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Post subject: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 3:40 pm
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My HRD III has a very low BIAS reading. Initially I thought it was the tubes that had gone, It was, but I think the misinformation being given by the low readings may have meant the BIAS was set to high. It was showing as near to 60 as I could get it but I suspect it was actually a lot higher and therefore the tubes burnt out.

I have just installed a new matched pair of JJ 6L6GC's and checked the BIAS reading. Once again I'm only getting a maximum reading of just under 60. When I set the adjuster to the half way point it reads just under 30. Fully counter clockwise gives me a reading of 15.

As far as tone is concerned it sounds fine set at the halfway point so I'm thinking, could there be something wrong which is only giving me half the true reading value?

As a point of interest, halfway is where the original setting was with the blob of red paint.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 3:45 pm
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How are you measuring the idle bias value? At TP30 or using an aftermarket tool? TP30 reads both 6L6GC and an aftermarket bias tool prolly one tube.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 3:47 pm
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BMW2002Ti wrote:
How are you measuring the idle bias value? At TP30 or using an aftermarket tool? TP30 reads both 6L6GC and an aftermarket bias tool prolly one tube.


I'm measuring at TP30.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 3:49 pm
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BMW2002Ti wrote:
How are you measuring the idle bias value? At TP30 or using an aftermarket tool? TP30 reads both 6L6GC and an aftermarket bias tool prolly one tube.



Plate Voltage is around 460 on each tube.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 3:59 pm
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You have one more pair of 6L6GC that you can try in the amp? Just to rule-out a bad new pair of JJ's.


Good luck!


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 4:15 pm
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BMW2002Ti wrote:
You have one more pair of 6L6GC that you can try in the amp? Just to rule-out a bad new pair of JJ's.


Good luck!



Thanks, I'll try that.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Mar 07, 2014 6:09 pm
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If the tubes prove OK, you may have a problem in the bias power supply.

Image

Measure the voltage at TP41. Also, check the voltage across the pot and R77 as you adjust the pot.

Other things to check are the screen grid resistors, R61/62. An open resistor will cause a very low bias reading.

Finally, check R59/60 and R64/65.

http://support.fender.com/schematics/gu ... _Rev-B.pdf

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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Sat Mar 08, 2014 10:13 am
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bluesky636 wrote:
If the tubes prove OK, you may have a problem in the bias power supply.

Image

Measure the voltage at TP41. Also, check the voltage across the pot and R77 as you adjust the pot.

Other things to check are the screen grid resistors, R61/62. An open resistor will cause a very low bias reading.

Finally, check R59/60 and R64/65.

http://support.fender.com/schematics/gu ... _Rev-B.pdf



Thanks bluesky636, I'll let you know how I get on.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Fri Nov 30, 2018 11:05 am
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Hi, the problem is R61 or R62 resistor is dead, so amp is working in single mode tube.


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Post subject: Re: Hot Rod Deluxe III BIAS very low reading
Posted: Sat Dec 08, 2018 4:44 pm
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:lol:

You're probably right wahoo, but this thread is four and a half years old.

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