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The 304TL Amplifier

The 304TL amplifier shown on this site is optimized for use with 8 to 16 ohm high-efficiency speakers and has the following features.

• A classic audio channel, in which half of a 6SN7GT dual triode drives a 304TL through an interstage transformer designed specifically for the 6SN7. With the 6SN7's 7,700 ohms plate resistance, the unloaded output of this interstage transformer is typically flat ±1/2 dB from 20 to 90,000 Hz. The amplifier uses no feedback.
• All transformers are manufactured to the highest standards of quality, using typical military-standard procedures. Visible transformers are styled with end-bells and have similar appearance to the Stancor, Triad, Thordarson or UTC units of the past. However, the output and the plate transformer end-bells are potted. (Detailed photo of an unpainted output transformer.) Transformers and inductors mounted below chassis use heavy machined aluminum frames and specially designed heavy-duty terminal strips.
• Direct current 10 V @ 26 A filament supply with choke-input filter (photograph).
• 1,000 VDC @ 300 mA high voltage power supply with:
  • 866A mercury vapor rectifiers.
  • Choke-input filter, three LC sections, and oil-filled capacitors.
  • 125 uF capacitor in third section. This capacitor stores 62.5 joules. (photo of the high voltage choke assembly with magnetic shield removed.)
• Autotransformer control of both filament and plate voltage. This allows the user to warm the filaments slowly and to operate the amplifier at reduced plate voltage and dissipation if desired. A mechanical interlock between the autotransformers allows the knobs to be rotated only in the correct sequence. Operation is fail-safe in the event of a power interruption.
• Front-mounted bias controls and plate current meters. This, together with control over the plate voltage, allows the operator to choose the quiescent operating point of the 304TLs.
• 12 gauge steel chassis, with extensive use of machined aluminum plates and mountings.
• 3/8 inch thick borosilicate (Pyrex) glass cover.
• Gain control and three inputs via pairs of XLR connectors, wired for unbalanced input in the Ampex tradition.
• Convection cooling and quiet operation, with attention paid to audible hum which can be mechanically generated by power components.
• Frequency response ±1/2 dB from 21 to 20,000 Hz. (graph)
• Output and damping capabilities:
  • 50 Vpp maximum.
  • 3 ohms effective source impedance.
  • 30 W/channel at 500 Hz and 10 W/channel at 20 Hz into 8 ohms at the 5% total harmonic distortion level.
• Voltage gain (no load) 10.
• Hum and noice less than 2 mVrms.
• Heat dissipation approximately 300 W plus 304TL plate dissipation, which is determined by the quiescent operation point chosen and is 300 W maximum.