Bias Mode Switching Question (guitar Amp)

Trout · 6th October 2008, 11:09 PM

Hey,

A buddy dropped off a homebrew amp that he wants a switch installed to select between Fixed adjustable bias and common Fender Cathode biasing.

I was wondering about floating the lead to the wiper on the bias pot.
Is it ok to let it float? Drawing below is basically the idea.

Discuss "

Trout

kevinkr · 7th October 2008, 03:50 AM

No problem with the electrical configuration, wiper can float. I would probably not recommend switching with the power on - could generate quite a pop. For improved reliability on the bias side I would probably put a 20K resistor across the switch contacts from the bias pot wiper to the pair of 220K grid bias resistors so if the contact becomes unreliable you do not have an open in either position of the switch. Other approaches to accomplish the same end should be equally fine, the key is to prevent an unbiased runaway condition.

porkchop61 · 7th October 2008, 12:09 PM

I have this arrangement on my "November" build from the AX84 site, and it works fine. As Kevin said, don't switch it with the power on

Look at the schematic here:
November AX84 amp schematic

Trout · 7th October 2008, 05:20 PM

Thanks Guys!

Looks like its good to go.

Trout

gingertube · 7th October 2008, 10:56 PM

Here is how I do it. Hot switching with no bangs, pops, thuds etc.

Borrowed from Kevin O'Connor.

Cheers,
Ian

MrTransistorm · 7th October 2008, 11:05 PM

Quote:

Originally posted by gingertube
Here is how I do it. Hot switching with no bangs, pops, thuds etc.

Borrowed from Kevin O'Connor.

Cheers,
Ian

I'd be worried about the life of that cathode cap. The switch shorts it to ground.

Wavebourn · 7th October 2008, 11:09 PM

Quote:

Originally posted by gingertube
Here is how I do it. Hot switching with no bangs, pops, thuds etc.

Borrowed from Kevin O'Connor.

Yep, a double bias between positions, also cheaper switch. Very clever.

gingertube · 7th October 2008, 11:15 PM

I actually don't use a cathode bypass capacitor.

2 of the above circuits used to switch either or both pairs of a parallel push pull 6V6 amp (Also has triode pentode switching on each pair).

If you want to use a cathode bypass cap then just add a series resistor in line with the bypass cap. Value approx 1/10th of the cathode bias resistor - this WILL change the sound a little in that it will supress some 3rd harmonic distortion in cathode bias mode.
I would suggest you actually try the amp without a cathode bypass cap. Seriously lovely.

Cheers,
Ian

Trout · 7th October 2008, 11:33 PM

I am not actually to worried about pops & bangs.
I mounted the switch in a location that would make hot switcjing impractical.

He wants this mod more as a way to alter the feel of the amp.

After talking with him, more than likely it will rarely be used. He just likes the cathode bias feel better, but wants to be able to switch back occasionally.

The KOC mod looks pretty good, but it would be a pretty tight fit in an already over crowded chassis.

Trout

7th October 2008, 03:50 AM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	No problem with the electrical configuration, wiper can float. I would probably not recommend switching with the power on - could generate quite a pop. For improved reliability on the bias side I would probably put a 20K resistor across the switch contacts from the bias pot wiper to the pair of 220K grid bias resistors so if the contact becomes unreliable you do not have an open in either position of the switch. Other approaches to accomplish the same end should be equally fine, the key is to prevent an unbiased runaway condition. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

7th October 2008, 12:09 PM	#3
porkchop61 diyAudio Member Join Date: Aug 2004 Location: Mansfield, Connecticut	I have this arrangement on my "November" build from the AX84 site, and it works fine. As Kevin said, don't switch it with the power on Look at the schematic here: November AX84 amp schematic __________________ "Imagination is more important than knowledge"

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7th October 2008, 05:20 PM	#4
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	Thanks Guys! Looks like its good to go. Trout

7th October 2008, 11:15 PM	#8
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	I actually don't use a cathode bypass capacitor. 2 of the above circuits used to switch either or both pairs of a parallel push pull 6V6 amp (Also has triode pentode switching on each pair). If you want to use a cathode bypass cap then just add a series resistor in line with the bypass cap. Value approx 1/10th of the cathode bias resistor - this WILL change the sound a little in that it will supress some 3rd harmonic distortion in cathode bias mode. I would suggest you actually try the amp without a cathode bypass cap. Seriously lovely. Cheers, Ian

7th October 2008, 11:33 PM	#9
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	I am not actually to worried about pops & bangs. I mounted the switch in a location that would make hot switcjing impractical. He wants this mod more as a way to alter the feel of the amp. After talking with him, more than likely it will rarely be used. He just likes the cathode bias feel better, but wants to be able to switch back occasionally. The KOC mod looks pretty good, but it would be a pretty tight fit in an already over crowded chassis. Trout