Question on using bias test points/adjustment

[mad dog]

Just had a fixed bias switch, test points and adjustment pot installed on my Sewell Wampus Cat. Never having done this before ... to check bias, do I need to do anything but hook up the DMM and have the switch set to the fixed side? Speaker remains plugged in? Any difference at all from normal operation? Thanks.

[Doug S]

If he built the jacks like I do, just set your meter to millivolts (assuming he used 1 ohm resistors between the cathodes and ground), leave the speaker in, etc., and let the amp just idle when you are reading the bias. You should be able to read the current in either cathode or fixed bias settings, but the fixed bias would be the mode for the (critical) bias adjustments. The cathode bias reading would just be somewhat informational, unless the tubes are way off for some reason.

--Doug Sewell
Sewell Amps

[mad dog]

Doug: Thanks for your reply. That is how I saw my tech do it. Just wasn't sure if speaker stayed plugged, or if anything plugged into the input.

I was going to email you on this. I'm reluctant to mod, but couldn't resist on the WC. The fixed/cathode switch makes an already versatile amp even more so. Using 7581 tubes now. Fixed mode a bit louder, a little more clarity. (I could see the extra power on my tech's scope, can hear it too.) By contrast, cathode is thicker, a little softer edge. Both sound so fine, I really can't decide which to use. A lotta tone in this amp ... Can't wait to get it on stage.

[Doug S]

The fixed/cathode is a good option for that model especially-- I do it on many amps. Your take on the different bias methods is spot on-- seems like the cathode "expands" the lower volume sweet spot a great deal at the expense of some headroom. 7581s for great for max. clean, a good alternative these days are SED 6L6GCs which have great distortion characteristics.

Feel free to email me anytime-- I'd like to refresh my records regarding your amp if you get a chance.

--Doug Sewell