Quick question on old schematic... Not sure what to use.

[tubedood]

Hi again folks.. A while back I asked a few questions on an old build I was about to revisit and I am about to finally dig in! On question I have is the author used a telephone 600 ohm to 600 ohm matching/coupling transformer as an inductor on a mid cut. He did so according to the text due to "good audio inductors being hard to find". The circuit should cut/boost at about 630 hz. I was wondering what the proper value inductor would be if I tried to find one instead of my old cheapie phone transformer? I used an online series LC calculator and plugged in the numbers of the .047 cap used and tried henry values till I got the 1.35 H that got the frequency to about the 630hz. Does anyone know who might have the right said inductor? I have attached the schematic for reference. Thanks in advance for any suggestions and your time!

Dale

[tubedood]

And also if any has a second to answer another question what is the purpose of the 470pf capacitor in series with the 470pf capacitor to ground in the tone shaping section? I am trying to wrap my head around some of the thoughts in this circuit.


Thanks again,
Dale

[Steve A.]

For guitars I used the Dan Torres mid-boost/cut control based on the same Mouser audio transformer that Craig Anderton used in his book on projects for the electric guitar:

Mouser Audio Transformer #42TL021
Outer P(rimary) terminals 1.5H

I removed the boost portion (on top of drawing) and wired the midcut to a pot in my 1978 LP 25-50 for a really slick control that cleaned up the sound. (I need to reverse engineer that mod to see what value resistor and cap I used.)

Wired as mid-boost/cut in my strats there was considerable signal in supposedly neutral middle position.

Steve A.

PDF of circuit:

dual_mid.pdf
.

[trobbins]

Hi Dale, It's probably easier to appreciate the role of the 470pF caps by thinking about the frequency response through that circuit if one of those 470pF was not involved (ie. the series 470pF was shorted), and then think about the response if instead, the other 470pF was not involved (ie. 470pF to ground was removed).

My view of the inductor is that it would need to be 'practical' - ie. cheap, available, somewhat screened, and able to be easily mounted, had an inductance that then didn't require an impractical capacitor to be found, and didn't have a very high DCR. I've used relay coil windings of about 0.1-0.2H for the same role, as I had the relays spare and could diy a coil in to a usable part. For a manufacturer it may be about buying in a batch of small transformers that nobody ever uses any more.

Ciao, Tim

[tubedood]

Thanks for the replies guys it's very appreciated! Steve your post is enlightening and interesting! ... the 1.5H is pretty close to the numbers I plugged into the LC calculator for an inductor in series with a cap that seemed to yield 1.35H for that frequency of 630Hz. I have so much to learn. Tim I am still at a lil lost about the caps in series and 1 to ground.. are they treated as 2 caps in series... yielding 235pf to ground per the old formula for series caps or does that actually split at the intersection? To my old eyes I see the first 470pf letting a frequency window pass thru then the other 470pf passing it to ground (removing it?) Sorry guys if these questions are novice... I am good at a few things in life but tubeamps is a hobby I am trying to grow in!
Thanks already for your time and input.

Dale

[trobbins]

If the series 470pF was shorted, then the circuit would be a low pass filter, with a 3.3/4.3 divider at low frequency, and a 340Hz corner frequency.

If the 470pF to ground was removed, then the circuit would be a high pass filter, with 3.3/4.3 divider at high frequency, and a 100Hz corner frequency.

RC filters start and finish nominally a decade below and above their corner frequencies. The two corner frequencies are quite close, so the response interacts, and a slight gain bump may be achieved in the 100-340Hz region.

But simplicity becomes complexity when all the surrounding circuits are included - such as the vol pot is not at max, and the tone stage is included, etc .

If you hooked your amp up to a soundcard and used software with a sweep (such as REW), it can be quite easy to see the response change with pot settings. Or you could simulate the circuit in LTSpice. Or you can use your ear.

[tubedood]

OK Tim that brings the idea to an understanding (somewhat hahaa)… so the overlap frequency of the high and low pass design is allowed to pass thru for the mid boost. Thanks again to you and Steve. Most appreciated! I am sure I will have plenty of other novice questions to ask this fine community.

Dale