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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Safety Modifications posted by jacksonwest on October 22, 2002 at 10:05:44:
1) How do I keep from getting horribly shocked by giant electrolytics that haven't discharged? What value resistors should I strap accross the filter capacitors? (for example 20mfd 600v = ?ohm ?watt)
--Avoid placing both hands on/in the unit at the same time (the "Keep one hand in your pocket" rule). For bleeding HV, I use 100 ohms per volt. The value is not critical so for 600 volts use 56K or 62K at 10 watts. Reason for the high wattage is if you forget to turn the power off it won't be destroyed. Also it is mechanically rugged. One end of the resistor should have a clip for connecting to chassis. The other end may be a spare meter probe. --
--Connect a small neon bulb with appropriate resistor to the B+ while troubleshooting, as a reminder that the voltage is present. Also the bulb fades as you discharge the caps.--
2) What happens if a bunch of DC starts flowing throught the speaker interconnects? How do I install a fuse between the speaker terminals and the output transformers? (for example 20w @ 8 ohms = ?amp)
--With tube amps this is not likely to happen but don't use those expensive vintage speakers for testing amps.--
3) How can I check the bias in a push-pull configuration with inexpensive tools (i.e. a voltmeter)? How would I install a meter in the amp to display the bias when powered up?
--Use a high-impedance voltmeter and measure on the tube socket pin for G1 (control) grid. If the voltage measures wrong, switch off power and make this repair first. You may install a permanent 50V meter on the amp, connected across the bias supply filter cap. --
4) Where would I put a standby switch to turn off the plate voltage while keeping the filaments warm? After the power transformer or after the filter section?
--Most switches will not withstand switching the high voltage AC or DC and will arc and keep arcing until it smells real bad and the smoke comes out. It's better to switch this on the primary side but this requires a dedicated transformer.--
5) What else can I do to reduce the risk of damage to my health/speakers/tubes (in that order)?
--Your bench work surface should be insulative wood or Formica, perhaps with anti-static mat. Do not use a conductive metal bench/table top. Use a series 75 watts lamp or a Variac to safely power up unknown equipment.--
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Follow Ups
- OK here goes.... - Tiny Daddy 10/22/0212:37:06 10/22/02 (0)
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