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FDP Forum / Amp Mods, Repairs, and Projects / drive a Twin Reverb with 5881?



Apr 30th, 2019 01:32 PM   Edit   Profile  

I thought I had asked this in one of the forums but can't find my possible post at present:

Can I swap the 6L6GC in my Twin Reverb Silver Face 135W against 5881. Do the same in my Bassman 135 amp?

Contributing Member


Apr 30th, 2019 04:13 PM   Edit   Profile  

“Real” 5881s or the Sovtek rendition?

I *think* B+ is too high, in those amps, for the real ones.



Apr 30th, 2019 05:32 PM   Edit   Profile  

I had a pair of original Tung Sol 5881's in a 1961 brown Concert running 470v on the plates. You just need to observe the maximum plate dissipation of 23 watts when biasing 5881's.


Too Near Atlanta GA

Amp Tech Emeritus
May 1st, 2019 09:05 AM   Edit   Profile  

Yes, and don't set the bias to mpd or that crazy 70% BS (JMHO)...original Tung Sol 5881's are truly great sounding tubes.




May 9th, 2019 05:28 AM   Edit   Profile  

The 135 watt amps have a high, stiff HT supply, usually >500V, so are tough on their power tubes.
They were designed around the Sylvania STR487, which was really the next generation of the 6L6, maybe equivilant to the 7581A.
To achieve the power output spec, a good set of those tubes are needed; modern tubes will lower the power output (maybe 80-100watts). A check of their idle dissipation is an extremely good idea, generally the magnitude of the bias voltage to the power tube grids needs increasing a bit to get things reasonable.
The regular Sovtek '5881' seems to be one tube that can cope, dunno about any other modern '5881'.



May 11th, 2019 08:54 AM   Edit   Profile  

So to recap (no pun intended):
1) Measure/determine the plate voltage of your amp.

2) For your new tubes, make sure that their plate voltage specs/rating can handle your amp. In other words, determine the maximum plate voltage for your new tubes.
3) Determine the maximum plate current that your new tubes will handle.

4) Install the new tubes and bias for a power dissipation at idle of no more than the maximum rating.
5) Measure the plate volatge and make sure that it is within spec for the new tubes.

Did I get it right?

(This message was last edited by ejm at 10:49 AM, May 12th, 2019)



May 11th, 2019 12:29 PM   Edit   Profile  

JJ make a '5881' too, it has plate limits of 500V and 23 watts, so may be a contender.

For it or any 5881 / 6L6GC, I suggest an idle cathode current of 30mA per tube should be safe for the tube and eliminate any chance of nasty (zero crossing) crossover distortion.

These amps have a bias balance adjustment control, but no overall bias level control. So without further info about ChristopherK's technical competence or the mod status of the amp, it's probably best to advise him to take the amp to a tech for the tubes to be fitted / amp adjusted to suit them.

A problem with 3) suggesting that the maximum plate current be determined is that it's a bit vague, it may be better to provide a method?

And with 4) if idling at the plate dissipation limit, the plates would surely overdissipate with signal?



May 12th, 2019 08:48 AM   Edit   Profile  

pdf64: Thanks for replying.
The reason that I posted the way that I did was to nail down the process and try to eliminate all of the generalities and swinging wildly in some other posts.

For #3 (and #2): I should have been specific and said to determine the maximum plate current by looking at the data sheets for the new tubes. Providing they're available.

For #4, I realized the potential conflict there. I left it as is hoping someone like you would post and try to clear it up.
I agree that you don't want to bias for the maximum plate current at idle. If you do, as soon as you start slamming those Pete Townshend windmill power chords, you're going to be drawing more current and dancing on that tightrope of eventual failure.
My understanding is that this is where the 70% or rated power dissipation at idle comes into play. If you bias for 70% at idle, then the remaining 30% will kick in when you actually start playing.

The other thing that was not mentioned is heater current. With tubes from the same gene pool, in this case 6L6, you're probably safe here. (I'd check it anyway.)
However, swapping in tubes like EL34s or KT66s or 6550s can get you into trouble here if all you do is rebias. Especially 6550s.

(This message was last edited by ejm at 10:51 AM, May 12th, 2019)



May 13th, 2019 08:50 AM   Edit   Profile  

I can't think of a (signal) tube data sheet that specifies a max plate current? That's more of a rectifier tube thing.
Of course for a given amp and its HT voltage, we can work out what current will result in the plate's dissipation limit being breached.

But yes, I think we seem to be 'singing from the same hymn sheet' with this :-)
And yes, the purpose of idling tubes in class AB at 70% or less is to allow them some slack, so that as signal level rises, their (average over time) dissipation doesn't exceed their limit.
The 70% idle thing is more of a 'limit' rather than a 'target' though; tubes don't have to dissipate some specific amount in order to work properly.
6L6GCs in a Fender are generally intended to idle a fair bit cooler than that, as willie implied above.


Too Near Atlanta GA

Amp Tech Emeritus
May 13th, 2019 10:58 AM   Edit   Profile  

Many times in life...as well as in class AB audio amps, less is better...up to a point. :) Finding the right point is quite variable...all part of the "magic"... :-)


FDP Forum / Amp Mods, Repairs, and Projects / drive a Twin Reverb with 5881?

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