UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Quad II classic - help with tips, valve rolling and what to look for?

PJL · 20th Aug 2017, 6:06 pm

The amplifiers were designed around the MOV KT66 and changing types and even manufacturers can cause damage to the amplifier as it will change the quiescent current.

The Quad II also uses a common cathode bias resistor, R12 (180 ohm), which requires the output valves to be fairly well matched. With this arrangement, if one valve takes less current the other takes more.

It looks like the output valves are new but it is still worth checking the voltage on R12 which should be about 26V. If it is significantly more than this report back as it may be worth changing the value.

20th Aug 2017, 6:06 pm	#22
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Quad II classic - help with tips, valve rolling and what to look for? The amplifiers were designed around the MOV KT66 and changing types and even manufacturers can cause damage to the amplifier as it will change the quiescent current. The Quad II also uses a common cathode bias resistor, R12 (180 ohm), which requires the output valves to be fairly well matched. With this arrangement, if one valve takes less current the other takes more. It looks like the output valves are new but it is still worth checking the voltage on R12 which should be about 26V. If it is significantly more than this report back as it may be worth changing the value.