Class A KT88 info

Tube Noob · 01:03 AM

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

artosalo · 07:06 AM

Here is quite complete information for KT88 including UL: http://www.mif.pg.gda.pl/homepages/f...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.

Tube Noob · 17th November 2011, 11:57 AM

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.

boywonder · 17th November 2011, 02:24 PM

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

kheper · 17th November 2011, 10:25 PM

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

Tube Noob · 18th November 2011, 12:05 AM

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?

kheper · 18th November 2011, 01:07 AM

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.

Tube Noob · 18th November 2011, 01:13 AM

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.

boywonder · 18th November 2011, 01:20 AM

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.

Tube Noob · 18th November 2011, 01:37 AM

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.

17th November 2011, 01:01 AM	#1
Tube Noob diyAudio Member Join Date: Feb 2008 Location: Cleveland, Ohio	Class A KT88 info 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 Last edited by Tube Noob; 17th November 2011 at 01:03 AM. Reason: content and grammar

17th November 2011, 07:04 AM	#2
artosalo diyAudio Member Join Date: Feb 2010	Here is quite complete information for KT88 including UL: http://www.mif.pg.gda.pl/homepages/f...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. Last edited by artosalo; 17th November 2011 at 07:06 AM.

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17th November 2011, 11:57 AM	#3
Tube Noob diyAudio Member Join Date: Feb 2008 Location: Cleveland, Ohio	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.

17th November 2011, 02:24 PM	#4
boywonder diyAudio Member Join Date: Jan 2008 Location: So.Cal.	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

17th November 2011, 10:25 PM	#5
kheper diyAudio Member Join Date: Jun 2007	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

18th November 2011, 12:05 AM	#6
Tube Noob diyAudio Member Join Date: Feb 2008 Location: Cleveland, Ohio	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?

18th November 2011, 01:07 AM	#7
kheper diyAudio Member Join Date: Jun 2007	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.

18th November 2011, 01:13 AM	#8
Tube Noob diyAudio Member Join Date: Feb 2008 Location: Cleveland, Ohio	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.

18th November 2011, 01:20 AM	#9
boywonder diyAudio Member Join Date: Jan 2008 Location: So.Cal.	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.

18th November 2011, 01:37 AM	#10
Tube Noob diyAudio Member Join Date: Feb 2008 Location: Cleveland, Ohio	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.

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