SWR Workingman's Bias control tip

[Chucky]

I would like to help out all SWR Workingman's 12 and 15 combo owners with a tech tip. I own both models and noticed my 15 was running way too hot compared to my WM12. After looking around the web I found that there is an internal bias trim pot to balance some transistors. After a visit to Fullcompass's service department this week here in Madison, WI my bias on the WM15 was properly adjusted. Problem solved and took 1 hours service time. Before I could not touch the heat sink on my WM15, now the heat is barely perceptible.

Photos are of my current home setup, SWR WM15 on one end and Acoustic Control Model 126 combo on the other end of my room connected with a Roland Micro Bass RX as preamp controller in stereo.

 

[BassmanPaul]

Most amplifiers have bias controls of one sort or another.

 

[T-Bird]

Hi.

Most amplifiers have bias controls of one sort or another.

^This.

And also most of the distortion and overheating problems can be remedied by just adjusting the said bias. Before the damage occurs.

BUT...

The the "modern" way of thinking that "only tube power amps with A/B output sections" (if even those ), can and need to be biased has led to the idea that SS output sections aren't biased at all, nor do they need the bias to be checked and adjusted periodically.

SWR is a prime example of the discretes circuit topology that absolutely needs the periodical biasing.
Blessing of a sorts, since one can usually buy a "malfunctioning" and overheating amp from a clueless seller for peanuts.

In all fairness, I suspected this thread to have the actual bias procedure, just like the one for the SM family, but the mere mention of it's importance will probably have to do .


Regards
Sam

 

[Chucky]

Hi Again,

I cannot vouch for this pdf file describing the bias procedure but here you have what I found.

Chuck

 

[T-Bird]

Hi.

Thanks for the attachment, I'm sure that it'll help anyone seeking for information about biasing their WM12 or WM15.

Looks OK to me, about the same as the good 'ol ST220 & SM400 bias procedure.

Regards
Sam