Can you replace a 6550 with a KT120?

[smemm]

Is this possible? If so has anyone tried this? From everything I have been able to determine this should be a feasible swap, with a rebias obviously needed. I am using a Traynor YBA200-2 btw. Thanks muchly.

[Lo-E]

I have not tried it, but there are some things you should consider before attempting it: The KT120 will draw more filament current (which will put more strain upon your power transformer), take up more physical space and run hotter than a 6550 or KT88. Also, to bias it correctly might require modifying the existing biasing circuit because it may fall outside of the normal range of a 6550. Lastly, the increased output of the KT120 might not get along so well with your output transformer.

While it may, in fact, be feasible to swap them, it doesn't seem like such a good idea to me.

KT88s, on the other hand, are a direct replacement for a 6550 and can be used with no change to the circuit.

[JGR]

you will get a tonal change and perhaps more clean headroom, but output is limited by the VA rating of the OT - more power is heavy and expensive.

[BassmanPaul]

Originally Posted by Lo-E

some things you should consider before attempting it: The KT120 will draw more filament current (which will put more strain upon your power transformer), take up more physical space and run hotter than a 6550 or KT88. Also, to bias it correctly might require modifying the existing biasing circuit because it may fall outside of the normal range of a 6550. Lastly, the increased output of the KT120 might not get along so well with your output transformer.

KT88s, on the other hand, are a direct replacement for a 6550 and can be used with no change to the circuit.

While the original spec'd KT88 and 6550 were similar the types have blurred more than a little by current manufacturers. Your point on heater current is well taken. The KT120 should be considered as having a 2A heater for safety reasons. The main limit will not be the OPT but the PT and power supply. Any increase in wattage will be inconsequential and not worth the effort