KT120

[ernie_jr]

Seeing the post about the new kt120 tube and reading up on it has got me thinking.
What power levels could I get using PP kt 120 with 560 VDC and a 4300 load?
was thinking of using a Sunn or ampeg b25 type preamp and phase inverter
thanks
ernie

[imaradiostar]

How much will that 560 sag under load?

Rough guess with that combo is around 80 watts. It'll drop off quickly if the plate or screen voltages sag. If you're really lucky and the planets align you could get up around 100-120 but I wouldn't bet on it without first breadboarding it to see if it would work.

If the power transformer is stout enough and the output transformer has the available taps you could try the Kt120 at a halved impedance mismatch. It has a huge plate structure so it can probably sink a lot of current with a nice low output impedance. I'd bet you'll get well over 100 watts that way if the power transformer is pretty well regulated. It's tough to say without plotting a load line or simply trying it. Theoretical math says if the tubes can sink the current you'll get power levels approaching 180 watts. Once again, even for a giant super tube, that feels a bit optimistic.

You'll probably need higher voltages for a tube like this to really shine though. I'd shoot for 700 or 800 on the plates and half that on the screens into your 4300 load. That should put you solidly into the 125 to 175 range. You could always get a cheap toroidal PT from Antek Inc and run it as a FWCT bridge setup to get both voltages. They have one that would be ideal with plate voltage around 770 and screens at half that.

I have an amp in the works like this and I plan to use a hiwatt style phase inverter. In my testing the Sunn style phase inverter had a more abrupt transition from usable to unusable when pushed. If I was using it for a bass amp I'd want it to hang in there a little longer at high volumes. I think the Hiwatt has a good rep for that.

jamie

[ernie_jr]

Pt has 200ma, was going to use sand rectors instead of tube on this one. this may be a fun project
thanks,
ernie

[imaradiostar]

200ma at 560 volts sounds like a better fit for 6ca7's or KT88's but probably not the KT120.

jamie

[PRR]

> 560 VDC and a 4300 load?

6550 specs http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf page 3.

600V and 5K gives 100W.

Your 560V is 93% of voltage, so 87% of power.

Your 4K3 is 116% down from 5K, so (if screen allows) 116% of power.

100W * 87%*116% = 100.9W.

Extrapolating up from 450V 4K UL specs gives 100.8W.

Bigger tube does NOT make bigger power.

And since you can buy 2 or at least 1.5 6550 for the price of KT120, it isn't economical.

KT120 makes sense where some over-hot-rodded chassis is eating 6550 for breakfast lunch and dinner, IF the KT120 actually lasts all day. (It might be wiser to figure and fix the toaster's problem.)

KT120 could be used in a *new* design, probably using custom iron, to give maybe 150% the power of 6550/KT88.... but at 1.5-2 times the bottle price, and a single-source tube of yet-unknown long-term availability.

Leave it for the rich boys who MUST have the biggest bottle.

As practical folks, if we need more power we use more standard-size bottles. A six-pack of 6L6/EL34 will make as much power as a couple KT120. Including air-space the 6-pack won't be much bigger than the two huge HOT bottles. If 1 of 6 tubes fails the show goes on; a half-dead 2-tube team gets very ill. And Saturday night in Podunk Valley, you probably can buy a replacement GT 6L6 or EL34, you won't be finding any KT120.

[ernie_jr]

good points prr, and 4 bottles look "bigger" as well
ernie

[Ritchie200]

Ernie,

PRR makes a good point.  I know folks with Majors had been used KT90's until the factory was bombed out, and KT100's and KT120's.  The only reason is that the current lineup of KT88's will not hold up in the Major.  Personally I did not like the KT90 as I thought it sounded like a bad tube......  I have been able to use the OLD KT88 Svetlana's with very good luck.  The Major has 630v on the plates and about 629.999v on the screens and beats the crap out of the tubes.  A KT100 and KT120 will live a long life in there - but that expense would be an investment born over the loss of several KT88's.  Again, a necessity, not necessarily a desired expense!

But hey, if you have some mad $$$ burning a hole - who knows, the 120's might be the next holy grail! Besides, big tubes look cool!  Just make sure they can be seen in your enclosure.  And, you can use them for supplemental heat during the winter....

Jim