Building the Hiwatt DR-103 clone
Intro
Right on the beginning I must say that building this amp was most
challenging and difficult thing I’ve done so far (as far as DIY goes).
When I decided to build something as complicated as “full-grown” 100W
guitar amp, I couldn’t even imagine what that meant. Only thing I’ve
done prior to this amp is a Big Muff Pi (which didn’t work at that time
- see the Big Muff Project for details) and rather poor sounding silicon
fuzz face which I threw in the garbage bin soon after it was built. So I
must have been out of my mind to start something that is zillion times
more complicated than poor (or not at all) functioning stompbox. Well,
not quite....
This is not a
Hiwatt clone in the true sense of that word. I wasn't trying to exactly
duplicate every component, let alone copy the looks of a real Hiwatt. I
didn't have nearly enough money to even try doing something like that.
This was, before anything else, a learning project, because my
primary intention was to (learn to) build an amplifier. There are two
good reasons why I decided to make a Hiwatt
1. The wealth of information on Hiwatt amps that Mark Huss has
compiled and published on his site (www.mhuss.com) made it very
attractive to me as a beginner, because reliable schematics and
layouts make a great start.
2. I love Pink Floyd, and especially David Gilmours playing
and sound he gets from his guitar. The tone gives another dimension
to his playing.
Betwean these two reasons, there's not much motivation to
make it look like a Hiwatt. Design of the enclousure is my own, and I am
very pleased how it came out in the end. Here is the final result (click
on a thumbnail for a larger view):
Parts
Output transformer
This is probably the
part that is most difficult to obtain, and surely most expensive. The
ones used in original Hiwatts are anything but standard. They have
frequency response that most HiFi amps can only wish for (-3dB points
bellow 5Hz and above 100kHz), let alone guitar amps which generally
don’t have a need to go below 50Hz.
That is one hefty
piece of iron, and finding something like that is very difficult.
Fortunately for those who live in America there are clones manufactured
today, but here in southeastern Europe there’s no such commodity.
I almost lost hope
for finding one, when people from local company, called Trafomatic (www.trafomatic.com), decided to
make me one for free, as a promotion of their new line of output
transformers. I gave them the specs, and in a weeks time they delivered
me my
present.
What is
peculiar about this transformer is that it is toroidal. It’s completely
isolated (dipped in some kind of plastic). It goes down to 10Hz easily,
and due to my lack of equipment, I couldn’t make proper measurements.
The fact that it is toroidal also means much less interference, and
greater efficiency. It is one hefty (more than 4kg) piece of iron. And
it sounds just right. Clean and crisp to the
bone.
Only
thing I forgot to include in the spec is multiple secondaries so I have
only 8 Ohm output. But it suits me fine, I just needed to make a minor
adjustments to global feedback return, because it normally comes from 16
Ohm tap. And I only have one cabinet so there’s not much need for
switching outputs anyway.
Tubes, caps and rest
Tubes I used
are standard, new production ones. For the output I've got 4 Electro
Harmonix EL34's. In the preamp I've got 3 Sovteks 12ax7's, and
one Ei 12at7 for the phase splitter. NOS tubes were (and still are)
way out of my budget and I am quite satisfied with the sound as it is,
so there's not much need for upgrade at this time.
I
started with plain, no name pollypropylene caps, because they were only
thing avaliable. Recently I've changed them all to Orange Drops and
Mica, and impact on the sound was amazing. It sounds much more "alive"
and way cleaner.
Resistors, elkos and everything else is pretty much standard. No carbon
comp resistors, just plain carbon film, and metal film. Elkos are of
inexpensive type, Samsung if I remember correctly, and instead of
16+32uf can, I used 3x22uf (22uf+2x22uf).
Schematic
I used
schematics and layouts from www.mhuss.com. First I built the Mid 70s
Four-Input Preamp, but using only two inputs, because my enclousure
didn't allow me enough space. This is very big mistake I made, but it
was forced one. I didn't have enough money to buy an enclousure so I
scavanged a steel door from an old refrigirator. It is little wider,
much to deep, and only 3cm's high. It was a real pain in the ass fitting
it all in, and it resulted in rather strange dimensions for a guitar
amp. That's the price i had to pay to keep it as cheep as possible. On
the other hand, it's very sturdy metal box, and I think it will stand to
any future abuse.
I will not post schematics here simply because you can get
them all (with many layouts) at www.mhuss.com.
Later on I
rearranged the preamp to an OL configuration for higher gain, but
keeping 34k resistors on the input for higher input impedance. That is
the arrangement I liked the most and decided to
stick to it. So in the end you could say that it is an almost OL model
with some mods.
I tried to follow
the layout in the begining, and if you look at the pictures bellow,
you can see the resemblance to an original layout. From the
aestethic point of view, compared to Joyces wiring, my amp
is a rats nest. That doesn't matter much to me, because after the amount
of thinkering I did on it, there's no wire in there that I couldn't be
able to recognize with my eyes closed.
There are few
modifications that are of greater significance. Number one would
be different grounding (some sort of star grounding but not
quite). I returned ground from every node to
the electrolitic cap that is closest to it (not physically
closest, but last in it's supply branch). Second one is the
improved bias circuit that Mark recomended.
I should
mention that, although this is a DR-103 output stage, most of the time I
use only two tubes in the output, making it closer to a DR-504. I only
have one cabinet, which has impedance of 16 Ohms, and i have only 8Ohm
tap on my OT. So 16Ohms load reflects as 3500Ohms on the plates of the
output tubes. That is exactly the amount needed for 2xEL34. If I were to
use 4xEL34, I would need 1750Ohm reflected across the primaries of the
OT, and that would mean a 8Ohm load on the secondaries.
Only drawback to this application is that the lowest response
frequency of the OT is doubled, but even that is not the problem
because OT I have is excellent, and it goes well bellow anything that
guitar (or this amp) can put in it.
Here are the pics of the guts:
This one was taken before I've done the mods, and replaced ordinary
caps for Orange Drops
And here's couple pictures of the enclousure:
That's it for now. I will update with more pics, and sound
samples later on.
Spice Simulations
I've done plenty of simualtions on Hiwatt circuits. Most interesting
of all is Tone Stack. It is one of the things that didn't change over
the years, and it is quite original compared to standard
Fender/Vox/Marshall Tone stacks. Here are some graphs I've done. I've
also done measurements with frequency analyzer, and it matched the
simulated results. Note that these are simulations of Tone stack
isolated from the rest of the circuit, and not the simulation of the
whole circuit.
And this is the whole circuit with controls on the middle.
Later, I will post the real mesured data for comparason. As soon as
I get my DIY frequency analyser working again :)
Questions and comments: cjokanov@f.bg.ac.yu
