Proper Transformers for Tung-Sol KT66 push-pull

[markmalin]

Hi, All


I'm building an amp using up some spare parts and have a few specific questions regarding a matched pair of Tung-Sol KT-66 power tubes I have on hand.  These are discontinued.  In doing some more reading, TheTubeStore had the following article:


"Tungsol KT66 (discontinued) - Not your average KT66 tube. In fact it’s more like an extremely rugged 6L6GC in a KT88 bottle. Heater current is the same as a 6L6 and the biasing is more like a 6L6. You might want them just for the look but the tone is all there. Big and open with very solid lows and smooth treble response. The mids are well balanced and don’t go to the extremes of sounding scooped or nasal. Like all the Tung Sol branded tubes, the build quality is top notch. The over sized base has the metal band you usually find on a KT88 tube, and the large bottle helps dissipate heat if you like to run your tubes hot or if you have cathode biasing in your amp. If your amp has “bear trap” tube retainers they will have to go because of the size of the tube's base but also the bear trap clamp will ground the metal base to the chassis and cause a malfunction. If your amp requires the tubes to pass through a hole in the chassis for socket insertion you’re also out of luck. Home audio is where I would use these bottles since they are really functional art objects. Bottles like this should be on display. If you have a Marshal head I think you might find them to be an alternative worth exploring. They will give you great tone but since they draw similar current as a 6L6 you will draw 1 amp less heater current in a 50 watt head and 2 amps less in a 100 watt head. Your power transformer will run cooler and you will get closer to the 6550 tone that many Marshall enthusiasts love."

Because these have a lower heater current draw than normal KT-66's (similar to 6L6's), I'm going to use a Hammond 290DX power transformer, which shows a 4A heater current rating.  I'll have 5 preamp tubes (300ma/each) and 2 of these KT-66's (900ma/each) for a total of 3.3A.  Let me know if my logic is correct here.   The secondary for the plate voltage is listed as 325V on this transformer (650VCT)

My biggest question is, what to use for an Output Transformer?   What should I spec for the primary impedance for the output transformer?  And do I treat these Tong-Sol KT-66's as 6L6's with respect to impedance, or KT-66's?

Here's something I found on TAG as a "rule of thumb" for example:

"pr 6V6--8Kqd 6V6--4Kpr 6L6--4Kqd 6L6--2Kpr EL84--8Kqd EL84--4Kpr EL34--3.5Kqd EL34--1.75K"



I appreciate your patience with me.  Again, the primary concern is with respect to impedance, do I assume these Tong-Sol's are just like 6L6's (per the article above), or do I assume their impedance is like a normal KT-66.  Help me understand how to determine this (or point me to a thread).HumblyMark

[pdf64]

Have a read of this thread, particularly Hotblueplates' posts https://el34world.com/Forum/index.php?topic=33118.0

Why build an amp around an obsolete valve type?

[markmalin]

Have a read of this thread, particularly Hotblueplates' posts https://el34world.com/Forum/index.php?topic=33118.0

Why build an amp around an obsolete valve type?

Thanks, I'll check this out.
Mark

[tubeswell]

Impedance (Z) is the (AC) load that your tubes need to ‘see’ to make optimum output power. The change in current and voltage makes power that is reliant on a load to work into.

Like harnessing a horse to a plow. Too-big-a-horse and too-light-a-plow, is not an optimal combination for all the feed, vet and farrier bills that go into maintaining the big horse. Might as well use a smaller beast. Other the big horse might get carried away and wreck the plow.

And so impedance is a function of voltage (V) and power (P) arranged under ohms law where:

For voltage V = I x R
and
For power W = V x I


Or - solving for ‘R’ using V and ‘W’, the ‘ideal’ Z is:


Z = V/(P/V)


It doesn’t matter what brand or model of tube you use. It’s always a function of plate dissipation and plate voltage.


But it’s not a linear relationship. For a reactive load, the optimal Z also changes with frequency/inductance/resonance.


So with all the frequencies found on a fretboard, the amp (including speaker) output will invariably peak more on some combinations of notes compared to others.