6П3С-Е Russian NOS: Gas Problem

[Rod Coleman]

Along with many amp builders, I've been shopping on ebay for Russian military NOS, and found them to be a good buy, on the whole.

I bought a dozen 6П3С-Е (6P3S-E, the coin-base version), 1977-83 manufacture. The look good, with big getters and a quality feel. They appear to be identical to the present day Sovtek 5881WGC.

THe Sovtek 5881WXT (same valve but with shouldered base) has been factory installed in the Fender reissue guitar amps for a while, so I thought I'd try some 6П3С-Е in there. The amps run pentode mode, Ua=Ug2=485V.

(Fixed) Biassed at -60V they run about 35mA (per pair) at turn on, but increasing to 65mA per pair after 1 hour - and still rising.

Suspecting gas, I looked at the 0.1uF/220K grid coupling, and dropped a second 220K on to the first, to make the grid circuit see 110K. Now the rise is limited to 44mA/pair (which sounds good) and seems to be stable.

Since these Beamers are gassy, I figured I'd change the grid circuit to 330n/630v and 68K.

Anyone comment on whether that ought to be stable? Do I risk getting arcovers on big peaks?? The Fenders put diodes on the anodes to keep the anode voltage below 2x B+.

[Globulator]

I recently ordered some GU-50s, on starting up via the variac I noticed one tube with a big purple haze: gas inside!!

However after a couple of minutes it all disappeared, so I wonder if it was a) left over from manufacture or b) just popping out of the metals upon first use.

I'm glad I started at low voltage however to allow it time for the getter to get the gas, but it has been fine since - so you may be just seeing an initial burn-off of gas like I did.

[OneyedK]

Originally Posted by Rod Coleman

Suspecting gas, I looked at the 0.1uF/220K grid coupling, and dropped a second 220K on to the first, to make the grid circuit see 110K. Now the rise is limited to 44mA/pair (which sounds good) and seems to be stable.

Since these Beamers are gassy, I figured I'd change the grid circuit to 330n/630v and 68K.

Anyone comment on whether that ought to be stable? Do I risk getting arcovers on big peaks?? The Fenders put diodes on the anodes to keep the anode voltage below 2x B+.

What type of Fender amp are you talking about?
12AT7 long tail phase splitter or 12AX7 not so long tail phase splitter?
Overloading those phase splitters with anything less than 220k kills the tone of those amps...

I'm guessing that the coupling caps are leaking
OR the 6P3S-E cannot stand 485V Ug2. As far as I know, they're rated 400V, on Ua and Ug2.
6L6GC does not have the same problem (520V).

You could lower Ug2...
But I'd start with replacing those coupling caps, most RI Fenders are stacked with crap caps...

[Rod Coleman]

It's a Hotrod Deville. I plan to swap the caps, just in case!

The 6P3S-E is the same valve as the 5881s used in the amp when it was new, so they are USUALLY OK, but I think these have leaked vacuum a bit

Thanks!

[Eli Duttman]

Rod,

Those Russkys are gassy. You need to "cook" them, before attempting any matching. Drop Jim McShane a line. He's been culling out sets for his customers.

[OneyedK]

Originally Posted by Rod Coleman

It's a Hotrod Deville. I plan to swap the caps, just in case!

The 6P3S-E is the same valve as the 5881s used in the amp when it was new, so they are USUALLY OK, but I think these have leaked vacuum a bit

This one has the 12AX7 not so long tail phase splitter, so you can't load them heavier than 220k. (almost a copy from the brownface and blonde, Fender stepped away from them in the Blackface series)
And yes, those "modern" Fenders often have low quality caps wich die rather young.

The "Fender" original tubes are marked 5881 but are, as far as I could see, 6L6GC's in a smaller glass container.
(that doesn't mean I don't believe those russian tubes are gassy)

What happened to the original tubes?

[Rod Coleman]

Originally Posted by OneyedK

This one has the 12AX7 not so long tail phase splitter, so you can't load them heavier than 220k. (almost a copy from the brownface and blonde, Fender stepped away from them in the Blackface series)
And yes, those "modern" Fenders often have low quality caps wich die rather young.

The "Fender" original tubes are marked 5881 but are, as far as I could see, 6L6GC's in a smaller glass container.
(that doesn't mean I don't believe those russian tubes are gassy)

What happened to the original tubes?

original tubes had burned-up getter!

But some old Svetlanas biased OK, with no drift.

You can see from the pictures that the 6P3S-e is the same as 5881WXT, exceot for the base...

THanks for the tip about loading the splitter! I forgot about that.


****

Eli, Thanks! I sent Jim some email.

[vega65]

I had gas trouble with 8 13E1's.
A night in the oven : 24 hours at 100 degrees C cured 7 of them.

[OneyedK]

Originally Posted by vega65

A night in the oven : 24 hours at 100 degrees C cured 7 of them.

What's the magic behind this treatment?

I tought tube "cooking" was to operate them at low voltage, positive grid to obtain full anode temperature (slight coloration), this for several hours.
?!

[Rod Coleman]

I cooked one today: 12 hours at Ua=485V, Ug2=440V Pa=25W.

At the start, Ig1 was 12uA, now it is about 3uA. I'm going to call that a success, unless anyone knows better!