Parallel effect loop

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

drz400
Posts: 509
Joined: Mon Nov 06, 2006 4:53 pm

Re: Parallel effect loop

Post by drz400 »

Bob-I wrote:Good Dr.... I'm not sure what's going on here. I tried the LNFB and the loop loses all output. Without the loop and with the proper loading on the CF I'm getting just about unity. As soon as I add the LNFB all output goes down dramatically.

I'll have to experiment more, perhaps different values for the load and LMFB resistors, but IMHO the simple gain stage is about neutral as to coloring and gain, and the effect drives nicely from the CF.
Use the cathode follower as your drive

The negative Feedback like in the Aiken diagram is for the return section and all the capacitors must be in place, the Negative feedback will reduce the return gain but creates a virtual ground point for the mix. With 100K input resistors and 1Meg in the feedback you will still have a gain of 10 whihc is just fine. I know all this works , I have done it many times it is also basically in the Kevin OConnor books. Can you draw a schematic of what you tried that didnt work?
User avatar
Bob-I
Posts: 3791
Joined: Thu Feb 02, 2006 10:06 pm
Location: Hillsborough NJ

Re: Parallel effect loop

Post by Bob-I »

drz400 wrote:
Bob-I wrote:Good Dr.... I'm not sure what's going on here. I tried the LNFB and the loop loses all output. Without the loop and with the proper loading on the CF I'm getting just about unity. As soon as I add the LNFB all output goes down dramatically.

I'll have to experiment more, perhaps different values for the load and LMFB resistors, but IMHO the simple gain stage is about neutral as to coloring and gain, and the effect drives nicely from the CF.
Use the cathode follower as your drive

The negative Feedback like in the Aiken diagram is for the return section and all the capacitors must be in place, the Negative feedback will reduce the return gain but creates a virtual ground point for the mix. With 100K input resistors and 1Meg in the feedback you will still have a gain of 10 whihc is just fine. I know all this works , I have done it many times it is also basically in the Kevin OConnor books. Can you draw a schematic of what you tried that didnt work?
Ok, maybe I'm missing one of the caps. I'll draw it up later and we can talk more. :D
User avatar
Bob-I
Posts: 3791
Joined: Thu Feb 02, 2006 10:06 pm
Location: Hillsborough NJ

Re: Parallel effect loop

Post by Bob-I »

The good Dr did it. I completed the ckt and although I have a ground loop hum somewhere it works really well.

I used the Aiken schematic for the mixer/feedback amp section and a simple CF drive split like on the Dumbleator. The return pot acts as a balance as expected, on 0 it's 100% dry and on 10 it's 100% wet... soaking wet as this really drives the Midiverb III well. There's just a tiny increase in gain, I'm thinking 2-3Db.

Thx everyone, this was a fun project. Now I plan to build this into a 1U rack chassis I E-bought for $35 awhile ago. Right now it's on terminal strips, in the chassis I'll use a turret board and fix the hum.
You do not have the required permissions to view the files attached to this post.
Last edited by Bob-I on Wed Feb 14, 2007 6:34 pm, edited 3 times in total.
User avatar
mhuss
Posts: 511
Joined: Wed Jan 19, 2005 3:09 am
Location: SE PA, USA
Contact:

Re: Parallel effect loop

Post by mhuss »

edit: deleted
Last edited by mhuss on Thu Feb 15, 2007 12:30 pm, edited 1 time in total.
groovtubin
Posts: 1114
Joined: Tue Feb 07, 2006 4:52 am

Re: Parallel effect loop

Post by groovtubin »

Sven wrote:Greetings,

OK, fine, now I understand. Well, the parallel effects loop seems to be the way to go if one does not wish to loose some of the qualities of the valve amp sound ... and even more so in designs where there is an attempt to get a special high quality sound, such as in Dumble models.

Having the entirire signal go through just about any pedal, that is, leaving it to the ¨mercy¨ of the pedal electronic network, is a loosing proposition in many cases. For that reason alone, one might as well use the pedal up front, and than run the signal from the pedal into an amplifier input, and forget about the whole FX loop deal, pretty much ....

Particularly with pedals that add delay or reverb, one is always compromising the final sound if one runs delayed and/or reverberated signals through the amp. In live concert and in studio situation, one would be much better off if one would run the dry amp sound and have the mixing engineer add delay/reverb effects. However, that is not always possible nor practical, even on the big stages and sometimes even in studios ...

Some guitar players who really care about tone very much, would run DI-out from one amp into another amp that would handle the delay-reverb effects (for example, that is what Brian May did). Such scenarios call for two amps, of course. So, in a one-amp scenario we are returning back to a parallel effects loop, as described in my first posting.

Can we look around for a solution of implementation of such a parallel FX loop, in a Dumble circuit.

One more question. Since pre-amp gives signal enough boost to drive any effects pedal (if guitar can do it, than pre amp boost can actually be too much), why is it neccessary to use valve amplification in FX loop of present (serial?) scheme, or is it for impendance matching and some other issues beyond the actual strength of a signal?!

But, having a parallel loop with ability to ¨mix-in¨ a bit of wet signal from delay and/or reverb pedal, is not really a cure-it-all solution, since even a 10% of mixed-in signal can spoil the dry signal in some ways due to phasing or whatever problems end up surfacing in the summed, final singal that goes to the output stage. In Dumbles, also, such an FX loop would have to be introduced after OD stage, since it would be somewhat weird to delay a clean signal and have the dry signal be overdriven ...

Once again, it would be nice to make a parallel loop for Dumble and see how it works and how it sounds overall ...

All the best,

Sven
i`ve built a Dator with a MIX control, just add a 47K plate to the cath follower stage ( PLATE side) , cap coupled, then 1m to grnd, to a MIXER pot, take the return sig to one end of pot, MIX sig to other end, wiper out to the RTN volume.....works like a freakin charm.......... found out that Zinky amps have a similar set-up..
User avatar
Bob-I
Posts: 3791
Joined: Thu Feb 02, 2006 10:06 pm
Location: Hillsborough NJ

Re: Parallel effect loop

Post by Bob-I »

groovtubin wrote:
i`ve built a Dator with a MIX control, just add a 47K plate to the cath follower stage ( PLATE side) , cap coupled, then 1m to grnd, to a MIXER pot, take the return sig to one end of pot, MIX sig to other end, wiper out to the RTN volume.....works like a freakin charm.......... found out that Zinky amps have a similar set-up..[/quote]

I think this is in English... I see a few words I know.. :wink: :lol:

On my schem the return acts as a mix control. In the final version I plan to use a trimmer for the send since it'll be set to the proper level for the effect.
groovtubin
Posts: 1114
Joined: Tue Feb 07, 2006 4:52 am

Re: Parallel effect loop

Post by groovtubin »

Bob-I wrote:
groovtubin wrote:
i`ve built a Dator with a MIX control, just add a 47K plate to the cath follower stage ( PLATE side) , cap coupled, then 1m to grnd, to a MIXER pot, take the return sig to one end of pot, MIX sig to other end, wiper out to the RTN volume.....works like a freakin charm.......... found out that Zinky amps have a similar set-up..
I think this is in English... I see a few words I know.. :wink: :lol:

On my schem the return acts as a mix control. In the final version I plan to use a trimmer for the send since it'll be set to the proper level for the effect.[/quote]Guess i`ve been at this TOO long.. any questions from NEWBIES fire away, But Bob i COULD scare you righteously with some stuff if need be..LOL!!
User avatar
mhuss
Posts: 511
Joined: Wed Jan 19, 2005 3:09 am
Location: SE PA, USA
Contact:

Re: Parallel effect loop

Post by mhuss »

hi Bob,

Hate to be a complainer :wink:, but I don't see any DC path to ground for the grid of the second tube -- either the schematic is wrong, or there must be a path to ground "off the schemo to the right" in the amp you have it implemented on. Getting rid of one .03 cap (in either branch) would fix this; I suggest the 'bottom' one, as the top one protects you from wayward effects.

--mark
User avatar
Bob-I
Posts: 3791
Joined: Thu Feb 02, 2006 10:06 pm
Location: Hillsborough NJ

Re: Parallel effect loop

Post by Bob-I »

mhuss wrote:hi Bob,

Hate to be a complainer :wink:, but I don't see any DC path to ground for the grid of the second tube -- either the schematic is wrong, or there must be a path to ground "off the schemo to the right" in the amp you have it implemented on. Getting rid of one .03 cap (in either branch) would fix this; I suggest the 'bottom' one, as the top one protects you from wayward effects.

--mark
Look at the Aiken schematic and drz400's explaination. There's a virtual ground point where the feedback resistor meets the dry input.
drz400
Posts: 509
Joined: Mon Nov 06, 2006 4:53 pm

Re: Parallel effect loop

Post by drz400 »

groovtubin wrote:
Bob-I wrote:
groovtubin wrote:
i`ve built a Dator with a MIX control, just add a 47K plate to the cath follower stage ( PLATE side) , cap coupled, then 1m to grnd, to a MIXER pot, take the return sig to one end of pot, MIX sig to other end, wiper out to the RTN volume.....works like a freakin charm.......... found out that Zinky amps have a similar set-up..

I think this is in English... I see a few words I know.. :wink: :lol:

On my schem the return acts as a mix control. In the final version I plan to use a trimmer for the send since it'll be set to the proper level for the effect.
Guess i`ve been at this TOO long.. any questions from NEWBIES fire away, But Bob i COULD scare you righteously with some stuff if need be..LOL!!
I'm no newbie unless 55 is a newbie but I'm confused too, can you sketch a schematic? Sounds like a passive mix to me and not low impedance either?
drz400
Posts: 509
Joined: Mon Nov 06, 2006 4:53 pm

Re: Parallel effect loop

Post by drz400 »

mhuss wrote:hi Bob,

Hate to be a complainer :wink:, but I don't see any DC path to ground for the grid of the second tube -- either the schematic is wrong, or there must be a path to ground "off the schemo to the right" in the amp you have it implemented on. Getting rid of one .03 cap (in either branch) would fix this; I suggest the 'bottom' one, as the top one protects you from wayward effects.

--mark
Note that the tube gets its grid DC bias reference from Rf and Rl. If the sum of the feedback resistor and the load resistor is very large, a 1Meg resistor should be added from the grid of the tube to ground, to keep the grid bias stable. This will not affect the gain, because the grid node is a "virtual ground" and has very little voltage on it. The voltage can be calculated as the error voltage E as follows (assuming an input voltage of 1V):
blah blah, it is on the Aiken explanation (easier to cut and paste) :wink:
Post Reply