FX Loop install

[Valvehead]

I have an effects loop kit i need to install . It runs on 250-300 vdc off the B+ .

The amp is a Bogner Brixton , but the board is meant for a traditional style Marshall etc..

heres a pic of the scem---trying to figure out the optimal insert point (the in/out...not the B+ ) for this circuit. Before or after the .022 circled in red ?? Or maybe after that 330k ?? Theres around 100vdc at these points. But 0vdc out of the Master volume.

[Enzo]

How about the circuit of the FX loop? We have no idea what it expects to connect to.

[Valvehead]

the instruction file is too big to attach (of course)

the loop pcb is basically designed to go into a Marshall 2203 / 2204 style amp , between the middle lug of the treble pot and the Master volume pot.

its a Metro Amp fx loop kit



https://store.metropoulos.net/produc...ss-fx-loop-kit

so im trying to figure out where to insert it in this slightly odd Bogner circuit , posted above , it only has a Gain pot and a volume pot , no tone controls .

[pdf64]

What are the 2 mushrooms in the sketch - vactrols?
I suggest to insert the loop immediately before the volume (surely 0Vdc there?); then at low/regular volume settings, any hiss from the recovery stage gets attenuated along with the signal, thereby achieving the best signal to noise ratio.
That would be most analogous to the intended use.
If placed after the volume, then low settings may make hiss noticeable.

[J M Fahey]

Your schematic snippet is very incomplete , we need the full amplifier (hint hint) but from what I see


the only tube you show looks like a Cathodyne inverter, and the Master volume feeding it actually works like a PPIMV , since the cathodyne has unity gain.
Which means that signal level there can easily reach 90V RMS and to boot is expected to clip ... the worst possible place for a loop or anything else.

We need an earlier stage where level is more reasonable.

From what I see the Metropoulos loop is specifically designed for Marshall amps where the preamp clips, more specifically the tone stack driver / cathode follower does, then signal level gets heavily knocked down because of passive tone stack loss (think some 20 dB or 10X in Volts, a HUGE loss) and it´s easy for that add on board circuit to handle and restore ... impossible at PPIMV levels.

The miniboard will later feed a LTPI which typically has gain from 20X to 40X ... in a classic Marshall amp that is, not in the Bogner.

[66 Kicks]

The Metropoulos FX loop is very versatile. It has a 100nF/250V blocking capacitor at the input. It has a very high input impedance. It has an input trimmer with a huge range. It has a +4/-20 switch that affects both the input and output.

This device can output 90V peak at clipping with a DC supply of 300V. The cathodyne probably clips at around 35V peak.

An insertion point immediately before the volume control looks good to me, then adjust the loop for whatever you are trying to achieve.

[Valvehead]

heres the earlier section of the amp...if theres a better insert point ?

[Enzo]

You show us a kit for a MArshall amp, with specific Marshall installation directions. Now amp circuits are all very similar. But what we need is a schematic of the Marshall circuit showing where the kit is tapped in. That is a schematic, not a photo of the board solder points. When we can see where they inserted it on the MArshall, we can then more easily find the equivalent points in your Bogner.

[Valvehead]

https://www.dropbox.com/s/jjx4zrqdqo...tions.pdf?dl=0

[Enzo]

I see instruction to put this here and that there, but no schematic.

I myself want to see the Marshall amp schematic with the insertion points designated.

[Valvehead]

ok, sorry . heres the marshall scem---the insert point circled in yellow

[Valvehead]

Thanks guys ---i got it installed--works great

[alexradium]

Thanks guys ---i got it installed--works great

which kind of transistors are used for the clipping network?
thanks

[J M Fahey]

Thanks guys ---i got it installed--works great

Cool.
Now tell us where did you install it on your amplifier
Show your "yellow inserion point" on the Bogner schematic, the Marshall way we already know.

[alexradium]

you could also post a couple of pics of that loop,is it rev.4?