tube effects loop on board

[tubenit]

This is a condensed edited version of a thread about installing an on board active effects loop.  It eventually got into the idea also of an outboard effects loop.

There is some additional information in an editable form here:  http://www.el34world.com/Forum/index.php?topic=4274.0

In response to what does an active effects loop add to an amp's tone:

I've never used an active effects loop so all I can say is reportly  that some active effects loops seem to eliminate an impedence problem that some pedals would have. For example, it seems like someone on the forum tried using a Holy Grail Reverb in a passive effects loop and it did not work properly. Perhaps it would with an active effects loop?

The other thing I see some people saying is that it improves the tone of the effects pedal giving it a more "3 dimensional"  feel/tone.

With respect, Tubenit

[Fresh_Start]

Below is in response to the cathode follower NOT having a resistor on the plate as was pointed out by a forum member.


I can't confirm whether or not there should be a plate resistor there for that particular circuit.  However, Merlin's discussion of an AC cathode follower (which that first stage is) does not include a plate resistor.  Also, there is a sample series effects loop at the bottom of that page with a very similar looking buffer except for the plate resistor.

The buffer stage Kevin O'Connor shows in his (modestly labelled) "best all tube effects loop" also does not have a plate resistor.  It does have a very different topology designed to provide a "constant source bias" which supposedly makes the buffer more transparent.

One last thought:  Merlin and KOC both recommend use of a 12AT7 for a tube-driven effects loop.

Chip

[tubenit]

Now I am debating between a parallel VS. series effects loop. I found where people seem to have preferences of why one is better then the other.

This will go into a Tweed Overdrive Special so I am somewhat hesitant about series because I don't really need any added gain. However, I have read that some series effects loops really don't "audibly" add any gain to the tone?            Tubenit

[kagliostro]

Can only say my son (technician of the sound) prefer parallel intakes

like in Koch Twintone II (that can do both serial and parallel)

http://www.koch-amps.com/index.php?option=com_content&view=article&id=92&Itemid=16

Kagliostro

[JayB]

I  build using one like this without the feedback. Seems to do the job well and I'm happy with it. I never liked parallel loops much or at least any of the current designed parallel loops out there.

I've been wanting to try a parallel loop using two buffers and one return driver but that would require one more tube and possibly wasting a triode to do it. I think it would be a better design for a parallel loop.

[JayB]

Jay - I don't "get" the topology of a parallel loop using two buffers.  Do you have a block diagram or something?

Chip

Something I haven't gotten around to trying yet.

[G._Hoffman]

I used an active effects loop (essentially) a Stout.  I lifted it out of KOC's TUT 1.  

Honestly, I'm not wild about it.  It makes the amp just a bit more sterile sounding - I'm not sure that's the best way to explain it, but it is very clearly different.  My next approach to it is going to be to take a tap off of the output transformer, and either go into a separate amp or back into the original amp on either a second channel, or just straight back into the power amp.  Obviously, this requires setting the delay to output only the delayed signal, with no dry signal.  My Boss DD-20 can do this, and the Empress Superdelay I want can do it as well.

Speaking of which, how would one go about figuring out the appropriate voltage divider to get the line out signal right?  I mean, it is basically similar to a negative feedback loop, but not negative.  
Gabriel

[LooseChange]

IMO the TUT parallel loop works very well. Gabriel may be right about being a bit sterile. But it does the job well.  I found the passive effects loop in the Dumble designs does not work well. It overloads my pedals.  So I built a Dumblator (Like Tubenit's design above). I modified it a bit to do parallel and removed the tone shaping parts. It did wonders for that amp and works well in every passive loop I have.

I hope I said something that helps. 

[JayB]

LC and others,   THANKS for all the replies. I've narrowed down my choices to something I will install and then tweak.

Question about grounding the effects loop components:

1) I am presuming all the components should be grounded on the buss wire with the rest of the preamp?  Correct?
    (meaning it's not separate & has it's own grounding point)

2) The 250k pots will be on the back of the chassis panel. Do I need to ground them on the buss wire also?

I sure appreciate all the comments and help.  With respect, Tubenit

1. I do and its not a problem.

2. I ground those with their own buss wire that is soldered to the back of the pots. It was just simple, easier to do for me and looked neater.

[Structo]

Check this out for a parallel loop.

The bottom one is the original showing the power requirements.

[tubenit]

Well, I got the tube effects loop in.  Here are my observations:

1) the effects loop is part of "the" tone that I have been hearing with Dumble type amps. I don't know if you have
    ever used a photoshop feature that enhances the color in a picture? This is sort of an audio version of that idea. The
    tone become much clearer including the overdrive. It has a creamier overdrive using the effects loop for some reason.

2) with the tube effects loop, the sustain is unbelievable, harmonics are lush and it has this huge 3-dimensional tone.
    since I don't use pedals much, I have not had much experience with delay pedals so I haven't had a good reference
    point of what they can or "should" sound like (prior to this).  

3) I can unplug the delay pedal and use the active loop to add another overdrive tone.

4) it was a royal pain to install since I've not done one before. One of the most time consuming tweaks I've ever done.
    AND it was worth it!  I think I could do another one in about 3hrs now that I know how.

5) I like a 5751 driving this loop real well. Using one of Hoffman's Sovtek 5751's and it sound really good there.

6) I like reverb alot.  I love the delay pedal in this tube effects loop. It's a cooler tone for what I am wanting.

7) after hearing the Boss digital delay within a tube effects loop, hearing it in a passive effects loop sounds pretty
    anemic in comparison (although I previously thought that sounded OK)

With respect, Tubenit

[Fresh_Start]

Rzenc - how does that polarity switch work?  It looks like you would need a special cable with signal & "ground" on separate conductors for the stereo jack and a separate shield.

Thanks,

Chip

[sluckey]

The polarity switch simply flips the transformer windings. In a balanced line there is no 'ground' line. You have two signals that are 180 degrees out of phase with each other, such as found on the secondary of the transformer. These two signals would then be applied to a twisted pair of conductors that have a shield wrapped around the pair. (Remember the STP cable I sent you a while back? Balanced line signal transmission is why that cable exists.) The other end of the twisted pair must connect to another transformer to convert back to an 'unbalanced' signal. Better yet, connect to a differential amplifier. A differential amp will amplify the difference of the signals on the twisted pair. Any outside interference (noise) would likely appear the same on each line of the twisted pair, and the diff. amp would effectively reject that noise since it only amplifies the difference signals. IE, if you have a positive 1 volt noise pulse on each line, the diff amp will subtract those pulses, leaving zero volts noise. Works the same with two AC noise voltages that are the same phase. This is called 'common mode rejection', and is a desirable characteristic of a differential amp. The higher the rejection factor, the better the diff amp.

BTW, the LTP PI is a differenial amplifier.

[rzenc]

Better yet, connect to a differential amplifier. A differential amp will amplify the difference of the signals on the twisted pair. Any outside interference (noise) would likely appear the same on each line of the twisted pair, and the diff. amp would effectively reject that noise since it only amplifies the difference signals. IE, if you have a positive 1 volt noise pulse on each line, the diff amp will subtract those pulses, leaving zero volts noise. Works the same with two AC noise voltages that are the same phase. This is called 'common mode rejection', and is a desirable characteristic of a differential amp. The higher the rejection factor, the better the diff amp.
BTW, the LTP PI is a differenial amplifier.

This means adding one more bottle to the plan to receive the rack return signal.
So your suggestion is to toss the input iron, grab tip and ring signals and input them to a diff. amp?
Like this?

Rzenc

[tubenit]

I am trying to figure out how to make an outboard tube effects loop similar to the one I installed on one of the TOS amps. I want to try to build it in a compact chassis.

Questions:

1)  Will using a regular amp PT work like this to get node B for the tube plates?

2)  Will using this in series create any problems with hum or a ground loop?

With respect, Tubenit

EDIT:  original schematic and layout are deleted here.  Correct ones are dated 12/06/10 further down in thread.

[sluckey]

1)  Will using a regular amp PT work like this to get node B for the tube plates?

I think you need a lot more filtering. I'd suggest a 3 node filter, ie, cap, choke, cap, resistor, cap. Use big caps if you got'em, especially on that first node.  (EDIT: my revised schematic now reflects the advice that Sluckey gave here -Tubenit)

2)  Will using this in series create any problems with hum or a ground loop?

That's always a possibility when you start adding more cables between more boxes. I'd just build it and tackle ground loop problems if they pop up.

So, is this right? You're gonna put a pair of preamp out and power amp in jacks on an amp (effectively a passive loop). Patch those jacks to another box that houses the active loop components. And finally patch to the actual effects boxes. Is this so you can share the active loop with different amps?

[topbrent]

Tubenit, as your loop is basically the Dumble-ator, here are some thread links from AmpGarage that have some pertinent data.

I know you visit Ampgarage also, so you I would presume you have already seen these threads during your studies, however for those following along this info may be helpful.

Henry (heisthl) has many threads over on AG about his internal and external D'lators.

Here is a nice external with schem: uses a fender stand alone reverb unit PT
http://ampgarage.com/forum/viewtopic.php?t=4748&highlight=dlator

Internal D'lator power supply questions: http://ampgarage.com/forum/viewtopic.php?t=3923&highlight=dlator

http://ampgarage.com/forum/viewtopic.php?t=15146&start=0&postdays=0&postorder=asc&highlight=

Parallel D'lator: http://ampgarage.com/forum/viewtopic.php?t=10111&highlight=dlator

Brandon's essay on the DumbleAtor:  http://ampgarage.com/forum/viewtopic.php?t=4084

[topbrent]

Here is Bludotone (funkaliciousgroove) Brandon's info: http://ampgarage.com/forum/viewtopic.php?t=4084

Very informative.

[tubenit]

TopBrent,

Hey, thanks!  No I had not seen all of that & I had actually done a search there. That was helpful.

With respect, Tubenit

[sluckey]

Gabriel suggested using a 12VAC wall wart that Hoffman carries.

It seems to me, if it is an outboard piece of kit, it may be a good place to use a 12VAC wall wart, and a step up transformer with your power supply.  For one tube, you can certainly use it, and it would probably save a bit of money over a normal amp PT.  I'm believe Doug sells just such a transformer. 

I asked how would that work & Sluckey replied below:

Hoffman's Champ PT is only $30

Better double that!  You wont need the filament transformer because you use the 12v to power the filaments. So just get two 12VAC wall warts. Bust one open and get the PT out.  Plug wart #1 into the wall. Connect the 12vac sides together (this is where you connect the 12A?7 filaments also). Use the 120v side of wart #2 to provide B+ thru a bridge or voltage doubler.

Doug has a toroidal transformer that's ideal for this project. Pick up a 12vac wart at Goodwill for real cheap. Might be worth checking out if you're wanting to keep this unit small.

Here's Dougs schematic showing how to use this method...

[G._Hoffman]

Below is a response to Sluckey's post above:

What he said.  Doug's Champ PT is $60 (the OT is $30).  His Wall Wart and the Toroidal transformer is $37.  Not a huge savings, and it requires a couple extra parts for the heater supply, but you can get the 7812 for less than a dollar each, the rectifier is pennies, and a couple 100-1000uF caps for maybe a dollar, and you're good.  If you are using it to heat a 12a_7 or two, I'd probably want to put a heat sink on the 7812, but again, they are pretty cheap.


Gabriel

[tubenit]

I thought about changing the previous layout I drew up to have the effects pedal plug into the front & the in/preamp and
out/ltpi be on the back panel along with an on/off, fuse and cord, etc......

The passive effects loop on the TOS is on the back of the head so I thought maybe this one makes more sense?

With respect, Tubenit

[joelap]

Jumping in after much discussion has already been ongoing.

I've been building a bass preamp with a series FX loop.  Admittedly, I'd spent more time with a Calculator and paper/pen figuring out the numbers than I spent actually constructing various loops.  A few observations on some of the schematics posted based off of Merlin's book on preamp design (which I highly highly recommend picking up if you havent yet).

Many of the schems posted have a 250kA send potentiometer...  isn't this high impedance almost negating the low impedance output of the cathode follower (and the reason you'd use one) in the first place?  What I have been using is a DC coupled cathode follower, with your normal Rk you would have chosen (usually 100k in most situations, 47k in mine) in series with a much lower resistance.  In mine, I'm fluctuating between 2.7k and 4.7k.  From there, the signal level and voltage is attenuated greatly, and you can use a lower voltage electrolytic cap to pass very low frequencies and have a low value pot as the send control, such as 5kA.  As my preamp is set up now, I've got 47k in series with 3k3, 50uF 50V // .1uf film for the coupling cap, and a 5kA send control.  Passes low frequencies down to 10Hz measured on my oscilloscope and has attenuated a hot (but still clean) preamp down to 3V pk-pk from a 500mV 500Hz sine wave at the input jack.

Merlin's book also describes a switchable series/parallel FX loop.  May be of interest to some of you.  I'm not a parallel loop fan, and I've seen many people who have converted parallel loops to series ones on big dollar mesa amps.  I'd rather have a series loop, and then modify whatever pedal I was using in the loop to have a blend control, thereby having control of the amount of wet/dry signal on a per-pedal basis.  Of course, this doesnt work for multiple pedals in series but nothing is ever a 100% perfect option, is it?

-JL

[JayB]

Many of the schems posted have a 250kA send potentiometer...  isn't this high impedance almost negating the low impedance output of the cathode follower (and the reason you'd use one) in the first place?  What I have been using is a DC coupled cathode follower, with your normal Rk you would have chosen (usually 100k in most situations, 47k in mine) in series with a much lower resistance.  In mine, I'm fluctuating between 2.7k and 4.7k.  From there, the signal level and voltage is attenuated greatly, and you can use a lower voltage electrolytic cap to pass very low frequencies and have a low value pot as the send control, such as 5kA.  As my preamp is set up now, I've got 47k in series with 3k3, 50uF 50V // .1uf film for the coupling cap, and a 5kA send control.  Passes low frequencies down to 10Hz measured on my oscilloscope and has attenuated a hot (but still clean) preamp down to 3V pk-pk from a 500mV 500Hz sine wave at the input jack.

That's why I use a 1 - 2.7 uf metal film cap for the coupling cap. For pedals, I'm down to maybe 3k on the 250k pot. About 100k for a effects processor that needs that pro-level signal. Trying to match the output to the effects input is the trick and I haven't come to a one size fits all solution yet.

[tubenit]

Reportedly this layout is from Valve Wizard's information. Check for errors!

http://www.freewebs.com/valvewizard/accf.html

With respect, Tubenit

[TIMBO]

Hi Tubenit,I'm not sure where you got the schem for that FX loop "TIMBO".That circuit was from http://www.el34world.com/Forum/index.php?topic=11645.0 and i'm not sure how well it works ( also that was from another schem that was not the final draft and the bypass switch is wired wrong. Tone Junkie built that amp without the FX loop but in the blurb that came with that post from metro amps, the guy that came up with the concept said that the FX loop could be used to add more gain to the amp circuit. I've drawn the schem with my own mods and i am in the process of the build. Thanks

[tubenit]

This is Doug's loop.

Tubenit