5f2a Planning -- Whadda ya got?

[JuneauMike]

I don't know if this is "right" or not -- but on a couple of ~5w SE builds I've had good luck
wiring one of those 5W ceramic resistors at a value of roughly 50ohm below spec in series
with a 100 ohm pot -- that way I can dial in any value from -50 / +50.

View attachment 1079895

That looks like exactly the fixture I need to bias an amp. Do you measure the resistance and then install an equivalent value resistor in place to finish?

 

[Lowerleftcoast]

Whoa! Stuff I've never bothered to learn or understand

This is where it gets good. RC filters are how we *voice* an amp. It is a techs parametric EQ so to speak.

Search for *Passive Low pass RC filters* and *Low pass RC filter calculators*. A low pass filter is what is used to filter out the *pulsed DC, AKA ripple, AKA AC content* in the DC supply.

Passive high pass filters use the same math as low pass filters to determine the *cutoff frequency, AKA corner frequency, AKA crossover frequency, AKA knee*. High pass filters are the filters used in most tone stacks and coupling caps are the *C* in RC filter. The *presence pot* is part of a RC filter. (And don't forget guitar tone controls.)

Also search for *Passive 2nd order RC filters*. This search will bring up 2nd order, 3rd, and 4th order filters.

Passive HiFi and PA speakers use high and low pass filters to separate the signal. Highs *pass* to the compression driver and lows *pass* to the woofer. The Crossover frequency is 3dB down... so when the compression driver and woofer are *crossed* there is not a volume bump at the crossover frequency. IOW, the speaker will have a *flat response* at the crossover point.

It may be beneficial to study passive speaker crossovers first and then apply that knowledge to the guitar related circuits.

 

[Jeru]

That looks like exactly the fixture I need to bias an amp. Do you measure the resistance and then install an equivalent value resistor in place to finish?

I don’t. I keep it as a kind of adjustable cathode bias. If a different 6V6 down the line wants a different cathode resistance value to be biased ideally I can adjust on the fly.

 

[Lingwendil]

Wanna have some more fun? Sub out the 6V6 for an EL84. Adjust the cathode resistor to suit and you'll have need the drive voltage for a given power output- lets you crank the crunch more.