes would last forever unless I'm missing something (and I'm sure I am!). Would love to hear your thoughts on this.

Thanks again everyone, Really appreciate your help.
Best,
Dan

[Chuck H]

High voltage for tubes should only be an issue if it's too high. Otherwise it's the actual WORK that the tube is doing that will determine it's life (current x voltage determining actual watts = work). But yes, at under 300Vp you'll likely never push the tubes to max outside of a failed bias circumstance. And that should equate to longer life. But...

You've got to get the amp running correctly. If for no other reason than to hear what it SHOULD sound like compared to how it sounds now. You can always mod it back to the way it is. And if the zener circuit is dropping too much voltage, why? And could this be a potential condition for other failures? A flashing rectifier tube might convince me to make the amp correct first and evaluate after.

[dango123]

Thanks Chuck, and totally agree with your thinking. I'll have a chance to do further troubleshooting late in the weekend and will report back on my findings (I'm very much hoping it's related to the Center Tap issue you raised earlier!).

Best,
Dan

[dango123]

Hi All - huge huge thanks for all the advice and guidance above!

Story seems to be ending well... with the introduction of a new meter I had tried 2 older multimeters that gave inconsistent results, so I hit the local radioshack and bought a new analog meter, and voila my voltages are back to what I would have expected - B+ at around 380vdc (340vdc on standby), so I think that explains alot.

Interesting observation - my original RCA 5y3 (which was giving me some arcing issues) runs the B+ at just shy of 400vdc - not sure why its 20v hotter than the new GE bottle.

In any case, I think this case is closed, and I sincerely appreciate your help and wisdom in troubleshooting this issue. Now on to more fun with this 5e3 (and no doubt more posts as I discover and screw up more things in the future ).

Best,
Dan