How to get a quote for a custom designed output transformer

This is a custom service for replacement or upgrade of output transformers in existing equipment and for new builds. We are not able to do rewinds.

We wind on a two chamber bobbin to ensure full geometric balance for push-pull amplifiers. A total of 14 separate winding sections are used to ensure minimal leakage inductance. A bandwidth of up to 100kHz is possible for low impedance transformers.

Insulation is guaranteed up to 2000V. This is made up of the HT rail plus the peak ac signal voltage. Peak voltage depends on the load impedance (Rl) and the Power (P).
Vp = SQRT(Rl * P) /1.41

Mumetal cores are not available for these transformers.

The price depends on the size which is governed by the power rating and the lowest frequency for full power. It is possible to increase the size of the transformer to reduce losses and/or increase the bandwidth.

All output transformers are built to order in about 5 weeks and we do not make any charge for design.

We need a precise specification to work from and we guarantee to performance to this specification. Below is a form which you can submit to us with your requirement. Once we have accepted the specification we will email you a Design Reference which you can use to order on this site or by telephone to Sales.

PUSH-PULL TRANSFORMERS

We normally design for full power at 20Hz . Other ratings are available for example, for guitar amplifiers our customers often request full power at 50Hz which greatly reduces the size of the transformer. As a guide, a transformer rated for 20 Hz would be about the same size as a transformer rated for approximately double the power at 50 Hz. The bandwidth will be from the specified full power frequency (e.g.20 Hz) to at least 25 kHz. bandwidth up to 80 kHz can sometimes be obtained for low impedance transformers of generous size. We will always optimize the design for bandwidth within the design constraints.

SINGLE-ENDED TRANSFORMERS

The key design parameters are the reflected load impedance (Rl), DC standing current, the power rating and the lowest frequency at which the rated power is specified. The transformer will be very large if you specify the full power frequency too low. The bandwidth depends strongly on the driving source impedance (rp). Triodes generally have a much lower rp than pentodes. If we know the driving source resistance we can optimize the primary inductance and the coupling factor which determine the low and high frequency cut off frequencies. It is possible to have a transformer with full rated power at say 40 Hz but a frequency response extending down to say 10 Hz.

PARAFEED TRANSFORMERS

A pareafeed transformer has to be isolated from the dc voltage on the anode by a capacitor. The design is essentially the same as a push pull transformer with no center tap or screen taps. The wire can be thinner because it only has to carry ac signal current. Parafeed transformers are normally used in conjunction with a plate choke.
The key design parameters are the reflected load impedance, power rating and lowest full power frequency. We normally design for an LF -3 dB point at 20 Hz maximum. The HF response depends on the reflected load impedance and the size of the transformer. We can optimise the bandwidth to your requirements.

REPLACEMENTS AND UPGRADES

Size. If you would like us to quote for a new transformer for a specific piece of equipment we first need to make sure it will fit into the space available. It is usually best to use a larger transformer to improve the performance and ensure it is properly rated for the job. Please choose the size and package style you need form our "Dimensions and Styles" page. You may find that your original transformer uses Metric dimensions in which case choose a size which is about the same volume or weight and then go to at least the next size up.

Primary load impedance Rla-a Next we need to know the reflected load impedance of the primary winding. Usually is is possible to get a schematic from the internet though often this is not specified. It is possible to determine the anode load for the output valves from the valve specification. If you forward the schematic to us at support@sowter.co.uk we will be pleased to help with this and any other points. If you have a working transformer you can measure the Rla-a quite easily with a low voltage ac supply and a voltmeter. See our FAQ.

Secondary impedance options You can specify various configurations. Please note that the values in the drop down list may be rounded and not exact. When configurable windings are specified it is necessary to use all windings either in series or parallel in order to get maximum bandwidth and to minimise losses.. Tapped windings have increased losses and reduced HF response when the taps are used. This is particularly true when for example a 4 ohm tap on a 16 ohm winding is used.

Feedback windings We can provide a feedback winding as an option for a small extra charge. The winding is normally one section in each chamber so the coupling factor is not as good as between primary and secondary but this is not normally a problem as the winding is normally lightly loaded. If the winding is used in the cathode circuit there is always the possibility of instability. We do not recommend using cathode feedback unless you have the necessary skills to ensure stability.

Submission Once you have determined size, package style and the specification please complete and submit the form below. We will analyses the specification and if we can make it we will send you a formal quotation. There is no obligation to order. Feel free to specify an alternative design point for us to quote if this helps you get the product you want.