How to add a mono Jack Passive FX loop to DC 30 EF86 Single Channel

[stinga11]

Greetings, I'm about to build a DC 30 for a friend. The project started as an Ac15 but has ended in another amplifier. The real matchless dc 30 has a passive effects loop but uses a Stereo Jack. I want to use two mono jacks but I don't know if I added it correctly. By the way, if you find other errors in the schematic, please correct me.

[pdf64]

Only the return jack needs to be a switching type; making both switching will work fine but introduces an additional failure point, and failing fx loop switching jacks are already a common source of signal loss. So adding another one (ie at the send jack) will double the likelihood of it happening 

[stinga11]

Only the return jack needs to be a switching type; making both switching will work fine but introduces an additional failure point, and failing fx loop switching jacks are already a common source of signal loss. So adding another one (ie at the send jack) will double the likelihood of it happening 

Thank you very much for answering. I just made the change in my schematic.

[tubeswell]

A problem with taking a passive loop after an EF86 is the high output impedance of the EF86, and you don't know how this will interact with 'whatever FX unit/pedal'. A possible alternative may be to take the FX send from the EF86 cathode and have the switching FX return jack into the other LTP grid (i.e so that it either functions as a normal LTP slave triode with nothing plugged in, or as an LTP input/mixing triode with a jack plugged in)

[stinga11]

A problem with taking a passive loop after an EF86 is the high output impedance of the EF86, and you don't know how this will interact with 'whatever FX unit/pedal'. A possible alternative may be to take the FX send from the EF86 cathode and have the switching FX return jack into the other LTP grid (i.e so that it either functions as a normal LTP slave triode with nothing plugged in, or as an LTP input/mixing triode with a jack plugged in)

Thanks for answering, where would I get the signal from, before or after the cathode resistor?

[tubeswell]

A problem with taking a passive loop after an EF86 is the high output impedance of the EF86, and you don't know how this will interact with 'whatever FX unit/pedal'. A possible alternative may be to take the FX send from the EF86 cathode and have the switching FX return jack into the other LTP grid (i.e so that it either functions as a normal LTP slave triode with nothing plugged in, or as an LTP input/mixing triode with a jack plugged in)

Thanks for answering, where would I get the signal from, before or after the cathode resistor?

Suggestion. Basic idea - you might want to add a 5-10kA FX send level control (after the 25uF cap) to fine-tune the signal hitting your FX unit input. This pot would double as a Ck gain/boost level control when the FX send was not in use. (Edit - amended attached schematic to show this)

[stinga11]

A problem with taking a passive loop after an EF86 is the high output impedance of the EF86, and you don't know how this will interact with 'whatever FX unit/pedal'. A possible alternative may be to take the FX send from the EF86 cathode and have the switching FX return jack into the other LTP grid (i.e so that it either functions as a normal LTP slave triode with nothing plugged in, or as an LTP input/mixing triode with a jack plugged in)

Thanks for answering, where would I get the signal from, before or after the cathode resistor?

Suggestion. Basic idea - you might want to add a 5kA FX send level control (after the 25uF cap) to fine-tune the signal hitting your FX unit input. (Also left out your MV on this)

Thank you very much, that will definitely help me.

[tubeswell]

Come to think of it, it may be slightly advantageous to run a more imbalanced LTP stage for that 'passive' FX loop suggestion I posted earlier - so that the FX return signal is slightly stronger than the dry signal in the differential amplifier mix. - Revision A


Even better would be to swap the EF86 for a 2-in-1 combined small signal pentode/triode tube (such as ECF80 or ECF82), with the pentode replacing the input stage, and the triode as a FX return gain stage prior to the LTP. Then you'd have a default 2nd input channel, or some extra zing if you just wanted to plug a cable from send directly to return. (See Revision B - You'd want a tube shield on the ECF80)

[stinga11]

Come to think of it, it may be slightly advantageous to run a more imbalanced LTP stage for that 'passive' FX loop suggestion I posted earlier - so that the FX return signal is slightly stronger than the dry signal in the differential amplifier mix. - Revision A


Even better would be to swap the EF86 for a 2-in-1 combined small signal pentode/triode tube (such as ECF80 or ECF82), with the pentode replacing the input stage, and the triode as a FX return gain stage prior to the LTP. Then you'd have a default 2nd input channel, or some extra zing if you just wanted to plug a cable from send directly to return. (See Revision B - You'd want a tube shield on the ECF80)

Thanks but now my friend wants the amp to have reverb. I am thinking of inserting the reverb signal through the other input of the phase inverter. What do you suggest in that case.

[tubeswell]

Suggestions

[stinga11]

Suggestions

You mean connect the Reverb Transformer to filter capacitor B?

[tubeswell]

Yes

[tubenit]

I'm NOT saying you should do this.  In fact, it may stray too far from the DC30 tone.  However, I thought I'd post this for anyone who might be interested and would like to try an active FX loop & a one tube reverb with this type of topology.

I use this same active FX on the HoSo56 "the minimalist" with mosfet CF and it works quite well to my liking.
I use this same "one tube reverb" on another amp.

I eliminated the crossline master volume which has been the worst sounding master volume I've ever tried.  The FX loop pots are now the master volume.

IF someone built this and it had too much gain, then use a 12AY7 in V2 and a 12AT7 for the LTPI.

With respect, Tubenit

[stinga11]

Yes

1.Some days ago I was curious to eliminate the 470k resistor to ground but I still had not decided but when you mentioned it I did not hesitate to eliminate it. Won't I get much gain now that I don't have it?

2. I was checking the schematics that Hoffman has and I found one of an AC30 with reverb and the recovery reverb tube part caught my attention. I put a schematic here for you to tell me what you think of using the two phases of the recovery reverb tube in this way.

[tubenit]

Parallel triodes get about 30% more gain with little to no increase in floor noise.  In this particular case, one triode for recovery I think would be more then sufficient to get strong reverb (even surf reverb) on as low as "3 to 4" on the reverb pot. 

with respect, Tubenit

[stinga11]

Parallel triodes get about 30% more gain with little to no increase in floor noise.  In this particular case, one triode for recovery I think would be more then sufficient to get strong reverb (even surf reverb) on as low as "3 to 4" on the reverb pot. 

with respect, Tubenit

In this particular case I want to use the two tube triodes so as not to leave half empty. I'm trying to stay in an area where the amp looks as much like a real Matchless as possible. This amp is for a very compulsive friend.

[shooter]

This amp is for a very compulsive friend.

you will NEVER build an amp quite right to satisfy them.  I got so frustrated with one guy, I just said it's yours, free, I'm leaving now 

[stinga11]

This amp is for a very compulsive friend.

you will NEVER build an amp quite right to satisfy them.  I got so frustrated with one guy, I just said it's yours, free, I'm leaving now 

[SnickSound]

Greetings, I'm about to build a DC 30 for a friend. The project started as an Ac15 but has ended in another amplifier. The real matchless dc 30 has a passive effects loop but uses a Stereo Jack. I want to use two mono jacks but I don't know if I added it correctly. By the way, if you find other errors in the schematic, please correct me.

My 2 cents: this is where you need to ask not just how, but why.

Driving a loop send from a plate has fairly high impedance, so if you have a typical 15-20' long guitar cable to your effects you will experience noticeable high end loss. Also, the signal will be too high for pedals (and if you reduce it then you have to reamplify it again on the way back). And a loop after just one stage has very little usefulness, you might simply put your pedals before the amp because the first stage isn't gonna overdrive much by itself (the point of a loop is typically to run time based effects after the distortion, but this amp doesn't have any preamp distortion)

So really, it only makes sense for using line level effects with short cable runs. If the buyer is hoping to use this to run some pedals in the loop, he'll likely experience nasty overdrives and treble loss. He should just put them before the amp.