As I mentioned in another thread, I am *attempting* to design my first output stage, a KT88 p-p ultralinear with fixed bias. Looking at the TDSL datasheets for the KT88, it would appear as though some information is missing? Only certain B+ and Vg1 rows are filled in.
1) Is there a reason that there is no Class A U/L information given?
2) For the different Class AB1 suggestions, am I correct to assume that the ones with less anode voltage (and less THD) are running closer to Class A?
3) Is the Zout that is listed the same as plate to plate load resistance? (like what I would want in the primary of the OPT?)
I have not decided yet if this will be a class A or AB1 amp yet, or how much feedback will be used in the design, but any input would be greatly appreciated.
Thank you
1) Is there a reason that there is no Class A U/L information given?
2) For the different Class AB1 suggestions, am I correct to assume that the ones with less anode voltage (and less THD) are running closer to Class A?
3) Is the Zout that is listed the same as plate to plate load resistance? (like what I would want in the primary of the OPT?)
I have not decided yet if this will be a class A or AB1 amp yet, or how much feedback will be used in the design, but any input would be greatly appreciated.
Thank you
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Here is quite complete information for KT88 including UL: http://www.mif.pg.gda.pl/homepages/frank/sheets/084/k/KT88.pdf
AB1 class amplifier is operating in class A when the used power level is less than maximum.
The transition between A and AB1 takes place depending on the used power level, bias, load impedance etc, but it is not sudden. You can assume that a 50 W KT88 AB1 amplifier works in pure class A when the power level is below 25 W.
Zout is not same as plate to plate load resistance.
Zout is the measured, real output impedance the amplifier has. It can be lowered with the use of negative feedback, which is typical.
Plate to plate load resistance is the optimum load that gives optimum distortion/ouput power combination.
AB1 class amplifier is operating in class A when the used power level is less than maximum.
The transition between A and AB1 takes place depending on the used power level, bias, load impedance etc, but it is not sudden. You can assume that a 50 W KT88 AB1 amplifier works in pure class A when the power level is below 25 W.
Zout is not same as plate to plate load resistance.
Zout is the measured, real output impedance the amplifier has. It can be lowered with the use of negative feedback, which is typical.
Plate to plate load resistance is the optimum load that gives optimum distortion/ouput power combination.
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Thank you for the response!
How could I calculate the power output of this stage at a given anode voltage into a 5k primary? I gather I would also have to take the bias into account as well.
How could I calculate the power output of this stage at a given anode voltage into a 5k primary? I gather I would also have to take the bias into account as well.
It has to do with what load line that you design for. Generally, less anode voltage, higher load and more current gives a greater percentage of operation in class A with less max output power. This is really easy to see by playing with John Broskies PP tube calculator software. The software has KT88's in the library, although it models everything as triodes.
Tube CAD Journal Companion Software
Another excellent resource is here:
loadmatch4-pp-beamtetrodes
Tube CAD Journal Companion Software
Another excellent resource is here:
loadmatch4-pp-beamtetrodes
I suspect the reason that PP Class A specs cannot be found in the manufacturers KT88 manuals is because it newer, than other tetrodes and pentodes. Old datasheets did contain specs for PP Class A pentodes and tetrodes, but they were not in UL mode. You could use the examples of PP Class A specs for the 6550, below, to create a KT88 amp, but again no UL mode.
NJ7P Tube Database Search
NJ7P Tube Database Search
Thanks boywonder and kheper,
I have purchased and downloaded John Broskie's PP calc, and will begin playing with it immediately.
How close will triode mode compare to UL linear mode? ...or is that a bad question to ask?
I have purchased and downloaded John Broskie's PP calc, and will begin playing with it immediately.
How close will triode mode compare to UL linear mode? ...or is that a bad question to ask?
PP triodes will produce around 1/2 the audio output of UL tetrodes - at the same voltage. Many (most?) people prefer the sound of triodes over tetrodes. Two KT88s strapped as triodes will produce 22-27W of audio power, enough to drive virtually any speaker. You can put in a switch to select between UL and PP triode mode, easily, then decide for yourself what you like.
The switch is a good idea. I will be doing a lot of experimenting when I build this circuit. I am a hands on kind of person, and I have been reading quite a bit, but I don't think I'll really"get it" until I have hooked some stuff up, made some measurements and some changes.
A couple of things....only switch modes when the amp is powered off..(keeps the smoke in), and the drive current requirements will be higher for triode connection.
Thanks, something else I have been trying to figure out is how to determine required drive voltage and current for this stage. I plan to drive the stage with lme family op amps (don't judge!) which I know are capable of pulling it off, but I am not sure yet how to design the stages.
I suppose that is my biggest concern, I have found some circuits that I could build, but I would really like to understand exactly why and how all of design choices are made. Which is a little more daunting than I anticipated.
This page "loadmatch4-pp-beam tetrodes" you suggested above has proven to be one of the most informative resources so far.
I suppose that is my biggest concern, I have found some circuits that I could build, but I would really like to understand exactly why and how all of design choices are made. Which is a little more daunting than I anticipated.
This page "loadmatch4-pp-beam tetrodes" you suggested above has proven to be one of the most informative resources so far.
Bruce Heran has a wonderful ClassA design with KT88 over on diyaudioprojects.com
I have had his amps in my house (he loaned to me so I could take them to a show for him) and I would love to own them. He is super friendly and helps everyone understand the schematic and the build, plus the project is already full described on the site.
Uriah
I have had his amps in my house (he loaned to me so I could take them to a show for him) and I would love to own them. He is super friendly and helps everyone understand the schematic and the build, plus the project is already full described on the site.
Uriah
The difference in output power is some 50 % as already said.
The difference of the open loop gain between triode mode and UL is quite noticeable. Therefore the sensitivity of the amplifier varies when switching between triode/UL. Also, if the amplifier is optimized with certain negative feedback, then the operating condition and the amount of NFB changes between the modes. The best result is found by selecting one of the modes only and to optimize the amplifier for that.
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