The above schematics show
my work on an integrated 30+30W UL
amp made in Victoria in in about 2003.
I won't say who made it, but the model name was "Tranquility",
but the buyer of this one didn't experience much tranquility when he tried
to use it, so it came to my work bench for re-engineering.
The original had about 1/3
of the parts count which I have used, and
produced noise, high THD, high Rout.
There were three 6SL7 tubes for input&drivers for both channels,
and 2 x KT66 for each PP UL output stage.
But only 2 triodes were used out of a possible 3, with one providing gain to
make needed Vg for one KT66, and other to invert the phase,
and no GNFB used. So without GNFB and one noisy 1/2 6SL7, the amp
had high THD, very poor damping factor, and lots of noise.
However, the 4 main iron-wound items, PT, filter choke, and both OPTs
were quite well made, and I saw potential for me to convert this pile of junk
to another example of best practice if I gutted the entire under-chassis wiring and
installed my new circuits.
Two years have gone by with no complaints from the owner.
There are 3 x 6SN7 tubes for each channel's SET input and LTP driver,
plus bjt CCS common cathode current for LTP. Critical damping networks
prevent any oscillations, and GNFB made a huge improvement to sound.
I don't have a picture of the top of the amp.
The transformers seemed to be copies of old A&R units from 1960,
with E&I and bell ends, with "crinkle tone" paint, very retro.
OPTs had sufficient bandwidth to allow unconditional stability with GNFB as I have
it in the schematic. I have no clue what lamination quality was in all wound items.
But PT ran quite hot, so its E&I could not have been GOSS. Appearance was fine.
But if the amp is dropped
off a bench the 19mm timber used for chassis sides turns
The owner took my advice to make a new bottom cover using 2mm aluminium.
He had access to a drill press for metalwork classes at a school, and he spent
an hour drilling the bottom cover with as many 8mm dia holes at 20mm c/c as possible
according to a plan I sent him. This allowed some ventilation of parts under the chassis.
I fitted the new cover to timber recess with many 50mm long wood screws for better
resistance to dropping. Finally, I fitted 15mm feet under bottom cover to allow air flow
up through holes in bottom plate. Such amps MUST NOT ever be placed on plush
carpets, and MUST only be placed on benches or a table.
The trend of using timber +
flat metal top&bottom plates is something many DIYers
do to avoid bending metal on the kitchen table to make a full chassis.
This amp had 2mm stainless steel top, good looking, but very difficult to drill.
The existing amp has its original screened IEC mains socket & fuse holder,
output terminals and input terminals on rear panel. Mounting all these nicely
in 19mm timber is much more difficult than through metal sheet.
Mains switch and volume pot, source switch are mounted through 19mm
front panel. These required recessing timber behind panel down to very thin
thickness to allow mounting. This meant I had to put in metal plates to hold
controls better because timber can only be 3mm thick, so fragile, and the pot and
switch could easily be pushed inwards breaking the timber.
DIYers and lazy stupid manufacturers do not consider the consequences of
I conclude timber surrounds on power amps is bad practice, wasting more
time than anyone wants, and it needs to have paint finishes applied, weakens
the amp structure, and does not give natural shielding, and does not improve
So, if YOU build an amp,
please use 30mm marine grade plywood at least.
AND put in metal 20mm x 20mm angles inside corner joins with 8 x 25mm x
4mm dia wood screws to hold corners together.
Top plate and bottom cover plate should have 4mm c/s wood screws at 100mm c/c
and there should be a perforated steel sheet cover over tubes.
If you don't like having a
power amp chassis made by a metalworker then for a chassis
which is all metal, easiest is to use 2mm flat plate aluminium for the top and bottom,
and then use use 75mm x 25mm x 3mm thick aluminium channels for the 4 sides
These can be cut with a hacksaw with channel held on bench with clamps with cloth
to prevent marking aluminium surfaces.
The cuts are 45 degrees and filed clean to make a nicely mitred join at corners.
The outside sharp corners may be filed to make a round to prevent you cutting
fingers each time you move the amp. The chassis parts can be held together
with M4 countersunk screws with nuts inside chassis space.
Such work is usually beyond many DIYers who try to build an amp after a
lifetime of never ever building anything or getting their hands dirty.
The amp had no top cover.
This ensures that when owners move their amps,
and put it on back seat of a car, and when they stop suddenly, the amp may roll
forward onto the floor, breaking a couple of tubes.
This amp had Chinese KT66, "similar but different" to original KT66 which
probably were meant to be used. The owner said the sound was just what
he wanted and expected, naturally smooth and warm, yet losing no definition
Back in about 1957,
Wireless World magazine published an article on how
to make a 400W amp with 10 x KT88. The finished amp had all its parts screwed
down onto a 25mm thick plywood sheet about 700mm long x 250mm wide.
There was a simple perforated steel cover which was screwed to edges
of plywood. A top cover could be 1.6mm perforated steel sheet with say
5mm holes and 50% open, bent and joined to form a box.
You end up with something that looks like it belongs in a back room
of 1950s stadium. Very retro.
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