Adjusted the bias on a Marshall DSL100HR

  • Thread starter Thread starter fek
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fek

fek

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I bought this new a year and a half ago and always really liked it. The past few months something sounded off and I thought that I was about ready to move it. Checked the bias and each side was at 60mv. Marshall recommends 70mv for this version of the head. Bumped it up to 74mv and there it is! Keeping it.

Not sure how the bias ended up down there but I was surprised at how big of a difference getting the bias back up in range made on the dirty tones. I have played with the bias in a few amps before (as long as they have external probes) and did not notice this big of a change. Going to be keeping an eye on that for a while to make sure I don't have a drifting issue.
 
Cold is pretty typical. They do it so the tubes last, but hold the amp back, at the same time.

I had one of those, and the DSL40C a couple times. Pretty solid amps, with a little help.

I had the older, pre "R" versions. Haven't heard the newer ones.
 
As tubes age, the bias will shift. It’s common on all tube amplifiers that are class A/B grid biased and not cathode biased.

You should keep an eye on it for tube drift but also note this is one of the signs of aging tubes. When the tube gets white around the boron coating internally or glows a deep blue when idling, the tube is gassy and has lost enough vacuum that it can fail taking out screen grid resistors and potentially damaging the OT.
 
glpg80":2smga3lw said:
As tubes age, the bias will shift. It’s common on all tube amplifiers that are class A/B grid biased and not cathode biased.

You should keep an eye on it for tube drift but also note this is one of the signs of aging tubes. When the tube gets white around the boron coating internally or glows a deep blue when idling, the tube is gassy and has lost enough vacuum that it can fail taking out screen grid resistors and potentially damaging the OT.
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Thanks for the heads up! I have not noticed anything like that on the tubes but will check next time I have it fired up for awhile.
 
I thought the blue was normal?

But yeah, power tubes can cause bias drift and usually downward right?
 
311splawndude":36hn0thd said:
I thought the blue was normal?

But yeah, power tubes can cause bias drift and usually downward right?
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At idle no.

During playing, yes :)
 
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