Power supply for KT-88s

[greiswig]

I'm puzzled by something: in most KT-88 designs, you hear that the screen voltage should be considerably lower than the plate voltage. So if you're running 500-600V on the plates, you want 350V on the screens.

How do you achieve that? With 6L6s and a 470 ohm screen resistor, there is little voltage difference between screen and plate. And it seems like you'd need a large value resistor between the first two nodes of the dropping string to get 350V at pin 4, or large value resistors between Pin 4 and that node.

[ampdork]

Use a totem pole for the plates, then take the screens from the node between the totem.
Check the MM stuff.

[Cliff Schecht]

I've done voltage dividers after the plate voltage to drop them down to about 350V from 530V. IIRC it was 22k series and 68k to ground, both 10W resistors, but the amp was only a SE KT88.

[greiswig]

Use a totem pole for the plates, then take the screens from the node between the totem.
Check the MM stuff.

Thanks, AD.

If I understand the totem pole concept, you're taking the screen feed from between the two series caps at the start of the PS? If so, that would always be half of the plate voltage, right?

Unless you use more series caps and resistors, I suppose...then you could get different multiples of the plate voltage.

Is the goal to get half of the plate voltage, or to get 350V on the screen?

[fperron_kt88]

Don't know if you want to consider this, but some amps simply run them ultra-linear with 40% taps on the OT primaries for the screens. As in a Marshall Major, for example.

More expensive OT and probably an amp that you would like for cleaner sounds, but I thought I'd mention it as another possible topology...

[talbany]

g- KT-88's sound sweet run around 500v and a bit less on the screens so why would you want to run the screens at half?..
Music Man Dumbles tap the series plate caps to get the 350 but they run @ 700v

Tony

[greiswig]

g- KT-88's sound sweet run around 500v and a bit less on the screens so why would you want to run the screens at half?..
Music Man Dumbles tap the series plate caps to get the 350 but they run @ 700v

Tony

I don't know why! I'm trying to understand what's best by design, but I have differing points of view on what "best" is. So what resistors would you put on pin4 if the amp has the usual 500ohm resistor between B1 and B2?

[Chris333]

Dunno about D-style builds, but my old Ampeg V-2 has 590v on plates and 585v on the screens and sounds nice w/88s...

[talbany]

g- KT-88's sound sweet run around 500v and a bit less on the screens so why would you want to run the screens at half?..
Music Man Dumbles tap the series plate caps to get the 350 but they run @ 700v

Tony

I don't know why! I'm trying to understand what's best by design, but I have differing points of view on what "best" is. So what resistors would you put on pin4 if the amp has the usual 500ohm resistor between B1 and B2?

g- 1K 5W should do nicely.. I assume you are subbing the 500 ohm resistor for the choke but you know what I am going to say (use a choke)..
Also 6550's and KT-88's like a bit faster bias recovery supply so I would use either 100k/150k for the bias feeders instead of the 220k's..
Also a suggestion if you want to get creative possibly do a fixed bias cathode bias switch as a pair of 88's run cathode bias @ 500v is really cool..Bloom

Hope this Helps!

Tony

[ToneMerc]

Tony, for this latest build I'm going to run KT88's as well. My PT should hit the plates at around 485-490V. I had planned on running 1.5K screens and the bias feeders are of course 100K.

Mike

[talbany]

Tony, for this latest build I'm going to run KT88's as well. My PT should hit the plates at around 485-490V. I had planned on running 1.5K screens and the bias feeders are of course 100K.

Mike

Mike
You doing a 3rd Gen loaded 88's.. OOHH BOING!!
Should be sweet.. Keep us posted..

Tony

[greiswig]

Also 6550's and KT-88's like a bit faster bias recovery supply so I would use either 100k/150k for the bias feeders instead of the 220k's..

Someone awhile back had actually suggested even lower...like 56k?

Also a suggestion if you want to get creative possibly do a fixed bias cathode bias switch as a pair of 88's run cathode bias @ 500v is really cool..Bloom

Hope this Helps!

Tony

Hmmm...interesting! Got a schematic you can point me to?

Thanks, Tony!

[ToneMerc]

Tony, for this latest build I'm going to run KT88's as well. My PT should hit the plates at around 485-490V. I had planned on running 1.5K screens and the bias feeders are of course 100K.

Mike

Mike
You doing a 3rd Gen loaded 88's.. OOHH BOING!!
Should be sweet.. Keep us posted..

Tony

It should be way huge, the OT is a Hammond 1650TA. I was going to have Heyboer wind me a double stacked Twin tranny, but I ran up on this at a price I couldn't refuse.

[67plexi]

Thank you tony on the 100k/150k bias feeder information.

My latest has 490 volts on the plates I may have to go for the KT88’s as well.


Steve.

[talbany]

Also 6550's and KT-88's like a bit faster bias recovery supply so I would use either 100k/150k for the bias feeders instead of the 220k's..

Someone awhile back had actually suggested even lower...like 56k?

Also a suggestion if you want to get creative possibly do a fixed bias cathode bias switch as a pair of 88's run cathode bias @ 500v is really cool..Bloom

Hope this Helps!

Tony

Hmmm...interesting! Got a schematic you can point me to?

Thanks, Tony!

Here is one w/bias probe jacks..
In order to find the right value cathode resistor after you get the amp set up you can start with roughly 300 ohm resistor then run it through this calculator to find the right value for that amps voltages and 88's etc etc..Let me know if you need help!!

http://www.webervst.com/tubes1/calcbias.htm

Tony