My next project -- Heavy

[emrr]

right

 

[CurtZHP]

Now, back to the HPF.  Every other schematic I've studied does the same thing (or similar enough).  I'm just having trouble getting my head around it.

Putting another, smaller, cap in series with the first stage coupling cap is supposed to create a HPF that isn't affected by the position of VR1, even though VR1 is part of that RC circuit.

When calculating the R in RC, do I just plug in the value of the pot (250K) or the value of the resistance between the wiper of the pot and the point where it connects to the cap (which would depend on the position of the wiper...)?

 

[CurtZHP]

OK, so after a good night's sleep (apart from the nightmare about falling through the stage...), I finally pulled my head out of my "low-end" as far as the HPF on this beast.  (I think...I hope...I pray....) 

I think I've got it.  In this version, the HPF should provide a roll-off at around 85Hz.  I decided against going as high as 100.

Still on the fence about substituting the plate resistor on the second stage with a pair of chokes.  This thing's going to weigh a ton as it is!  (Which is why the working title of this project is "Heavy!")  ;D

 

[Heikki]

In this version, the HPF should provide a roll-off at around 85Hz.

65Hz

 

[CurtZHP]

65Hz

GAAAA!!!

 

[PRR]

>>     around 85Hz.
> 65Hz

I get 72Hz.

Are we accounting for the 12AX7+100K acting like a ~~39K source impedance?

 

[CurtZHP]

>>     around 85Hz.
> 65Hz

I get 72Hz.

Are we accounting for the 12AX7+100K acting like a ~~39K source impedance?

Couldn't find a formula that took that into account.  How would I account for that?

 

[emrr]

I would account for it with actual measurements on the bench.

 

[abbey road d enfer]

Couldn't find a formula that took that into account.  How would I account for that?

Use a simulator. LTspice is good for that. It agrees with ca. 65Hz. F=1/2pi.R.C with C=8.2n and R=250k+39k

 

[Heikki]

>>     around 85Hz.
> 65Hz

I get 72Hz.

Are we accounting for the 12AX7+100K acting like a ~~39K source impedance?

My ballpark figure was 50k for source impedance, but for some reason I thought C10 was 1u not 0.1u. So 50k+250k and 0.1u+0.0082u is ~70Hz.

Edit: By C10 I mean C1. I got confused because the component annotation and value are so close to each other.

 

[CurtZHP]

I think, as long as the roll-off doesn't get too close to 100Hz, I'll be fine with it.

At this point, I think I've got enough to get started putting something together on the bench, and we'll see what floats.

 

[CurtZHP]

Sorry for the long absence.  Got insanely busy on actual food-on-the-table work related projects, and a man's gotta eat!

Started amassing parts for this Frankenstein's Monster, and naturally can't leave well enough alone with what I have so far. 

Came across a pair of UTC LS-21s, which I understand to be interstage transformers, which I think I understand are put between amplifier stages in lieu of the usual (de)coupling capacitor.  Sort of like a choke, but not really.

Anyway, I got to wondering if it was worth the aggravation to try it.  Looked up the pinout, and it didn't make much sense, so I'd have to find out more about it.

Any upside?  The one downside I see so far is that this thing's going to end up being so darn heavy I'll have to leave it on the floor!

Maybe just stick to emrr's idea of using a choke instead.