Tube Buffered Effects Loop

[NewReligion]

I am so surprised at how great this endeavor to work out a Tube Buffered Effects Loop turned out.

Now other guitarists like my self have yet another option for an Effects Loop.

I am a huge fan of the buffered Metropoulos Zero Loss Effects Loop.

I also stock PCB to assemble Headfirst amplification buffered Effect Loops which are consistent or very close to what Dave Friedman uses which is a high quality circuit.

Then there is the passive loop which is not buffered & severely degrades the guitar signal with 20’ to 40’ cables runs from amp to floor effects & back to the return.

This loop is fine for guys like my self or Neddi Stanz that use rack mount effects which are buffered to send the signal as it should be.

I experienced one buffered loop from Ceriatone that need more R & D IMO. I have worked on no less than 7 Ceriatone amps since then. One just in March oh this year (2023). It had a passive loop in this 100 watt King Kong. It appeared that more than the loop changed. This was dark & muddy with few harmonics. I suspect transformers .

The 2018 & previous King Kong’s I worked on were blistering. But looks like Nik is back to passive loops.

I rarely install a passive loop unless asked to and the guitarist is aware of the issue I have discussed.

I have tried a few tube loops which have came across my bench but always met with disappointing results. But after studying & borrowing some ideas from my memory I was able to complete my goal of a beautiful lush tube driven Effects Loop.

Thanks goes out to Alex Catlow for working it all out then sending me the Gerber file for production.

 

[NewReligion]

EDIT: I tried to load a photo but it was too large.

 

[66 Kicks]

Can you post a schematic this FX Loop?

 

[StingRay85]

Here's one

 

[NewReligion]

Can you post a schematic this FX Loop?

I guess maybe I should not have started a post on this as it is new & proprietary. I looked every where fit data to no avail. It took a lot of work to get this circuit right.

Here's one

View attachment 130404

Thank you. The values look very familiar. The one I just finished has four trim pots.

It can be dialed in to present a really vintage edgy sizzle on top that mimics some of the greatest Marshall + 25 watt green backs I’ve heard.

There is a lot here to work with as it’s applications will be manny.

Thanks for posting.

David

 

[66 Kicks]

... as it is new & proprietary.

Exactly what I was expecting.

 

[NewReligion]

Exactly what I was expecting.

Well for one it’s not quite where I want it as it adds some positive anomalies that can be useful in other applications I need to work out how & why.

I have no problem posting schematics & layouts of some common work but not new R & D.

I went to my studio office to work with the new loop.

Tweaked the four trim pots after testing values & limits.

It certainly is NOT transparent. It comes up to unity gain but has not only a loss of lows but adds a really nice broken edgy hairy (not fizz) 25 watt Greenback sound.

🤔 almost like EVH DOA but better to my ear. Point is it’s cool but not providing the proper function.

This will be a bit of a rabbit hole dive later but I have amps to get turned around & two more to build.

Any thoughts are very welcomed. Below, you can hear the difference in the circuit after the tube buffered loop is added.

It took this circuit below.


And turned it into this circuit below.

 

[Pete Farrington]

Sharp eared folk might be able to detect the flip in signal polarity that accompanies many such circuits
eg due to asymmetry in the response of guitar speakers.

 

[NewReligion]

Sharp eared folk might be able to detect the flip in signal polarity that accompanies many such circuits
eg due to asymmetry in the response of guitar speakers.

Oscilloscope

 

[NewReligion]

Exactly what I was expecting.

This is a good piece.

 

[NewReligion]

Here's one

View attachment 130404

Thank you for the data @StingRay85

 

[66 Kicks]

This is a good piece.

This is how it looks like it ended up:


The switch could have been wired so that R100/220k gets grounded when the loop is bypassed.

R100/220k plus R89/68k add up to 288k. It would have been better if they added up to 1M so that the tone stack is loaded the same as it was without the loop. Could have made R100=750k and R89=240k.

The biasing of the cathode follower was missing on their schematic. It makes a big difference when it comes to the ability to swing the signal with a 16.7k AC load (Send Level maxed with a 25k device connected to the Send jack).

If the effects device has an input impedance of 25k and the Send Level is maxed, then C50/47nF gives a corner frequency of 169Hz. Too high.

The worst case source impedance at the Send jack is right around 25k. This is too high for long cable runs.

I would have taken the switch on the Return jack from the junction of the C50/47nF capacitor and the 100k-L Send pot. That way you could have an adjustable pre-amp out that doesn't affect the rest of the amp. It would also give you a set level at the recovery so that you could design a definitive unity gain loop.

 

[66 Kicks]

Here is one with unity gain that takes the Send signal from the cathode of a cathode follower (or nearly the cathode).

 

[StingRay85]

Here is one with unity gain that takes the Send signal from the cathode of a cathode follower (or nearly the cathode).

View attachment 130438

That's awesome. Where did you get this schematic from? I got the #6 from EL34world.

 

[66 Kicks]

That's awesome. Where did you get this schematic from? I got the #6 from EL34world.

I designed it, tested it, and drew it. I have 34 loops for 2204/2203 drawn up. 14 of them are tube and 20 of them are solid state.

 

[StingRay85]

And that's the best one?

 

[66 Kicks]

And that's the best one?

Naw. This one is better because it has a Return Level control.


They can both drive an entire spool of cable, but other loops may have other attributes that they excel in making it hard to say that any one loop is better than another.

 

[NewReligion]

Naw. This one is better because it has a Return Level control.
View attachment 130449

They can both drive an entire spool of cable, but other loops may have other attributes that they excel in making it hard to say that any one loop is better than another.

Great & generous work. I have hewn installing Metropoulos loops for 14 years now & Jason’s Headfirst for a couple.

I am working on a tube buffered as the OP states.

There is a lot of loop & additional data available on line. Check out the ENGL E765 with the optional Noise Gate.

It is easy to find.

Engl Schematics - Tube amp Schematics

Engl Tube amp schematics, Hoffman amps tube amp schematic library

el34world.com

 

[StingRay85]

Naw. This one is better because it has a Return Level control.
View attachment 130449

They can both drive an entire spool of cable, but other loops may have other attributes that they excel in making it hard to say that any one loop is better than another.

Cool. I need one for my Sound City 2204 build, and will take this one then. Will report back in a week or so

 

[66 Kicks]

Check out the ENGL E765

Merlin has one like that.