Last word on Sovtek 5881 plate voltages, pleeeeeease.

[Gaz]

I'm trying to replace the power tubes in my Russian Red Bear MK 60, but I can't figure out a proper replacement. The stock tubes are Russian 6N3C-E (better known as the Sovtek '5881'). I thought it'd be in my best interest to switch to 6l6GCs - and just adjust the bias accordingly - until I found 'information' suggesting that the amp wouldn't sound 'good' with 6L6s, and that the Russian '5881' tubes can handle much high plate voltages.

According to every spec sheet I've found for the Sovtek 5881/ 6N3C-E, the maximum plate voltage is 250 volts, much lower than the plate voltage in my properly functioning Red Bear that has a plate voltage of 482 volts!

How could this be? Could the maximum plate voltage rating be rated at half of what's it's capable of? They are using these tubes in so many amps, and I can't believe the maximum plate voltage is actually that low. Most 6L6s can handle 500 volts on the plate, and I don't understand how the Russian 6N3C-E are supposed to have higher plate tolerances (BTW, the infamous Lord Valve stated that in his catalog).

Clarification would be a godsend.

Thanks.

[txstrat]

Just for the ratings you might go to:
http://thetubestore.com/tungsol5881.html which go up to 400v.
or: http://thetubestore.com/sovtek5881.html
or for a overview: http://thetubestore.com/6l65881types.html
There might be some fitting tubes...

[Chuck H]

I did not find a data sheet for the 6N3C-E tube. But from what I've read in real world tests that tube can take up to 600V on the anode and has a max diss of about 40W. Perhaps the data sheet spec is for class A operation. From what I found you are not the only one to be confused by the data sheet. That tube is commonly used near 500V on the plate.

I think that tube is still available if you like the way your amp has always sounded. Otherwise the 6l6gc will "work" but I can't comment on the tone. Based on the tonee of those amps it would probably sound good with el34's too. But I don't know if the filament wind on the PT can handle the extra current. It probably can. That may require a modification to the bias circuit.

Chuck

[Gaz]

Chuck,

Thanks so much for the response. I concur that from all I've read "real world" tests have taken much higher plate voltages. I never thought about the limited charts I viewed only showing class A operation, so thanks for that thought as well.

Yeah, the EL34s would sound good, but I already have a Marshall 2203, and I've rewired the Red Bear's tone stack to a 5F6-A Bassman's because I wanted it to sound a little differt

By the way, where do you live in the PNW? I'm in Olympia, and about to move to Portland.

[Chuck H]

I gotta say, everyone I've known that goes to Portland never leaves. Not in a bad way. People seem to like it there. I have to say that just driving through on I-5 I don't get the charm. But I've never taken any time to get to know Portland.

I'm on Whidbey Island.

Since you've already revoiced the amp a little I suppose that the reports of 6l6 tubes not being a good choice for that amp don't really apply to you anymore. They will operate fine in that amp. And since your looking for something NOT Marshall... If you can access a set for free that would be ideal. Then you could know before you buy.

Chuck

[Gaz]

Yes, people seem to love Portland, especially young people like myself. All my band mates currently live there, and Olympia has less charm than a Russian 5881 (Sorry, but I thought I better keep this tube related).

Anyways, I was wondering because I was think about going to the Mike & Key Amateur Radio Club electronics flea market in Puyallup on the 7th. Ever been? Have any good finds?

Needless to say, I'm looking for some 6L6s, a capacitance tester, and maybe a deal on a functional tube tester...

[soundmasterg]

Welcome to Portland, a little in advance. Its a great place. Send me a PM when you gt into town and I can clue you in on some good stores and good places to jam at.

The Sovtek 5881 can take tons of voltage...probably up to 550v or more with a plate dissipation of 25- 30 watts. KG runs 12 5881's in his BAGA amp at very high plate voltages. They tend to sound better when whipped with high voltage and high bias too. They were made for Soviet military purposes and are the same as the Sovtek 6L6GB except in a different bottle. In fact, I have a couple of those 6L6GB Sovteks around, NIB, that I'd sell you cheaper than retail if you want them?

Greg

[Gaz]

Thanks for the welcome!

So I'm starting to believe that these tubes can handle very high voltages, and the only things that's told me otherwise is the actual spec sheet

The spec sheet, which is apparently good for nothing, also states that the maximum plate dissapation is 20 watts. soundmasterg, so you're saying it's more like 25-30 watts? I totally trust you, but I'm curious as to how you arrived at this conclusion. Did you keep getting the tubes hotter until signs of red plating, backed off a little, read the bias, and did the math from there? That's the only way this newb can think of finding your own limits on a tube.

[Steve Conner]

These were military grade tubes designed to last a long time. They can take a lot of abuse in guitar amps. Groove Tubes used to supply them, and Fender used to use them in their reissue amps.

Ken Gilbert tried to fry them with up to 600 volts on the plate, IIRC, but he couldn't break them. I wouldn't care to run them at that claimed 40W plate dissipation for long, though, unless maybe they were in some crazy Peavey style wind tunnel.

The spec sheet may well be correct, if the numbers on the front page were maximum operating conditions for one tube in single-ended Class-A service, not limiting values. If you're used to the "headline" page of a modern semiconductor datasheet, tube datasheets are laid out quite differently. The headline numbers tended to be these Class-A figures, which are also the ones that you had to set your tube tester to, if you wanted to check the operation of the tube. That is why these numbers got reprinted in the "Vademecum" tube guides and eventually found their way onto the intertubes.

But on a transistor datasheet the headline numbers are the ones that will blow it up if exceeded.

[Gaz]

Thanks for the input, Steve. I'd already read a few posts discussing this particular tube in which you clarified a few things for me

So when doing my 70% biasing math, where is a reasonable spot to start with plate dissipation? I was previously following the spec sheet, which stated the maximum plate dissipation at about 20 watts, and doing the math as follows:

20w/482vdc x 70% = 29ma

Would 30 watts plate dissipation be more like it?

Any advice would be great. In addition, would 6L6GCs rated at 500vdc max. dis. be okay with this plate voltage (482vdc)? Do you think the tone would benefit, or should I stick with the Russian tubes the amp was designed with?

Thanks again.

[Steve Conner]

I would consider the max plate dissipation about 25-30W on these tubes. I've seen the original RCA 6L6GCs, and the Sovteks look a good deal beefier, they have more plate surface area and a bigger bottle. But they don't look quite as hefty as a 6550 with its 35W rating.

29mA per tube will probably sound a little thin, I'd crank it to 35-40. The B+ will sag down somewhat under the extra current draw, so the plate dissipation won't be nearly as much as you think. Of course depending on your musical preferences, the thin tone may be perfectly OK, and it makes the tubes last longer.

Again, any Russian or Chinese 6L6GC type tube would probably work fine.

BTW, the Sovtek tubes go under a few different codes because of the transliteration from Cyrillic letters to English. I know this tube as the 6Pi3SE. Something to bear in mind when googling for datasheets.

[Robert M. Martinelli]

As Steve said, 5881s were intended as a "beefier" military version of the 6L6, however, be aware that nowadays, some of the 5881s around are just "re-branded" 6L6s. I recently re-tubed and re-biased a JCM900 SL-X belonging to a friend, and when I opened the boxes I found that the Sovtek 5881s inside had "long sockets" and were mechanically and visually identical to some Sovtek 6L6s I had lying around. "Real" 5881s should have "beefier" plates and a very "short" socket.

Hope this helps

Best regards

Bob

[Gaz]

I biased the Red Bear at 35ma today from about 25ma where it was at. Two words: tone town. Thanks for all the advice. BTW, fixed a friend's Sovtek Mig 100 today as well, which uses the same tubes, and the plate voltage measured at 545vdc. So yeah, these tubes can take a beating and sound just fine in both amps for loudloud rock music. Nothing 'harsh' or 'flabby' to my ear as some have said elsewhere about those tubes.

[drewl]

FWIW I rebuilt a couple Red Bear amps for a friend and he had found a bunch of the odd output tubes and the strange preamp tubes for me to install.
After all was said and done, the amp didn't sound much different from any of the ones converted to run on 6L6's and 12AX7's.

[Gaz]

Yeah, i did the 12ax7 conversion, but that was just for ease of replacement and to have a little variety. I concur, it didn't change the sound much,