Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Re: Safety Modifications

Posted by jacksonwest on October 25, 2002 at 09:52:34

In Reply to: Safety Modifications posted by jacksonwest on October 22, 2002 at 10:05:44:

Thanks so much guys! Between rec.audio.tubes and audioasylum.com I got some really good feedback. Tonight I'm going to have to break out the camera and start logging my Heathkit W4 restoration/modfication project so that I can put a page up for other newbies.

My distilled to-do list:

1) I'm going to make my own schematics for the two amps to check that the circuit is identical (since neither was factory wired, and one has already been worked on a little), and then replace every capacitor and tube in their chassis with brand new so that I can be reasonably sure I've got identical monoblocks driving my two speakers - and I get two amps worth of working backup parts.

2) The capacitors that replace the four electrolytics in the old can will be strapped with 56k/10w resistors, the other electrolytics with their stock bleeder resistors.

3) Standby switch is scrapped. Instead I will be installing a new power cord (the stock ones are really cheap) and heavy-duty switch. Then I'm going to shunt the pre-amp power switch (by putting the SPST heavy duty switch across the wires connected to pins 6 and 7 of the umbilical cord socket) so that I can turn the power amp on and off without the preamp.

4) A pilot light (LED) will be installed across the filament supply with a series resistor and diode. This way I will know the amp is on even if the tubes are gone, and I won't horribly burn/shock myself.

5) After replacing the caps, installing extra resistors, improving the power supply and putting in the pilot light I'll plug it in tubeless to test the circuit. Assuming the fuse doesn't blow, I'll put the tubes in one by one. Once everything seems to be working, I'll use my multimeter to test the milliamps running to the cathode of each 6L6 output tube and even them out using the balance pot.

By that time I should have the equivalent of two brand new 20w 6L6 PP Williamson amplifiers. Cost rundown:

2 Heathkit W4 Power Amps @ $100/ea $200
2 Heathkit WAP2 Preamps @ $25/ea $50
Shipping from eBay sellers $50
Tubes, caps from Triode Electronics $155 w/shipping
(14 lytics, 8 poly, 8 other, Matched Quad Svetlana 6l6s, 4 Sovtek 6SN7, 2 5V4G Russian NOS)
Assorted parts from Radio Shack $45
Power Cord, Diodes, Resistors, RCA and Bannana Jacks, Preamp knobs
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Total: $500

Future Purchases: $75 for 4 12ax7 and 2 12au7 and caps for the preamps; $25 for wood, bolts, brackets and feet for protective enclosures.

It seemed like a bargain when I started!!! Anybody looking for original stock Heathkit parts, Tungsol Tubes (hehe)?

Jackson West

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

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