Real World 6P3S-EV/Sovtek "5881" test

[tunghaichuan]

I've seen claims that the 6P3S-EV/Sovtek "5881" can dissipate 40W on the net, made by Ken Gilbert, I believe.

I decided to test this out using a 5F2-A tweed Princeton circuit that I built a while back. This amp has Hammond 270EX and 125ESE transformers. I have a 2.7k/.68uF combination on the first cathode of the 12AX7 for more of a Marshall vibe. The second cathode has 1.5k/10uF.

I used a 201ohm/7w cathode bias resistor. This biased the tube at 39w, close enough for RnR. Cathode to ground voltage was 21.88v, plate to cathode voltage was 362v. Actual bias current was about 109mA.

I tried two different "5881" tubes from my tube stash. Both have the "coin" base. Sovtek sells one with a full base called the "5881WXT". The first tube started to get a hint of red on the plate (anode for Dr. Phil, LOL) after about 5 minutes. Not good.

I put in the other "5881" and no red plating. My guess is the quality control of the Russian tubes is pretty spotty. I can't really recommend biasing the tubes this hot, but really only two tubes isn't really that big of a sample. The actual data sheet for the Russian 6P3S-EV gives the maximum plate voltage as 250v, which is way under what it can really take. I had a pair in a Soldano designed Yamaha amp which had 500v on the plate (yeah, deep in my heart, I'm just a hick) in fixed bias.

I tried both 2.5K and 5K for the primary on the OT. I couldn't hear a difference one way or the other.

The tone was less distorted when cranked than a 6V6. It really responded to picking dynamics, though. Dig in and the amp distorts, play softer and the amp cleans up. It didn't seem that much louder than a single 6V6 or Groove Tubes KT66HP tube in the amp.

Neither the OT or PT got even slightly warm after about 20 minutes of playing with the amp cranked. I believe the 125ESE is more robust than the official Hammond specs say and can handle a single 6550 or KT88. Try this at your own risk, though. YMMV.

tung

[ontariomaximus]

As I mentioned over at ax84, a little redplating ain't bad at all with 39 watts dissipation. Should be able to get 15 watts from 1 tube single ended. Thats 6550/KT88 territory.

[zaphod_phil]

Thanks for posting these test results!

My guess is the quality control of the Russian tubes is pretty spotty.

You're right about that.

It didn't seem that much louder than a single 6V6 or Groove Tubes KT66HP tube in the amp.

It probably needed to run at higher voltage and with more drive on the grid to get the extra power out. In any case, we know that you really need 10 times more power out, to sound twice as loud.

I believe the 125ESE is more robust than the official Hammond specs say and can handle a single 6550 or KT88.

That's correct. A Hammond 125ESE can handle a KT88 or 6550 pretty easily for guitar, and that's what gets used in the P1eX, as well as in KurtL's 6550 power amp for the HO. For bass amps, you really need something bigger.