Unconstant constants

[Reeltarded]

In my new amp I used a 1.8k/.68 that looks like 130Hz. I want to keep that but the 1.8k is a little too hot.

I want to add 1k on that without changing the constant.. so I just string a 1k on it between it and the tube, right? Seems I will keep the 130Hz curve, but have an effective 2.8k cathode.

Either I am starting to understand this stuff enough to think for myself, ormI don't get it enough to consider myself anything better than a monkey.

It's ok. I like 'nanners.

[roberto]

10k between the 1k8//680n pair and the tube?
Look at my thread about how gain stages sound. You will find some indications on frequency shape.

[Reeltarded]

Very good then. Thanks Roberto, you would like this amp!

[tubeswell]

In my new amp I used a 1.8k/.68 that looks like 130Hz. I want to keep that but the 1.8k is a little too hot.

I want to add 1k on that without changing the constant.. so I just string a 1k on it between it and the tube, right? Seems I will keep the 130Hz curve, but have an effective 2.8k cathode.

Either I am starting to understand this stuff enough to think for myself, ormI don't get it enough to consider myself anything better than a monkey.

It's ok. I like 'nanners.

To keep the frequency roll-off the same, a simple way to remember it is that if you double the R, you halve the C, or vice versa.

Seeing as how 2k8 = 1.55 x 1k8, then you need to multiply the C by the reciprocal of 1.55 (i.e.: 1/1.55) to keep things the same, which in this case is the same as saying .68/1.55 (which is .44)

To test this using f(half-boost) = 1/(2π.Rk.Ck):

1/[(44/7)x1800Rx0.00000068F] = 130Hz

1/[(44/7)x2800Rx0.00000044F] = 129Hz

There you go

[Reeltarded]

I see, but what I mean is adding the 1k between the bypassed 1.8k and pin 3, not bypass the full 2.8-ish k value in total.

[surfsup]

I believe you will set up to attenuate highs above 250Hz and pass low frequencies below that, which would effectively boost bass...

?

[roberto]

To keep the frequency roll-off the same, a simple way to remember it is that if you double the R, you halve the C, or vice versa. There you go

No, it's not that easy. You have to take care of the frequency shape (so the shelving effect) of bypassed and unbypassed range. An higher cathode resistor will sound brighter than a fat low value resistor, also if the RC is the same. Please look at my thread, it's in the stiky thread and it's all explained with graphs of different combinations. Also plate resistor change the shape of the shelving effect.


As for the 1k8//680n plus 1k in series, You change shelving of the treble boost, output impedance of the stage, local feedback and so on.

You can use this trick also to add a little bit of positive feedback on that stage.

[Reeltarded]

Thank you again for doing the work. I experimented for a few hours tonight. You could spend a decade tweaking one stage.

[roberto]

At least.

So what's your favourite configuration?
Have you tried the positive feedback trick?

[VacuumVoodoo]

What positive feedback "trick"?

[roberto]

In fact it's not a trick, but the word "trick" always put forum people on a positive mood to try it.

I can't write anything at the moment, but just figure out that Rk//Ck 1k8//680nF and 1k to ground. Then add an in phase signal coming from following stages to the point between 1k8 and 1k, and you'll have positive feedback.

I've used sometimes in lead channels. Of course you have to add just a little bit, otherwise you'll obtain only whistles.

[VacuumVoodoo]

OK. That's the one that can really trick some SPICE simulators into outputting very optimistic results. Like 55db gain with a single ECC83 triode..
Personally, I like bootstrapping anode resistor better. It can be seen as a form of PF too.

[roberto]

In fact you make me poit out that I've never SPICEd that PF configuration.
I've just done some try - write down - mod by ear.

[tubeswell]

To keep the frequency roll-off the same, a simple way to remember it is that if you double the R, you halve the C, or vice versa. There you go

No, it's not that easy.

I said it was a simple way to remember, and it is. (I was merely discussing the basic concept underlying how R and C affect f.)

You have to take care of the frequency shape (so the shelving effect) of bypassed and unbypassed range.

Ok - so I understood Reeltarded to be asking about how he would keep the same 130Hz rolloff gotten from bypassing 1k8 with .68uF, if he was wanting to bypass 2k8 instead. (However he later further explained that this wasn't what he was wanting to know)

An higher cathode resistor will sound brighter than a fat low value resistor, also if the RC is the same. Please look at my thread, it's in the stiky thread and it's all explained with graphs of different combinations. Also plate resistor change the shape of the shelving effect..

Well I wasn't getting into a discussion about the effect of changing the operating-bias point relative to the transconductance of the tube or changing the load line. Those are opening another can of worms. I was merely talking about f in terms of Rk||Ck.

[Reeltarded]

No need for feedback at this point. It's 4 stages with a clipping diode arrangment.

1.8//.68
9.75//.1 <--- this one is crazy to deal with
10k
820//.68+250


I had to shunt 250p at first mixer and 500p at last mixer. I am going to tinker with values on that 1.8 stage today and find a happy place.