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 Post subject: Fender 5C1 Champ Guitar Amplifier Gain Stage Bias
PostPosted: Oct Tue 05, 2021 1:29 pm 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 380
Location: Springwater, NY
The Antique Wireless Association has a Fender 5C1 Champ inspired printed circuit board called the AWA '48 Special. It offers the builder the option of using grid leak bias on the 6SJ7 preamp, like the original 5C1, or cathode bias. Here is some information on the PCB:

https://www.antiquewireless.org/homepage/pcb-item-7/

and the original Fender 5C1 schematic:

Attachment:
Fender 5C1 Schematic.jpg
Fender 5C1 Schematic.jpg [ 238.44 KiB | Viewed 399 times ]


Is there some functional reason, other than the cost of implementing cathode bias over grid leak bias, why Fender (and Gibson GA-5 and others) chose to use grid leak bias way back when?

I built an AWA '48 Special with cathode bias. It went together quickly and easily and is really quiet. It sounds good clean and goes into breakup nicely at higher volume using a guitar or bass.

It's said that harmonica players prefer grid leak bias on this type of amp, but I don't know why.


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 Post subject: Re: Fender 5C1 Champ Guitar Amplifier Gain Stage Bias
PostPosted: Oct Tue 05, 2021 8:26 pm 
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Joined: Jul Fri 18, 2008 10:02 am
Posts: 2172
Location: near ST Louis Mo 62002
Bean counting , they saved a capacitor / and or a higher wattage resistor .

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 Post subject: Re: Fender 5C1 Champ Guitar Amplifier Gain Stage Bias
PostPosted: Oct Wed 06, 2021 3:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Eagan, Minnesota, USA
Wow! didn't know this was available from AWA! I'll be ordering one of these to build. I'm a Fender fan for sure.

This is a hell of a good way to get lots of interest in AWA and tube electronics. This is something very useful today.

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 Post subject: Re: Fender 5C1 Champ Guitar Amplifier Gain Stage Bias
PostPosted: Oct Wed 06, 2021 3:23 am 
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Joined: Jul Mon 08, 2013 2:48 pm
Posts: 818
battradio@ wrote:
Bean counting , they saved a capacitor / and or a higher wattage resistor .


I can't see any other technically valid reasons either.

Grid bias is a cheap way of doing things and is not recommended, there are no advantages of using it (except for saving a couple of parts).
Grid leak bias is very tube dependent and unstable, contrary to cathode (automatic or self) bias there is no operating point stabilisation effect and the results are somewhat unpredictable.
It works, but with higher distorsion due to lower overload margin (it's hard to get over 1.5V bias with this method), but maybe some guitarists are considering this defect (also called front-end clipping) as an advantage .


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 Post subject: Re: Fender 5C1 Champ Guitar Amplifier Gain Stage Bias
PostPosted: Oct Thu 07, 2021 2:15 pm 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 12353
Location: Ohio 45177
Why not add a switch to bypass the cathode bias and A/B compare them? BTW not much of a challenge to handwire that circuit and save 30$ on your project. Besides fanatics adore hand wired circuits. Then if you tell them it has cloth covered wire, they will be delighted. I got the tubes and transformers for a Champ around here, just have to get around to punching a chassis out. Make sure the coupling caps are leaky bumblebees.

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