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Bias instability in Heathkit W-5M amp
Bias instability in Heathkit W-5M amp
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Old 26th September 2007, 06:55 PM   #1
ltarini is offline ltarini  United States
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Default Bias instability in Heathkit W-5M amp

Hello, I have a Heathkit W-5M that will not set bias. The W-5M does not have a bias control to set overall current, only a balance control to match the current in the two KT66 (China) output tubes. However, as the amp warms up no amount of adjustment will keep the balance at 0, clearly one tube is drawing too much current, so I shut the amp off immediately.

The amp has been modified by a prior owner, so I need to check if any cathode resistors or grid resistors have changed value in a way that might create excess current. However, there is some evidence of tar leakage from the Peerless 16458 output transormer, so I wonder if there might be something amiss inside there. I don't know a good way to check that without removing the transformer.

I have heard that too high a grid resistor value can cause grid current (electrons flowing from grid to plate) which makes the bias on the grid less negative, causing current to increase, and I have also heard that some Chinese tubes are particularly susceptible to this. Should I try lowering the grid resistor values? Or, alternately, try raising the cathode resistor values? Most of all, the leaked tar from the output worries me and I would like to know how I can test the transformer, even if it involves removing it from the amp.

Thanks for your suggestions.

Schematic for the Heathkit W-5M can be found here:

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Old 27th September 2007, 02:44 PM   #2
kstagger is offline kstagger  United States
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I've heard a pair of Heath W5s a few years ago that was converted to fixed bias. They added a small transformer, diodes and capacitors to provide the negative voltage.

If you have another pair of output tubes, you may want to try those - you could have some seriously mismatched output tubes and/or some improper modifications.

and Hello fellow michigander!
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Old 27th September 2007, 03:02 PM   #3
Tom Bavis is offline Tom Bavis
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Location: Macedon NY
Check the output transformer with an ohmmeter - if the plate section opens, the tube will still draw current thought the screen, which will overheat...

It's also possible that there's a high-frequency oscillation... hard to tell without a 'scope. Grid resistors in this are pretty low - I wouldn't expect any problem there. But as mentioned, tubes could be a problem.

I have the assembly manual on my page: http://www.audiophool.cjb.net/hifistuff.html
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