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FAQ - Common Problems and Known Issues

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Old 6th January 2006, 09:21 PM
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Default WAD KEL34 KIT88 KEL84 KEL80 Known Problems

KEL34 and KIT88

These 2 amps use very similar circuits. Both offer a high quality sound and have proven to be very reliable. KEL34 was introduced in 1998 and KIT88 in 2000.

Time has shown issues 1,2,3 with KIT88 and issue 3 with KEL34.

1) Red heat spots on KT88 plates

KT88 plate dissipation was originally set a little high at around 40W. This should be OK according to the data sheets but in some high mains voltage areas some valves will show red spots on the anode (plate). 2 fixes are possible; from the published circuit, change the 150R cathode resistors to 220R which increases the amount of cathode bias by 2v or, as WAD did later, change R3 to 18k from 13k to increase grid bias about -2v. Either way works, dropping plate dissipation to around 32W and stopping the plates glowing.

2) Hum

KIT88 was unstable into some cable and speaker loads. This may show as stable into a resistor load on the bench but hum when connected to a cable and speaker load. Larger grid stopper values of 15k for 6AU6 and 4k7 for KT88 settle this down from the 6k8 and 1k ones shown in the original circuit. No one has reported problems using these and some users can get away with less (or the original ones) if they wish to try.

3) Low-level rustling sound leading to distortion later is sometimes due to carbon resistors breaking down

KEL34 and KIT88

The ECC82 anode resistors dissipate approx .7W. They are 1W carbon film which should be OK on paper but they may break down after a couple of years. This causes them to read high when hot and shows as a low level rustling sound. (They are particularly tricky to pin down in the early stages of failure as they sound like a dirty contact somewhere.) Simply changing them to 2W values fixes this. The original ohmic values can be used to construct them or it may be more convenient to buy just 2 ohmic values as follows, R7/20/24/19 anode R`s to 2W carbon film, 47k`s are RS 131 671 or similar and 36k`s can be made up from 47k again, paralleled with 150k RS 135 443.


It is worth checking or replacing the following in this amp with larger wattage resistors. As standard they are carbon film resistors. They are correctly rated but due to the enclosed conditions may start to break down after many hours.
R10/11 100K 1W - continually dissipate nearly .5W - change to 2W
R12/13/14/15 27K .5W - continually dissipate nearly .24W - change to 1W


Check similar resistors on the anodes of the first 2 valves.
Supersonic oscillation often causing the output valve anodes to glow dull red. This appears to be a slight change in output transformer characteristics due to age and can be eliminated by fitting a Zobel network consisting a 0.1µF 250V capacitor in series with a 10R 1 Watt resistor between the positive to negative loudspeaker terminals on each channel.

Last edited by John Caswell; 1st February 2008 at 06:01 PM. Reason: extra info KEL 80

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