• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL-34 to 6P3S-E

Laila2

Member
2009-12-19 1:53 pm
Hi,

I have a´ pair of Sentec M30 mono amplifiers constructed
for running two EL-34 type tubes in p-p. Can I put in 6p3s-e
in place of EL-34 without to much risk ? The OPT:s primary
impedans is 6kOhm when your speaker have 8 Ohm. The
Anode voltage is about 400V.

Thank you !
 

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Rod Coleman

Member
2004-07-25 6:55 pm
UK
Compared to modern production EL34s, I'd say that the 6П3С-Е (6P3S-E, the coin-base version) is tougher and more reliable. Don't use the ordinary 6П3С though, that's a 6L6GB analogue, and only rated for 270V on the screens, 19W anode dissipation.

The bias voltage is very different though, and you will have to correct this before installing a 6П3С-Е. Measure the cathode current using those resistors in the cathode leads, then look up the bias voltage needed against the screen voltage the amp is running. Watch out though, the sample-to-sample variation on these is very wide, and your amp specifies matched pairs!

Many NOS 6П3С-Е are gassy, and you need to test for this, and monitor the tubes' cathode current for a hour or more before you're sure it's stable. A slow roast at 100 deg C for 12 hours will often fix the gas.

The 6П3С-Е will work in a EL34 socket with no connexion changes.
 

Laila2

Member
2009-12-19 1:53 pm
Compared to modern production EL34s, I'd say that the 6П3С-Е (6P3S-E, the coin-base version) is tougher and more reliable. Don't use the ordinary 6П3С though, that's a 6L6GB analogue, and only rated for 270V on the screens, 19W anode dissipation.

The bias voltage is very different though, and you will have to correct this before installing a 6П3С-Е. Measure the cathode current using those resistors in the cathode leads, then look up the bias voltage needed against the screen voltage the amp is running. Watch out though, the sample-to-sample variation on these is very wide, and your amp specifies matched pairs!

Many NOS 6П3С-Е are gassy, and you need to test for this, and monitor the tubes' cathode current for a hour or more before you're sure it's stable. A slow roast at 100 deg C for 12 hours will often fix the gas.

The 6П3С-Е will work in a EL34 socket with no connexion changes.

Thank´s for your detailed answer Rod Coleman,

Any idea on which cathode current that should be the "optimal"
setting whit 6P3S-E ? EL-34 needs about 52mA "optimal"
as you can see in the drawing. Thank´s again !
 

Rod Coleman

Member
2004-07-25 6:55 pm
UK
52mA with 400V is a shade over 20W.

6П3С-Е will withstand 20W better than a modern EL34 can. Why not start there, at 50mA? then you can compare the sound.

If you modify your bias circuit so that it can adjust between -45V to -35V, approximately, it will be OK. start at -45V and adjust until the current is correct (1.0V at the test point).

You have two adjusters, so a matched pair is not too important, despite the schematic note.
 
Neither do I, but I do have some 6P3S-E:s . . .:)

Me too. :D

I'm afraid 390V on screen grids of 6П3С-Е is lethal for them. But you have your right to try. ;)

Also, the amp has deep enough feedback and frequency compensation that is adjusted according to EL-34 gain and properties of output transformer. You will need to tweak it, anyway...
 

Rod Coleman

Member
2004-07-25 6:55 pm
UK
Me too. :D

I'm afraid 390V on screen grids of 6П3С-Е is lethal for them. But you have your right to try. ;)

Also, the amp has deep enough feedback and frequency compensation that is adjusted according to EL-34 gain and properties of output transformer. You will need to tweak it, anyway...

Well, I know the original spec for these is 300V max VG2, but this exact valve [the -E version] is relabelled as the Sovtek 5881WXT, and used as an OEM component in the Fender guitar amps, starting in the 1990s or earlier. Sometimes they are labelled Fender. These run pentode mode with Va=Vg2= 470V to 510V depending on the model, and they have better reliability than any other 6L6GC variant, that I have witnessed. And they get some heavy beating!

They are very cheap. Just make sure you have fast fuses in the HT if you are worried - 500V T500mA type should be a starting spec.
 

Rod Coleman

Member
2004-07-25 6:55 pm
UK
THe other precaution to take when you are working with high screen voltage, is to make sure the screen resistor is large enough.

Fenders use 470-ohm, and this is a good starting point at 400V. You can use higher values, too, but compare the sound in every case.

I can't read the value on your schema....
 

Laila2

Member
2009-12-19 1:53 pm
Well, I know the original spec for these is 300V max VG2, but this exact valve [the -E version] is relabelled as the Sovtek 5881WXT, and used as an OEM component in the Fender guitar amps, starting in the 1990s or earlier. Sometimes they are labelled Fender. These run pentode mode with Va=Vg2= 470V to 510V depending on the model, and they have better reliability than any other 6L6GC variant, that I have witnessed. And they get some heavy beating!

I currenly use them in a Guitar amp. built for 6L6GC/5881
(SovtecMig-50) that has 535V B+ and it seems to work
quite well.
 

Laila2

Member
2009-12-19 1:53 pm
THe other precaution to take when you are working with high screen voltage, is to make sure the screen resistor is large enough.

Fenders use 470-ohm, and this is a good starting point at 400V. You can use higher values, too, but compare the sound in every case.

I can't read the value on your schema....

I think it´s just 100-Ohm, but I´m not quite sure . .