ValveWizard FX loop problem

[fossilshark]

Hi all, I am trying to botch ValveWizard's FX loop circuit into my LA100BL clone and it is not going so well. After installation the amp sounds super farty both when the circuit is in "bypass" (SEND and RETURN switched together via nothing plugged into the jacks) and with a known working preamp plugged into RETURN.

I have a feeling the plate voltages are not correct but I am unsure what they should be. Also, Merlin mentions using a resistor network on the output of the FX loop to achieve the proper level for the phase inverter, but again I am unsure what that level should be.   

I tried it with and without C4, and with and without R11

I have attatched the schematic for the LA100BL but that schematic excludes the master volume and the B+ voltages are not whats on my amp (on account of not wanting to nuke new production output tubes).

[AlNewman]

Looks like you have R2 and R3 reversed from Merlin's drawing.  The 1M should be the grid leak.
I also don't see the bypass to the return jack from the send stage.
You have 2 cathode followers in a row, 1 into your tone stack, and the first stage of the effects loop.  I'm not sure how well that would work?
To determine what's needed at the output, you can measure signal voltage off the back of your tone stack, that would be what was originally going into your PI.
You need C4 to keep dc off your pot.

[Merlin]

Your circuit doesn't show a power supply decoupling cap connected to R11, which might explain your problem

[kagliostro]

Excuse me Merlin, English isn't my language so may be I misunderstand

You say decoupling cap but I don't understand you

May be you are referring to a PS filter cap ?

Thanks

Franco

[Merlin]

You say coupling cap but I don't understand you
May be you are referring to a PS filter cap ?

Decoupling cap = a cap to ground. So yes, a PS filter cap.

[kagliostro]

Thanks, I was meaning decoupling but I wrote coupling

However I was wrong because C4 and C5 are decoupling (or coupling) caps, to me

Thanks

Franco

[Merlin]

However I was wrong because C4 and C5 are decoupling (or coupling) caps, to me

Coupling caps transmit the AC signal to the next stage. Decoupling caps shunt the signal to ground.

[Dave]

English is a psychotic language. Most of us "native" speakers don't get it either. Don't worry about that.

Dave

[shooter]

while I admit, English was were I caught up on all the sleep I missed the night before, all the brain power I used up in physics, math n science
it truly is

psychotic

[kagliostro]

Many thank for clarifying the matter

I try to convert correctly in Italian what I read but, for sure, I'm not omniscient

Franco

[kagliostro]

I would like someone will clarify to me

a Decoupling cap is a Decoupling cap and has some purpose, like to shunt to ground some frequencies

a Filtering cap (on the Power Supply) is both a Filtering cap & a decoupling cap because performs this two different performance ?

Am I correct ?

Thanks

Franco

[Merlin]

a Filtering cap (on the Power Supply) is both a Filtering cap & a decoupling cap because performs this two different performance ?

Yes. 'Filter' is a general term. Both of the circuits below are 'filters' and therefore both capacitors might be called 'filter capacitors'. But the first one can also be called a coupling cap, the second one can be called a decoupling cap.



"...further smoothing is necessary. This is achieved with a chain of LC or RC (low-pass) filters, variously referred to as smoothing, bypass, or decoupling filters. These alternative names derive from the fact that there are really three interrelated jobs to perform:
1: Smooth/filter out residual ripple voltage;
2: Bypass/provide a local energy supply for sudden current demands;
3: Decouple/isolate each amplifier stage from the rest."

https://valvewizard.co.uk/smoothing.html

[kagliostro]

Thanks Merlin

Franco

[shooter]

since I have a monkey with a wrench;


might have missed these, just in case
you may see/hear "series pass and series shunt" also, same as above, ....different school, different time 
then after you convert it all to math and the teacher gives you a passing grade, move on to parallel LC, RC,......


it all ends when you get your 1st fluke meter and a Tektronix scope and you have to go make a living with them