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.
50 watt (DR504, etc.) 100 watt (DR103, etc.) 200 watt (DR201, etc.) 400 watt (DR405, etc.) 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 original 50 and 100 watt amps.
 
A quick-and-dirty way to add a bias adjustment to a DR504 or DR103. NOTE: This will not help much, if at all, on a DR103 with the old original Partridges, because these do NOT have enough bias voltage for this to work well.
 
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:

The original Hylight machine screws were British Imperial #2 (2BA) thread, and these can be hard to find, particularly outside of the U.K.

The US 10-32 thread size is close, as is the metric size M5 x .08. In either of these latter cases, I'd recommend getting a tap of the appropriate size and running it through the chassis nuts so the that the replacement screws do not bind. The tap barely touches the threads, so it's very easy to do.

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. VHR is a good 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 old Partridges is available at IG Präzisions-Wickeltechnik: Partridge Transformers

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. Again, VHR is a good 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. VHR & Heyboer are currently developing a drop-in replacement.

Another possibility is to use a JMP100 Marshall 100 watt replacement like the Hammond 290HX. NOTE: If you choose this option do NOT ground the secondary center tap, but rather tape and stow it unconnected. These specific Marshall replacement transformers are designed to be used with the Marshall "voltage doubler" power supply, and so the voltages also work out for a DR103. You will need also to adjust the bias supply resistor, as the bias supply on these is twice the stock Hiwatt value.

• 356 VAC @ 420mA (HT)
• 6.3 VAC CT @ 7A (Heaters)
• 100 VAC @ 50mA (bias)

For a 50 watt, a generic Marshall 50watt replacement transformer such as the Hammond 290GX will work fine:

• 300-0-300 VAC @ 250mA (HT)
• 6.3 VAC CT @ 5A (Heaters)