Adding FX loop to Matchless Spitfire and other similar tube guitar amps

[Potato Cakes]

Hello, everyone,

I've been building tube guitar amps for my studio and the latest one is a Matchless Spitfire with a number of component variations. I love the way it sounds and when doing tests I find myself enjoying playing guitar again. It's also dead quiet as far as 15W guitar all tube amps go.

I am wanting to implement an effects loop as I get much better results regarding dynamic response and tone variations by plugging a guitar directly into the input of the amp rather than through a pedal board and then the amp. I have tried two different buffered effects loop boards from different vendors and each one increases the noise floor (hiss and hum) dramatically when engaged. Both designs are based around a LND150 for the in and out. I've gone through numerous ground wiring scheme for these boards but I get no improvement. When I disengage the loop it's dead quiet again. I don't know if I should just do a passive approach for the effects loop. I've attached a schematic of the Spitfire and where I am taking signal to and from the buffered loop boards that I am using. To me, it sounds like the issue is impedance related, but I don't have a schematic on these boards to see if others could spot the issue.

I've built tons of studio gear but am relatively new to tube guitar amps. Again, I love the way this amp sounds and would like to apply effects after the preamp stage. Doing searches on the internet regarding effects loops in amp generates lots of not so helpful information so I am coming here to try to get more objective information on implementing a loop and experiences people have had doing so.

Thanks!

Paul

 

[dmp]

If you can add a tube, you can do the send with a cathode follower and have a triode for return. A tube driver circuit will have a high impedance, so can be driven at the point you want to connect, after vol/tone. Lot's of info here
If you need to go steady state for size, you could do it, but you need a high impedance input to the send circuit and you need a separate voltage supply. If the board you are using is a FET input, it should be high impedance. I wonder if the noise you are getting is due to a poor power supply to the effects loop boards? How are you powering them? Can you try a clean bench power supply to rule that out? The other issue could be the effects board is having to add a lot of gain, bringing up the noise floor. Can you test how much gain the effects board is adding, both on the send side and the return side?

 

[Mworthin]

Hello, everyone,

I've been building tube guitar amps for my studio and the latest one is a Matchless Spitfire with a number of component variations. I love the way it sounds and when doing tests I find myself enjoying playing guitar again. It's also dead quiet as far as 15W guitar all tube amps go.

I am wanting to implement an effects loop as I get much better results regarding dynamic response and tone variations by plugging a guitar directly into the input of the amp rather than through a pedal board and then the amp. I have tried two different buffered effects loop boards from different vendors and each one increases the noise floor (hiss and hum) dramatically when engaged. Both designs are based around a LND150 for the in and out. I've gone through numerous ground wiring scheme for these boards but I get no improvement. When I disengage the loop it's dead quiet again. I don't know if I should just do a passive approach for the effects loop. I've attached a schematic of the Spitfire and where I am taking signal to and from the buffered loop boards that I am using. To me, it sounds like the issue is impedance related, but I don't have a schematic on these boards to see if others could spot the issue.

I've built tons of studio gear but am relatively new to tube guitar amps. Again, I love the way this amp sounds and would like to apply effects after the preamp stage. Doing searches on the internet regarding effects loops in amp generates lots of not so helpful information so I am coming here to try to get more objective information on implementing a loop and experiences people have had doing so.

Thanks!

Paul

This is how Matchless does it in the DC30; just a passive insert, and the Spitfire is essentially channel 1 of the DC30 with 2 instead of 4 EL84s and a different tone control. DMP makes lots of valid points

I did this as a passive effects loop with my Trinity TC15 kit, which is a lower-power DC30 clone, and it works, but you will need the active circuit for the send and return, either inboard or outboard.

I do it outboard and use a Fuchs Plush FX Loop pedal, which is essentially a Dumblator Clone (pedal, surprisingly uncolored, needs its own dedicated power supply), or a V941 Vox Effects Loop Buffer (noisy, rack mount, but higher output, colored, cheaply made, but has its own charm--no longer made).

You could get an effects loop kit to build in to the chassis if you want to modify your amp further--and there are several out there that will put the send and return amps (either solid state or vacuum tube) in there, but you may not want to do that to your amp or may not have room, since it is point-to-point construction. You also do not want to degrade the value of the amp.

Playing with the passive loop, the choice of effects loop amp does color the sound. I like it better than having it built in, because it gives me options. But you need something to buffer and make up the gain.

I made mine with shielded coax to and from the input and output jacks, and jacks with switches that break the connection to the phase inverter when you plug it in.

Since this is an amp of some value, you want to make sure that you do it right, and may wish to have a Matchless authorized repair shop, such as Nashvegas guitars in Franklin weigh in, since it looks like you are in Nashville. The mod itself is trivial--two switched jacks and some shielded coax--but you want it done right. They have probably done that in a Spitfire before.

My 2 cents. Love that circuit...it's the amp I play more than any other.

 

[Potato Cakes]

This is how Matchless does it in the DC30; just a passive insert, and the Spitfire is essentially channel 1 of the DC30 with 2 instead of 4 EL84s and a different tone control. DMP makes lots of valid points

I did this as a passive effects loop with my Trinity TC15 kit, which is a lower-power DC30 clone, and it works, but you will need the active circuit for the send and return, either inboard or outboard.

I do it outboard and use a Fuchs Plush FX Loop pedal, which is essentially a Dumblator Clone (pedal, surprisingly uncolored, needs its own dedicated power supply), or a V941 Vox Effects Loop Buffer (noisy, rack mount, but higher output, colored, cheaply made, but has its own charm--no longer made).

You could get an effects loop kit to build in to the chassis if you want to modify your amp further--and there are several out there that will put the send and return amps (either solid state or vacuum tube) in there, but you may not want to do that to your amp or may not have room, since it is point-to-point construction. You also do not want to degrade the value of the amp.

Playing with the passive loop, the choice of effects loop amp does color the sound. I like it better than having it built in, because it gives me options. But you need something to buffer and make up the gain.

I made mine with shielded coax to and from the input and output jacks, and jacks with switches that break the connection to the phase inverter when you plug it in.

Since this is an amp of some value, you want to make sure that you do it right, and may wish to have a Matchless authorized repair shop, such as Nashvegas guitars in Franklin weigh in, since it looks like you are in Nashville. The mod itself is trivial--two switched jacks and some shielded coax--but you want it done right. They have probably done that in a Spitfire before.

My 2 cents. Love that circuit...it's the amp I play more than any other.

I was leaning towards the passive option and then see how much the sound of the amp changed. My volume pedal has a buffer built into it so maybe that will help. I'm also using very short cables for the send/return connections. Later this week I am going to switch out the power switch so I will wire up a passive loop and see how it actually sounds. If I find the results undesirable I may hit up Nashvegas guitars. But I have found that repair places have been so swamped that my friends are coming to me to repair their random studio pieces and guitar amps.

If I find I do need an active buffer, I would like to stay with solid state option. There is a spot drilled in the chassis for another tube and the power transformer could handle the additional load, I just don't want to deal with adding another tube if I can help it.

Thanks!

Paul

 

[Mworthin]

Sounds like a good plan…you can always convert passive to active later, if you like. And you will know right away if your outboard pedal is what you need in a buffer or if you need something designed for the purpose.

 

[Potato Cakes]

I wired up up passively and there was no added noise as before. I also could not hear a loss in clarity or body of the guitar tone with the loop inserted or not. I also use Mogami 2791 to build all of my guitar cables soldering both conductors for the tip connection, so I'm not adding the level of impedance and noise that most players do with the cheap cable quality that prevalent with musicians. I might still repurpose the effects loop switch for a 0,-3, and -6dB pad as when I started opening up the volume of the amp, there were moments where I thought the ICs in the reverb pedal being used for testing started to every so slightly become overdriven. I will have to do more testing with my full pedal board but it's probably not a bad idea to incorporate some sort of pad. There is one other buffered board option I may try just for fun but right now this amp is perfect.

Thanks!

Paul