Newsgroups: rec.music.makers.guitar,alt.guitar,alt.guitar.amps,rec.audio.tubes
Subject: Dr. Z Prescription Review
Date: Mon, 13 May 1996 06:51:32 -0400
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Xref: geraldo.cc.utexas.edu rec.music.makers.guitar:93103 alt.guitar:94101 alt.guitar.amps:14969 rec.audio.tubes:9661
Dr. Z Rx Review
I recently borrowed a Dr. Z Prescription guitar amp. After a week of
playing it, I decided to write a review of it in order to share the
information with other network folks.
Description:
The amp has 3 12AX-7 preamp tubes, 4 EL-84 power tubes running in
class-A with no negative feedback, and a GZ-34 rectifier tube. As stock,
my evaluation unit had 2 NOS Phillips 12AX7s in the first gain stage and
phase inverter, and a Sovtek 12AX7 powering the 2nd and 3rd gain stages.
Rectification was handled by a Groove Tubes "Reactive" solid state
rectifier but may also be ordered with a GZ-34 rectifier tube. The power
stage is handled by 4 Tesla EL-84 tubes which put out about 45 watts.
The controls on the front of the unit from left to right consist of:
Bass, Mid, Treble, Volume, Expand Switch, high input, low input. The
expand switch bridges a capacitor across the first gain stage cathode
resistor for an extra thick tone. There is no reverb or effects loop.
Dr. Z was going for a design with an emphasis on tone and felt that
reverb or an effects loop would compromise the tone and integrity of the
signal path.
The controls on the back of the unit consist of 3 jacks for 4,8 or 16
ohms.
The head-cabinet is a very small, sexy enclosure measuring 9.5" tall,
19.5" wide and 9.5" deep. I really like this design and it seems
equally at home on top of a 1x12 as well as a 4x12 cabinet. Very groovy.
Design:
The Dr. Z Prescription is a brand new design. Dr. Z does not make retro
or copy amps and this amp is no exception. While it certainly borrows
concepts from classic Western Electric circuit designs, you won’t find
its schematic in the Tube Amp Book. The amp employs a fairly novel
conjunctive filter circuit before the output stage which seems to really
juice the amp up and give it a very original tone.
The amp is built almost entirely of parts designed by Dr. Z himself. Dr.
Z has his own special formulation (Sprague) capacitors which are not
available commercially. They are designed to react in a similar way as
the old "Black-Cat" capacitors which were used in many classic early
60’s amps. Transformers are manufactured to his specifications as well.
The output transformer in the prescription is huge. It has 11
interleaves, paper bobbins, etc. Though the amp puts out about 45 watts,
the output transformer looks like something you’d see in a 100 watt amp.
(For those of you doubting the output rating, I compared it to my 50
watt Plexi Marshall which is really loud and the Z amp kept up with it
pretty well.
Resistors are NOS Allen Bradley carbon composition. Pots are CTS, jacks
are SwitchCraft. Wire used throughout is 600V, solid core. The
components are hand wired on custom made glass boards using turret
posts. Turret posts make better electrical connections, are more
reliable, and less prone to cold solder joints than eyelet boards. The
tubes, while not designed by Dr. Z, are hand selected and matched in
order to make the amp as quiet as possible with the best tone possible.
Dr. Z is extremely committed to doing that and I’ve seen him
painstakingly going through phase inverter tubes and measuring the
voltage of each triode until he finds a tube which is a match. (A 12AX-7
consists of 2 triodes in one glass bottle).
The actual mechanical structure is simply wonderful. The chassis is
solid aluminum with a special plating. This material increases the
conductivity to that of solid copper, yet retains the strength of
aluminum. This is an important factor in reducing hum and maximizing
tone. All wire runs are as short as possible and bent to right angles.
Dr. Z is very meticulous about wire routing and I noticed that extreme
care was taken to keep AC voltage wires away from signal grid wires.
There are no loose wires or silicone goo holding parts together.
Whenever possible, wires are twisted together and/or wire wrapped with
cable ties. No parts are hanging in the air either. Soldering is very
carefully done as no wires showed signs of insulation burning, and all
solder joints looked very solid, with just the right amount of solder
applied. This amp is built for the long haul.
Sound
First of all, let me say that the amp sounds amazing. It’s dripping with
tone. From the very first note, you can hear and feel it.. The amp’s
tonal range encompasses many facets. The first thing I noticed was that
the amp sounds radically different depending on what range the volume
control is set to. The volume control seems to be calibrated into 3
distinct areas: Clean-Sheen, Bluesy/Stingy/Crunch, and High-Gain
Clean-Sheen
Beautifully clean and chimey. The tone was bell-like and glassy with a
piano-like clarity on the low notes. Note separation was very good.
Playing complex jazz chords with ½ step voicings, I was able to clearly
discern each individual voice of the chord. Playing riffs across the
guitar from low strings to high strings demonstrates the evenness of the
amps tone across a wide frequency range. I was able to get a really full
bodied jazz tone as well as a really sweet maple neck Strat tone.
Bluesy/Stingy/Crunch
Using a Strat and with the volume about 1/3 the way up, I could get a
really great Stevie Ray Vaughan tone. At this volume level, overdrive
and compression are controlled by the guitar’s volume control. With the
guitar’s volume partially down, I was able to get that really sweet,
clean, wispy Strat tone that I’ve been looking for forever, and with the
guitar’s volume pegged, the amp is just starting to distort. If you’ve
heard SRV’s Texas Flood CD, you know the tone I’m talking about.
Switching to a 335 brought out another completely different personality
in the amp. With the guitar’s volume down and gradually increasing the
amp’s volume, the sound went from slight compression, to crunch, to a
really bluesy/stingy/cutting tone. Turning the guitar full up yielded a
very rich, bluesy, distorted tone. Not the least bit buzzy or unnatural.
Playing on the treble pickup yielded a really rich, smooth overdrive,
not unlike Eric Clapton’s live Crossroads tone.
High-Gain
Turning the amp up past halfway increased the gain and harmonics
considerably. Here, the amp started to take on the character of the Eric
Johnson violin tone. With my buddies Les Paul and Pearly Gates pickups,
the amp yielded a very fine early ZZ-Top tone. I was able to get
harmonic feedback, squeals and squawks on just about every note and the
incredible touch sensitivity was astounding. The only other time I’ve
had a ride on such a wild beast was playing a fully cranked 100 watt
Plexi-Marshall through two 4x12 cabinets!
Tone Controls
The tone controls on this amp are wonderful. They are specially tuned to
enhance the bass, mid, and treble bands that most effect the guitar’s
tone. When the controls are at the 12 O’clock position, the amp sounds
very natural. The effect is not unlike having a detent position and
being able to add or subtract from the zero point. Sweeping the tone
controls up or down yielded clearly audible, yet musical results. This
was very pleasing to me since so many amps today suffer from tone
controls which either do nothing, or are so severe that the majority of
their range is unusable. Again, this is not the case with the
Prescription. In fact, I could not find a combination of tone settings
that was not extremely musical.
And More…
At this point, you’re probably wondering if this amp suffers from
multiple personality disorders. I’m here to tell you that Sybil has
nothing on this amp. Read on…
A few days ago, I picked up a blues CD which had some really nice tweed
guitar sounds on it. It was a recent recording but done in the style of
the old Howling Wolf or Muddy Waters sessions. I really loved the guitar
sounds and it inspired me to try to get those sounds out of the
Prescription just for kicks. I decided to change the preamp and
rectifier tubes to see if I could get more of a brown/tweed sound out of
it.
I replaced the first preamp tube in the Prescription with a GE 12AY-7.
(This is the tube the ’59 bassman had in this position). I replaced the
phase inverter tube with a GE 5751 which is a lower gain than a 12AX-7
but higher than an 12AT-7. The last thing I did was to substitute a
Sovtek 5U4 rectifier tube. Because of it’s higher internal resistance,
the Sovtek tube lowers the plate voltages on the EL-84s as well as gives
the amp more sag. Additionally, having seen many tweed amp circuits, I
decided to leave the "enhance" switch in the off position since many
tweed amps employed no bypass cap for the cathode of the first preamp
tube.
Voila ! Instant tweed. I could not believe the difference. With my
Strat, the amp sounded incredibly like the tones I heard on the blues
CD. With the amp on 4 and the volume turned down on the guitar, the tone
was full bodied and clear with lots of sustain, and with the guitar full
up, the amp just ripped. It sounded very woody and reminded me of many
tweed amps I had played and heard on records.
I plugged my 335 in played a few notes and once again my jaw dropped.
With these tubes and the amp up about ½ way, the amp sounded almost
exactly like Eric Clapton’s tone on the John Mayall Bluesbreaker
recordings. It was fatter, darker, less distorted, and less harmonically
rich, but it had the touch, tone, and feel of the record. Twisting the
treble control, I found that I could make the amp transition from ’59
Bassman to ’64 JTM-45. I’ve played through several original ’59
bassman’s as well as original JTM-45s and I would have to say that this
amp holds its own with any of them. I suggested to Dr. Z that he make
available a "Tweed Upgrade Kit" consisting of the 12AY-7, 5751, and 5U4
rectifier tube. This seems like a simple way to add a whole plethora of
sounds to the amps palette.
Closing
In closing, I would have to say that this amp produces the widest array
of tones I’ve ever experienced from a single amp. It’s possible to go
>from 50’s tweed, to 60’s plexi, to 90’s violin-tone. It’s well built,
well designed and well thought out. The designer Dr. Z, personally
builds and inspects every amp. He does not employ students to build his
amps and he does not have a factory. He works out of his basement and
has resisted attempts to be bought by larger companies. He’s very
committed to making hand-built amplifiers for guitar players who are fed
up with paying exorbitant prices for vintage or retro gear. His
philosophy seems to be very simple: Hand-build a top quality, original
product, designed to last a lifetime which will become an extension of
the player’s personality and sound.
If you recall, Dr. Z was listed in Guitar Player™ Magazine’s 50 best
buys for 1995. With a price tag of under $1000.00, the Prescription
ought to shoot Dr. Z to the top of that list for 1996 and beyond. If you
want an amp with interchangeable modules, multiple channels, progressive
diode expansion, midi switching, LED’s, aged tweed, etc., look
elsewhere. If you want an amp with damn good tone, check out the Dr. Z
Prescription amp. You’ll be glad you did.
You can contact Dr. Z at (216) 581-0052 or email him at mzait--(at)--ol.com
He’s had some problems getting email recently so if you cannot get hold
of him electronically, feel free to call him. He’s a very nice guy who
is more than happy assisting you in getting to tone-heaven.
Jack A. Zucker © 1996
ja--(at)--n.com
From craig--(at)--ps.com Tue Sep 10 11:28:14 CDT 1996
Article: 107475 of rec.music.makers.guitar
From: Craig Wu
Newsgroups: rec.music.makers.guitar
Subject: More on Dr. Z Prescription
Date: Tue, 10 Sep 1996 01:00:47 -0700
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I was going to wait until I had a few more gigs with the Prescription
before posting a full review, but I couldn't hold out anymore...
My Prescription has a 3 position bright, an expand switch, and a rear
jack for a footswitch boost. It came with three Sino (Chinese) 12AX7WAs
and a quartet of N.O.S. Phillips EL84/6BQ5 tubes. The Phillips aren't
the normal stock tubes. The head was designed with Ruby Tesla EL84 Czech
tubes (according to Mike Zaite a.k.a Dr. Z, the Prescription puts more
voltage on the EL84 plates than Russian EL84s can handle reliably).
I've been playing the Prescription through a THD 2X12 rear-vented cab
loaded with THD Longhorns (essentially Celestion G12-80 Classic Lead
speakers "aged" by THD). The guitars have been a PRS EG-3 with Fralin
"Zero Hum" single coils and a G&L Fullerton strat with Texas Specials.
While my solos are definitely in the pentatonic realm, my band plays
original music which could by typified as "Modern Rock" ala Live, REM,
Jane's Addition, Pixies, Replacements, etc. So I am not a blues purist
by any stretch of the imagination. I do try to avoid "cowboy" chords and
power cords, and generally make up most of the cords, using a lot of
suspended, augmented, and diminished type extensions. The tones I use
generally range from clean and shimmery, to medium overdrive to heavy
crunch (often in the same song). With a Mesa DC-5, I could get a decent
clean, and a decent "metallish" crunch, but it was difficult to get a
natural overdriven tone due to the abundance of master volumes. In
addition, I don't play very loud with the band (although my girlfriend
would strongly disagree). I generally play clubs and theaters with house
P.A., so we try to keep a modest stage volume and let the P.A. do the
mix.
The Prescription fits my needs very well. While it is very stripped down
in terms of controls (bass, mid, treble, volume and the aforementioned
switches) it has a wide array of excellent tones. The clean is
absolutely luscious, shimmering and DEEP. It transitions from clean to
medium overdrive (and everything in between) like it was reading your
mind. Just the slightest variance in pick attack will yield infinite
tone shades from clean to smooth rich harmonic overdrive. The
Prescripition will do pretty heavy crunch when cranked open or nearly
open, but I find the volume too much at this level. I'm not much of a
volume knob player, although I will definitely work on this aspect of
playing...the Prescription likes to be driven hard. For heavy distortion
and sustain, I rely on a Fulltone 69 pedal. For now I keep the volume
around 4 (10 o'clock).
If you've made it this far, I'll go into some specific impressions on the
amp. When I ordered the amp, I was a little wary of the lack of reverb.
But, it absolutely doesn't need it, although reverb has never been
crucial for me. It's got plenty of richness and depth without reverb. I
didn't find the footswitch boost to be too useful for my needs. It
basically provides a 6db volume boost in all frequencies, but I have
pedals that accomplish the same thing. Maybe it would be more useful if
you could set the boost level. The tone controls are very subtle, but it
sounds good no matter how they're set (I keep them more or less at 12
o'clock except for the mids which I set at 10 o'clock). The most
dramatic tone shaping occurs with the bright and expand switches. I
prefer the expand off as to my ears it increases the mids where its too
"honky", although Dr. Z told me I might like it better with humbuckers.
I like the bright switch set to the single-coil position, the amp seems
to lose sparkle without the bright switch. The amp never sounds thin or
"ice-picky" with any setting. The only niggle I have with the amp is the
rear tube panel/vent. It's fit makes it flimsy in an otherwise
impeccably constructed amp. It also doesn't have a convenient place to
stash the AC cord, which isn't a big concern if you don't play out much,
put for those that do, a detatchable AC cord would be a big improvement.
Just tonight I put in a set of Ruby Tesla EL84s to see how the
Prescription sounded with the tubes it was designed for. My initial
impressions...the Rubys sound crisper and brighter, with more clean
headroom and more volume at the same setting. In contrast the N.O.S.
Phillips are warmer and rounder with a smoother distortion at lower
volumes. I also noticed the Rubys brought out a dramatic difference
between the low and high inputs. With the Rubys I definitely preferred
the high input, whereas with the Phillips, the difference between the two
were more subtle, although I still prefer the high input.
As for how it performs on a gig, I'll post some more comments after this
weekend's gig. Last week was in a small club in a rare un-miked gig in
which it had more than enough volume. This weekend is a 500 seat theater
with awesome house sound. In addition, the theater just put in 24 track
digital recording capabilities, so I'll be able to report on how well it
records (granted this is not a studio environment by any means).
Overall, its a fantastic amp at a very fair price (I ordered mine when it
was still $950 with shipping). Dr. Z is one of the nicest guys I've
talked to in the boutique gear business. Never condescending, always
patient to a fault, and very committed to providing a great sounding amp
at a great value. He even offers spare/replacement Ruby Tesla EL84s for
Prescription owners for a mere $30 with shipping).
BTW, my stock broker and I both agree it looks great beside the BMW or in
the living room next to my Ethan Allen furniture...just kidding! :-)
Craig
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