Adding effects loop onboard

[Vlada]

Hi all, I have read a lot topics about effects loop here, and would like to add one to my next project (hot rodded jcm800). Main reason is so I can use noise suppresor pedal the way it should be used. I have found that most loops are tube or LND150 based. Marshall loop is what looks very interesting to me because of its simplicity, and I can easily draw PCB layout using Eagle
https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Plexi-Superlead-Reissue-50W-1987X-Schematic.pdf
Are there any drawbacks with op amp loops? Should I rather look for tube loop?
Thanks

[shooter]

chips work, just make sure you get DC voltages right.


here's an archive of the tube FX, didn't look through it this time but I think I recall some chips in there 


https://el34world.com/Forum/index.php?topic=10208.0

[Vlada]

I reconsidered, and I think it would be easier to build something like this: FX loop for 2204 that 2deaf posted.
What plate voltage should I aim for?

[2deaf]

Updated version

[2deaf]

What plate voltage should I aim for?

300V or more on the cathode follower for the older one.

#13 was tested down to 200V and still handled everything a 2203 could throw at it.

[Vlada]

Thats great, thank you
Would using 12at7 be an improvent over 12ax7 for loop?
I also need to plan where to fit one more tube and other components 

[tubenit]

Another option for Marshall style amps especially if you don't have much room for another tube is the Metro Zero Loss FX loop.

https://store.metropoulos.net/products/zero-loss-fx-loop-kit

With respect, Tubenit

[2deaf]

Thats great, thank you
Would using 12at7 be an improvent over 12ax7 for loop?

There are pros and cons to every design and it always comes down to some kind of compromise.  The 12AT7 consumes more current and has less gain for the recovery stage as compared to the 12AX7.  But then, the 12AT7 has higher transconductance and lower internal plate resistance making the internal cathode resistance lower so that it is capable of a lower output impedance in a cathode follower configuration.  But then again, the #13 loop can drive 90 meters of 100pF cable at a cutoff frequency of 15KHz with a 12AX7.  That seems sufficient, so I see no reason to use more current and sacrifice gain by using a 12AT7.   

[pdf64]

Updated version

What's the benefit of moving the CF's output from its cathode, to the 1M-10k-820 node?
Or of reducing the 'send' 0V reference resistor from 56k to 22k?
I guess the CF's hotter bias helps to reduce the source impedance?
Ta

[2deaf]

What's the benefit of moving the CF's output from its cathode, to the 1M-10k-820 node?
Or of reducing the 'send' 0V reference resistor from 56k to 22k?
I guess the CF's hotter bias helps to reduce the source impedance?

I caught a ration for connecting a capacitive load (cable) directly to the cathode.

When the amp is in standby mode, both ends of the 2u2 Send capacitor are referenced to ground so that there is no charge across that capacitor.  When the standby goes off, the DC cathode voltage suddenly jumps up taking both ends of the 2u2 capacitor with it and putting a DC voltage on the Send jack.  I didn't keep my calculations, but I'm sure that I made the 'send' 0V reference as small as possible to bleed off this DC voltage as fast as possible while still meeting the other criteria.  The 10K tail resistor is that small to also minimize the DC voltage spike.

The hotter bias is a trade-off between the AC voltage swing I need and the current I'm willing to spend.  I'm not worried about reducing the source impedance when the thing can already drive 295 feet of cable.