Mesa Nomad 55 and Substi-Tubes EL84 converter

[Green Manalishi]

I recently picked up a Nomad 55, and I am using the Groove Tubes Substitubes in it with EL84s, which really complement the nature of this amp. I find it to be more balanced now, and ofcourse a little more Brit sounding, which is my preference. (I liked the Nomad better running EL34s in it)
Anyway, the Substi-tubes don't require biasing, I guess it's cathode biased, I could be wrong.

My question is does it make a difference if I have the amp in 6L6 or EL34 bias setting when using the Substi-tubes?

These are like Yellow Jackets, but are made better and sound better, I've owned both.

 

[Jonom]

I am about to do the same thing. Did you notice any difference that you could hear when switching between the 6L6 and El34 modes that you could hear? I've tried 6L6's and EL34's and also prefer EL34's too. I know one more pin is utilized on the EL 34's so my guess is that would be the mode to set the selector switch when using EL84's with the groove tube adaptors.

 

[RickyLee]

I have some of the GT Substitubes and the YellowJacket types stashed awy. When I get a chance I will dig them out and try them in my Nomad 100. I will let you know what is going on with the bias as well. I am wondering the same thing, and thinking maybe the adapters are cathode biased internally.

 

[the recluse]

I'm curious about these. I think I'm going to pick up a pair of yellow jackets for my Nomad 100. I want to put them in the outer tube sockets so when I have it in 100W mode it will run around 70W with 2 El84s and 2 E34Ls, then when I switch it to 50W mode it will run at 20W with just the EL84s.

Incidentally, if anyone has a pair of YJs that they're looking to get rid of, let me know.

 

[Jonom]

I recently put a couple of GT 928-PR's (GT Power reducers aka substitutes) in my Nomad 55 12" combo and absolutely love the sound! EL-84's are rich and buttery sounding great for nice clean comping and Landau-ish soloing on the red channel. I can hear a difference between the 6L6 setting and the EL-34 setting and prefer the EL-34 setting.

 

[the recluse]

I managed to get my hands on a duet of Substitubes and stuck them in my Nomad 100 today (after removing the tube retainers for the outer tubes).

After putting it through the paces for about 45 minutes or so I am very happy to have made the switch. I've been running the Nomad with E34Ls since last January, and I liked those better than the 6L6s, but there was still a bit of stiffness that I couldn't quite dial out.

Now with a pair of EL84s and a pair of E34Ls it's pretty much exactly where I wanted it to be. There's a bit more warmth and sweetness in the highs and mids, and the low end is just a tad looser. All around it's a nice change.

The other reason I wanted to put these into my amp was to see what the viability of a 20 or so watt amp would be in my band situation. I put the EL84s in the outer slots so when I kick the Nomad into 50 watt mode it is only running the EL84s at around 20 watts. The real surprise is that switching between the two modes there really isn't that much difference in volume. The clean channel definitely dirties up a lot quicker, and there's less thud in channels two and three, but none of it is bad, just different. Kicking it back to 100 watt mode doesn't raise the volume as much as it adds some oomph back into the low end.

I'll definitely need to try out 20 watt mode playing with my band to make sure that output power is viable (I think it might be now). I can already feel the Mini-rec gas starting.

 

[RickyLee]

I managed to get my hands on a duet of Substitubes and stuck them in my Nomad 100 today (after removing the tube retainers for the outer tubes).

After putting it through the paces for about 45 minutes or so I am very happy to have made the switch. I've been running the Nomad with E34Ls since last January, and I liked those better than the 6L6s, but there was still a bit of stiffness that I couldn't quite dial out.

Now with a pair of EL84s and a pair of E34Ls it's pretty much exactly where I wanted it to be. There's a bit more warmth and sweetness in the highs and mids, and the low end is just a tad looser. All around it's a nice change.

The other reason I wanted to put these into my amp was to see what the viability of a 20 or so watt amp would be in my band situation. I put the EL84s in the outer slots so when I kick the Nomad into 50 watt mode it is only running the EL84s at around 20 watts. The real surprise is that switching between the two modes there really isn't that much difference in volume. The clean channel definitely dirties up a lot quicker, and there's less thud in channels two and three, but none of it is bad, just different. Kicking it back to 100 watt mode doesn't raise the volume as much as it adds some oomph back into the low end.

I'll definitely need to try out 20 watt mode playing with my band to make sure that output power is viable (I think it might be now). I can already feel the Mini-rec gas starting.

When you put the amp into half power mode, did you adjust the impedance setting on the amp to be half of what the cab/speaker load is? If not, that would contribute to not hearing much of a change. Plus, theoretically there's not a huge difference in dB levels going from 100W to 50W. But then two EL84's should be around 20W as well.

 

[the recluse]

I did not change the speaker to the 4 Ohm output when I dropped it into 50Watt mode. I'll give that a shot too, but mostly I was surprised at the minimal perceived volume drop.

 

[the recluse]

And doing some further reading, it looks like I've basically turned my Nomad into a pseudo Simul-class amp. Nice!

 

[RickyLee]

I recently picked up a Nomad 55, and I am using the Groove Tubes Substitubes in it with EL84s, which really complement the nature of this amp. I find it to be more balanced now, and ofcourse a little more Brit sounding, which is my preference. (I liked the Nomad better running EL34s in it)
Anyway, the Substi-tubes don't require biasing, I guess it's cathode biased, I could be wrong.

My question is does it make a difference if I have the amp in 6L6 or EL34 bias setting when using the Substi-tubes?

These are like Yellow Jackets, but are made better and sound better, I've owned both.

I tried the GT EL84 power reducers in my Nomad 100 this evening. I pulled the two JJ 6V6S valves from my Marshall Studio 15 and stuck them in the Nomad 100 along with two GT Power Reducer adapters running a pair of GT EL84S valves, which should basically be JJ's.

So being it was quite late, I just did a quick sound check but did not fire up the amp with the 6L6's to compare. But I liked what I heard. I ran the amp at half power trying it with just the two 6V6's and then with just the two GT EL84 adapters. The 6V6's sound great by themselves but the EL84's were not that impressive by themselves - but the GT EL84 knocked down A LOT of volume.

What was scary with these GT adapters, was I measured a reduction of aprox. 100V on the plates, but had 36mA on each EL84 measured by my 1 Ohm bias resistors in the amp. I am figuring that should be accurate checking them with an external bias resistor though. I had 335V on the plates, so that works out to a 100% dissipation as these JJ's are rated at 12 watts.

And it does not matter if you set the bias switch to 6L6 or EL34, as the control grid voltage does not affect these adapters.

 

[RickyLee]

OK. Regarding the Groove Tube Substi-Tube Power Reducers model 928PR:

For anyone that is interested, or may be running these or own them, I was able to get some official Voltage readings on these while they were installed and running.

I used the VariAC to bring the voltage down a bit and put these back in the Mesa Nomad 100 for the test. The Nomad 100 would normally be running 480V plate voltage. But with the VariAC, I lowered the MAINS voltage a bit, down to 425V. Heaters were at 5.8VAC.

Here's the readings directly off of the EL84's AFTER the 928PR adapters. The plate voltage coming into/feeding the adapters was 425V.

Pin 7 Plate: 425VDC

Pin 9 Screen: 415VDC

Pin 3 Cathode: 16.9VDC

Pin 2 Control Grid 82mVDC

Power Dissipation/Bias Idle:

16.9V drop across 600R cathode = 28mA

425V - 16.9V = 408.1V

408.1V X .028A = 11.5W





A word of caution: These adapters are not reducing the plate voltage at all. And only knocking aprox 8V - 10V off of the screens. And even JJ EL84S valves that are the highest rated I have found, are only rated for 300V plate and screens. So use these at your own risk. I don't know of any amp running EL34, 6L6 ect. that runs anywhere near that low of a voltage.

 

[Tommy_G]

A word of caution: These adapters are not reducing the plate voltage at all. And only knocking aprox 8V - 10V off of the screens. And even JJ EL84S valves that are the highest rated I have found, are only rated for 300V plate and screens.

I had unofficial clearance by an adapter manufacturer to run the JJ EL84's at 365v plate in Class AB (fixed bias). Your thoughts? Is this crazy talk?

 

[RickyLee]

A word of caution: These adapters are not reducing the plate voltage at all. And only knocking aprox 8V - 10V off of the screens. And even JJ EL84S valves that are the highest rated I have found, are only rated for 300V plate and screens.

I had unofficial clearance by an adapter manufacturer to run the JJ EL84's at 365v plate in Class AB (fixed bias). Your thoughts? Is this crazy talk?

Even though they are only rated at 300V plate and screen, they seem to be built very well. I was just cranking them at full tilt in my Marshall Studio 15 at around 360V on the plates, and pushed them beyond the 12W dissipation.

And to add to that, this past week I ran these same valves in my Nomad 100 at both 435V & 480V on the plates. AND . . . that was after I accidentally red plated them for about 4 seconds!

This set I have is amazing.

I would think that pushing them beyond specs might just shorten the life of them. If gigging with them above rated specs, have a backup set on hand.

 

[Tommy_G]

Cool....just read a thread on another forum where under certain circumstances these JJ EL84's can be pushed to 500V plate voltages and it appears that 365V is well within the experiential envelope.

How good do they sound compared to other EL84's....only curious, I don't have many options that can take high plate voltages.

 

[RickyLee]

Cool....just read a thread on another forum where under certain circumstances these JJ EL84's can be pushed to 500V plate voltages and it appears that 365V is well within the experiential envelope.

How good do they sound compared to other EL84's....only curious, I don't have many options that can take high plate voltages.

All of the JJ power valves I have tried are awesome tonally. I have tried their KT88, KT77, KT66, 6L6GC, EL34, E34L, 7591S, 6V6S and EL84.

They all stack up to other older brands of power valves. I pulled out some GE 6V6's and compared them to the JJ 6V6S and the JJ's sounded just as good. But the JJ's were slightly louder as well LOL.

I do not have any other older EL84's, but the JJ's are much richer sounding than the Russian EL84's.