Sovtek 5881WXT/Quad II

[7N7]

Hello all.

You might have noticed I have been asking questions about 6L6 and its cousins hereabouts.

I have a pair of Croft modified Quad IIs; in place of the KT66s the amps are fitted with EL34s; additionally there is regulated voltage with 85A2/12AX7/6080. I have not yet found any diodes so perhaps half of the 6080 is used as a rectifier?

Anyway having learned quickly just how hot the mains transformer gets, I decided that as EL34 wants 1.5A for its heater compared with 0.9A for 6L6 I decided to try to save 1.2A. I have received a pair of Sovtek 5881WXT; now it is said that 6L6 is a good idea in the Quad II, but that the bias resistor should be changed from 180 ohms (with KT66) to appx 220. I was surprised that the resistor is 180 even with the EL34s.

So having adjusted HT voltage to the specified 340V (anodes of EL34s) I plugged in the 5881s and was very surprised to see 22V at the cathodes: identical to that measured with the EL34s. Screen voltage is just over 330V; so far the 5881s have not gone "phut".

All in all, the easiest mod I have ever done.

7N7

[gingertube]

22 Volts across 180 Ohms => 122mA or 61mA per tube.

At 340 - 22V across the tube => Vak = 318V

Pa = 318 x 0.061 = 19.4 Watts per tube.

That is a very conservative running point for 5881. They should last for years.

Cheers,
Ian

[7N7]

Hello,

Yes, just like the original KT66s they are really being under-run.

It was the lack of data for the Sovtek WXT that was just a bit un-settling - in particular if the WXT could cope with 330V on g2, but I have since found out that Fender and Marshall use them in guitar amplifiers and recalling the voltages usually found in these I have no worries at all.

I have read quite enough about eastern fireworks displays, though Tubelab's experiments were reassuring!

7N7