Notes
on power transformers for sale, Dec 2008.
You will need to write a list of what supply voltages and currents you
wish to use for your project.
Perhaps you'd like to study the web pages at
http://www.turneraudio.com.au/powersupplies.html
http://www.turneraudio.com.au/powertranschokes.htm
When you have built a preamp to get some practice at building tube
operated electronics, perhaps you may know enough to design a power amp.
For those unable to trust their minds to design, I can arrange to supply a compatible set of
iron wound parts including a power transformer, an output
transformer and choke for B+ filtering to suit a monoblock or dual
channel chassis.
I do not supply full kits for amplifiers. Diyers must be able to work
out fine details.
I can recommend the choice of schematics i have elsewhere at this
website.
About the transformers for sale......
Most transformers have
GOSS double C-cores, formerly made by AEM in Sth
Australia. These low loss cores tend to run with a low rise in
operational temperature.
After inspecting each transformer type carefully and measuring wire
guages used I have given current ratings for wire used in
windings at 3Amps per square
millimetre of copper section area, eg, for 1.0mm Cu dia wire the rating
is for 2.36 Ac.
The C-cored transformers are high quality items for audiophile grade
amplifier construction.
They all have carefully layer wound wire with at least 0.15 Nomex or
mylar
insulation between every layer of wire.
There is a bobbin with 3mm base wall thickness with ends of wire layers
all kept back 3mm from the edge of the insulation to maximize creepage
distance.
Precautions.....
Buyers should have slow blow fuses to all primary and secondary
windings. Fuse values should be no more than 2 twice the expected
maximum RMS current.
So if a primary has 1 Amp of input current, the fuse should be a slow
blow 2 Amp type, or just above the value where there would be
occasional nuisance fuse blowings.
There are thermal fuses inside the windings of these transformers.
Because I have
no control over how anyone might use the transformers being offered,
I cannot offer a warranty included if you do not construct your
amplifier to high standards of safety and protection against bias
failure or any other fault.
There are schematics elsewhere at this website for active protection
against bias failure and excessive cathode currents. All tube amps need
such protection fitted!!!
Some transformers with E&I laminations were made in
the 1960s by A&R and Ferguson and have been in use in other
equipment but are still in excellent condition.
Buyers should carefully design
their amp schematics with regard for anode supply voltages and anode
load values and allow the B+ winding to cope with at least 33% more
current delivery than required at the idle condition.
They should allow for some flexibility for the anode supply
voltage of say +20% and -5% before finalising their design. Its easier
to use a series resistance in the B+ circuit to the OPT input to reduce
the B+ to enable the right Iadc for class A working with the load
wanted.
The best outcome for the hobbyist is to design the amplifier around the
output and power transformers available, rather than insist that the
amplifier conform to yet another configuration for which it is
impossible to find a perfectly suitable transformer. Buyers should have
the iron wound parts in their posession before
proceding to make a chassis or to purchase one so as to optimally
arrange the parts for spacious and well proportioned layout.
Voltages....
All
transformers have
nominal 240V primaries only to suit Australian buyers, or where mains
voltages are between 220V and 250V.
The secondary voltages stated are
nominal working voltages with 240V applied to the primary.
The B+ anode supply voltages are
caculated at 1.35 x Vac and for
working Idc and for capacitor input filters.
Bias windings can give Vdc = 1.35 x
Vac, or 2.7 x Vac depending on rectifiers being single diode, diode
bridge, or doubler configuration.
For those wanting to use tube
rectifiers, the B+ Vdc at the reservoir cap will be between about 1.0
and 1.2 x Vac.
For choke input filters, the B+ Vdc
at the cap after the choke will be approximately 0.8 x Vac at the
working Idc, and when the choke winding resistance is low.
The input VA rating is
that of the input winding only.
The total of wanted secondary voltages x secondary currents plus 10%
for winding losses and all measured at idle MUST NOT exceed the input
VA rating.
Some windings for B+ have a higher VA rating than required for idle
conditions because one must allow for temporary higher anode currents
because of class AB working and and occasional tube bias faults.
In general, the core VA rating for C-cores is higher than the copper
winding VA. The C-cores have a larger ratio between winding window area
and the
centre leg area when compared to wasteless pattern E&I laminations.
Thus winding losses in C-core
transformers are lower than when using standard wasteless
pattern E&I laminations for the same VA.
The C-cored transformers will be supplied with mounting brackets
attatched, and all supplied with full information.
Power
transformers for sale
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