Help with fixed/cathode bias switch

[lain]

Can you gentlemen help me with wiring up a fixed/cathode bias switch to my SF Champ? I have the fixed bias board completed and wired to the rectifier. I'm just a little confused as to how to hook up a switch. In theory, I believe the switch has to short the cathode R/cap to ground while passing negative bias voltage to the 220K grid R. I'm at a loss as to the best way to hook this up, or what switch to use.

[MWJB]

go to Tube Amp Kits, Tube Amp Books, Tube Amplifiers by London Power, search through the FAQ for the cathode fixed switch, there's a schem there.

[Bob M.]

Fixed/Cathode biased Champ...

Iain,

You probably need a two pole switch: 1st pole connects pin 8 of the power tube to either ground (for fixed bias) or to a cathode resistor and bypass cap (for cathode bias). Keep these two components separated by around 2~3". The 2nd pole will connect a grid load resistor on the power tube to ground or to a negative voltage, probably derived from the rectifier output (see Princeton Amp schematic to see how).

Having said this, this is alot of work, alot of parts and I'm not sure how much of a big change you might hear. I've done this on one of my homebrews (not a Champ) and cannot tell the difference between the two modes. But it's always good to try new stuff...

Bob M.

[tubeswell]

Revised switchable bias circuit (shorts the rk instead of lifting it between modes)

[mooreamps]

If you used combination bias, then there is no need to short out rk....



-g

[FUCHSAUDIO]

Except combination bias sounds like crap.

[pdf64]

'Except combination bias sounds like crap.'
Could I draw your attention to the vox AC50, which uses (the much maligned) combination bias? The several examples I've worked on sound excellent, with a room filling 3D tone that belies their performance on the test bench.
The Sept 2006 Tone Quest Report gave a glowing review of their AC50.
In the light of this, perhaps we could conclude that rather than combination bias being crap per se, it's how it 's implemented that matters?

[FUCHSAUDIO]

'Except combination bias sounds like crap.'
Could I draw your attention to the vox AC50, which uses (the much maligned) combination bias? The several examples I've worked on sound excellent, with a room filling 3D tone that belies their performance on the test bench.
The Sept 2006 Tone Quest Report gave a glowing review of their AC50.
In the light of this, perhaps we could conclude that rather than combination bias being crap per se, it's how it 's implemented that matters?

Well, those wacky SF Fenders weren't particularly worth writing home about. I do agree, perhaps Vox has managed to make it work well. I never much cared for it myself.

[mooreamps]

Except combination bias sounds like crap.

Maybe better if you keep a check on your "kneejerk" replies. I know you don't like me. But I ain't no 19 year old punk, either.


-g

[FUCHSAUDIO]