
GONZO-X
Well-known member
bought a pair of these off of Ebay, just installed them last night.
Groove Tubes SUBSTI-TUBE 928-PR
Duet Power Reducer Class A converter
Octal to 9-pin adapter for the use of EL-84 tubes in an octal socket.
This converts class A/B amplifiers to class A for lower power.
'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.'

i've only played them for one night, so i'll save a full report for later....
but at first blush, i was amazed at how it changed (for the better) the voicing of my mesa boogie mark2b head.
the boogie is a 60 watt head, so it has (2) 6L6's.....
so both of the power tubes are replaced by this adapter with a little EL84, cuts the power about in half.
These are groove tube EL84s, for those of you that are into tubes... they're rated at 4, out of a scale of 1-10 (1-3 is rated for quick breakup, 4-7 is rated for normal breakup, 8-10 for almost no breakup)
the sound:
well, i'm having to tame a 60 watt beast, to be able to record at lower volumes with it.
i've found that the amp really only does it's magic, when the master volume is at least halfway.
which is grounds for divorce, eviction, incarceration.

i know the boogie amp well.... i've had it since 1981, and gigged full time with it for the first 5 years of it's life...
the 6L6 sound, is a classic american tube sound....
but can be harsh with certain settings at high gain, and with presence dialed in.
the first thing i noticed about the new 'substitubes', was that the high end sounded smoother.....
sweeter....chimey...
more dialed in, and in a word, lush.
these tubes are also more 'responsive' than the 6l6's on idle were...
this could be because my 6l6's are simply getting old, but i don't think so.
this is a tone that's specific to the EL84, i believe. it doesn't have quite the frequency range of the 6l6, especially in the low end, but to my ears, sounds better, in a more 'guitar-friendly' frequency range.
the only other amp that i've played in recent years, that struck me as to having that greasy, smoother, darker, creamier sound, was a Vox AC-30.
very similar to that....
but, with the circuitry of the mesa being more high gain than the vox, the mesa has more of the tone i'm looking for.
if you could morph an older boogie tone, to a vox ac-30 tone, that's what i have now.
there's more presence at low volume, but without sounding 'trebly'.
on the lead channel, set at low gain, it almost sounds like a DR. Z MAZ 18.
it's hard to explain.
it's like, more saturation, but at the same time, more clarity.
i think that with the wide range of groove tube distortion/character ratings, you could dial in exactly the amount of power breakup, to match the gain settings and tone you like on the amp.
i dig the #4 rated tubes that are in it now, but of course, i haven't a/b'd that to anything yet.
on both humbuckers, and single coil sounds, the amp has more clarity than i've ever heard out of it before.
i'll follow up on this 'experiment' with some sound bites, and some more info on how the amp responds to different guitars, the effects loop (the sound of it changed too) and pedals in front of the amp.....
i'm extremely happy with the sound, i've just got to really push em hard for a while, and see if there are any weaknesses.
my next step in the tone search for low volume recording:
a Weber Mass.
~~~~~~
here's a link, to a comparison between the GT 928-PR's, and the THD yellowjackets, which were the other power tube adapter i was looking at...
http://www.groovetubes.com/assets/1778_GT928-PR_vs_THDYJSD_Test.pdf

Groove Tubes SUBSTI-TUBE 928-PR
Duet Power Reducer Class A converter
Octal to 9-pin adapter for the use of EL-84 tubes in an octal socket.
This converts class A/B amplifiers to class A for lower power.
'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.'

i've only played them for one night, so i'll save a full report for later....
but at first blush, i was amazed at how it changed (for the better) the voicing of my mesa boogie mark2b head.
the boogie is a 60 watt head, so it has (2) 6L6's.....
so both of the power tubes are replaced by this adapter with a little EL84, cuts the power about in half.
These are groove tube EL84s, for those of you that are into tubes... they're rated at 4, out of a scale of 1-10 (1-3 is rated for quick breakup, 4-7 is rated for normal breakup, 8-10 for almost no breakup)
the sound:
well, i'm having to tame a 60 watt beast, to be able to record at lower volumes with it.
i've found that the amp really only does it's magic, when the master volume is at least halfway.
which is grounds for divorce, eviction, incarceration.

i know the boogie amp well.... i've had it since 1981, and gigged full time with it for the first 5 years of it's life...
the 6L6 sound, is a classic american tube sound....
but can be harsh with certain settings at high gain, and with presence dialed in.
the first thing i noticed about the new 'substitubes', was that the high end sounded smoother.....
sweeter....chimey...
more dialed in, and in a word, lush.
these tubes are also more 'responsive' than the 6l6's on idle were...
this could be because my 6l6's are simply getting old, but i don't think so.
this is a tone that's specific to the EL84, i believe. it doesn't have quite the frequency range of the 6l6, especially in the low end, but to my ears, sounds better, in a more 'guitar-friendly' frequency range.
the only other amp that i've played in recent years, that struck me as to having that greasy, smoother, darker, creamier sound, was a Vox AC-30.
very similar to that....
but, with the circuitry of the mesa being more high gain than the vox, the mesa has more of the tone i'm looking for.
if you could morph an older boogie tone, to a vox ac-30 tone, that's what i have now.
there's more presence at low volume, but without sounding 'trebly'.
on the lead channel, set at low gain, it almost sounds like a DR. Z MAZ 18.
it's hard to explain.
it's like, more saturation, but at the same time, more clarity.
i think that with the wide range of groove tube distortion/character ratings, you could dial in exactly the amount of power breakup, to match the gain settings and tone you like on the amp.
i dig the #4 rated tubes that are in it now, but of course, i haven't a/b'd that to anything yet.
on both humbuckers, and single coil sounds, the amp has more clarity than i've ever heard out of it before.
i'll follow up on this 'experiment' with some sound bites, and some more info on how the amp responds to different guitars, the effects loop (the sound of it changed too) and pedals in front of the amp.....
i'm extremely happy with the sound, i've just got to really push em hard for a while, and see if there are any weaknesses.
my next step in the tone search for low volume recording:
a Weber Mass.
~~~~~~
here's a link, to a comparison between the GT 928-PR's, and the THD yellowjackets, which were the other power tube adapter i was looking at...
http://www.groovetubes.com/assets/1778_GT928-PR_vs_THDYJSD_Test.pdf