Transformer Mounts and Bias Circuits

This weekend I worked on the following:

• power transformer chassis mounts
• fabricated from angled aluminum stock I picked up at the local metal supply (Blue Collar Supply)

The power transformer I intend to use for this project is meant to be mounted semi-flush with the chassis. In it's stock form, a rectangular shaped hole is meant to be made to allow the bell-end of the transformer casing to fit into, and the transformer laminations rest on the chassis. I do not particularly enjoy metal work, probably mostly because I do not posses the tools to easily manipulate the pieces. I decided to fabricate some mounts for the transformer, so I can escape machining a large hole in the chassis. This way, I only have to drill 4 holes for mounting, and cut some angled aluminum. It went quite well.

I haven't made the holes for mounting yet. I plan to draw up a simple outline in CADSTD to have a visual size representation to work with on the chassis.

• Output tube bias circuit
• Adjustable, negative voltage power supply, full wave bridge, PI filter into 2 sealed trim potentiometers

On the turret board layout I made, I kept the output tube grid supply resistors on separate lines, with the intention that I should have provisions for separate bias supplies, to allow for compensation for mismatched tubes. I began with a voltage doubler circuit that I ended up not using, because the range of voltages was much too negative than I needed for the Fisher output stage. According to the 500-C schematic, the bias voltage in stock form is a set -17 V. I was getting anywhere from -40V to -102V DC from the first circuit.

I started over with a full wave bridge rectified power supply, grounding the positive side of the bridge. With this circuit I was able to obtain a range of -13V to -35V DC. I will have to recheck when the amp is powered up, as there was no other load on the power transformer.

I used a variable AC power transformer to "ramp" up the primary of the transformer that fed the power supply circuit. I did this because I am using old capacitors (from an old tube oscilloscope) and wanted to make sure I did not damage the electrolytic capacitors.

Here are some pics of the second version bias circuit:

 

• Output Transformer wiring
• Extended the wiring for the output transformer

Because the output transformer I am using is an old, used unit, the wires were somewhat shorter than I would like to work with. I found some spare wire and extended the existing wiring. I had to open up the transformer casing and expose the wires connected to the core.

• Recycled some parts
• I have some old terminal strip boards from previous junk-yard sales and such, and decided to put them to use. I desoldered all the old wiring/component residue from them and cleaned them up. Good as 60 years ago!

(8HRS)