KT88 in an Express???

[mcrracer]

I just read some info on the Express that says with the proper bias adjustment, KT66, KT88, 6550 and 5881 tubes can be used. Has anyone here tried KT88, KT66 or 6550 tubes in one? Any sound clips? What are your impressions of the differences from 6V6 or EL34 tubes as used in the Express? Thanks

[billyz]

might want to check the Wattage Rating of the OT if you plan on using 6550 or the KT88's. They are capable of 42w per tube. I think at 400 V on the plate you might get 60 watts out of a pair. They do break up later than the el34. The amps I have used them in sounded good though, very robust sounding , maybe muscular would be a good description.

[Roe]

42w refers to heat diss, not output. new gold lion kt88s sounds good in my express but not as good as siemens/rtf el34s

[dobbhill]

42 watt heat output? Please explain.

[guitarsnguns04]

I love the kt77s in my express

[gearhead]

42 watt heat output? Please explain.

On your charts, each tube has a maximum rating of 35 watts on the plates (anodes). That means the current thru and voltage at the plate cannot be a multiplication (power) of more than 35 watts (voltage times current equals power). The heat put out by reaching such a limit is the byproduct of running such high current/voltage, not an end unto itself.

It's in the upfront tables and if you look on the first two charts, you will see a curve labeled "Pa=35w". The two axies are Anode current (Ia) and Anode Voltage (Va). If you look anywhere on that curve and trace to the corresponding voltage and current, multiplying those values equals 35 watts. This curve gives you an upper boundary when designing an amp using that tube type.

[dobbhill]

thanks, Gearhead.

[billyz]

Does this data sheet show a pair of KT 88's putting out 100W ( P out) ? I realize that is with 560 V on the Anode. It does make me think a pair is capable of 60 W at 400V on the plate. I have seen the charts showing 35 w plate dissipation on very early tubes and also 40 W and 42 W plate dissipation. Which one is correct ? all of them? I realize that is not actual Power output, but the V x A rating of the plate, or the design center. Is it the recommended operating condition?

[jaysg]

It does make me think a pair is capable of 60 W at 400V on the plate.

The power supply would have to be capable of 60W. I don't recall whether there's a mismatch between the TW iron...seems like the PT is over-rated, so that set might create trouble for the OT.

fwiw, it seems like the TW concept is based on cranked output tube distortion with a percolating pre-amp...a little from here, a little from there, etc. If you use a 6550, KT88, KT90, you'd need to tweak the PI area.