njcanuck
aka: MacGyver
Just spent a little time doing a basic restore of my Scott 299 (2nd version). As it was only through the generous help offered in previous AK threads that I was able to accomplish this, I am documenting my restore so that it may help others. This is my first one!
I acquired my 299 in very nice cosmetic shape but non-functional. The original Scott 7189 power tubes were shot and when new tubes were substituted, one would immediately red-plate causing me to suspect bad coupling caps.
Besides basic recapping, I decided to install bias sensing resistors, test jacks and individual channel adjustment pots. Additionally, to tighten bass response, I raised the values of the coupling caps by 50% as well as the main power filters to be able to take advantage of this mod (most of this info was derived from an old NOS Valves posts regarding a 299B – thanks Craig!). I chose Sonicaps Gen II for coupling caps – great reviews and reasonable price.
Rather than change everything at once, I proceeded in sections and tested the amp after each section. The first order of business was to get it working so that I could test after each change. I used a Fluke 111 meter for all measurements.
Here is the process I followed in order:
• Power tube coupling caps - replaced Pyramid .1uF/400V with Sonicap .15uF/600V. Plugged in 6P14P’s and warmed it up on the variac. Red-plating gone! Whoops, not so fast – it’s back. Old caps tested within spec so it wasn’t that. After much frigging around (and some tube wiggling) I discovered a bad pin 2 socket that wasn’t making proper contact. I swapped it with an unused socket (they come out from the top) and it really was fixed!
• Selenium rectifier – replaced with a RadioShack bridge 400V 8A and changed the first 10Ω dropping resistor to a 22Ω 5W. At 117 line voltage, this gives me -45.5V at the first 12AX7 or approx -11.4V for each tube.
• Test jacks and Bias pots – added a terminal strip and lifted all pin 3 grounds to the strip and then ran matched 10Ω 1/4W resistors from the strip to pin 3’s. The tip jacks were installed on the top of the rear panel and wired to the pin 3’s. The ground jack is wired to the ground terminal strip. Two additional 10KΩ linear bias pots (to control bias individually to each channel) were installed next to the DC balance pots. The circuit mimics the 233 schematic but takes its voltage supply from the original bias pot and uses two 15KΩ resistors for the ground reference.
• Power supply caps – I restuffed the original cans to retain the original appearance. The two 30uF/20uF/475V cans were upped to 47uF/33uF/450V Nichicons (the old caps all tested slightly higher than spec). The two 75uF/75uF/10uF/75V cans were stuffed with 75uF/80V Nichicons and 10uF/150V Sprague Atoms. (The old 75’s all tested very poorly at around 9uF! The 10’s were ok). The two 10’s were strapped together for a little extra bias filtering (originally, one was unused). The 20uF/20uF/450V-25uF/25uF/25V can was stuffed with 33uF/450V Nichicons and 25uF/25V Sprague Atoms.
• Signal and Inverter caps – The two .022uF/400V Pyramid inverter caps were replaced with .022uF/600V Sonicaps (the old tested at .032uF and open!). The .047uF/400V signal coupling caps were replaced with .068uF/600V Sonicaps (old all tested within spec). The two .0012uF Ceracaps were replaced with .0012uF/600V Sonicaps (old tested .011 and .003).
• Misc caps – The 10uF/25V in the bias circuit was replaced with a Sprague Atom 10uF/25V (old tested at 1uF). The 4uF/250V was replaced with a Sprague Atom 4uF/250V (old tested at 5uF).
• Spike caps and line voltage – The two .001uF/2500V spike caps were replaced with Sprague .001uF/1600V. To adjust the line voltage down from 120V I installed a terminal strip and a CL-80 thermistor on each leg of the power line. This brought the 120V down to 116.5V and provides a nice soft power-up.
• Voltage Measurements – At 117 input voltage (measured after thermistors pushed with a variac) the regular 6P14P tubes are biased at .23V, plate voltages average 376V and screens average 324V.
She’s been up and running through my Klipsch Chorus II’s for about 6 hours now and sounding very, very good. She’s also very quiet with these very sensitive speakers (except for one damned flickering neon on the right channel – fortunately not heard at normal listening levels – gotta fix that next). Sometime, down the road, I’ll address the control caps but they are sounding pretty good for now. All key resistors were within spec but I’ll also monitor those going forward.
If any of our AK experts sees any mistakes or improvements to the above, please chime in. That being said, I think I did pretty well for my first effort! Hope this adds to the AK body of knowledge and helps someone else with a future restore.
OK, time for a drink! :beer:
I acquired my 299 in very nice cosmetic shape but non-functional. The original Scott 7189 power tubes were shot and when new tubes were substituted, one would immediately red-plate causing me to suspect bad coupling caps.
Besides basic recapping, I decided to install bias sensing resistors, test jacks and individual channel adjustment pots. Additionally, to tighten bass response, I raised the values of the coupling caps by 50% as well as the main power filters to be able to take advantage of this mod (most of this info was derived from an old NOS Valves posts regarding a 299B – thanks Craig!). I chose Sonicaps Gen II for coupling caps – great reviews and reasonable price.
Rather than change everything at once, I proceeded in sections and tested the amp after each section. The first order of business was to get it working so that I could test after each change. I used a Fluke 111 meter for all measurements.
Here is the process I followed in order:
• Power tube coupling caps - replaced Pyramid .1uF/400V with Sonicap .15uF/600V. Plugged in 6P14P’s and warmed it up on the variac. Red-plating gone! Whoops, not so fast – it’s back. Old caps tested within spec so it wasn’t that. After much frigging around (and some tube wiggling) I discovered a bad pin 2 socket that wasn’t making proper contact. I swapped it with an unused socket (they come out from the top) and it really was fixed!
• Selenium rectifier – replaced with a RadioShack bridge 400V 8A and changed the first 10Ω dropping resistor to a 22Ω 5W. At 117 line voltage, this gives me -45.5V at the first 12AX7 or approx -11.4V for each tube.
• Test jacks and Bias pots – added a terminal strip and lifted all pin 3 grounds to the strip and then ran matched 10Ω 1/4W resistors from the strip to pin 3’s. The tip jacks were installed on the top of the rear panel and wired to the pin 3’s. The ground jack is wired to the ground terminal strip. Two additional 10KΩ linear bias pots (to control bias individually to each channel) were installed next to the DC balance pots. The circuit mimics the 233 schematic but takes its voltage supply from the original bias pot and uses two 15KΩ resistors for the ground reference.
• Power supply caps – I restuffed the original cans to retain the original appearance. The two 30uF/20uF/475V cans were upped to 47uF/33uF/450V Nichicons (the old caps all tested slightly higher than spec). The two 75uF/75uF/10uF/75V cans were stuffed with 75uF/80V Nichicons and 10uF/150V Sprague Atoms. (The old 75’s all tested very poorly at around 9uF! The 10’s were ok). The two 10’s were strapped together for a little extra bias filtering (originally, one was unused). The 20uF/20uF/450V-25uF/25uF/25V can was stuffed with 33uF/450V Nichicons and 25uF/25V Sprague Atoms.
• Signal and Inverter caps – The two .022uF/400V Pyramid inverter caps were replaced with .022uF/600V Sonicaps (the old tested at .032uF and open!). The .047uF/400V signal coupling caps were replaced with .068uF/600V Sonicaps (old all tested within spec). The two .0012uF Ceracaps were replaced with .0012uF/600V Sonicaps (old tested .011 and .003).
• Misc caps – The 10uF/25V in the bias circuit was replaced with a Sprague Atom 10uF/25V (old tested at 1uF). The 4uF/250V was replaced with a Sprague Atom 4uF/250V (old tested at 5uF).
• Spike caps and line voltage – The two .001uF/2500V spike caps were replaced with Sprague .001uF/1600V. To adjust the line voltage down from 120V I installed a terminal strip and a CL-80 thermistor on each leg of the power line. This brought the 120V down to 116.5V and provides a nice soft power-up.
• Voltage Measurements – At 117 input voltage (measured after thermistors pushed with a variac) the regular 6P14P tubes are biased at .23V, plate voltages average 376V and screens average 324V.
She’s been up and running through my Klipsch Chorus II’s for about 6 hours now and sounding very, very good. She’s also very quiet with these very sensitive speakers (except for one damned flickering neon on the right channel – fortunately not heard at normal listening levels – gotta fix that next). Sometime, down the road, I’ll address the control caps but they are sounding pretty good for now. All key resistors were within spec but I’ll also monitor those going forward.
If any of our AK experts sees any mistakes or improvements to the above, please chime in. That being said, I think I did pretty well for my first effort! Hope this adds to the AK body of knowledge and helps someone else with a future restore.
OK, time for a drink! :beer:
