Power supply for KT-88s

[greiswig]

Cool! Thanks, Tony. That looks easy enough to make it well worthwhile. Much of the fun of tweaking this amp is just learning what effect things have.

Since in fixed bias mode the amp tends to sound best biased a bit on the cool side (although not as much with KT88s as with 6L6s, IMHO), what plate dissipation would you recommend shooting for in cathode bias mode?

[talbany]

Cool! Thanks, Tony. That looks easy enough to make it well worthwhile. Much of the fun of tweaking this amp is just learning what effect things have.

Since in fixed bias mode the amp tends to sound best biased a bit on the cool side (although not as much with KT88s as with 6L6s, IMHO), what plate dissipation would you recommend shooting for in cathode bias mode?

g- I like to leave me a little cushion so I usually shoot for around 90% max plate dissipation in CB mode (of coarse this will change 88 to 88) also try to use a 150v bypass cap there..so if they red plate it won't explode... 25uf if you want it fat and juicy and 10uf if you want it a bit leaner and tighter..

Good Luck
Tony

[greiswig]

Saint Tony, or anyone else for that matter...

Okay, two more questions:

1. I tried a choke in there last night, not mounted, just wired in. I like the "authority" that it gives to the notes! They bark nicely. Trouble is, this is a D'Lite chassis, and there is very little room between the PT and OT for a choke to fit. It will fit, but it will be in close proximity to both transformers. Is that an issue? (EDIT: See picture...is this an acceptable mounting? Too close? Wrong orientation?)

2. What grid resistors would you recommend for the KT88s? (That's what you call the R on Pin 4, right?)

Thanks!

[glasman]

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 was kinda thinking the same thing. KT88's are rated at 800V on the plates and 600v on the screen. Crank'r til she blows boys... .

I am putting together a 600V supply for my 88 project and I am going to decide the screen voltage after the fact by running the screens on an external regulated supply and let me ears and screen dissipation make the final call.

Gary

[Structo]

George, I put a choke in my D'lite.

I put it where you have yours aligned the same way as the OT.

Right in the middle of the space.

[greiswig]

Thanks, Tom. I'll do it thataway.

Re. Cathode bias: before I jump into those deep, dark waters, do I have it right that at 90% the tubes would be putting out around 30W of heat even qt idle? Seems like a recipe for some serious heat buildup! Mebbe that's why you hear about cathode bias mostly in lower wattage amps?

[talbany]

Saint Tony, or anyone else for that matter...

Okay, two more questions:

1. I tried a choke in there last night, not mounted, just wired in. I like the "authority" that it gives to the notes! They bark nicely. Trouble is, this is a D'Lite chassis, and there is very little room between the PT and OT for a choke to fit. It will fit, but it will be in close proximity to both transformers. Is that an issue? (EDIT: See picture...is this an acceptable mounting? Too close? Wrong orientation?)

2. What grid resistors would you recommend for the KT88s? (That's what you call the R on Pin 4, right?)

Thanks!

g- Like Tom said you can mount that choke 90 degrees to PT and OPT. I think you will be fine.. Noise is not a big issue w/chokes..
Grid Stoppers- Marshall didn't even use grid resistors with their JMP loaded 6550's but if you want to play it safe you can go with something small like 1k or 1k5 should be fine
The problem I see is those 88's are pretty close together..I don't think it would be to much of an issue with just 2 but I certainly wouldn't mount 4 that close together.. Make sure it's got decent ventilation..
In cathode bias the tubes run their hottest at idle and yes the amp will get hotter in cathode setting once again adequate ventilation you should be fine..

FWIW..KT88s were originally rated for ultralinear and triode use, so the screens can stand quite a high voltage.
6550s can't stand as much screen voltage, but they have more "inner mu", so they don't need as much screen voltage to generate the same power as KT88s. This makes them more suited to tetrode operation at high power perhaps you got the 2 confused in your opening post..
Hope this helps

Tony

[greiswig]

So I temporarily wired the amp for cathode bias. I only had a 470ohm 5W power resistor, so I was limited to 24W dissipation as best I can determine. I am using 600 ohm screen resistors, 56k bias feeders, and 3.3k on the control grids. Measured voltage between plate and cathode was 468V.

Some observations:

A. Nice! It's a different flavor, for sure. Cleans in particular take on a nice quality, with a bit of added dimension. OD sounded more raw, but I'm not convinced that's what I'm after there. I kind of like the control that the stock power section puts on the OD. It didn't result in a huge amount more bloom. But I can easily see where this relatively easy modification would make it so you can have your sundae with chocolate sauce or caramel, whatever your preference at the time. But...

2. Man, that's hot! Even underbiased and without a front to the head cab, I don't think this chassis could handle the KT-88s in cathode bias mode for long without a fan. Break out the Ove-Glove.

III. Man, that's hot! A 5W resistor is NOT enough. Shoulda guessed that.

[talbany]

So I temporarily wired the amp for cathode bias. I only had a 470ohm 5W power resistor, so I was limited to 24W dissipation as best I can determine. I am using 600 ohm screen resistors, 56k bias feeders, and 3.3k on the control grids. Measured voltage between plate and cathode was 468V.

Some observations:

A. Nice! It's a different flavor, for sure. Cleans in particular take on a nice quality, with a bit of added dimension. OD sounded more raw, but I'm not convinced that's what I'm after there. I kind of like the control that the stock power section puts on the OD. It didn't result in a huge amount more bloom. But I can easily see where this relatively easy modification would make it so you can have your sundae with chocolate sauce or caramel, whatever your preference at the time. But...

2. Man, that's hot! Even underbiased and without a front to the head cab, I don't think this chassis could handle the KT-88s in cathode bias mode for long without a fan. Break out the Ove-Glove.

III. Man, that's hot! A 5W resistor is NOT enough. Shoulda guessed that.

Svetlana shows a 370 ohm @560v and a 25w cathode resistor.. Biased pretty cold like you have it they will throw some grit on you when you crank it.. Where the cathode bias really shines is at lower volumes or doing really small clubs sounds bigger at lower volumes.. If you can get it into a situation where you can crank it fixed bias is always better IMO.. In a Dinner club situation where the 1st set is always mellow and volume is a concern I like the CB setting.. Later on after the place livens up and the band gets louder I always go to fixed bias..

I played a top mount AC 30 combo for years and the thing ran so hot I couldn't sit on it.. Never once crapped out.. A fan is always good..

Good Luck!!
Tony

[greiswig]

Thanks, Tony! You're a saint in my book.