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DR103 Specifications (Audio Brothers)
Input Line Voltages 100v, 117.5v, 225v, 250v AC
Power consumption 330 watts maximum
Fuse Ratings Mains (100 or 117.5v) 6.3A (T) slow blow
(225 or 250v) 3.15A (T) slow blow
H. T. (all) 3.15A (T) slow blow
Output Valves 4 matched EL34 (6CA7)
Preamp Valves V1, V2, V3 ECC83 (12AX7)(7025)
Preamp Valve V4 ECC81 (12AT7)(6201)
Output Power 100w RMS (min) into 8Ω
Loudspeaker Impedance 4Ω, 8Ω, or 16Ω
Dimensions W: 640mm (25.2"), D: 288mm (11.3"), H: 269mm (10.6")
Weight 20.5kg (45 pounds)
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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
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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)
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