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Thread: New 5881's in a BF Pro Reverb

  1. #1
    Supporting Member Randall's Avatar
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    New 5881's in a BF Pro Reverb

    Customer has a flaky 15 year old Sovtek 5881 that needs replacing. The amp sounds marvelous as it is, so he decided to stay with 5881's. I have read interesting things about the new Tung-Sol brand, so I suggested them to him. They sound a lot more midrange-y to me, with much less of the Fender scoop, so I don't know how he is going to react. I may end up buying them back from him. But, this is not my issue.

    I didn't notice until after they arrived that max plate V is 400v, and the amp is putting 485 on the plates with it biased to 31mA. I THINK that is right, but I am unsure if the data sheet is vintage or modern. At any rate, I expect that some will say we run tubes past their design parameters all the time, and some will say it's too much, shorter tube life, etc. Tung-Sol has an STR version that is 30 watts I think, and rated 500v plates, but I wonder if, provided he likes the sound of these, it will be OK to run them at 485v?
    It's weird, because it WAS working fine.....

  2. #2
    Supporting Member The Dude's Avatar
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    I wouldn't worry about it a bunch. You might consider a different rectifier (5Y3) to lower B+ a bit and give you a little more cushion. When those amps were made line voltage was lower. It won't help filament voltage (I'm betting your filament voltage is a little high, too), but unless it's ridiculously high, I wouldn't sweat it.

    This might help if you choose to go that route.

    Rectifier Tube Voltage Drop Chart - 300guitars.com

    Edit: Be aware that some of the alternative rectifiers draw more filament current, but it's not normally an issue with the old Fenders. They have pretty hefty PT's.
    Last edited by The Dude; 12-08-2017 at 03:46 AM.
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  3. #3
    Supporting Member Randall's Avatar
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    I don't want to get into changing rectifiers. This guy will not handle that. I doubt he is even going to like this tube in this amp anyway. I think it is more of a Marshall type tube, not as much for a BF Fender.
    It's weird, because it WAS working fine.....

  4. #4
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    Those Sovtek 5881WXTs are nothing like a vintage 5881. They're closer to a Super-6L6GC. We're not exactly sure how much punishment they can take, but they go in 7027-equipped Ampegs all the time. They're also cheap, and if the amp sounds "marvelous" with the Sovteks, I'd just stick with them - they'll probably take anything the amp can throw at them. Why mess with a good thing?

    I only tried the TS "reissue" 5881s once, in my Prosonic. They redplated in a few minutes. The Sovtek 5881s went right back in. I later tried the TSes in one of my builds, and heard nothing better than the JJ 6V6s & 6L6GCs that I also tried in it...

    One thing I DID notice, in perusing Fender schema: they only used the 5881 in the F-series Tweeds, and the A-series Brown/White amps. All the others were 6L6GCs, even some of the earlier tweeds and the time in between those two short spans, and all amps after. That kind of limited and only sporadic use tells me something: maybe they occasionally got a good deal on them, and that's why they used them. Otherwise, why would they go from a 19W tube (6L6GB) to the 5881 (23W) then to 30W, then back to the 5881, before ultimately landing back on the 30W 6L6GC?

    All my opinion, and lots of speculation...

    Justin
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  5. #5
    Old Timer Leo_Gnardo's Avatar
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    You may want to consider JJ 5881. I don't know how they dis-resemble the original item but I can say I've used plenty of them, upwards of 100, in the past 2 years and they have all worked perfectly well. Not only that, but in most cases deliver more power than you would expect. Not a single amp has come back with an output tube failure, neither has there been a customer complaint about tone or dynamic behavior (early compression, noises, etc.) How will they fare in the long run? Only time can tell. But so far - so good!
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    Senior Member Enzo's Avatar
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    BAck in the old days, like a couple generations ago, the 5881 was an industrial tube related to a 6L6, but had lower specs. These days, anything made with a 5881 on it, is pretty much just another 6L6. The data sheets from the old days don't apply to the new tubes.
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  7. #7
    Supporting Member Jazz P Bass's Avatar
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    "They sound a lot more midrange-y to me, with much less of the Fender scoop"

    An output tube can cause this??
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  8. #8
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    Agree with the others, if he likes the Sovteks, stick with them. They are not like any other 5881's or really 6L6 either. Pretty much bulletproof though, which is a good thing.
    As far as what Enzo mentioned about the modern ones being 6L6's, that seems to have been changing lately. Some of the 5881 reissues ("tung-sol")are now claiming 23W max. and cautioning about biasing accordingly and not using in applications with high plate voltages.
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  9. #9
    g1
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    Quote Originally Posted by Jazz P Bass View Post
    "They sound a lot more midrange-y to me, with much less of the Fender scoop"

    An output tube can cause this??
    I think a change in the output tube type could, and basically, the way they are throwing the type names around you do not know what type you are actually getting. The Sovteks he is comparing them to are not actual 6L6 or 5881, but some Russian number that has similar but not same characteristics.
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    Quote Originally Posted by Jazz P Bass View Post
    "They sound a lot more midrange-y to me, with much less of the Fender scoop"

    An output tube can cause this??
    Maybe -
    1. even with their regular headline characteristics, tube variance from bogey may be significant, especially recent / current production, which seems to be pretty much of a free market, ie no industry body to hold manufacturers to account over their product quality.

    2. Guitar amps, especially non-master volume types, are typically used beyond their linear range. Under large signal conditions, screen grid HT nodes can sag A LOT.
    And who ever checks screen grid characteristic variance? Tube charts hold screen grids under tight control, guitar amps don't. It seems feasible to me that different screen grid design construction may lead to different characteristics, and things would perform differently in real amps.

    3. Continuing from above, how about the forward bias characteristics of the g1-k diode, eg what the heck is the type bogey, never mind how do different brands conform to that. I'm not aware of any info, yet this is surely the most significant point of signal clipping in a guitar amp.

    Hence it seems reasonable to me that differently manufactured output tubes might well perform / sound different.

  11. #11
    Supporting Member Jazz P Bass's Avatar
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    I still do not see how an output tube (2 no less) could influence a specific frequency band.

    Which brings up: "with much less of the Fender scoop".

    Doesn't make any sense to me.

  12. #12
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    I still do not see how an output tube (2 no less) could influence a specific frequency band.

    Which brings up: "with much less of the Fender scoop".

    Doesn't make any sense to me.
    Very interesting thread. I've never stopped long enough to really get my head around all of the dynamics in the different power tube types, let alone how each manufacturer creates the innards that fit into the glass cylinder and gets closed up under vacuum , then interfaces with different output transformers of different manufacturer's designs & builds, to finally pass signal to and thru the O/T and onto the speaker. I've always thought one of the big differences between Marshall and Fender amps is their magnetics, when you get back to the most basic circuits using the Fender 59 bassman tone stack and similar power amp designs.

    I doubt if you'd find the clues in steady-state signal conditions (Sine wave drive). Quantifying the drive behavior under dynamic conditions (both into and out of the Output Transformer, recording both current and voltage) would no doubt reveal a lot more. Our ears tell us there are very noticable differences. Very difficult to quantify, as most of our benches are set up with steady state instrumentation.

    I've had one local tube vendor (ARS Electronics Van Nuys, CA) tell me there's no difference in the tubes they have marked as 6L6GC, 5881, 7027 and others in that same class. He said it's mostly a specsmanship game. I'll plead ignorance to it, not knowing nor having taken the time to learn for certain.
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  13. #13
    Supporting Member Randall's Avatar
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    Well, to my ears the sound that way compared to the old Sovteks. This amp has no mid control, only a fixed resistor, so maybe that has something to do with it. I actually took a chance on them because I read in one of the vendors reviews that the TS's were more "chimey in the mids", blah, blah, blah. Turns out they seem to be that, which doesn't suit this amp so much.
    It's weird, because it WAS working fine.....

  14. #14
    g1
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    Quote Originally Posted by Jazz P Bass View Post
    I still do not see how an output tube (2 no less) could influence a specific frequency band.

    Which brings up: "with much less of the Fender scoop".

    Doesn't make any sense to me.
    This may be an over-exaggeration, but what if I said "I changed from 6L6 to EL34 power tubes and it doesn't sound the same".
    Wouldn't you say "well, what did you expect"?
    In this case, we are going from a Russian 6*** (insert russian hieroglyphics here ) to some modern manufacturers idea of a vintage 5881. I expect there to be at least some difference, even if it's just plate resistance, which should do something to the freq. response.
    But I guess you are taking issue with why it would show up as a midrange thing?
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  15. #15
    Supporting Member Jazz P Bass's Avatar
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    Totally agree .
    Different tubes.

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