gmoon
11-29-2007, 03:04 AM
Hi! First-timer here!
I've a old (1961) Kay 503A amp that needs some upgrading. It's a low-wattage practice amp with 'radio' tubes--12AU6, 50L6, 35Z5. It has good tone, but is fairly noisy at times, and has some (unpleasant) breakup at higher volumes I can't quite blame on the speaker.
It's all original (including tubes), so I'm planning the the usual stuff: recap, new tubes, speaker replacement. But I've a couple questions first...
--Never done a 'recap' job. Is it customary to leave the old multi-cap 'tin can' in place for looks? (From reading here and other places, I'm aware of the dangers of adding too much capacitance in the rectifier filtering.)
--Should the filter caps voltage rating be upped? I ask because the previous owner had already bought caps and they are rated 150V (highest voltage in the circuit is 130V, I believe.) While the original caps were rated 150V, this seems a little skimpy to me.
--The amp circuitry is isolated from the line, but not the heaters. They're connected directly to the line, in series with a 150 ohm 5 watt resistor. I know it isn't ideal, but it's better than no iso transformer at all. Is this reasonably safe? Or should I build a separate power supply for the heaters? (DC?)
--The amp appears to have an error in the original wiring (The three inputs each have an inline 22K resistor. Between them and the .05 cap should be a 22K res to GND. Instead the 22K resistor is connected to GND BEFORE one of the input resistors on INPUT 1.)
Since I need to correct this anyway, I've found several schematics for amps with nearly identical tubes, but some wiring variation ('Alamo Fiesta', 'Airline GDR-8511A', 'Harmony 303A', 'Kent 2189', 'Silvertone 1430, 1448' and more...) I plan to try some of those variations in the preamp stage. I've done enough experimentation to see that the amp is capable of a much 'brighter' sound. Any thoughts?
I've a old (1961) Kay 503A amp that needs some upgrading. It's a low-wattage practice amp with 'radio' tubes--12AU6, 50L6, 35Z5. It has good tone, but is fairly noisy at times, and has some (unpleasant) breakup at higher volumes I can't quite blame on the speaker.
It's all original (including tubes), so I'm planning the the usual stuff: recap, new tubes, speaker replacement. But I've a couple questions first...
--Never done a 'recap' job. Is it customary to leave the old multi-cap 'tin can' in place for looks? (From reading here and other places, I'm aware of the dangers of adding too much capacitance in the rectifier filtering.)
--Should the filter caps voltage rating be upped? I ask because the previous owner had already bought caps and they are rated 150V (highest voltage in the circuit is 130V, I believe.) While the original caps were rated 150V, this seems a little skimpy to me.
--The amp circuitry is isolated from the line, but not the heaters. They're connected directly to the line, in series with a 150 ohm 5 watt resistor. I know it isn't ideal, but it's better than no iso transformer at all. Is this reasonably safe? Or should I build a separate power supply for the heaters? (DC?)
--The amp appears to have an error in the original wiring (The three inputs each have an inline 22K resistor. Between them and the .05 cap should be a 22K res to GND. Instead the 22K resistor is connected to GND BEFORE one of the input resistors on INPUT 1.)
Since I need to correct this anyway, I've found several schematics for amps with nearly identical tubes, but some wiring variation ('Alamo Fiesta', 'Airline GDR-8511A', 'Harmony 303A', 'Kent 2189', 'Silvertone 1430, 1448' and more...) I plan to try some of those variations in the preamp stage. I've done enough experimentation to see that the amp is capable of a much 'brighter' sound. Any thoughts?