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Post subject: Logarithmic Bias Pot???
Posted: Sun Jul 01, 2012 10:08 am
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Hi guys

I have this problem biasing my Hot Rod Deluxe amp, from the measurements I'm getting at the bias point it looks as if the bias pot is a logarithmic pot and not a linear pot!!! The problem is that the tubes would either run way too hot or not run at all at the prescribed range of 70-90 mV.

I've checked out some videos on youtube of guys biasing their amps and they make it look like a walk in the park, being able to set the bias at fractions of a millivolt. With the bias pot in my amp that is not going to happen...

You can literally go from the tubes not being on (0V) to over 300mV by JUST TOUCHING the screwdriver, that's less than a degree turn to go from 0-300. SO you have the option of running the tubes way too cold with crappy sound to frying them and buying tubes every month.

Just some info about the amp, it is a limited addition amp made in Mexico serial number B-358094 and model type PR-246. Any ideas suggestions? I assume I can pull the main board an replace the pot with a linear pot? What value would you suggest for such a pot to run 6L6's??? Thanks in advance....

Cheers Ettienne


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Post subject: Re: Logarithmic Bias Pot???
Posted: Sun Jul 01, 2012 10:23 am
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Highly unlikely that the wrong bias pot was installed. More likely that you have a problem in the overall bias circuit or other part of the power tube circuit. Unless you are competent at diagnosing and repairing problems in a tube amp, I suggest you take the amp to a professional amp tech for repair.

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Post subject: Re: Logarithmic Bias Pot???
Posted: Sun Jul 01, 2012 10:33 am
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+1 bluesky636

The fact that the bias goes from 0 to 300 mV that quickly tells me that the pot wiper is not making good contact, or you have a solder joint problem.

Edit:
Just something simple to try, no guarantees, but with the amp off and unplugged from electricity, you might be able to clean the bias pot by rotating it back and forth many times, then try biasing again.

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Post subject: Re: Logarithmic Bias Pot???
Posted: Mon Jul 02, 2012 6:10 am
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Thanks for the feedback, I'll do as you propose and check whether the wiper is maybe not connecting properly. I'll be replacing the caps next weekend and will check the solder joints then.

I already had the amp at an electronic repair shop here in Atlanta to have it biased but from the lifetime that I got from those tubes (about 2 months) it was running too hot so they also didn't get it right. The original tubes that came with the amp lasted about 3 years!!!


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Post subject: Re: Logarithmic Bias Pot???
Posted: Mon Jul 02, 2012 8:31 am
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et1hugo, I'd look into changing that pot. It's a real cheapie plastic housing jobbie. A substitute metal jobbie (multi-turn) would really be nice. You'd need to make a raised PCB platform for it. But, you'd get years and years of reliable service.

http://www.bourns.com/data/global/pdfs/3549h_mds.pdf


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Post subject: Re: Logarithmic Bias Pot???
Posted: Thu Jul 05, 2012 8:54 pm
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Hey guys

Thanks for the feedback, sure as hell, that pot was a logarithmic pot. Ordered a linear pot from Darren Riley's and installed it now it is really easy to get the proper bias voltage. Works like a charm.

Cheers.


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Post subject: Re: Logarithmic Bias Pot???
Posted: Thu Jul 05, 2012 9:15 pm
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et1hugo wrote:
Hey guys

Thanks for the feedback, sure as hell, that pot was a logarithmic pot. Ordered a linear pot from Darren Riley's and installed it now it is really easy to get the proper bias voltage. Works like a charm.

Cheers.


And you determined that the pot was logrithmic how? Did you compare part numbers on the old and new pots? If it was logrithmic, how come your original tubes lasted 3years? Obviously, they were biased properly. They would not have lasted that long, indeed, they would have failed as quickly as your new tubes did if the pot was logrithmic. Did the pot fail somehow? Clearly it did. Was it ever logrithmic? No, the failure caused it to act that way.

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