Question for Heathkit W-5M owners - diyAudio
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Old 16th October 2007, 02:39 PM   #1
ltarini is offline ltarini  United States
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Default Question for Heathkit W-5M owners

Hello,

Two schematics exist for the W-5M, one has R30 listed as 15K and one has it as 6.8K. The 15K schematic specifies plate voltages of 255 on the 2nd 12AU7, while the schematic with 6.8K does not list plate voltages for the 12AU7.

I am very interested to know which version is older and what are the 12AU7 plate voltages for the 6.8K version.

If you have a W-5M amp I would appreciate it if you would let me know the serial number and which R30 resistor you have. If you don't recall the color code for resistors off hand, just give me the sequence of colors with the silver band last.

If you happen to know your plate voltages on the 2nd 12AU7 that would also be of interest.

Thanks!

Larry

P.S.

The motivation is: I am trying to solve a bias balance problem on a W-5M. All resistors are within spec and matched where indicated, but V3 consistently gives at least 10 mA more current than V4. The closest I can come to zero balance is .4VDC, that's with the entire resistance of the pot on the V4 side. My 12AU7 voltages are 295, not 255, so I'm wondering if the 15K R30 would bring them down. Next step is probably to take out the 1 uF signal caps and check them.
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Old 16th October 2007, 03:28 PM   #2
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I suspect that R30 was increased to avoid a low-frequency oscillation, as it drops the cutoff frequency of the RC filter by half. The higher value will lower the voltage on the phase splitter.

Bias is unrelated. The coupling caps certainly could be the problem - disconnect one end of each and recheck bias balance - if it has changed, caps areresponsible.
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Old 17th October 2007, 02:03 AM   #3
ltarini is offline ltarini  United States
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It makes sense that 6.8K is the older version of R30, because my amps also have the older transformer, the 16458. If anyone has a 16309 unit with a 15K resistor in R30 position, that would be consistent with the timeline, if not actually conclusive. Should I upgrade to the 15K resistor, then?
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Old 17th October 2007, 03:35 AM   #4
Trout is offline Trout  United States
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I just looked at Mine and R30 is 15K, My original manual/book has a date of 1959, But I beleive my units are 1961.

Trout

Edit,

BTW, Mr Bavis is correct on suspecting the coupling caps. I had the coupling caps in both units cause serious bias issues. After replacing them it was smooth sailing.

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Old 17th October 2007, 03:48 AM   #5
ltarini is offline ltarini  United States
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I found the following info. I'm going to do all the changes except the surgistor.

Look at:

http://www.geocities.com/mnowlen/heathw5.htm

"W-5M Service Bulletin

The Heathkit W-5M Amplifier, a "standard" in our audio amplifier kit line for many years, has been discontinued. Style changes and new concepts in circuit design plus the increased trend towards stereo, contributed to the discontinuance of the W-5M. During the production of the W-5M, a few minor changes were made in its circuitry to improve its performance. These changes are listed below:

Page 19, Step 12 - Change "15k ohm, 1 watt resistor (medium body, brown-green-orange)" to "15k ohm, 2watt resistor (large body, brown-green-orange)" R-28.

Page 19, Step 13 - Change "6.8k ohm, 2 watt resistor (large body, blue-grey-red)" to "15k ohm, 2 watt resistor (large body, brown-green-orange)" R-30.

Page 20, Step 12 - Change "22k ohm, 1/2 watt restitor (small body, red-red-orange)" to "22k ohm, 1 watt matched pair of reststors (medium body, red-red-orange)" R-10.

Page 20, Step 13 - Change "22k ohm, 1/2 watt resistor (small body, red-red-orange)" to "remaining 22k ohm, 1 watt matched pair of resistors (medium body, red-red-orange)" R-9.

Page 21, Step 1 - Change "47k ohm, 1 watt resistor (medium body, yellow-violet-orange)" to "47k ohm, 2watt resistor (large body, yellow-violet-orange)" R-8.

Also a Surgistor was added to limit the high voltage surge that may occur when the amplifier is first turned on. This will help prolong the life of the components used in the amplifier, where line voltage or transformer voltages may be somewhat higher than normal but still within their rated specifications. An information sheet regarding the installation of the Surgistor is enclosed with this component."
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