I purchased 2x Quad II mono-blocks with a Quad 202 pre-amp/controller a while back and they were dropped off yesterday by Karel's partner Louis . :clap:
http://www.keith-snook.info/quad-ii-valve-power-amplifier.html
They arrived with one block having 2x GEC KT66's , 2x dissimilar EF86's and a Zaerix GZ32 ; the other with 2x Sovtek KT66's , 2x dissimilar EF86's and a Sovtek 5AR4 - I knew this was the case when buying the amps . Karel had tested the amps , and found that one channel faded after a while of playing - he guessed that this was probably the fault of the pre-amp . Not wanting to use the pre , I immediately proceeded to install RCA plugs . Long story short - the GEC KT66's were spent ( methodically swopping valves between the blocks to find the culprits ) .
Not having a spare set of KT66's , I decided to try a set of 6P3S's . :sweat: In about 45 minutes ( yes , I watched them nervously all the time ) red spots started forming on the plates , so I switched off the amps to install a cathode bias resistor mod I had read about on the 'net - this entails changing the cathode bias resistor from 180e 3watt to 220e 3watt . Not having a 3watt 220e resistor , I decided to parallel 4x 1 watt 1k resistors , so-as to end up with a 250e 4 watt resistor . I started the amps up upside down to measure , and found that one block had 380V on the anodes and the other 360V . I then installed 2x 5C3S rectifiers ( approxiamate Russian equivalent of GZ34 ) . Thankfully voltages now settled down to 340V ( Quad II spec ) . The amps sound very good , although they are now biased to Class A/B instead of class A . Noise floor is better than before the change - this probably due to the spent valves .
Here are some calculations derived from my measurements , followed by Quad II schematic re-drawn by Kieth Snook .
http://www.keith-snook.info/quad-ii-valve-power-amplifier.html
They arrived with one block having 2x GEC KT66's , 2x dissimilar EF86's and a Zaerix GZ32 ; the other with 2x Sovtek KT66's , 2x dissimilar EF86's and a Sovtek 5AR4 - I knew this was the case when buying the amps . Karel had tested the amps , and found that one channel faded after a while of playing - he guessed that this was probably the fault of the pre-amp . Not wanting to use the pre , I immediately proceeded to install RCA plugs . Long story short - the GEC KT66's were spent ( methodically swopping valves between the blocks to find the culprits ) .
Not having a spare set of KT66's , I decided to try a set of 6P3S's . :sweat: In about 45 minutes ( yes , I watched them nervously all the time ) red spots started forming on the plates , so I switched off the amps to install a cathode bias resistor mod I had read about on the 'net - this entails changing the cathode bias resistor from 180e 3watt to 220e 3watt . Not having a 3watt 220e resistor , I decided to parallel 4x 1 watt 1k resistors , so-as to end up with a 250e 4 watt resistor . I started the amps up upside down to measure , and found that one block had 380V on the anodes and the other 360V . I then installed 2x 5C3S rectifiers ( approxiamate Russian equivalent of GZ34 ) . Thankfully voltages now settled down to 340V ( Quad II spec ) . The amps sound very good , although they are now biased to Class A/B instead of class A . Noise floor is better than before the change - this probably due to the spent valves .
Here are some calculations derived from my measurements , followed by Quad II schematic re-drawn by Kieth Snook .
