Output trans impedance for KT88
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funkgang49
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Output trans impedance for KT88
Thinking about building a low wattage (5-10 watts) single-ended amp using a KT88. What is the preferred Output transformer impedance for that power tube?
Re: Output trans impedance for KT88
Zout = Vp/(Pmax/Vp)
Where:
Zout = desired load impedance for centre-bias Class A
Vp = the plate-to-cathode voltage
Pmax = the maximum plate dissipation
(This equation will also be affected by the screen voltage (Vg2) and bias point*, so you want to have some idea about what you are aiming for in that regard)
*In Center-bias, you want the bias point to be such that the plate idle voltage is sitting in the middle of the load line w.r.t. the grid curves. The spacing between the grid curves, and hence the overall ‘height’ of the grid curves, is affected by your chosen Vg2. If the load line is too ‘flat’, you can set the Vg2 lower to avoid overdissipating the screen. (This is why we see screen voltages up 50v lower than plate voltage in some cathode biased output stages from yesteryear).
Or copy a proven design like the AX84 SE power amp that uses a KT88.
Where:
Zout = desired load impedance for centre-bias Class A
Vp = the plate-to-cathode voltage
Pmax = the maximum plate dissipation
(This equation will also be affected by the screen voltage (Vg2) and bias point*, so you want to have some idea about what you are aiming for in that regard)
*In Center-bias, you want the bias point to be such that the plate idle voltage is sitting in the middle of the load line w.r.t. the grid curves. The spacing between the grid curves, and hence the overall ‘height’ of the grid curves, is affected by your chosen Vg2. If the load line is too ‘flat’, you can set the Vg2 lower to avoid overdissipating the screen. (This is why we see screen voltages up 50v lower than plate voltage in some cathode biased output stages from yesteryear).
Or copy a proven design like the AX84 SE power amp that uses a KT88.
He who dies with the most tubes... wins
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Stevem
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Re: Output trans impedance for KT88
If your going to spend the money on a big ass 42 watt KT88 tube then you need to make the amp 15 to 20 wats other wise save some 25 bucks and use a 6L6GC or EL34 for only 8 to 11 watts as it will sound better since your driving the tube enough to get get some of its tonal caractor out of it.
Anyway for a KT88 and 20 watts here's what you need.
OT= 3000 ohms
Plate voltage 400
Screen voltage 225
Bias voltage -16.5
Peak audio drive signal 16.5
Idle current 87 ma
Peak current 105 ma @ max signal drive.
A EL34 will need that same 3K output Transformer and make 8 watts
A 6L6GC will want a 4200K output transformer and make 11 watts.
Note that the voltages needed are different for these two tubes then the KT88/ 6550A voItages I posted.
Anyway for a KT88 and 20 watts here's what you need.
OT= 3000 ohms
Plate voltage 400
Screen voltage 225
Bias voltage -16.5
Peak audio drive signal 16.5
Idle current 87 ma
Peak current 105 ma @ max signal drive.
A EL34 will need that same 3K output Transformer and make 8 watts
A 6L6GC will want a 4200K output transformer and make 11 watts.
Note that the voltages needed are different for these two tubes then the KT88/ 6550A voItages I posted.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Tony Bones
- Posts: 228
- Joined: Thu Oct 30, 2014 8:24 pm
Re: Output trans impedance for KT88
There is no "right" primary impedance. If nothing else, it's going to depend a lot on the operating conditions (i.e. plate and screen voltage.) Even with that determined there's still a lot of room.funkgang49 wrote: ↑Sun Jul 14, 2019 11:45 am Thinking about building a low wattage (5-10 watts) single-ended amp using a KT88. What is the preferred Output transformer impedance for that power tube?
My suggestion is to buy a universal SE output transformer good for at least 15W. These have a bunch of taps that will allow you to experiment after the amp is built. Pick the one(s) you like based entirely on sound. Don't worry, changing the load will not cause the amp to blow up or burst into flames.