Thank you for the update.
I am simply seeing if an EL86 work, rather not red plate it.
Richard been trying to get me to, but I been very stubborn about the lack of at least distortion figures (did ask) and fell in love with this Elektor Labs amp, which I modified. Still not bought components, getting generous amount of help on designing a power supply first. Plan is a 8k OPT with 43 percent UL.
Regardless of the circuit in use the 6CW5 / EL86 does not like anything over 200 volts on the screen grid (G2). They work best in the 150 to 175 voltage range. The plate voltage spec is 250 volts which can be violated provided that the screen is kept below 170 volts. I have used 6CW5's in the SPP with 150 volts on G2 and 340 volts on the plate into a 3300 ohm OPT to get about 30 watts per channel. I also made a guitar amp that used UL84's at 330 on the plate and 150 on G2 for about 25 watts, but the real power output was probably higher as I used an Antek power (mains) toroid for the OPT.
Cathode bias was used in both cases with about 20 volts across the resistor and maybe 10 volts across the OPT for a bit over 300 volts across the tube. I believe that the cathode resistors were about 270 ohms, but I'm not sure because both amps are long gone. You want about 25 to 35 mA of idle current per tube. There are many different variations of these tubes and they don't all bias up the same. Some may red plate well below their published 12 watt plate dissipation spec. I found some Canadian GE 6CW5 tubes with tiny plates that glowed at 10 watts. I have some French made UL84's that eat 15 watts without issue. They went into the guitar amp.
Cathode bias was used in both cases with about 20 volts across the resistor and maybe 10 volts across the OPT for a bit over 300 volts across the tube. I believe that the cathode resistors were about 270 ohms, but I'm not sure because both amps are long gone. You want about 25 to 35 mA of idle current per tube. There are many different variations of these tubes and they don't all bias up the same. Some may red plate well below their published 12 watt plate dissipation spec. I found some Canadian GE 6CW5 tubes with tiny plates that glowed at 10 watts. I have some French made UL84's that eat 15 watts without issue. They went into the guitar amp.
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This will only work with EL84's, but not with EL86's, unless you're satisfied with some tiny output power. 8 k primary impedance with a pair of EL86's would call for more than 400 V plate supply voltage and will result in an emormous output power for these little tubes, but, as others, especialy George, already mentioned, UL will be a no-go with this voltage also at the screens. And you'll need a bigger OPT.
Best regards!
For EL86 finals you ideally need 3-5k output transformers. As others had pointed out, UL will only work at relatively low B+ with EL86, not to exceed the screen max, providing low output power, probably <6-8 watts. (Perhaps enough for your use?)
If you already have 8k output transformers you could halve the loading to 4k primaries for use with EL86 if you connect 8 Ohm speakers to the 16 Ohm transformer taps, or 8 ohm speakers on the 4 Ohm transformer taps.
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