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Post subject: questions about redplating EL84 in Pro Jr
Posted: Tue Sep 08, 2009 9:03 am
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Aspiring Musician
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so i received a new Pro J a few days back, and so far it seems like i may have found a winner...but now here's another concern, in the form of a very long post.

one of the stock sovtek EL84s redplated last night after only a few hours use. it happened while at a rehearsal. i haven't "diagnosed" the problem yet, as in if it's tube-related or amp-related. i'll see what i can find later.

i have a fresh set of JJ's on hand, so i'll be sticking them in. Eurotubes advocate cleaning your sockets before installing new tubes. i don't want the JJ's to redplate, so i may pick up some contact cleaner before installing the JJ tubes. if i don't use cleaner, and i simply just stick the tubes in and out of the sockets several times, will that be enough to avoid killing/redplating them?

also, i'm getting mixed signals with this... the Pro Junior is fixed bias, and everyone knows it runs hot. so there's a possibility that maybe it's too hot and as a result the EL84 mentioned above redplated? at the same time, Bob from eurotubes disagrees with Bill M about cooling off the bias in the Pro Jr. (sorry for the news Bill, you probably knew already. please don't be sore at me, i'm just the messenger). if i read the eurotubes website correctly, they suggest that re-biasing an EL84 amp isn't mandatory. The quote below is what suggests this to me (but i may be misunderstanding):

"Q: Do I Need to Worry About It?
A: Yes and no. All tube hi-fi amps need to be biased properly. Some have trim pots for each tube and some are cathode biased. Always stick with what the amp maker recommends. Guitar amps are a different story. Amps using EL84/6BQ5 tubes are almost always fixed bias and because of the type of circuit used are usually O.K. On the other hand, any guitar amp that is not cathode biased and uses 6L6, EL34, 6550 or KT88 power tubes NEEDS to be properly biased "

anyway, i could switch the sovtek el84s around and see if the same tube redplates in the other socket. at that point it'd be safe to say "oh, just a bad tube". but what if it IS the amp? how do i know that after replacing the power tubes, the redplating won't re-occur in the new tube that is in the same location as the one that fried?

i guess another way to ask this question is this...if an amp is biased too hot (or the amp is faulty in some other way), will redplating occur immediately, or does it only (or can it) occur after playing through the amp for awhile? i guess what i seek through this question is "do i need to re-bias or not?"
(recall that i have played through this amp for only several hours...most of that being quiet clean playing at home. i have only played at louder volume last night at rehearsal, for 2-3 hours. even then, the tones were generally clean, i wasn't really overdriving at all.)

you can see i'm a novice here with valid concerns. i simply want to be sure i don't do anything that will both screw up my new tubes, and void my warranty. bad tube? bad amp? something else? hopefully you can guide me. thanks alot.


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Post subject:
Posted: Tue Sep 08, 2009 12:51 pm
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Swap your existing tubes to see if it's still red plating. If the new tube red plates, it's the amp. Your tubes should not red plate period. The tubes won't last long. You need to isolate the problem. The amp or the tube.

No offense to Billm, but you CAN hear a difference in a tube amp that is biased too cold. If you can't, I guess it's just something you can't hear. I can and it bothers the crap out of me.

I wouldn't even bother modding these amps just because the PCB is of marginal quality and the traces can be easily lifted. I'd look for a different amp.


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Posted: Tue Sep 08, 2009 6:55 pm
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Aspiring Musician
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well, as of right now, i'm confused, to put it simply. i switched the power tubes locations. sitting in a dark room, i've just played through the amp for about an hour. i let it idle for about 10 minutes before plugging in and playing the guitar. i played about as loud as is considered "house tolerable". if no one was home, i'd crank it, seeing as that'd be the ideal testing condition.

anyhow, haven't noticed anything so far. no redplate, no strange noise, or lack of noise. i watched the tubes as i played, didn't see anything weird. the only "weird" thing was a blue haze which seemed to reside mainly in the area inside the plates. it wasn't too prominent, but noticeable. frankly, the amp sounds great. don't get me wrong, i'm ecstatic that nothing bad has happened so far, but i was expecting to see the problem tube flare up by now. so now, i don't know what to make of this. i wasn't imagining things, because the GT label is burned a bit...just very weird.

but maybe i didn't push the tube hard enough to induce any redplating? that's the only thing i can think of. agree/disagree?

i'm still gonna stick JJ's in there, as i'm planning to use this amp at a gig this weekend, and have more faith in those right now (needless to say i'll probably bring another amp with me as backup, which kinda defeats the PJ's advantageous portability). i'm going to hold onto the sovtek's rather than trash them. if anything, i'll give em a harder test if i get the chance to play louder. you might say "why not use the gig as a test?" well, perhaps i'll bring the sovtek's along, but i don't know if i'll be able to mess around there. this is going to be a well-paying gig, don't wanna clown around too much.

once again, i lay on the table the fact that i know nothing, which may be the cause of my incessant questioning.


Last edited by msvolpe on Tue Sep 08, 2009 7:24 pm, edited 2 times in total.

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Post subject:
Posted: Tue Sep 08, 2009 7:01 pm
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GT's for me were always pretty much garbage. I would pull the panel and look around for any scorched parts just for peace of mind. Pushing the amp should't cause red plating. If that were the case, all my power tubes in my amps would red plate.


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Posted: Tue Sep 08, 2009 7:04 pm
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Aim a small fan into the back of the amp to keep things cool. Ive done that with all my tube amps and I beleave its a good thing to do for any tube amp.


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Posted: Tue Sep 08, 2009 7:36 pm
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I added a bias pot to one of my tube amps.Biased it up and played about 20-30 minutes when one of the tubes red plated.I shut it down let it cool and switched the tubes (6L6) with each other seating them firm then rechecked the bias and have been playing it off and on for several weeks now with no problems.
I have no idea what the deal was? Usually it spells disaster of some sort guess we both got lucky and it may have been a tube not seated properly?


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Posted: Tue Sep 08, 2009 7:45 pm
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Aspiring Musician
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thanks so far guys.

thought i'd add that the "weird furnace noise" has returned in this amp, for those who recall my thread from a couple weeks back regarding that issue. although, it might be best described as a static-y white noise, and it occurs periodically, not constantly. this might serve to prove that the power tubes are in fact on the way out, perhaps i should see this as associated with the red plate mystery.


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Posted: Tue Sep 08, 2009 8:22 pm
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Professional Musician
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Yeah man you shouldn't hear any static through your amp. That would usually indicate arcing, in your case probably just a faulty tube. But Supro has a good point about inspecting the board for signs of burnt traces or components. If the tube sockets are mounted on the board, then you need to take care when replacing tubes. These newer amps and tubes seem to be kind of delicate, compared to the old bruisers. IMHO, ART


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