This all sounds scary to me the kt88 tube runs at a much higher voltage than the 6550 and the bias current is different also. They might sound good but i do not think that this setup will last long unless he ran the kt88 tubes at a lower voltage it would shorten the life of the tubes(6550) significantly. Also the fact that the amp is auto bias with a different tube in it eeven scares me more but like i said he could have designed it that way to work with both tubes ask him to be sure.
Red Plating??
I have a CLASS A KT88 amp that has an autobias circuit. The company rep recommended SED 6550C Winged C tubes. I purchased a quad of them. They bias OK and they sound wonderful but I just noticed that they red plate a bit. If I can figure out how to do it I will attach a picture.
The red plating is just in the inner corner of one of the plates on all 4 tubes. It looks as though I am looking at the filament through a translucent plate but I know that its red plating.
https://imgur.com/a/RVLWPcf
Can I operate like this safely? The bias does hold steady and does not fluctuate. The red plating does not change. As a Class A amp this status should hold steady. Of course, it does run hot. If all that it hurts is tube life, thats OK. These are my best sounding tubes without a doubt.
The red plating is just in the inner corner of one of the plates on all 4 tubes. It looks as though I am looking at the filament through a translucent plate but I know that its red plating.
https://imgur.com/a/RVLWPcf
Can I operate like this safely? The bias does hold steady and does not fluctuate. The red plating does not change. As a Class A amp this status should hold steady. Of course, it does run hot. If all that it hurts is tube life, thats OK. These are my best sounding tubes without a doubt.