Lowering feedback in KiT88 and KeL34

[FAQ Team]

The circuit design of KiT/KaT88 & KeL34 are very similar. The feedback circuit is basically the same.

The 6AU6 cathode resistor and the amps feedback resistor are the same IE: R38 & R39 have a dual function of setting overall loop feedback and setting the operation condition for the input 6AU6.

If we are attempting to lower feedback by changing the value of R38 & R39 then we will change the operating conditions for the 6AU6. So to ensure that the operating conditions of the 6AU6 remain the same and additional resistor needs to be added from Pin7 (cathode) of the 6AU6 to ground.

In other words, in order change the amount of feedback, the resistance between Pin 7 of the 6AU6 to the Red speaker post needs to be changed but Pin 7 of the 6AU6 to ground resistance must stay the same at 330R.

To reduce the feedback by approx. 50% R38 and R39 can be substituted with a resistor value of 680R, then to keep the operation conditions of the 6AU6 the same a parallel resistor value of 680R can be taken from the cathode end of R38 & 39 to earth. Paralleling two resistors like this will result in a cathode resistance of 340R which is close enough to the original 330R value as not to worry about (3% larger).

In addition to this we also need to change the HF compensating capacitor C17 & C18. Their function is critical in controlling HF feedback, by calculation and test their value needs to be changed to 2.2nF for the above reduction in feedback.

To summarize:

Reducing the overall feedback by approx. 50% in KiT/KaT88 & KeL34 can be achieved by changing R38/39 to 680R 1/2W and changing C19/20 to 3nF with UO82 Sowter OPTs or 2n2 with std OPTs. To keep the operating conditions of the input 6AU6 the same an additional resistor value of 680R needs to be added from the cathode end of R38 & 39 to ground.

Standard setting with 330R at R38/39 and 3n9 cap Kit88/KEL34 (5n6 Sowters) gives 22dB fb
680R at R38/39 with 680R cathode to ground and 2n2 cap Kit88/KEL34 (2n7 Sowters) gives 17dB fb
1k2 at R38/39 with 470R cathode to ground and approx 1n5 Kit88/KEL34 (1n8 Sowters) gives 13dB fb
Connecting feedback leads straight to black post gives zero fb
How do we do this?

Substitute R38 & R39 for 680R ? watt resistors and substitute C19 & C20 for 2.2nF capacitors. Then on the valve base side of the PCB board fit another of the same 680R resistors between the ends of R38/39 nearest the 6AU6 bases and the Earth solder pads of C17/18.





PCB track side view showing location of additional 680R.

The results.

The sensitivity of the amp will have increased slightly from 400mV for KiT/KaT88 to approx. 220mV. Bass response should be better and there should be more of a ‘fluidity’ to the sound with greater ‘air’ around instruments and vocals. 2nd harmonic distortion will rise slightly but this is benign, frequency response will remain the same and power will be unaffected.

Measuring the level of feedback.

To find the level of feedback measure the input sensitivity of the amplifier with no feedback. Then find
it with feedback. The difference, expressed in dB, is the level of feedback. Using WAD`s 300B PSE as an example it has 380mV sensitivity without feedback and 1000mV with, so the level of feedback is determined by:


log(1000mV divided by 380mV) x 20 = 8.4dB