How Reliable is This Bias Circuit

[SilverFox]

Carvin obtains the source voltage for the Bias on a number of amps, by tapping of the HV, 350 VAC to the right of C45. WOW! What if that cap fails? Red Plate? Burn out the output tubes?

Is this a good way to obtain the Bias source? Should there be a fail safe or does it already have one built in? WIMA double insulated cap?

silverfox.

[martin manning]

Marshall does that too, and if you have a FWB and no dedicated bias winding it's necessary. Typically it will be spec'd as a Class X capacitor.

[Stevem]

If this amp is yours you can get less ripple on the bias voltage by rebuilding that bias circuit and taping the voltage feed off of both legs of the v+ winding.

[Andy Le Blanc]

ampeg did this a lot... works fine... its the same thing as a speed selector on a fan.

the larger the cap value the greater the current , so you can tweak it.

[frankdrebin]

I have a 20+ years Carvin,no problem with the cap but with the pot,it burned and started redplating.
Better substitute that with cermet type.

[Structo]

That's good to know Frank.

A lot of people own Carvin amps, myself included with a Belair.

[pdf64]

That arrangement (and the regular Fender one ) can be made more resilient to pot failure by wiring the bias trimmer as a variable resistor, with the wiper linked to the 'other' track end.
Part values will need to be tweaked to accommodate this.

[jaysg]

I can't find it, but a while back I looked into the subject. If you get 10uF Solens to replace the electrolytic(s), it takes a lot of space, but would be more reliable.

[Structo]

I've seen guys use the film caps, like Solens, for bypass caps.
They are larger but if you incorporate them into your design for the circuit board you can accommodate them.

I'm curious how they compare to electrolytic caps sound wise.
If you like them they should last forever.

Also, apparently you can use motor run caps for the power supply but I never looked into it.

I think everything from the power supply to the preamp will affect the tone or overall sound of the amp as well as playability (stiff, fluid, sag, etc) for your style.

So when we build our heroes amps, we should first build them as was intended using the same components where possible.

Or not.

[tictac]

also jumper the pot input to wiper with a x10 value resistor, 220k in this case, to prevent loss of bias V in case the wiper loses contact while adjusting, or for any reason adds a little peace of mind.

(or rebuild bias circuit like Marshall which yields max bias V if wiper fails)

TT

[Roe]

If this amp is yours you can get less ripple on the bias voltage by rebuilding that bias circuit and taping the voltage feed off of both legs of the v+ winding.

I plan to try this on a underfiltered amp which suffers from ghosting when running flat out (it's a 67 dual rectifier 100w plexi, aka black flag 100w)

[SilverFox]

See pg. 6 for the bias circuit in question. The question I have is: Should the bridge rectifier fail will it take out the output tube or some other catastrophic failure mode? Doesn't appear so.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

silverfox.