Notes
on power transformers for sale.
In 2008, obtained the hundreds of power and output transformers,
and filter chokes from a friend who
made many more amps than I ever did. He always ordered more
transformers than he needed because he
was able to get a cheaper price for 4 rather than only 1, and this
came in handy when he had a repeat order
for the same amplifier. But because he costom made amps to order,
his stocks built up to occupy a large space
in a spare room of a house he rented. When he was forced to move
from house to apartment there was no room,
and he and I agreed we could store them at my house, and I could
advertize them for sale here, and whenever
I used any for amps I made and sold I would pay him. Total weight
of stocks were about 1 tonne, and between
2008 and 2017, I sold about 70Kg total for OPTs and PTs.
Perhaps only 4 of 13 people who bought transformers managed to
make complete working amps. Of the 4,
the fastest time anyone took was Mr Zel, in
18 months.
I have since retired myself from building amplifiers.
I will NOT spend a huge amount of time pile of time sorting out
what you want unless you can convince me you
are serious about building something.
You need to tell me about your project, and give all details of
schematic of the amp and PSU. Please try to send
me a schematic which is well drawn and less than 200kB in file
size, .pdf or .gif, .jpg, Or send me a link to a
website which has a schematic and description of your project.
I can recommend the choice of schematics I have elsewhere at this
website, but I cannot guarantee that I'd have
an OPT to suit it exactly because some popular OPTs and PTs have
been sold.
Perhaps you'd like to study my informative web pages at powertranschokes.htm and
powersupplies.htm
Those DIYers with little experience should always build an
integrated line level pre-amp before moving to
much more challenging tubed power amps.
For those unable to fully design any amp, I would be able to
select a compatible set of parts including a power
transformer, output transformer and filter choke and probably be
able to supply a schematic suitable for almost any
output tubes you can think of between a tiny 6AQ5 up to an 833.
I suggest all DIY hobbyists complete all work and get their
amplifier projects basically operational before
taking it to someone skilled to check it out especially where NFB
has been used. There are not many fellows
who undertsand tube amps and NFB.
About the transformers for sale......
Most transformers have GOSS double C-cores, formerly made by AEM
in Sth Australia. These low loss cores run
with a low rise in operational temperature. After inspecting each
transformer type and measuring wire gauges where
possible I have given current ratings for each secondary winding
based on 3Amps per square millimetre of copper
section area, eg, for 1.0mm copper dia wire the rating is 2.36
Amps.
Most transformers have carefully layer wound wire with at least
0.15mm thick Nomex or polyester 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 operational maximum RMS current. So if a
primary has 1 Amp rms maximum of input current
while charging capacitors, the mains fuse should be a slow blow 2
Amp type, or just above the value where there
would be occasional nuisance fuse blowings.
There are NO 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 should have such protection fitted!!!
Some transformers with E&I laminations were made in the 1960s
by A&R or 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 finalizing their
design. It is easier to use a series resistance in the B+ circuit
to the OPT input to slightly reduce the B+ to enable
the right Iadc for class A working with the load wanted. It is
impossible to increase the B+ voltage easily if the HT
winding has a voltage which was too low for the project. The best
outcome for the hobbyist is to design the amplifier
and its PSU and power tranny around the output transformers
available, rather than insist that the amplifier conform
to yet another configuration for which it is impossible to find a
perfectly suitable output transformer which will always
be harder to find than a suitable power transformer. Buyers should
have the iron wound parts in their possession
before proceeding to make a chassis or to purchase one so as to
optimally arrange the parts for spacious and well
proportioned layout.
Mains Voltages....
All power transformers have nominal 240V primaries 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 calculated 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 silicon diode, silicon 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 Vrms
of HT winding, with the B+ being highest factor used where Idc is
small, as it may be for a preamp.
For choke input filters,
the B+ Vdc at the cap after the choke will be approximately 0.8 x
Vrms at the working Idc, and only if the choke
winding resistance is low.
The input VA rating is that of the input winding only, and VA =
Input Vrms x Irms. This input VA rating will be equal
to the sum of all VA ratings for all windings plus 10% winding and
core heat losses. A user may find that he can extract
more current from a HT winding than the VA rating indicates but
this means he must then extract less current from other
windings to keep the total VA under the transformer input VA
rating.
Some HT 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 iron 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 permeability of GOSS C-cores is higher than
for non oriented Si Fe cores. Heat losses for GOSS
is often negligible because the iron permeability is very high,
usually above 7,000. GOSS Toroidal cores have the highest
permeability because there are no joins in the sheet metal strip
used to wind the spiral core, so toroidal cores run coolest
of all, and this is needed because the core is suspended within
the many insulated layers of wire and core heat cannot
escape easily.
The C-cored transformers will be supplied with full information,
but without mounting brackets or potting.
For sale power
transformers
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