Help understanding buffered effects loop?

[mark2]

I'm creating a hybrid amp with a tube preamp (Frogpedals) and a TDA2030 power amp. 12VDC for the power amp, upped to 185V by an on-board SMPS for the preamp.

I'd like to add an effects loop, and am considering this: https://www.diyguitaramp.com/fxloop/


1. How do I figure out what pins those leads correspond to on the 12AU7?

2. What specs do I need for the other components? e.g. resistor wattage and V capacity of the caps?

3. (I think I can eventually figure this one out, but...) how or where is it switching off the effects loop? Or do you have to patch it with a cable when not in use?

Thanks a lot for any help or pointers.

[PRR]

> How do I figure out what pins those leads correspond to on the 12AU7?

Read the data-sheet.
http://www.mif.pg.gda.pl/homepages/frank/vs.html
Search "12AU7". (Don't have any stray spaces before or after- this searcher can't handle it.)
General Electric and Tung-Sol are usually good reliable datasheets. RCA also, except sometimes you get a chunk of tubes beside the one you asked for. ("RCA 12AU7" really starts on page 12AB5.) Philips is excellent for European tubes. If you compare all for a given tube, you'll see they usually all copy words&data from the original tube-proposer, but some copy clearer than others.

I have doubts about the bias on the first tube but it will pass signal.

The Return jack is drawn funny, you will figure it out.

[mark2]

Thanks.
From the datasheet you can figure out which of the grid, plate, cathode each of those leads is. It's not discernable to me, though, which is section 1 and which is section 2.

Or does it not matter?

And does anyone know if I need to be using 1W resistors and 250V caps?

[tubegeek]

Section 1 and section 2 are identical and interchangeable on any "Dual Triode" tube like 12AX7 etc. Where it matters is on a "Dual Dissimilar Triode" such as 12FM7 etc. but those tubes are less often used in commonly found circuits for guitar.

The bias on the 1st (cathode follower) section will certainly depend on what exactly the "From Tone Stack" node is connected to - what is the rest of the design?

[mark2]

Thanks for confirming about the sections.
This is the preamp:

[merlinb]

I'm just gonna leave this here:
http://valvewizard.co.uk/fxlooppcb.html

[mark2]

Thanks, I came across this but my problem with this and to a lesser extent the schematic above (which has more values filled in) is that I'm not sure what values to use for the caps and resistors.  I also don't know what rating they need to be (e.g. 250V and 1watt? Or is 1/4 watt and 100 V just fine?)

Any help or tips would be appreciated.  My fallback plan is to simply 1. use a zero-component simple unbuffered loop and see what happens, 2. omit it, or 3. (ouch) buy one of those $90 metro boards.

[PRR]

> It's not discernable to me, though, which is section 1 and which is section 2.

The GE (most US) datasheets say TWIN Triode. Both halves are identical twins. Like if you could buy beer in 2-packs: both cans taste/work the same.

(Yes, many Euro sheets say "dual" and some other US Sheets say "dissimilar" when the pack two different types in one bottle.)

[merlinb]

Thanks, I came across this but my problem with this and to a lesser extent the schematic above (which has more values filled in) is that I'm not sure what values to use for the caps and resistors.  I also don't know what rating they need to be (e.g. 250V and 1watt? Or is 1/4 watt and 100 V just fine?)

Here:

If your HT is 185V then you can use 200V (or more) rated caps.

[mark2]

Hi Merlin. That's really going above and beyond, and I sincerely appreciate your help.

Knowing the ratings of those caps and resistors not only gives me the confidence to build it, but now I can also work backwards to truly understand why. I'll spend some time studying this tonight.

I've done a ton of googling to find a thoroughly documented fx loop schematic like this, and didn't find much so hopefully others in the same situation will come across this and find it helpful too.