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SSP/6CW5 OT Question

Assumng they are 2.5k output transformers then they are not optimum ...
They would probably work, but the tricky part is with the 6CW5 /EL86 is the B+ and screen supplies. As you can see the tube is designed for lower voltage and higher current than a EL84. The plate supply can be somewhat higher, but the 200V screen limit needs to be hnoured.

Do your transformers have different secondary taps, e.g. 4R, 8R and 16R? If so there might be some creative options using dfferent secondaries for the speakers you have.

Screenshot 2023-08-13 220949.png
As stated above, the 6CW5 needs a screen voltage in the 170 to 200 volt range. The plate in most 6CW5 tubes is identical to the plate in the 6BQ5 / EL84. It can eat lots of voltage depending on the load impedance. Run a 6600 ohm load and you can feed the plates up to 400 volts. I have actually been to 430 volts without issue, but the screen voltage, screen dissipation, plate dissipation, and peak cathode current (about 175 mA) must be respected. There will be a point somewhere around 375 volts where the idle current needed to clear up any crossover distortion results in too much plate dissipation. Going any higher in plate voltage is futile. This is true of many tubes.

You want to use a 2500 ohm load. The first thing to do is to look at the impedance VS frequency curve for your intended speakers. If the curve is significantly higher than 8 ohms across the low bass region where the amp is called on for most of its power, then just use the 3000 ohm recommendations. My GE 6CW5 data sheet shows 200 volts on the screen and 250 volts on the plates with an idle current of about 45 mA per tube into a 3000 ohm load for 1.0% THD at 25 watts.

I built a guitar amp almost 10 years ago that ran a pair of UL84's ( a 6CW5 with a 45 volt heater) on 340 volts of plate voltage and 170 volts on the screen into a 2700 ohm Antek power toroid being used for the OPT. It made about 18 watts with the toroid OPT and just over 20 watts with a real guitar amp OPT rated for 6600 ohms but miswired to present a 3300 ohm load to the tubes. That amp still has the original tubes in it and still works fine, though it doesn't see much use because it's too loud!