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Post subject: Very low mV when biasing Hot Rod Deluxe
Posted: Tue Apr 03, 2018 6:59 am
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Joined: Tue Apr 03, 2018 6:52 am
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Hi everyone,

I'm new to the forum and need some help. My 2002 Hot Rod Deluxe has developed an intermittent crackling noise. Whilst investigating what might be the cause of this (unsuccessfully so far), I thought I’d change the valves (tubes). When I went to check the bias using a multimeter, I was surprised to find that the mV readings are nowhere near where they should be.

With the blue variable resistor (R82) turned to its lowest setting, I get a reading of 15mV; in the centre position 30mV and 43mV at its highest setting. As I understand things, the range of readings should be between circa 60mV - 110mV! What is going on with my amp? I have successfully checked and changed the bias on my Marshall DSL50 using the same multimeter, so (I think) I know what I’m doing, but these low readings are weird!

Some years back, one of the 6L6 tubes blew, taking out the screen grid resistor (it fried it!) I replaced that with an identical resistor and all has been well since. I did not check the bias when I replaced both 6L6 tubes after the Harma tube blew (dumb, I know).

Today, I set the bias to 40mV and my amp still sounds great, especially when cranked up loud. My only criticism would be it can be a bit too trebly, but I just turn down the tone knob. It’s always been a very bright amp.

Any help or advice with regard to the very low bias readings would be greatly appreciated.

Update: last night, as it got dark, I noticed that only one of the 6L6 tubes was lighting up or glowing. I guess I've got a blown tube.

Replaced 6L6 tubes and all is well - normal bias readings and the amp sounds great! :D


Last edited by Easy East on Wed Apr 11, 2018 6:25 am, edited 1 time in total.

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Post subject: Re: Very low mV when biasing Hot Rod Deluxe
Posted: Wed Apr 04, 2018 4:08 pm
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Joined: Fri Feb 05, 2010 4:31 am
Posts: 14047
Location: Province de Québec, Canada
You must use only know good tubes.


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Post subject: Re: Very low mV when biasing Hot Rod Deluxe
Posted: Fri Apr 27, 2018 6:49 am
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Howdy, I’ve been away for a long time but here’s my two cents. I would swap the tube that glows with the one that doesn’t and see if that one doesn't glow to rule out the tube, then look for cold solder joints or fractured joints on the tube sockets and do a visual on for any scorched components. I had a HRDlx way back that had that same issue. The tube sockets were notorious for being trouble spots.

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