W5M Power Xfrmr Question

[Jack Crenshaw]

First, it's not a matter of age.  The power transformer in my W5M failed 
ca. 1963.  And the filter caps weren't leaking.

It's worth remembering that this is, after all, sort of a souped-up 
20-watt amplifier, designed around the 6L6-class KT66's. To get 25 watts 
out of them, the KT66's were run at over 500v.  The power supply uses 
300v electrolytics in totem pole configuration, because there were no 
(and still aren't) 600v electrolytics to be had.

For another clue, look at a picture of the amp itself. All those parts 
are packed in there very tightly. Trust me when I tell you that the 
power transformer -- in fact, the entire power supply circuitry -- was 
being pushed hard, and tranformer failures were, while not common, at 
least not rare, either.

Apparently, they're still happening.  See, for example,

http://www.audioasylum.com/cgi/m.pl?forum=vintage&n=165054&highlight=W5M&r=&session=

I currently have six W5Ms, waiting for refurbishment. At least two of 
them have replacement transformers.  On that note, I should also note 
that the replacements are larger, and don't allow the dust cover to be 
fitted on.

Jack

Wilson Lamb wrote:
> You may have a bad filter cap that is drawing current, or a bias 
> setting of the PA causing excess current.  It would be good to put in 
> solid diodes to get the rectifier filament load off the xfmr.
>
> I don't think it was designed to run as hot as you describe.
> WL
> ----- Original Message ----- From: "Alex [Temple-Heath List]" 
> <temple.heath1 at MIWWW.COM>
> To: <HEATH at LISTS.TEMPE.GOV>
> Sent: Saturday, June 28, 2008 3:59 PM
> Subject: Re: W5M Power Xfrmr Question
>
>
>> I don't have much experience with Heath audio amps, but I do have a 
>> Fisher X202B late 60's vintage tube amp. After several hours of 
>> listening the power transformer gradually gets quite hot, you can't 
>> leave your hand on it for more then a few seconds.
>>
>> I'm guessing the manufactures of the time thought that the savings of 
>> using a somewhat underrated transformer would be offset by making it 
>> easy for the customer to order a new one if it ever burned out. But 
>> what they never though of is that nowadays anyone would still be 
>> using 60-year plus amplifiers, with weakened transformers due to so 
>> many hot-cold cycles, and no easy way to find a replacement.
>>
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