# Flaw in the 5F6A tweed bassman plan - Bias supply voltage



## Lincoln (Jun 2, 2008)

I was well on my way to building a tweed bassman 5F6A using a huge old power transformer from a Hammond organ amp (AO20880). I was to the point of running wires from the board to the chassis when I realized I had a problem. The transformer I want to use has no 50 volt winding to power the bias circuit. (Now I know why tweed bassman PT's are more expensive than most other Fender PT's)

So......am I hooped? Do I have any options? I see Hammond makes a 166G50 isolation transformer with 50V secondary that's only about $20. I could hide that somewhere.

Open to suggestions :Smiley-fart:


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## epis (Feb 25, 2012)

There is no problem. Take a look at this diagram : https://hotbottles.files.wordpress.com/2010/01/jcm800_2204.gif


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## WCGill (Mar 27, 2009)

http://ejjamps.com/articles/bias-circuits.html


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## Lincoln (Jun 2, 2008)

epis said:


> There is no problem. Take a look at this diagram : https://hotbottles.files.wordpress.com/2010/01/jcm800_2204.gif


3 resistors, 2 caps and a diode......I can do that! Great solution, thank you


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## Lincoln (Jun 2, 2008)

WCGill said:


> http://ejjamps.com/articles/bias-circuits.html


Thank you for the explanation WC


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## jb welder (Sep 14, 2010)

Just a reminder these circuits will only work in a power supply with a grounded PT center-tap.
As this is a 5F6A, it should be no problem, I just thought I'd mention it for future reference with other amps.


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## Church-Audio (Sep 27, 2014)

Make it cathode biased it will sound better IMO have more sustain and work great. Check out the old tweed super amps.

very easy to do works great. 
https://sites.google.com/site/yourtubeamp/mods-and-maintenance/bias/convert-to-cathode-bias


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## Lincoln (Jun 2, 2008)

Amprepair said:


> Make it cathode biased it will sound better IMO have more sustain and work great. Check out the old tweed super amps.
> 
> very easy to do works great.
> https://sites.google.com/site/yourtubeamp/mods-and-maintenance/bias/convert-to-cathode-bias


That would be the easy way out for sure.

Crisis? What Crisis?


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