Sano 50 WRT - convert 8417 to 6550 or 6L6

[KRKozBassman]

I want to convert my Sano 50 WRT from 8417's to 6550 mainly because 8417's are scarce and I already have 6550's. One of the two 8417's was replaced by the previous owner with a 6L6 and I want to drop in a pair of 6550's instead. I am a 35 year electronics veteran, but haven't worked with tubes or circuits in this detail for 25 years! From what I see by the tube specs and the amp schematic it looks like it should work - it works now with the mismatched 8417/6L6, but I want it better/reliable if that's even possible. I am not worried about watts or perfection, I just want to make it reliable so I can sell it without feeling like I am ripping anyone off. In case it's possible or easier I do also have an extra 6L6 to use so it would be 2 6L6's but the tube data doesn't look as compatible as the 6550.

[Leo_Gnardo]

I believe we covered this just a couple of days ago.

One more time...

A pair of 6L6 will work just fine, I've done it with several Sano's & no complaints.

With the self bias mode, you may find bias current runs to the high side. You might bump the Rk up to 250 or 270 ohms 10W, otoh that may show you some obvious crossover distortion as you approach the power limit. Also no point in chasing every last watt, these really turn out more like 25W at clip, in spite of the product designation.

[g1]

Agree that 6L6 are best solution here (as with Bogen's that use 8417), but I think he has some 6550's he would like to use.
Individual screen resistors (1K 5W) at each socket would be good to add. You will need to adjust the bias via the cathode resistor and make any adjustments for pin-out differences.
If the power transformer can not support the extra heater current you will need to use an aux. transformer for the heaters.

[Leo_Gnardo]

If the power transformer can not support the extra heater current you will need to use an aux. transformer for the heaters.

Spec sheet shows 1.6A filament current for both 6550 and 8417, so if KRKoz goes 6550 no problem there.

[Jazz P Bass]

[QUOTE=g1;450100]Agree that 6L6 are best solution here (as with Bogen's that use 8417), but I think he has some 6550's he would like to use.
Individual screen resistors (1K 5W) at each socket would be good to add."

The screens are already at a lower potential than the plates via the 2K resistor.
Are we suggesting adding another screen resistor to each tube?
Curious.

[g1]

Yes, seems to be common practice even when screen node already has dropping resistor from plate feed.