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Post subject: Fixed bias to Cathode bias conversion
Posted: Thu Nov 20, 2014 12:07 am
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Just thinkin' about changing the output tube bias configuration in my Bassman amp. The amp is an AB165 reverted to an AA864 (totally stripped- rewired & repopulated) with Merc '63 clone X-formers, PIO caps...etc. It's lodged in a 2X10 combo cab(Weber spkrs- one ceramic, the other an alnico). I'd like to mellow it's voice and power output. Considered a tube rectifier conversion(and PT). But a cathode biasing situation(no NFB) keeps coming to mind. Has anyone messed around with this ? The math and converting this amp is not an issue, for me. Just wondering if any one, around here, has tried this with a 2 6L6 Fender amp!!! Art

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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Fri Nov 21, 2014 8:59 am
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Art, you can arrange the output stage like some early tweed 6L6 amps which used cathode bias. I away from home computer & all the schemo links. Will lok it up when I get home later tonight.


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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Fri Nov 21, 2014 1:07 pm
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What about the rect. conversion (5Y3 maybe) and then use 6V6's?


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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Fri Nov 21, 2014 8:59 pm
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Here's the cathode-bias arrangement that Fender used in their 5B6 Bassman. A shared 250-ohm, 10-watt cathode-to-ground resistor. With a 25mfd/25VDC bypass cap. This amp used a 6L6G or -GA tube. You can use the 6L6GC. You may need to play with that bypass cap. I've seen anywhere from a 25mfd to 250mfd bypass cap. Use 50VDC or higher VDC rating.

I'd use a wire-wound 12-watt Mills resistor for the cathode-to-ground. Very stable under heated conditions.

The higher the mfd value, the more bass is bypassed to the output tranny = more bass. Too little mfd = weak bass and transients. Too much mfd = excess bass distortion, doubling, muddy tone.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf


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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Sat Nov 22, 2014 9:26 am
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upnorth wrote:
What about the rect. conversion (5Y3 maybe) and then use 6V6's?


Not sure if the 5Y3GT will pass enough current to drive the two 6L6GC's. Yes, you may need to drop the plate voltages if you use the same PT. You could drop the voltage with an inline resistor. It would need to be pretty high wattage. Which may add another heat factor. Plus, adversely effect the tone and transient response.


I would go with another PT. Perhaps, a Hammond equivalent to the PT used by Fender in their 6G9-B transition-era Tremolux. A 5U4GB in this amp will make around 360-365VDC onto the plates. This is just right for a Class "A" cathode-biased, push-pull 6L6GC amp.

http://bmamps.com/Schematics/fender/tre ... _schem.pdf


I agree... remove the solid-state diode rectification. Most good Class "A" guitar amp is built around a good tube rectifier. Classic old-school, Bluesy tones are what you are after. One wants nice sag. With the nice bass generated with a 6L6GC amp.

Here is the Matchless Clubman amp schemo. Matchless uses separate cathode-tied resistors and bypass caps on the EL34's. The values should be close for the 6L6GC, as the two tube types share similar output wattage values. Separate cathode-to-ground resistors and caps allow each output tube to self-level. Perhaps, leading to a better "matched" output.

http://bmamps.com/Schematics/Matchless/ ... lubman.pdf

In either case, I would run 1k-ohm, 5 watt wire-wound screen resistors. Class "A" amps run hot --- you need a reliable, neat resistant voltage supply on those screens.

One needs to physically separate the bypass cap from those 12- or 15-watt cathode resistors. They generate quite a bit of heat. As well as the tube, which will be running about 75-80% total dissipation, while at idle (or near true Class A).




Good luck! Keep in informed. :D :mrgreen: :D


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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Sat Nov 22, 2014 9:31 am
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OT... just spent the week up in Monterey & Big Sur with my new (old) ride! 2007 Honda S2000. Pics of vacation to follow.


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Post subject: Re: Fixed bias to Cathode bias conversion
Posted: Sat Nov 22, 2014 4:30 pm
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Thanks for the suggestions guys !!! I'll probably go with another PT and tube (GZ34)rectifier. But with much lower B+ voltages. That and the seperate cathode resistor bias should bring it closer to a Class A operating amp. The sweetest sounding amp I've had was a'95 Clubman 35. The Bassman uses NOS Tungsol 5881's now. Guess I'll build the output to their specs. A project seems like a good idea!! Keep Pickin' !!! Art

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