Vintage Type 45 Stereo Amplifier
- Stereo
- 2 Watt/channel
- Single-Ended
- Class A Triode
Specifications:
Type:
Single-ended, Class A Triode
Mode:
Stereo (2-channels)
Power Output:
2 Watts rms/channel
Hum & Noise:
Less than 1mV
Output Impedance:
8 or 16 ohms
Circuit type:
Interstage transformer coupled
Output Tubes (2):
Type 45 Made in USA
Driver/Gain Tubes:
6EA7 gain/driver NOS Made in USA
Rectifier Tube:
5V4G Made in USA
PS Filter Type:
Choke input, linear phase
Frequency Response:
20Hz to 20kHz +0/-1 dB
Input Voltage:
.4 Volts rms for full output
Feedback:
Zero
Biasing:
Self-Biasing / Cathode Biased
Coupling Caps:
NOS Paper in oil
Input jacks:
RCA type Gold plated
Output jacks:
5-way binding posts, accepts 12-gauge wire
Speaker Sensitivity:
94 dB or higher is ideal
Chassis Size:
11 x 17 inches
Height:
6.5 inches
Weight:
23 lbs.
GENERAL INFORMATION
Type 45 SE tube amplifiers are identified by their Type 45 output tubes which are popular for their airy highs not found in any other tube. The tubes are rated at 2 Watts power output, adequate for high efficiency speaker systems.
REINVENTION OF TYPE 45 AMP CIRCUITRY
Type 45 filamentary tubes run on AC filament and have a residual hum in the output signal. The simple and easy thing to do is use a center tapped filament transformer to connect the RC bias components. To get a much lower residual hum the American Tube Amp Type 45 SE amp design incorporates balance pots to null the residual hum.
AMPLIFIER DESCRIPTION
Power supply: The power supply design eliminates any influence in the sound characteristics of the amp as it doesn’t ring caused by a load change. A choke input filter provides better regulation with a changing load which occurs at very low frequencies. This minimizes the peak charging current lessening the strain on the rectifier tube. The first cap is a 35uF oil cap, thru a 1.7H choke to a 470uF, 450volt output cap. This very large value cap also minimizes inter-channel crosstalk at low frequencies. Components used in this prototype model including the tubes are vintage that test new or near new or are NOS components.
Output transformers: The transformers have an impedance of ~5,000 ohms ideal for type 45 tubes and they measure flat to beyond the audio band. Output impedance is 8 and 16 ohms. They handle full power down to around 35 Hz. Speaker systems with 94dB efficiency or higher with extended low end response will perform very well with this amplifier without a separate subwoofer.
Driver stage: The driver stage is transformer coupled to the output stage. The low resistance windings cause this amplifier design to recover almost instantaneously when clipped without a noticeable interruption in the audio signal. American Tube Amp pioneered this design concept over a decade ago. This design causes the interstage transformer to yield a pristine performance. A low plate resistance tube drives the primary to achieve superb wideband performance. This technique extends the low-end performance to quite low but causes the interstage transformer to ring quite considerably slightly higher than 10 kHz. By capacitor coupling the secondary back to the primary and the low rp of the driver tube the 10kHz ring is swamped out completely. We call this technique a bridging cap. The design features paper in oil caps. The 6EA7 driver tube has a gain stage (mu=70) and a power stage with a rp of approximately a thousand ohms yielding flat frequency well beyond 20 kHz at very low distortion. The RCA brand tube performs better in this service than do other brand USA tubes. (Driver stage is unique to American Tube Amp.)
