Groove Tubes "Substi-Tube" 928-PRs / THD YellowJackets


Epic Member
Found these on site; ostensibly you can pull a 6L6 duet out of an amp and switch it to a lower power Class A amp. Anyone try these out have any comments? Lab report test on link at bottom of page looks impressive.
Very much the same as the "yellowjacket", except about triple the price.

However... some noted fluff in their ad-copy should be pointed out.

(Groove Tube stated)

An adapter which allows class A/B amplifiers such as Marshall and Fender, to replace their output tubes with the EL-84 tube, and operate in class A.  This reduces the power of the amplifier.  In the case of a 100 watt amp, the output will be about 40 watts.  In a typical 50 watt amp, the power will be about 20-25 watts.

-CB- notes that plate dissipation for a quartet of EL84's in Class A operation is 30 watts,  and a pair can dissipate 15 watts.

These are used in pairs or quads, not individually.  These are NOT for use in Class A amplifiers.  For Class A amplifiers, please use the 928, not this 928-PR.

These will reduce the load on your power transformer, and allow your amp to reach distortion, with a nice AC-30 or EL-84 sound and feel at levels that are lower and more friendly to recording and venues where your amp will not feel, play, or distort at the amplifier's normal higher sound levels.

-CB- notes that the load on the power transformer will be, for a two tube 50 watt amp, the same as running that amp at about 30watts output.... no matter how loud you play...because even at soft volume, class A operation has constant transformer load.  So, if you're playing softer with the Substitube, you're still using the transformers (all of them) as if you were running normally about #7 or so on teh dial.

(end of their ad-copy)
Dont be....  They work!~  Well, at least the yellow jacket does. 

What you have is a capacitor input to the grid on the EL84, that blocks the bias voltage coming on the grid of the 6L6 socket.  The plate is connected to plate, on each.  Cathode on the EL84 has a cathode resistor in series to ground - where the cathode pin on the 6L6 is already grounded.  Heaters are heaters.... no issues there.  So, some little bit of rewiring, a capacitor and resistor, two sockets and something to hold them in... almost $125..?????  Ouch.  The yellow jacket ( is the same, and is much cheaper (last time I checked at least).

Running EL84's that way is a nice alternative.  The result is class A push pull - one might argue why not go class A/B and get more oomph..... but then you'd have to fiddle with bias circuits and such.

The other model they have, simply has the cathode resistor deleted, since on a class A amp you'll have one anyway.  There are problems with that... since on an EL84 you use a rather small cathode resistor, and on something like a 6L6 or 6V6 you use about 470 ohms.  So the model for Class A/B amps is the "one to get" for best results.
I don't know, been happy to date with GrooveTubes; a duet set can be had online for about a C-note; seems like if they work it's not a bad investment that could give you the versatility of having an extra couple of amps in the house if you switched them out for the existing power tubes; in the house the biggest issue with my 40 or 75 watt amp is being able to crank them loud enough to get the tube sound I want.

While I also was looking at the wattage claims somewhat askance theoretically, doesn't really make a lot of difference. I would stipulate that if you're at home playing/rehearsing with backing tracks or recording and use efficient speakers, you don't really need more than about 5 watts of power anyway, that's going to translate into somewhere in the vicinity of 106dB.

The "holy grail" I seek is the sound of like a Blackface Fender Bassman or Super Reverb at that 106dB level. Am doing some experimentation with the little 5w Blackheart I just switching tubes out on it; this seemed like another good alternative. Will probably buy a set a check it out.

I checked the Yellow Jackets out, but looks like they run about the same price bundled with a matched duet of EL84s, around 99 dollars, were you just referring to the adapter sockets CB?
oh man... they used to be about $60 per pair - WITH tubes.

guess they gone up bit

does the groove tube give you tubes too? has best price - 89.95 for duet, just ordered a set to test in my Blues Deluxe and Fender 75. Should be interesting to see what the 75 sounds like when the "15 watts" step down switch in engaged...
The GT Substi-Tubes still on back order, so I bought the set of THD YellowJackets off Ildar that wouldn't fit his amp; he had accidentally broke one of the EL84s (no-names from the remaining good one he sent), so I got a Groove Tubes GT-EL84-S duet to use for tubes.

Plugged in lieu of the stock 6L6's in my Blues Deluxe, and they work like a champ! One of the kids had left a Blues Jr. amp in the band room, which I think is a very similar circuit, but with EL84s instead of 6L6 power tubes, side by side. The Blues Deluxe through the clean channel actually sounds better (Blues Jr, has one channel more akin to the drive channel on the Blues Deluxe); that could be partly due to the Celestion Vintage 30 in it vs. the "Special Fender Design" speaker stock in the Blues Jr., but tone is a bit cleaner dimed out.

Would definitely recommend these; obviously do the trick to turn a 40-50 watt amp into a 12-15 watter more useful at home or at small gigs, as when you crank the Blues Deluxe up to 8 with the 6L6s in it, it's pretty f'ing loud.

So for around $100 bucks online, it's like having a second, lower powered amp that fits in the compartment of your guitar case, not a bad deal at all....

One caveat - avoid Ildar's issue; these converters with the EL84 are 4.5" tall, a good inch taller than most 6L6 tubes. They should fit OK in most open back combo in which the tubes point down from the chassis, but measure and make sure you have clearance to install them, particularly in a head rather than a combo amp. Accounting for the pins as well you'll need to have at least 5.25-5.5" clearance to install them.
Next mod to the Blues Deluxe with the Yellow Jackets/GT-EL84-S replacing the stock 6L6s (really Sovtek 5881s); tried replacing the 12AX7 preamp tubes with 12AX7WB's and a old Fender 12AT7 for the existing 12AX7 phase inverter; sounds bitching, but with the 12AT7 it's actually louder, WTF?
Old Fender 7025 subbed out for 12AT7 in phase inverter socket, that's the ticket! Now it sounds more "Marshally" than " Fenderish" if you dial it past 4....
The backordered Groove Tubes 928-PRs finally came in today; compared them to the THD YellowJackets in the Blues Deluxe; both converters have GT-EL84-S in them; Sovtek 12AX&WBs in preamp slots, vintage Fender 7025 in phase inverter slot.

1.) See pic below; GT-928-PR converters are almost 3/8" shorter than the YellowJackets, might fit in some heads with limited space the YellowJackets won't.
2.) The Groove Tubes sound a little different than the YellowJackets; YellowJackets sound a little darker, GTs a little brighter. I was going to resell whichever pair I didn't like as much, but like both and the difference in tone is enough to keep both.
3.) As a function of how these work in a 6L6 based amp, you get "max volume" on the YellowJackets at about 5 on the Blues Deluxe volume dial, about 6 with the GT-928-PRs. You can turn the volume all the way up, but it doesn't get appreciably louder past that point - CB can probably explain why if you care...

Hard to recommend one over the other; one's brighter, one's darker; max volume is the same and through a Celestion Vintage 30 slightly louder than a stock Fender Blues Jr.

Also note that with the stock tubes in the Blues Deluxe I've never the "Drive" channel other than playing around with it the first couple days I had the amp, as I REALLY didn't like the distorted sound; I've always just used the "Normal" channel, and know a lot of people feel the same way about the Blues Deluxe/Blues Deville/Hot Rod Deluxe/Deville. With either of the EL84 converters subbed for the 6L6, it kicks ass, dial the normal channel down a bit for a cleaner sound and kick into "Drive" at near max or max for lead Crunch City. If you own one of this series of Fender tube amps you need a set of these converters, great for all the smaller gigs/venues. The sound level meter we use at work (I'm a little dubious of it's accuracy at volumes over 100dB) shows 111.2 dB with the amp pegged with converters; by way of comparison the stock Blues Jr, reads 109.8 dimed out.

After spending the evening playing a variety of guitars and pickup sets through the modded rig, I doubt the 6L6's will ever go back in the Blues Deluxe....
jackthehack said:
2.) The Groove Tubes sound a little different than the YellowJackets; YellowJackets sound a little darker, GTs a little brighter. I was going to resell whichever pair I didn't like as much, but like both and the difference in tone is enough to keep both.

Probably because the YJs use JJ EL84s.  Not sure what brand GT uses (and re-labels)...

EL84s compress and distort earlier than many other output tubes.

I have a pair of YJs in my Marshall JTM45RI... those coupled with the Celestion Alnico Gold speaker and Seth Lovers in my geetar give me chime for days.

It's a Marshallvox.  :icon_biggrin:
"Probably because the YJs use JJ EL84s.  Not sure what brand GT uses (and re-labels)..."

Have the same GT Duet sets in both converters
jackthehack said:
"Probably because the YJs use JJ EL84s.  Not sure what brand GT uses (and re-labels)..."

Have the same GT Duet sets in both converters

Ah.  My YJ's came with JJ's... which are generally somewhat darker than typical; especially their 12AX7s.
I have some Yellowjackets that I've used in my Marshalls before. They do what their supposed to but I don't really like them more than stock EL34's. You lose some low end with the YJ's/84's & headroom. Which is understandable. They're fun to play around with.