Technical Information

Note: The layouts and schematics have been been moved to their own page.

Hiwatt Fuse Locations and Values Note that fuse values are not an exact science. For example, a 5 or 6.3A fuse will do in place of the "6A" specified, and a 3.15 will do in place of the "3A" specified. All of the values below are normal "fast" fuses unless otherwise indicated, and were taken from a circa 1978 Hiwatt catalog.
DR504 DR103 DR201 DR405 HT 1A 3A 3A 3A 120v Mains 3A 6A 10A 10A SLOW 240v Mains 1A 3A 5A 5A SLOW

Tech Trivia

Improved bias circuits for the 50 and 100 watt amps.
 
A page describing the (in)famous Canadian mod, and how to restore one of these amps to stock.
 
A page describing a mod to reduce the "spitty" blocking distortion on the standard two-input amps.
 
A page describing the input voltage selector.
 
A speaker cabinet layout diagram.
 
Chassis Screws:
My Biacrown DR504 screws are 1 inch 10/32 Phillips heads with 3/4 inch washers.

DR103 Specifications (Audio Brothers)

The DR103 Partridge Output Transformer

This is a wide bandwidth "hi fi" transformer, much more so than the normal guitar amp output transformers in wide use.

Primary Impedance (Raa)	1750Ω
Frequency Response (-3db)	3.6 hz - 112 khz (!)
Weight	3.2kg (7 pounds)

Heyboer Transformers in Michigan, USA, is now making Hiwatt-style OTs, using high-grade lams and winding patterns like the original. Mojo is their retail outlet for these.

Hammond is now making output transformers with normal 0-4-8-16 secondaries! Their model 1650TA is good for a 100 watt and 1650KA for a 50 watt.

Another possibility for new builds (not a drop-in replacement) is a generic Marshall replacement transformer like the New Sensor MOP-100.

Detailed research on this OT is still available at Shinrock: Vintage Partridge Transformer Analysis

The DR103 Partridge Power Transformer

For a DR103, the power transformer is less critical, and any quality unit which provides the following will work fine:

• 350 VAC @300mA (HT)
• 6.3 VAC CT @ 7.5A (Heaters)
• 50 VAC @ 50mA (Bias)

Unfortunately, the HT winding is an unusual voltage value, meant for a full wave bridge, and separate bias windings are also rare (as opposed to a Fender-style bias tap off the main HT secondary).

Heyboer Transformers in Michigan, USA, is now making Hiwatt-style PTs which have the same set of secondary windings as the original, but with an improved (higher) bias voltage winding. Their part number is HTS-8206. Mojo is their retail outlet for these. Note that this is currently available in either single 120v primary or dual 120v (switchable 120 to 240 volt only), and is therefore not a drop-in replacement.

Another possibility for new builds (not a drop-in replacement) is a generic Marshall replacement transformer like the New Sensor MPT-100. In this case, you'd need to convert the power supply to a center-tapped full wave (exactly like the 50w supply, shown here). Note that the HT filter cap and shunt resistor values are identical on the 50 and 100 watt power supplies.