Jolida 502 KT120 tube swapping

The Jolida is basically a Mullard 5-20 design with different tubes. Don't spend the money on the tubes. IMHO. Replace the 15k resistor in the 12at7 tail with a Compact Cascode current source kit from http://www.kandkaudio.com/accessories.html

Then replace the Plate resistors on the 12at7's. Match them at either 48k or 56k. Adjust for about 3mA per side or 6mA current across each resistor. Remember do this to both channels.

Look on the net for the Jolida schematic and you will see the resistors I'm talking about. You will thank me. This will make a larger improvement than the output tubes.

The resistors I mentioned was not part of the upgrade. I'm sorry I was just trying to help you improve the amp.

I would be careful putting the KT120's in this amp. First they draw more current and this could hurt the power transformer with extended use. The current draw from a KT88 is 1.6A. According to the Tungsol data sheets they draw between 1.7A and 1.95A. It doesn't sound like much but it could be enough to fry the transformer over time.

Secondly these are not the same tubes. KT88 and KT120 have different operating conditions. Yes they both have 6.3v heaters and the tube pins are the same. But there is more to a tubes operation. If your amp is not set-up to run the KT120 it will not be running these tubes at their best operating point.

There are enough new KT88's to choose from. Also your input tubes can both be improved on. IMHO Roll a few and find out.