Is ANY red/orange plating acceptable in EL84 amps?

[Eddie]

Hey all,

I have noticed something with my MAZ Jr. If I am playing in a dark room, I notice a _very_ slight orange glow running along the seams of both EL84 tubes. I swapped out the tubes and can see it with EHX, Sovtek, and a set of Russian 7189 tubes. This isn't full-on red-plating, but there is certainly a glow on the plates themselves. This is with a 5AR4 rectifier.

If I run the amp with a 5Y3 rectifier, the orange line up the sides of each tube goes completely away.  

Is this just a part of an EL84 amp that runs hot by design, or has the bias maybe drifted (bought the amp new in 2006) ?


Thanks!
Eddie

[troy]

Eddie,

I am curious to read what others think about this.  I had a similar problem with a different brand amp running EL84s, and a reputable tech told me that any red-plating like that is bad news. 

[Deleted]

It can't be good for the tubes. Seems it would cut (tube) life expectancy.

[Eddie]

Myles sent me a response via FB, so I am pasting that here too:



Eddie,
If you send the link I will address the post there or feel free to copy and paste.
Your EL84s are drawing a bit too much current. They are starting to red plate. If it is only in a dark room with the 5AR4 you are OK. The amp will run a touch hot, tube life will be a bit shorter and the tone a bit more harsh but there is really no need for concern.
If you have some sort of rating on your output tubes or number you just need to tell your supplier what is happening so they can send you a lower number by a few digits next time around and the problem will be gone.





Thanks, Myles!

[digiTED aka 'Ted']

Great info; thanks for sharing Eddie+ Myles

Any thoughts on how to measure the plate current with 9 pins in a cathode-bias circuit? It would be interesting to compare the draw between various EL84 in a given circuit to gain a deeper understanding of the tones to expect.

I know how to do it w/ my Z28, but use an octal adapter at the first output tube.

[Deleted]

That's good info. Thanks for sharing, Eddie.

[DeanG]

Read a lot threads about low frequency hum and hiss level increase to a point of amp performance concern. For my case, I experienced this after 30 minute warm up of my Maz 18 NR. Where the noise level was at library whisper level until power tubes warmed up significantly (all knobs at noon, no input plugin connections), then an increase of hum and hiss noise reaching to low talking level (subjective). A 5AR4 rectifier is installed.

Pulling the PI (V3) tube, the Hum reduced but not eliminated.

Here is a picture of the Russian 6n14n, where the right (V5) EL84 is depicting more plate glow. Moving the right EL84 to V4 (left) position depicted the same intensity of plate glow. Problem followed the EL84. The plate glow is only visible in a dark room, the Android camera's CCD does enhance the IR somewhat. V5 red/orange glow observed by the naked eye (although, not a full bore total red plate). These Russian 6n14n have about 60hrs of use on them.



I had a set of backup JJ EL84's with about 30hrs of use on them. Installed them, hum/noise level remains at library whisper level after the 30 minute warm up. No red/orange plate glow observed by the naked eye. Usable performance level. Suspect the JJ's are operating at a more matched plate current...



Below is a picture of the Russian 6n14n after V5 (right EL84) was replaced with a matched EL84, same V4 EL84 is present as first pictured above. In the dark, no red/orange glow is observed, EL84 plates are black. The Android camera's CCD is capturing the IR in the image. Now with the replaced V5 EL84 no hum (all knobs facing noon, no input plugin connections), improvement noted as compared to the JJ's. The hiss is at library whisper level. 



As mention in another post, keeping a trusted spare power tube set will assist in detecting performance problems that are contributed from the power tube section of the AMP. Also, you can use the spares to verify if the current EL84 set are degrading.

Also, if one begins their gig with a quiet AMP, and at the end, hum/noise has increased, the above experience may be useful. 


Without the OP, I would not have attempted to monitor the power tubes in the dark, which assisted to highlight the defective EL84.

Thanks, Eddie.