Adding fx loop to Kustom 5H

[Esppse]

Hello,

I'd like to add an FX Loop to this Kustom 5H so I can just use the power amp.

http://www.rh-tech.org/public/Kustom_Defender_v5/Kustom_Defender_V5.pdf

I have no idea what I am looking at with this schematic. Do you guys think you can help me find the point I break between the preamp and the power amp?

Thank you

[imJonWain]

Do you want an actual loop or a single input that connects directly to the poweramp and removes the preamp?  Don't those amps have a single volume knob? Is Vr2 a trimpot?

[Esppse]

Oh I would love to just get rid of the preamp section, I'd only like to use it for the power section.

Here are some pictures:
https://ibb.co/d5hDO7
https://ibb.co/e4Cf37
https://ibb.co/ehp4Vn
https://ibb.co/b4L037
https://ibb.co/j4Cf37


I could not locate VR2. I think there is only one potentiometer in it

[mth5044]

Do you want an actual loop or a single input that connects directly to the poweramp and removes the preamp?  Don't those amps have a single volume knob? Is Vr2 a trimpot?

I see R17 and C15 that are supposed to be on either side of it, and the strange part is the PCB has R17 visible, but not the value. If you got to the next picture, you can see a value ending in 8k. Schematic says R17 is supposed to be 100ohms. Perhaps this is a 68k or so resistor in place of VR2 and the prescribed 100ohm R17?

[thermionix]

Yeah that schematic isn't a 100% match to the actual amp.  But to use just the power amp, wouldn't you want to come in at the volume wiper, but disconnect C14?

[imJonWain]

nah, further along.  It's a SE amp so no phase inverter.  You want a switching jack that disconnects the preamp signal from the power tube just before the power tube.  You could do it directly at the tube but you'd want a larger value resistor to ground there (current at grid needs to go somewhere), you don't "need" it but a grid resistor wouldn't hurt, and possibly a blocking cap at your input incase you plug something stupid outputting DC into the power amp input.

So you could... Remove C15, VR2, R17. Change R4 (now your grid stopper) to 1.5K to 5.6K or so. and wire your switching jack between C2 and r8. Maybe increase R8, maybe not.


you could just wire the input Between C2 and R8 which will work but your signal would be cut/filtered a bit by that stuff after it. 

[Esppse]

Oh thank you very much, I'll try this out as soon as possible. May I ask which side of the switching jack each goes to (C2 or R8). And is a 1/4 watt resistor ok to use for this application? I'm very new to working on amplifiers. Quite scary too!

[imJonWain]

you want to be at the junction of C2 and R8 somewhere, I'm not looking at he pcb images right now.  The switching jack needs to be wired in series so that when normally closed (no plug inserted) C2 connects to R8 but when a plug is inserted that connection is broken (C2 is now isolated/floating on that side) and the tip of the jack connects to the R8/R4 junction.  So you'll need to lift the side of C2 facing R8/R4 and connect it to said jack. 

1/4w should be fine, looks like what they use everywhere in there.

Obviously it can be dangerous working in there while powered on or the caps are charged (aka you could possibly kill yourself) so take your time and be careful, I'm not responsible for anything bad that may happen haha.

[thermionix]

nah, further along.  It's a SE amp so no phase inverter.

Not a phase inverter but a driver.  I realize it's an EL84 so it has some gain and might not need it.  I have basically no experience with FX loops.

[PRR]

[thermionix]

Hmmm...I've got Jon to the right of me and Paul to the left.  They're both more knowledgeable than me so...good luck OP!

[PRR]

> junction of C2 and R8 somewhere

The level required there (EL84 grid) is 8V-12V. Rather high for most audio sources.

I'm chopping one stage to the left. Level here may be 0.2V. A hot effects level or a cool line level. Including the Volume control allows hot sources to be turned-down to taste.

I'm assuming he wants "only" the power amp section; that preamplification and modification is done elsewhere. If this is truly an Insert job, then around the Volume control is IMHO still a good starter-chop, but additional level control may be needed so everything plats together well.

[imJonWain]

PRR has fogotten more than I'll ever know haha.... so I side step and take his lead haha. Sorry for the bad info, wasn't aware of the drive level needing to be that high.

[Esppse]

Ooh just to clarify, is it safe to proceed with the earlier instructions in Reply#7 or was there another step to the procedure? Is C2-R8 the best splice point? Heh got my soldering iron ready, just wanted to double check. Thank you

[imJonWain]

Safe, ya.      Usable, probably not.  So don't do what I said earlier.

PRR & thermonix pointed out that you need a much larger drive signal at the grid then a preamp (assuming that's what your plugging in) can output.  So you need to put your jack "earlier" in the schematic.
Per PRR's suggestion you could remove R3 (which should be a jumper) and wire your switching jack there so that when it's NC R3 is a short and when a plug is inserted C1 is isolated and your signal goes into the R15/VR1 junction.  You also get to use the volume control this way.

[Esppse]

Ah ok, so this is what I do:

1. Remove R3 jumper
2. Solder jack, so when plugged in, guitar goes to R15/VR1
3. When plugged out, C1 does not connect to R15/VR1 like before.

So basically the switching jack never connects C1 to R15, when either plugged in or out?

And do I still have to change to value if R4 or just leave it alone?

And may I ask is that volume knob for the power amp section or a preamp gain knob?

[PRR]

Value of R4 is fairly unimportant.

If "R3" (jumper) is accessible, snip it and wire jack there.

The volume knob is letting stuff into the Power Amp. You will probably have/want a volume knob earlier in the system (in your unspecified preamp).

[imJonWain]

@ Esppse: Not quite...You want it wired so that when nothing is plugged into the Jack C1 is connected to R15/VR1 via the normally closed contact of the switch ( the amp works as normal) and when something is plugged into the jack the contact is broken and your signal goes to R15/VR1.  So your tip goes towards R15/VR1 and the Switching contact goes towards C1,  sleeve is GND. 

[Esppse]

OK thanks so much guys, I'll report back after I get a chance to do it!

[Esppse]

Works perfectly, thank you everyone so much!