Notes on power transformers for sale, March 2011.

Before I can sell you a power transformer which will satisfy the technical needs of your project,
I may need to know a few things about the proposed project.

Most DIY hobbyists do not have years of experience building amplifiers and
may make understandable errors when choosing power transformers,
output transformers or chokes for their project.
My advice may ensure the best choice is made for a project.

Please email a link to a website which has a schematic and description of your project.

Schematics with tube types and output transformer details and load impedances may be sent to me in
.pdf or .jpg or .gif form and preferably less than 400kB, and must be fully readable.

I can recommend the choice of schematics i have elsewhere at this website.

Perhaps you'd like to study my informative web pages at
powersupplies.htm
powertranschokes.htm

I would always think that after you have successfully built a preamp to get some practice at building tube
operated electronics, perhaps you may become experienced enough to design and build power amp.

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 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.

Should someone wish to have me draw up a schematic and design something special and offer information
support as construction progresses, there may be a moderate cost involved.

Any practical support I maybe asked to give can be expensive depending on what sort of workmanship a hobbyist
has achieved before he hands his work to me to inspect, repair, adjust, modify.
I suggest all DIY hobbyist complete all work and get his project operational before bringing it to me
for any final adjustments which might include critical damping network adjustment where NFB is
applied, because some years of experience are needed to get this right.
 
I have a certain price for supply of new hand made amplifiers, and it is far higher than the price of a few transformers.
So I offer no cheap deals to build a DIYer's project because he cannot and will not learn or devote enough time
to his project.

I do not supply full kits of parts for amplifiers. DIYers must be able to work out fine details,
and source all minor parts such as tubes, diodes, resistors, capacitors, solder, hookup wire, terminals, switches,
chassis, LEDs, SCRs, BJTs, or anything else.
There are plenty of good suppliers online for tubes and for small parts I suggest wescomponents.com.au and evatco.com.au
Usually DIYers realise that If I were to supply minor parts, they would have to pay much more for them for me to
spend time dealing with an order, working out the order, and supplying in small quantities.
And I don't have any credit card and will not make online purchases unless a supplier accepts a direct bank account deposit.

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 and measuring wire guages 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 0.15 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 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 finalising 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 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 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 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 Vrms of HT winding.

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 only if the choke winding resistance is low.

The input VA rating  is that of the input winding only, and = 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 permability of GOSS C-cores is higher than for non oriented SiFe cores, and heat losses for GOSS is often negligible as it is in
toroidal cores which must run cool 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 mounting brackets attatched, and all supplied with full information.

Power transformers for sale

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