need to add an Fx loop to valve amp ?

[blindboybenton]

Hi all

Ive got a customer who wants me to fit an Fx loop to his amp. I can see on the net there are many different circuits does anyone have any advise
as ive never added one before.



Carlsbro 100 PA/R Amp Schematic


many thanks

BBB

[Gingertube]

BBB,
No one has responded cause it is not a simple cut this wire and shove a couple of sockets in there type problem.

Ideally you want a send level of about 0dBV (2.8Vpk-pk) and an output impedance of say less than 10 KOhms. That would require adding a dedicated buffer.

Ideally you want to accommodate a return level of between -20dBV (280mV Pk-PK) and 0dBV. That may require a deicated recive stage and gain switching.

Without looking at the signal levels with an oscilloscope it is difficult to say how simple we can go even when we compromise.

For a simple as possible which MAY work:

Break the connection of the 0.047uF cap and the volume control, wire the cap output to the send jack hot.
Wire the top of the 100K volume control to the return jack hot.
Use switch jacks for both and wire the 2 switch circuits together such that with nothing plugged into either socket the signal goes straight through.
Additional protection (for the external equipment) on the send jack - add back to back green leds or 3v3 zener diodes across hot and 0V to limit maximum output voltage and suppress HV spikes when turning the amp on, you may need to also add some series resistance in the hot lead for these protection devices to work against.
AND
if the signal level at the top of the 100K volume pot is more than about 4 volts pk to pk this will all turn to poo anyway and won't work. Then you have to think about adding another tube or generating new power supplies for some solid state stuff etc to implement proper send, return and mix circuits..

Sorry to be brief, but the question is a bit "how long is a piece of string?" type question.

For your customer, you could quote him for one of these kits
Effects Loops
and add 3 or 4 hours labour cost and see if he still wants it.

Cheers,
Ian

[Enzo]

And Ted Weber sells a tube FX loop kit for $40, there is the sheet from that kit.

https://taweber.powweb.com/store/el_schem.jpg


So price of a kit plus some markup, time to wire it up, then to wire it in, and also time to find a spot and mount the jacks. Going to need some holes in your chassis. I think a three or four hour labor estimate is fair, like ginger says.

[pdf64]

My view is that it depends on what the customer wants to achieve.
Looking at the scheme, the signal level at the LTP phase splitter input at clipping may not be very high. If it's around 1Vrms / 0dB, and the fx will accomodate that signal level, then a simple insert series type send/return pair of jack sockets (which just break into the signal path between the master vol wiper and LTP input) may work fine.
However, unless the customer is setting the controls so as to overdrive the pre-amp (eg channel volume very high, master low) then there may not be much benefit from an fx loop.
Just arrange things to drive the fx at instrument level, it could have its own input / channel volume.
If the customer is adamant that they want an fx loop and the simple type above won't do, then kits are available which use high voltage mosfets eg Zero-Loss FX Loop Kit - $95.00 : MetroAmp, Amplifier Kits and Parts Online Store to avoid fitting more tubes.
Pete

[blindboybenton]

Hi

Many thanks for the reply

I have since the post fitted an fx loop. But had some issues with noise. Ive had to put it down for a bit.



I had spare holes for ecc83 and as this amp is a pa amp used for guitar. I used some of the spare inputs
I will have to check this all again and add some info on the noise

BBB

My post has gone a bit strange and i cant delete the 2 pictures but the one ive used is the one on the right ( Dark grey)
Im aware that the circuit has some connections missing

[Gregg]

And Ted Weber sells a tube FX loop kit for $40, there is the sheet from that kit.

https://taweber.powweb.com/store/el_schem.jpg

I don't see any use for this "loop". You can't just buffer the signal from the TS to your effects unless you need some crazy fuzz sounds.
If you lower your send level to say 0dB you need to boost it after that before feeding it to the PI. Also 100k is too much for a send level pot.

[pdf64]

Just noticed Tube Town kits https://www.tube-town.net/ttstore/in...Misc-Kits.html from Germany.
Pete

[Mick Bailey]

I've used this design with good results;

http://www.blueguitar.org/new/schem/...od/fx_loop.gif