can you mimic a FAC knob with a graphic eq?

[justinhedrick]

well?

[FullCustom]

You should be able to.  The FAC knob is low frequency roll off. 

[justinhedrick]

You should be able to.  The FAC knob is low frequency roll off. 

is that all it is?

[Mr. Foxen]

Where in the circuit makes some difference too. Depends where you can patch in your EQ. Don't actually know where the FAC is.

[Hemisaurus]

FAC position seems to vary by design, on some it's right at the input, on others it's after the first valve stage. You could easily make a FAC box, to plug into the input of your amp, it's all entirely passive, you just need a rotary switch and some capacitors.

I think Orange FG have some schematics

[Hemisaurus]

Just imagine the switch and caps in a separate box.

[VOLVO)))]

Paging Nick, he's got a good FAC design.

[justinhedrick]

where does nick put his?

i thought they almost all came first in line?

[Hemisaurus]

Not on the 200's or the Matamps, or at least some of them

British tube amps are very much a mix and match affair, this preamp with that power section, these mods made for these tubes on export models, etc. Use any schematic as a guide, but don't be surprised if it doesn't match up

[justinhedrick]

Not on the 200's or the Matamps, or at least some of them

British tube amps are very much a mix and match affair, this preamp with that power section, these mods made for these tubes on export models, etc. Use any schematic as a guide, but don't be surprised if it doesn't match up

well, a buddy of mine is going to take a stab at building an OR120 type head and i'm going to be testing it for him, but it got me thinking about the FAC knob. then i saw on nicks amps he has a swirl knob. not sure what that does.

I also think he should put an EQ defeat switch on it . . . thoughts?

[Hemisaurus]

EQ defeat on an amp with a passive tonestack is going to scare you. A tonestack has about a 20dB insertion loss, taking that out is going to make the amp very loud and distorty

You might play with the idea of switching out the tonestack and puttin in a pad of some kind, to compensate for the volume jump, not all of it, you may want some of that extra gain, but at least some of it might be necessary.

The key to Orange is the Bax stack and the FAC, don't be tempted to fit a mid knob a la the Weber, it won't sound the same. See previous thread.

[dunwichamps]

I use the FAC like a matamp, before the Bax.

It has an adjustable RC cutoff. Theres a few other controls which are similar, a Shelf Filter will also be a decent approximation, it operates a little differently than a FAC but it can achieve a similar setup. 

[bitter]

Not speaking about any particular amps, but is there a difference between an external FAC setup right before the input and an onboard FAC right after the input?

[justinhedrick]

Not speaking about any particular amps, but is there a difference between an external FAC setup right before the input and an onboard FAC right after the input?

not if it comes before the first gain stage.

[dunwichamps]

Not speaking about any particular amps, but is there a difference between an external FAC setup right before the input and an onboard FAC right after the input?

the idea of the control is the same but ud have to adjust the values of the cap to achieve the same RC cutoffs, different source and output impedances

[justinhedrick]

Not speaking about any particular amps, but is there a difference between an external FAC setup right before the input and an onboard FAC right after the input?

the idea of the control is the same but ud have to adjust the values of the cap to achieve the same RC cutoffs, different source and output impedances

so nick, since you use yours after the first gain stage, how did you decide on the values of the caps/resistors?

[dunwichamps]

Depends on how you wire it, Series or Parallel style

The R value is fixed by the impedance of the first stage, and the resistance of the gain pot (usually 500k or 1M)

In parallel style, I choose standard values, for latest amp, 47n, 22n, 10n, 4.7n, 2.2n, 1n. (all standard Mallory 150 values) with a 68nF cap right off the first stage plate resistor.

Here is how i wire it in parallel (this is a dual FAC)



If I was wiring it for a single FAC I could increase the number of selections beyond 6, up to 12 so I could extend the number of caps with values down to 220pF if I wanted.

In series wiring now you have a 1/c series relationship so the response is different. I just pick things which I like experimentally rather than calculate it.

Series wiring here