Talking From The Rough Sound Of KT88

The most common use of output power tubes in tube amplifier making are triode tube 300B, pentode tube EL34, and beem tube KT88. They are famous generations and have own characteristics. But if the design is not properly, the advantages will gone, and someone may be very disappointed with them.

The famous beem tube KT88 was first developed by UK GEC in 1957. It almost has the best specifications in audio industry then. It has high transconductance, low inner resistance, easy to drive, and low distortion. But many of the commodities in market are not satisfactory. The main problem is that these commodities¡¯ sound is rough, dim, and less vivid sense. So many audiophiles would think that is KT88¡¯s sound and negate it thoroughly. But, in fact, if the design is suitable, the KT88¡¯s sound will be smooth, wide, full, and dynamic. It¡¯s almost a overall performing tube.
Though it is good, many of using KT88 commodities are not successful at present. It si said that it¡¯s easy to speak out loudly, but hard to speak out gracefully. Is it true? Of course not. But it is still a difficult work to design a good work. Some problems are negative in amplifier design. Let¡¯s analyse them one by one. I hope it¡¯ll be helpful for you to design a good work.

It¡¯s important to stablize the voltage on screen grid when using beem tube. If the voltage could be stable on screen grid, the will be reduced. The stability of the amplifier would strengthened and negative feedback could be led more deeper. When KT88 is working on more powerful as class AB1 amplifying, the voltage on anode is normally at 400v to 600v. The voltage on screen grid shouldn¡¯t be over 300v. So many design uses resistances series connection to power supply. In view of screen grid eletricity will be changing greatly on working, for example, the output power is 55w, the anode voltage is 400v, screen grid voltage is 275v, the screen grid current intensity will be working on 18~88mA with singal change, if using resistances series connection, it will cause the work of screen grid voltage rising and falling greatly, the will be enlarged, dynamic will be compressed, the obviouse consequence is causing the sound roughly and less vividly. This the very problem ignored by many people. I would advise using isolation power supply to screen grid to dismiss the influence.
There is another problem would be ignored by using KT88. It¡¯s the grid resistance.
For tube amplifier, because the heating temperature will cause left gas be ionised or grid is heating to launch electron to cause reverse grid current, this current will cause bias reducing at grid resistance. If the grid resistance value is too higher, the situation will be more serious. At last, it will enlarge anode current unnormally, and the tube is working unstable, balance is damaged. Even overpower cause red tube to shorten its life. This situation is more serious for tubes like KT88 whose output power is large and working temperature is also higher. This problem is not importantly attention widely in design, even some foreign famous brand designers would make this low class mistaken. For example, when using stablization bias, some adopt 100k¦¸ grid resistance, so as the problem came out, and no correct solution. When using stablization bias, we must strictly follow the rules of grid resistance value < 47k¦¸, when using cathode self bias, grid resistance value < 270k¦¸.

The load impedance value of power tube as it¡¯s working will affect the output power and nonlinear distortion. The best load impedance value depends on its static characteristic curve. It¡¯s about 0.25~0.1 times of anode inner resistance. If the load impedance value is larger, though the second harmonic will be reduced, obviously the third harmonic affect tune greatly. It¡¯s very important to control the third harmonic to cause distortion. So when the beem tube is used on pull-push amplifying, its load impedance value shoud be less on single-ended amplifying. Though it¡¯ll enlarge the second harmonic, the pull-push circuit will dimiss it, and the third harmonic will reduce, the total harmonic distortion will be reduced greatly. Normally, when the KT88 is working on pull-push amplifying, it anode load impedance value is 4.5k¦¸.
The anode voltage of KT88 should not be lower to get better performance. When the anode voltage is more over 450v, the sound would be more transparent and active. Otherwise, the sound will be more dimmer.
When KT88 is working, its heating is larger. For lengthening its using life, we should make sure it has a better condition of air ventilation. Also its filament voltage should be strictly controled at 6.3v¡À5%(5.9~6.6v). if the filament voltage is over ¡À10%(5.7~6.9), its using life will be shortened greatly. Some area that AC power supply is unstable or over 220v to ¡À10%, this problem should be more attention importantly.

The KT88 can be replaced directly by 6550, 6550A, KT90, and kt100, etc. 6550 was developed by American Tung-Sol in 1955. 6550A was improved from 6550 by GEC in 1970s, its biggest specifications is little higher than the original model. KT90 was developed by Yugoslavia Ei, its power dissipation is little over than KT88. KT100 was developed by German Siemens, its power dissipation is little over KT90. The tone colour of KT88 and 6550 is different. 6550 is softer than KT88. Now the best KT88 which is still be working is the Russian Svetlana.
The successful design of KT88 amplifier has powerful dynamic, smooth, soft and wide sound field, these character. When we use it to listen to music, we would be attracted easily and be exciting strongly.

Extracted from¡¶HIGH FIDELITY¡·2009-4 Author£ºTang Daoji