Parallel effects loop for Marshall style amps

novosibir · Post by **novosibir** » Fri Aug 29, 2008 2:17 pm

I've also drafted a "pop free" schem of the loop w/ true bypass. It requires additional a DPDT switch, one .022 cap and two 2.2M resistors.

But actually you don't need a true bypass, there's no need to take the loop out of the circuitry and it's maybe the only loop, which doesn't influence the sound - even with the amp dimed.

Larry

hotguit · Post by **hotguit** » Fri Sep 05, 2008 6:37 am

Hi Larry
is it still possible to have a schem/layout for your loop?
I'd like to replace the one on my 1987x reissue.
thank you
bye
Hot

novosibir · Post by **novosibir** » Fri Sep 05, 2008 8:47 am

hotguit wrote:is it still possible to have a schem/layout for your loop?

Yes - when you request for it via PM AND if you tell me your email address in this PM - as I wrote in the 1-st post of this thread.

Larry

franc · Post by **franc** » Sat Oct 04, 2008 2:09 am

I'm wondering if I could put this loop in a 20W Lead & Bass aka 2061 type circuit?

I'm not quite sure how to, because there's two volume/tone stacks that use a decoupled signal before coming together into the PI....

I was also working on an idea to build the loop inside a box of its own, self-powered and then feed it with preamp-out/poweramp in signals from/to the amp. That way you only have to make a small modification to the amp (cut the signal between preamp and poweramp) and you could use it for different amps.

Anyone tried these ideas yet?

jbrew73 · Post by **jbrew73** » Wed Nov 12, 2008 2:27 am

would any of the cap values need to be changed if this effects loop was used in a bass guitar amp?

novosibir · Post by **novosibir** » Wed Nov 12, 2008 4:13 am

No!

Just keep the value of the PI input cap, if it was different before!

Larry

emartine · Post by **emartine** » Wed Nov 12, 2008 8:44 am

novosibir wrote:I've also drafted a "pop free" schem of the loop w/ true bypass. It requires additional a DPDT switch, one .022 cap and two 2.2M resistors.

But actually you don't need a true bypass, there's no need to take the loop out of the circuitry and it's maybe the only loop, which doesn't influence the sound - even with the amp dimed.

Larry

Whats the purpose of that 0.022 cap there, Larry???

jbrew73 · Post by **jbrew73** » Wed Nov 12, 2008 6:14 pm

novosibir wrote:No!

Just keep the value of the PI input cap, if it was different before!

Larry

cool just wondering. my math comes up with roughly 300hz cutoff using a .470uf cap and 900 ohm z for the cathode follower. maybe my math is bad(that would not surprise me) or that cutoff is not that important. i see some bass amps with .1uf cap coming off of c.f.'s driving the output tubes.

could you explain that so i can get a better grasp on the situation please?

novosibir · Post by **novosibir** » Wed Nov 12, 2008 7:32 pm

The 900 ohms is the source impedance, not the load impedance.
And you don't feed backwards from the load through the cap into the source - it's vice versa

Larry

jbrew73 · Post by **jbrew73** » Wed Nov 12, 2008 7:43 pm

ok
i thought you had to add the 900 and 220k load resistor in parallel.

i got his quote from the r/c time constant sticky thread.

*****You can also calculate the frequency determined by coupling caps and the equivalent resistance in the circuit. In the first stage of a Marshall you have a .022 or a .0022 with a 100k plate resistor and 1M to ground via the volume pot. Your equivalent circuit resistance is 91k.*****

so this is not the same thing then?

novosibir · Post by **novosibir** » Wed Nov 12, 2008 8:00 pm

The coupling cap is in series w/ the load and only these both factors are forming the high pass filter.

The source is in parallel to the load and only useful to calculate the amplification of the previous stage.

The source impedance hasn't be set in the equation for calculating the high pass knee frequency!

It might be, that it's written different in the mentioned post/thread.
Also Dave Funk wrote something like that in his Tube Amp Workbook.

BUT IT'S SIMPLY WRONG!

Larry

jbrew73 · Post by **jbrew73** » Wed Nov 12, 2008 8:11 pm

thanks for the clarification on that larry. your knowledge is a great asset to the tube d.i.y community.

nasoftz · Post by **nasoftz** » Fri Dec 05, 2008 12:10 pm

Larry,many many thanks for your help and your great work!...I really appreciate it!
I 'm going to make the pcb as soon as possible!I love my 2203 but often I need to connect an mfx pedal board on this amp couse I play a lot's of rage against the machine..oh a silly queston :p
Will your FXloop work on my Peavy classic 20,too?
Best regards!
Maurizio

novosibir · Post by **novosibir** » Fri Dec 05, 2008 3:07 pm

Maurizio, the loop will work in any amp, when placed on the right spot in the circuit.
Put it in the Peavey Classic 20 right before or after the Master.

Larry

thinlizzy · Post by **thinlizzy** » Fri Dec 12, 2008 11:25 am

Hi Larry,

I've been using youre loop now for a long time and it suits me well.
I installed it in a 2204 after the master control.
Every time I'm altering the master I have to adjust the send and return from the loop otherwise my signal to the pedals gets to hot.
Can I install the loop before the master and will it work fine, this way I dont have to reajust the looplevels when I adjust the master...

Thanks
Regards Marcel

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Parallel effects loop for Marshall style amps

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