Active fx loop

[Bugman3183]

Good afternoon all.   I am in the process of building a yeti 50 using the layout  provided below.  This amp has a passive fx loop and I had questions on how tho add an active one.   First, I'd like to know if the one on the layout below (provided by tubenit) would work, and if so, where would I connect the 1meg resistor that's at the junction of the 1.8k and 27k resistors on tubenits layout. (Located at v2 pin 7 grid)  I would guess the right log of mv, but then, I know nothing about fx loop which its why I'm here asking inn three first place.  Any info would be greatly appreciated.   Thanks.

[purpletele]

Good afternoon all.   I am in the process of building a yeti 50 using the layout  provided below.  This amp has a passive fx loop and I had questions on how tho add an active one.   First, I'd like to know if the one on the layout below (provided by tubenit) would work, and if so, where would I connect the 1meg resistor that's at the junction of the 1.8k and 27k resistors on tubenits layout. (Located at v2 pin 7 grid)  I would guess the right log of mv, but then, I know nothing about fx loop which its why I'm here asking inn three first place.  Any info would be greatly appreciated.   Thanks.

An FX Loop is part of my next project which I will post below.  I wanted to learn about how it works and such.

That drawing that Tubenit put together looks to me like a Passive Loop, where the signal is being inserted right into the circuit.  In the HoSo that I am building the signal is processed through a tube and then into the circuit.

I may be incorrect in the description, but maybe someone can verify the difference.

[2deaf]

You guys need to post schematics. 

Some effect loops are striving to be transparent and some are an integral part of the amplifier's sound.  I can comment on the transparent ones, but the integral-sound ones are a matter of taste and opinion. 

[pdf64]

...I'd like to know if the one on the layout below (provided by tubenit) would work, and if so, where would I connect the 1meg resistor that's at the junction of the 1.8k and 27k resistors on tubenits layout. (Located at v2 pin 7 grid)  I would guess the right log of mv, but then, I know nothing about fx loop which its why I'm here asking inn three first place...

Layouts are for building stuff, but for design / analysis, a circuit diagram aka schematic is necessary.
I think that the likelihood of ever understanding properly how a circuit works from a layout is low.

[Bugman3183]

 Here's a schematic of the layout in my original post of tubenit's amp.  Thanks.

[tubenit]

Since you are asking about adding an active FX loop to the Ceriatone ............. we need to see a schematic of the Yeti   (not Ryan's amp which already has a proven schematic and layout design).

Reportedly,  the schematic in the first post of this thread is similar to the Yeti.  However,  I'd advise you look over the Chupacabra schematic and make note of differences between it and the Yeti and post a proper Yeti schematic. 

http://el34world.com/Forum/index.php?topic=20664.0

Here's a schematic I found for on the Net for the Chupacabra, which other than slightly different voicing is the same as the Yeti.

Before I ever attempt any amp build,  I always have both a schematic and layout on hand.   You also might want to consider the Metro FX loop for the Yeti?

BTW, I found the Chupacabra schematic and thread by googling  "schematic Yeti amp".

With respect, Tubenit

[tubenit]

OK,  I attached an editable SCH schematic that will head you in the right direction to be the 1st person on the internet to post an accurate schematic of the Ceriatone YETI !!   

Step 1:         Print off a layout of the Chupra from the Ceriaton website and have next to a layout of the Yeti.  Mark differences
                    on the Chupra layout that are different from the Yeti.

Step 2:         Print off 2 copies  the Chupra schematic

Step 3:         Carefully edit one of the copies of the Chupra to match the layout of the YETI.  Yes, this will take you some time
                    but you will greatly benefit from it.  You can do this in pencil noting the differences which reportedly are minor
                    ones.

Step 4:         Now, pull up the editable SCH schematic I've provided and make it match your edited Chupra to Yeti schematic. 
                    Once you've done that, post it on the forum.  There is a second page showing PT & B+ rail and bias.

Now, when someone googles Yeti schematic,  ............ it will be you that will be "that guy" that did ALL the work to provide the amp building communities a reliable accurate Yeti schematic & one that can be edited for the tweakers that will surely be out there. How cool would that be!

Think about this MUCH of the work has already been done for you.  You just have to finish it.

With respect, Tubenit

[pdf64]

That's a great suggestion!
With regard to fx loop design considerations, see http://www.aikenamps.com/index.php/technical-q-a scroll about halfway down.

When weighing up the benefits of an active loop, bear in mind that the detail of how you wish to use the loop matters; if using short leads between amp and +4dBu fx unit, eg a rack delay put on top/under the amp, there may be almost no benefit in an active loop over a passive loop.
Whereas in the case of long leads back to pedalboard fx units, the potentially higher output impedance of a passive loop may roll off a significant amount of treble, possibly overdrive the pedals due to excessive signal level, and the return signal may be rather weak.

[Bugman3183]

That actually is a great suggestion, I'm sure there's a lots to be learned by doing so as well.  Currently I'm just using a cell phone and as such,I do not have access to the programs necessary to open the editable schematic.   At some point I'll visit my brother and knock it out over there.   Right now, I just finished building the amp according to the layout with the exception of a couple of mods.  I converted it to cathode biased and it's set up for 4 6v6s or 2 5881 tubes, also added a ppimv. it seems I have some bugs to work out because it doesn't operate as expected (fairly low volume and possibly some parasitic oscillation.  Time to start troubleshooting.  Good times!  I'll report back if I find anything out of the ordinary and afterwards once it's functioning properly I'll get started on that schematic.   

[Bugman3183]

Btw:  the one thing I have found so far is that with the presence pot grounded with the preamp as shown in layout, when you turn down the presence or resonance pot down it completely kills the signal. Don't know if that's normal or not but when I moved the ground to the power section they seem to function properly.  The couple of amps i have built so far didn't have presence or resonance so that's a new one for me.

[2deaf]

The conventional wisdom seems to be to get an amplifier functioning properly before attempting further mods. 

An active loop can be installed between the Treble wiper and the PI just like any other Plexi.

[nandrewjackson]

This is from my latest build. Works great. I have output send at about 1 O'clock for pedals in the loop. I haven't had to use the return control, it just stays all the way up.


With nothing plugged in the jacks, both pots need to be turned up all the way.


I've heard its overkill, but I've seen more complex ones out there.

[2deaf]

With nothing plugged in the jacks, both pots need to be turned up all the way.

If you connect the switch on the Return jack to the junction of the 25KA Send potentiometer and the 1uF capacitor, you won't need to fool with the Send pot with nothing plugged in.  Using the switch on the Send jack never seemed necessary to me and it doubles the chance of switching jack failure.

[nandrewjackson]

Thanks 2deaf!

[2deaf]

First, I'd like to know if the one on the layout below (provided by tubenit) would work, and if so, where would I connect the 1meg resistor that's at the junction of the 1.8k and 27k resistors on tubenits layout. (Located at v2 pin 7 grid)

I'm sure that it will work, but it may or may not be what you want.  Ryan may be hitting the cathode follower with a very large signal and then setting the 250K send pot very low so that he gets a low impedance output at the Send jack.  You might have a much smaller signal hitting the cathode follower with the tone stack attenuating the signal (especially if the Zeners are engaged).  You might have to turn the 250K pot up to the point that the output impedance at the Send jack is too high for use with longer cables.

Then there is the frequency response shaping that you may or may not like.  I would presume that this shaping is matched to the preceding circuit, but you do not have this circuit in your amp.  Still, it may work out well.

Attached is the way Ryan's loop would typically be inserted in a Plexi style amp.  Also attached is an alternate idea with better transparency and a low impedance Send.  With the alternate loop in the circuit, the Master Volume becomes the Send Level and the Return Level becomes the Master Volume.   

[pdf64]

I wonder why those designs put the tone stack after the master vol?
The tonal balance / control range will vary significantly, according to the MV setting, eg there will be more treble at extremes of the MV, more muffled at the electrical mid point.