The "EF86 Xtra" and the passive FX Loop

[JMPGuitars]

I recently completed my EF86 Xtra amp build, which includes the passive FX Loop we'd been discussing. The EF86 Xtra is an EF86 front end, with an extra gain stage. Originally based on the EF86 Lite, but I completely changed the EF86's setup. For anybody interested in building the EF86 Xtra, the docs are in the High Gainers and EF86 sections of the Download area.

You can check out the demo here: https://youtu.be/WK7U1Na9sPg

1. Guitar directly plugged into amp (no effects): 00:00 - 05:35
2. Guitar directly plugged into amp (pedal board through FX loop): 05:36 - 06:59
3. Guitar plugged into pedal board: 07:00 - END

The first section is just me making noise to hear different tones and overdrive levels, as well as touch-sensitivity from the amp.
Section 2 has my pedalboard and JamMan looper plugged in through the FX loop.
Section 3 has the board plugged in to the front of the amp.

As I mentioned in the video description, with the JamMan looper in the FX loop, I can raise the preamp gain to distort the guitar without affecting the looper sound. But, with the pedal board/looper pedal plugged in to the front of the amp, I have to keep the preamp gain clean enough to make the loop/backing track sound good. And obviously the difference in noise from the digital stuff is significant.

Brace yourself ZP, there's a couple jazz chords in there for like 2 seconds.

Thanks,
Josh

PS. Side note: my build has a 2.2µF cathode cap, and I think a 1.2K or 1.5K cathode resistor on V2, for reduced bass, and reduced distortion (I included options in the Tweaker's Table). This brought the plate voltage on V2 up to 165VDC.

[Bieworm]

Awesome amp Josh!!! People would be surprised what that 3-knob amp can deliver... hats off

[JMPGuitars]

Awesome amp Josh!!! People would be surprised what that 3-knob amp can deliver... hats off

Thanks! I really like it. If you look at the new docs, I completely changed the EF86 setup, and I think it sounds great/feels really responsive. It also gives some great distortion tones at apartment levels without needing an attenuator...as much.

[zaphod_phil]

You can check out the demo here: https://youtu.be/WK7U1Na9sPg

1. Guitar directly plugged into amp (no effects): 00:00 - 05:35
2. Guitar directly plugged into amp (pedal board through FX loop): 05:36 - 06:59
3. Guitar plugged into pedal board: 07:00 - END

The first section is just me making noise to hear different tones and overdrive levels, as well as touch-sensitivity from the amp.
Section 2 has my pedalboard and JamMan looper plugged in through the FX loop.
Section 3 has the board plugged in to the front of the amp.
Brace yourself ZP, there's a couple jazz chords in there for like 2 seconds.

That amp sounds great!!! It's got a nice throaty EF86 richness, along with the typical 18W crisp tones. Cleans up nicely too I honestly didn't think much of the jazzy chords - they just sounded kind of very blah and dull to me. Still, I understand that's a good way to show off what the amp can do. As you said, you were just making noise to hear different tones and overdrive levels OTOH I loved your solo during the FX loop section - very Dave Gilmour!

PS: Side note: my build has a 2.2µF cathode cap, and I think a 1.2K or 1.5K cathode resistor on V2, for reduced bass, and reduced gain (I included options in the Tweaker's Table). This brought the plate voltage on V2 up to 165VDC.

I get it that the 2.2uF cathode cap is what's giving you less boomy bass, relative to the previous 50uF one. EDIT: Having larger cathode resistors will change a preamp stage's headroom, but not reduce stage gain if there's a cathode cap present.

[JMPGuitars]

That amp sounds great!!! It's got a nice throaty EF86 richness, along with the typical 18W crisp tones. Cleans up nicely too I honestly didn't think much of the jazzy chords - they just sounded kind of very blah and dull to me. Still, I understand that's a good way to show off what the amp can do. As you said, you were just making noise to hear different tones and overdrive levels OTOH I loved your solo during the FX loop section - very Dave Gilmour!

Thanks!

EDIT: I just realized that's kinda funny. Gilmour is one of my influences, but it's funny because you mentioned the FX loop section. The distortion I used in the latter section through the front end is a Violet BMP, which I chose to build because that was what Gilmour used on past recordings with Pink Floyd.

I get it that the 2.2uF cathode cap is what's giving you less boomy bass, relative to the previous 50uF one. EDIT: Having larger cathode resistors will change a preamp stage's headroom, but not reduce stage gain if there's a cathode cap present.

Oops, I meant distortion not gain. OP updated.

[Riffmonster]

Sounds super nice Josh, great work!

[Crabman]

Nice job Josh. Some great tones in the demo. I’m intrigued by the EF86 and may have to give this a try.

I recently listened to a demo of a TEX amp EF86+2x12AX7+2xEL84 with volume plus tone control only. Yikes … what a nice sounding amp. Do you know anything about these amps? I’m wondering if something like this may have enticed you to add the additional 12AX7 in the pre-amp?

Also I see your anode resistor in the EF86 produces a load line that is more in keeping with where Merlin says it should be. Did you decide on this after trying 220K there and finding it wasn’t going to work? I know Merlin harps on about how the Vox configuration for an EF86 should not be used in any amp. I assume you use those values in the EF86 lite to properly drive the phase inverter?? I’ve never played with an EF86 so I am curious.

Anyway, great job and thanks for the effort you put into the documentation. Really top notch.

Dave

[JMPGuitars]

Nice job Josh. Some great tones in the demo. I’m intrigued by the EF86 and may have to give this a try.

I recently listened to a demo of a TEX amp EF86+2x12AX7+2xEL84 with volume plus tone control only. Yikes … what a nice sounding amp. Do you know anything about these amps? I’m wondering if something like this may have enticed you to add the additional 12AX7 in the pre-amp?

Also I see your anode resistor in the EF86 produces a load line that is more in keeping with where Merlin says it should be. Did you decide on this after trying 220K there and finding it wasn’t going to work? I know Merlin harps on about how the Vox configuration for an EF86 should not be used in any amp. I assume you use those values in the EF86 lite to properly drive the phase inverter?? I’ve never played with an EF86 so I am curious.

Anyway, great job and thanks for the effort you put into the documentation. Really top notch.

Dave

Thanks!

Yes, I did reference Merlin's EF86 thoughts, as well as a few others while I was experimenting with the values for the EF86 part of the circuit. Zaphod_Phil had actually pushed me in that direction too, a few years ago before I started this concept (for modifying the Lite).

My final values were based on the tone, and the amount of overdrive, along with voltages that looked happy to me. A lot of what you see in the circuit is for taming the signal, as it can be insanely overdriven. It's possible it could handle even more than what I ended up with, as I was actually working with a defective OT while developing this.

I'm not familiar with the other amps you mentioned. The reason I created this is because I made a custom EF86 lite for somebody...and while he loved the overall tone of the amp, he said it didn't have enough distortion for his taste (he never told me, he told a friend in common - and I don't think he had an attenuator). That bugged me, so this idea was born. More dirt!

For me, and my attenuator usage, I never found the EF86 Lite lacking in dirt. But truthfully, I would likely build this version of the EF86 if/when I build another Lite.

Thanks,
Josh

[zaphod_phil]

Thanks!

Yes, I did reference Merlin's EF86 thoughts, as well as a few others, while I was experimenting with the values for the EF86 part of the circuit. Zaphod_Phil had actually pushed me in that direction too, a few years ago, before I started this concept (for modifying the Lite).

That was because the very high gain of Vox's EF86 setup can be somewhat uncontrollable, emphasizing the EF86 tendency for microphonics. Otherwise, just to set the story straight, I don't necessarily go by everything Merlin says. I think sometimes he gets anal about some quite trivial things. However, his web site has excellent information for guitar amp builders. Let's hear it for Valve Wizard Merlin!

My final values were based on the tone, and the amount of overdrive, along with voltages that looked happy to me.

IMO that was the perfect approach. Well done, Josh!

[JMPGuitars]

That was because the very high gain of Vox's EF86 setup can be somewhat uncontrollable, emphasizing the EF86 tendency for microphonics. Otherwise, just to set the story straight, I don't necessarily go by everything Merlin says. I think sometimes he gets anal about some quite trivial things. However, his web site has excellent information for guitar amp builders. Let's hear it for Valve Wizard Merlin!

Yes! It actually does sound less microphonic, and I like the character of the distortion better.

My final values were based on the tone, and the amount of overdrive, along with voltages that looked happy to me.

IMO that was the perfect approach. Well done, Josh!

Thanks!

[Crabman]

Quick question regarding the grid stoppers on the EL84s. What made you go with 56K? Was this to combat blocking distortion or is this strictly aimed at taming treble content? Or could it be a typo and we are looking at 5.6K?

With my 18W TMB I did try larger grid stoppers to try and eliminate a bit of fizz but found that it removed too much of the shimmering glassy treble content so I reverted to 8.2K. If larger grid stoppers are to be used to reduce blocking then from a design standpoint I guess it could help to allow more treble content through earlier in the pre-amp?

I'm interested to know how much experimenting you had to do with this to get it right.

Dave

[JMPGuitars]

Quick question regarding the grid stoppers on the EL84s. What made you go with 56K? Was this to combat blocking distortion or is this strictly aimed at taming treble content? Or could it be a typo and we are looking at 5.6K?

I used the 56K to combat what I assumed might be blocking distortion. Those values were chosen while I was still using a defective OT. It sounded great with those values when the new OT was put in, so I left them in. Whether or not they're necessary is another story, but I certainly don't think the amp lacks shimmer or top end. You could always use a lower value and see how it sounds if you decide to build this circuit.

Thanks,
Josh

[JMPGuitars]

EF86ScreenVoltages.jpg

Check that out, I actually bothered doing a tiny bit of math. I remember reading somewhere to target the screen between 60 to 70% of the plate, so I went for 65% in the end. I totally forgot about that.