5U4GB tube rectifier flash

[nsubulysses]

Hi all. I am experiencing a tube rectifier phenomenon that is a little perplexing. Have never really troubleshot something like this before.

I received a Mesa Boogie Dual Rectifier with 1 of its 2 5U4GB rectifiers not glowing. The customer said the amp sounded weak and weird, and I would agree it did sound weak and weird. Switching it to the silicone rectifier option it played normally. On the scope it was putting out 100W with the silicone rectifier, 63W with the tube rectifiers.

When I put in a new 5U4GB it flashed on turn on. The flash occurred between about 10-15 seconds of the amp being turned on, standby still engaged (standby on, no high voltage). The tube lights up and has not flashed again after about 30 minutes. I have a rectifier "plug" which is a silicone rectifier in a tube socket. When I put that in place of the 5U4GB the amp puts out 100W on the tube rectifier setting and works normally.

The Q I have is, why does the 5U4GB light up and test good in my tester, but the clean power output drops to 63W when I use it, signaling that it doesn't work, just like the 5U4GB that was in there earlier that didn't light up? I thought a rectifier either just works or it doesn't. Since the amp works as it should with the tube rectifier "plug" it leads me to believe i just got faulty 5U4GB. Is the tube toasted now? Was the flash a spec of dust in the envelope that got vaporized? It is not arcing, certainly more of a flash towards the top of the tube, and a one time event (so far).

[pdf64]

Has a fuse blown in the amp?
Maybe the 5U4 is fine at the lower voltages that the tube tester uses, but not the tougher conditions that the amp subjects it to?

[g1]

Maybe try putting the good working rectifier tube in one socket at a time (without the SS rect.) and see if there is any socket issue.
If it is the same in either socket, maybe the new replacement was bad.

[nsubulysses]

Hey all, the fuse has not blown. Also I switched the two 5U4s back and forth and got the same results (weak, asymmetrical output). Only when I sub the new 5U4 i got with the solid state rectifier plug do I get normal 100W symmetrical sine wave output from the amp. This tube has to be a defect out of the box. The original 5U4 works in both sockets, without flashing.

[Jazz P Bass]

Let me guess.
The 'new' tube is an EH.

I went round & round with a dual 5U4 with EH's in it.
The amp arrived with one of them being a fuse blower.
Put a new EH in & it lasted an hour.
Flick this.
I installed NOS tubes that I snagged on EBay.
That was 2 years ago & the guy plays the amp 3 days a week.

[nsubulysses]

I am not sure what tubes Mesa uses but it is actually a Mesa branded 5U4GB. Coincidentally that was what I could get my hands on locally and I thought the guy would like it since it matched his other rectifier tube

[pdf64]

Is the amp specified to put out 100W when tube rectified?
It seems unlikely to me, so unless it's explicitly stated, I wouldn't be concerned.
As long as it produced more power with two 5U4 than one.

[nsubulysses]

I would have thought you should get the same power from tube and solid state rectification but the manual says this

9.) Choosing the proper Rectifier mode with the RECTIFIER SELECT switch (located on the Rear Panel) that best suits the sound style you are trying to dial can greatly enhance the performance. VACUUM TUBES produce a more elastic feel and softer clipping characteristics which is especially great for single note soloing. SILICON DIODES produce more power and headroom (my emphasis) and can also track better for extreme gain settings while staying tighter in the bottom end.

I will have to give this a listen tomorrow and see if it puts out a good sounding 63W. With the old, unlit rectifier it put out a bad sounding 63W, so when I put the new one in and the output didn't change I thought the rectifier wasn't working. I was in a rush when I gave this a quick diagnosis on Friday.

[Jazz P Bass]

I would look at the B+ voltage in both conditions.
SS mode & rectifier mode.

The 5U4's are in parallel, so the amp will 'function' with one tube.
Try a test with one rectifier and setup the output to produce 50 watts into a dummy load.
Then try the other rectifier.
They should both be the same output & B+ voltage wise.

The SS mode will most certainly produce a higher B+ voltage.
Though 63 watts compared to 100 watts does sound a bit off.

[km6xz]

With no other changes, the tube rectifiers should put out about 50 volts less than the solid-state diode. That means the bias point will change and power drop will be noticeable. If you want the most power, consider setting up for GZ-34 tubes that only have about 10 volts drop. The Mesa does not put out 100 watts with tube rectifiers but 70 would still be respectable.

[nsubulysses]

Cool thanks Stan. I actually just got to give this a little listening test and it does sound normal with the tube rectifier, albeit slighly less ballsy, so I think that is within the realm of normal. I gotta go get some contact cleaner for the sockets and I'll come back and do some voltage checks. IIRC from a few days back I think the plate voltage with SS rectifier was about 467 and with the tube rectifier it was around 430.

The new 5U4 is still going strong and no more flashes yet at this point too, so I think I'm in the clear.

thanks, everyone

[nsubulysses]

Well, the amp sounds good again. Here are my results in case anyone was curious, or has to work on a Mesa Rectifier series in the future. I used my DMM to judge power output rather than guesstimating pk-pk on the scope screen and converting to RMS.

Tube rectifier mode:
Biased to 30mA at 437Vp
Power output tested 68.5W at clipping into 4 ohm load

SS rectifier mode:
Biased to 46mA at 459 Vp
Power output tested 98.5W at clipping into 4 ohm load

I was fortunate and didn't have to bias it. The new 6L6s worked pretty well where the bias was set -- in tube rectifier mode just under 50% plate dissipation and in SS rectifier mode just at 70%. Guess that's not too bad as far as an over/under. Funny, because if I did do a slight adjustment and dial it in I wouldn't be able to get either setting within the 50-70% range.

The only weirdness I can still wonder about is that the new rectifier has a little cloud towards the base of the tube. Not sure if it was like this before I got it, or if the flash affected this. As you can see also the original rectifier has more of a rounded top (JJ?) and the new 5U4 has more of a flat top. Hope it's not an EH :/




For some reason these uploaded upside down wtf

Thank you all for your input

[pdf64]

It would be interesting to know the VB+when the amp is putting those power levels, in both rectifier modes?

[nsubulysses]

I will let you know because I actually made an error in my bias calculation and will have to re-check it today.

[Audiotexan]

The only weirdness I can still wonder about is that the new rectifier has a little cloud towards the base of the tube. Not sure if it was like this before I got it, or if the flash affected this. As you can see also the original rectifier has more of a rounded top (JJ?) and the new 5U4 has more of a flat top. Hope it's not an EH :/

[ATTACH=CONFIG]31516[/ATTACH]
[ATTACH=CONFIG]31517[/ATTACH]

That 'cloud' kind of looks like the old "wear marks" you sometimes see on old 6L6's that have the 'blue flame' when they've aged and had alot of hours on them...

Perhaps the evacuation/flashing process isn't taking properly on a given batch of 5U4's, and this is one out of that batch as well? Also, could it be some leftover flashing material (from an overfill) that didn't flash at the top, and landed on the bottom, then flashed once it was fired up in an amp (assuming while on the bench)?
Hard to say, but odd all the same.

For some reason these uploaded upside down wtf

Thank you all for your input

Wierder still? They loaded in my browser shifted 90 deg. to the left (from posted). Something odd is definitely going on.