KT88 construction problem

[A.N. Beal]

Dear Friends,

I have just completed wiring up a new KT88 XL amplifier and have a problem. I have set it up with dummy load resistors across the speaker terminals as directed and checked voltages. The left channel measures and behaves OK.

However on the right channel a gentle buzz develops after a minute or so and the dummy load resistors overheat. (I switch off when this happens.)
I have checked voltages and all are correct except for U2 pins 3 and 6, which measure 88-90v (left channel 84v) and U3 and U4 pins 8 which measure 48v (left channel 40v).

I have checked my wiring and resoldered a couple of joints but the problem remains. Any suggestions what the problem might be and what to check?

[Richard]

Hello

Heating of the load resistor shows an AC current is going through it. This may be from an external signal that is being amplified (RF) or an oscillation within the amp circuit itself.

1) Short the inputs with a shorted RCA plug or by attaching a CD player etc. I have had this myself with a Kit88 and yes the resistor gets hot by itself as you find.

2) It may be a bad valve causing oscillation. Swap U2 across channels and see if the problem stays or moves with the valve to the other channel. Swap U3 and U4, for a similar check, and then U1 also.

3) It could be oscillation within the amp itself. Check carefully that the output transformer wires are connected correctly and the feedback wire and components are correctly connected.

[A.N. Beal]

Dear Richard,
Thanks for your helpful reply.
1. I tried shorting the inputs - problem didn't change.
2. I tried swapping valves U2 and then swapping valves U3 and U4 but it was still the left channel load resistor that started to smoke. There is only one valve U1, so I couldn't swap this.
3. I checked continuity of all the output transformer and feedback wiring against the instructions using a multimeter and it all checked OK.
So no joy so far. Any further suggestions?
Regards,
Alasdair

[A.N. Beal]

Extra information: I checked it with the volume control turned down to zero, which effectively short circuits the inputs. The left channel load resistor still overheated. A little before the load resistor starts smoking, as well as gently buzzing the amplifier also starts to emit a tone (I estimate the frequency as in the region of 500Hz).
Any help in tracking down the cause and solution would be welcome.
Regards,
Alasdair

[A.N. Beal]

I have just measured the AC voltages across the dummy load terminals: a fraction of a volt on the right channel, 7 volts on the left channel.

[John Caswell]

Hi,
With the amplifier turned off and the KT88s removed measure from Pin 5 of each KT88 to earth. Should read 220K or thereabouts. If not you have a wiring fault. Correct this and try again.
If you still have a problem disconnect the feedback link to C10/R18 on the faulty channel, if the problem goes away then the phase of the feedback is wrong and can be corrected in one of several ways. IF you are sure that you have wired the secondary of the output tx correctly then it is possible that you have the primary wired incorrectly or the two anodes feeds of the 5687 driver "**** about face" In which case swap one or the other reconnect feedback and try again. I would choose the 5687 anode feeds as the simplest method
The variations on U2 I suggest are within limits of ±10%.
Let us know how you get on.
Have you spoken to Matthew about this? as I am involved indirectly

John