5F6-A Adding a switch for fixed adj & cathode bias

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PhatTele
Junior tube assistant
Posts: 6
(2/10/04 3:21 pm)
 5F6-A Adding a switch for fixed adj & cathode bias modes
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 I'm working on my first 5F6-A Hoffman style circuit.

I've heard of people adding a switch to the original Fender layout to swap between fixed bias and cathode bias. I was wondering if this was possible with the Hoffman style 5F6-A layout which incorporates a fixed adjustable bias in its design? Specifically, I was wondering if this switching can be done with a single DPDT switch?

I've compared several drawings as models, but I'm not seeing a solution. Is this a situation (using the Hoffman layout) where it's either one or the other but not both? Thanx...
 
bluetonene
Junior tube assistant
Posts: 22
(2/10/04 5:01 pm)
 Re: 5F6-A Adding a switch for fixed adj & cathode bias m
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 I too am interested in this mod. Perhaps the switch could be located in the "ground" switch hole in the chassis' that Doug sells.
 
Wanabejimi
Junior tube assistant
Posts: 39
(2/10/04 5:33 pm)
 switch
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 Go to the www.ax84.com site. Surf around projects until you find the "November" schematic. What you are trying to do is on there. Of course, you'll need to adjust the cathode resistor for your tubes, but the drawing is there. I've got it on mine. Its a really cool switch. The cathode bias distorts a little earlier and is a bit creamier. I mostly use the cathode, but switch to fixed on occaision.
 
LooseChange
Senior tube assistant
Posts: 205
(2/10/04 5:35 pm)
 Re: switch
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 And try this link for another approach...

www.londonpower.com/cath-fix.htm
 
ganzonimx
Hey get your own solder
Posts: 364
(2/10/04 5:37 pm)
 Re: 5F6-A Adding a switch for fixed adj & cathode bias m
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 There are several ways to do that, you find schematics from Kevin O'Connor's book on his site here.. Scroll down the page 'till "Information (miscellaneous)" and have a look at the three schematics.

Cris
 
PhatTele
Junior tube assistant
Posts: 7
(2/10/04 7:02 pm)
 It figures...
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 ...I only have volumes 1 and 3 by Kevin O'Conner.

I pulled down that information, and I'll also take a look at those November projects. In the meantime, I'm going to get this working as originally designed and then try the cathode bias switching as a mod.

Thanx...
 
pabloxyz
Hey get your own solder
Posts: 201
(2/11/04 1:58 am)
  re: fixed/cathode bias
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 Hiya Loose Change,

the pot shown in O'Connor's fixed/cathode bias page (fig.1) is a bias balance control like the later silver face Fenders, which is cool. But, it's a center tapped pot labeled 10KTB. How would you work it using a plain ole pot with no center tap?
 
ganzonimx
Hey get your own solder
Posts: 367
(2/11/04 11:39 am)
  Re: re: fixed/cathode bias
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 I think you could use a normal 10K lin pot, connect the wiper to the switch and the outer legs of the pot to the 220K resistors. The 15K resistor to ground falls away.

To close the voltage divider circuit you need a additional resistor between the wiper and ground, could be a 25K pot, so you could regulate bias voltage and balance.

As always, I hope I didn't mess it up...

Cris

 
 
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