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Ampeg produced the PF-20T-U as part of the Portaflex line, positioning it as an all-tube bass amplifier head rated at 20 watts. The unit carried forward the Portaflex heritage in a compact form factor, intended to deliver the harmonic character associated with all-tube amplifier designs.
The PF-20T-U runs an all-tube signal path with two 12AX7 preamp tubes. It provides 20 watts of output power at 8 ohms (10% THD) and offers a three-band tone stack: bass at +8/−12 dB at 50 Hz, midrange at +5/−15 dB at 550 Hz, and treble at +13/−20 dB at 4 kHz. Maximum input gain is 54 dB, and the signal-to-noise ratio is 60 dB (20 Hz–20 kHz, unweighted). The head includes a user-adjustable bias control to facilitate tube replacement without requiring a technician for that step. Both a preamp direct output and a transformer-balanced direct output are fitted, and the design does not require a speaker load connected for use — allowing silent recording or use as a dedicated tube DI. Power requirements cover 120VAC/60 Hz, 100VAC/50–60 Hz, and 220–240VAC/50–60 Hz, all at 50W draw. The unit measures 7.3 × 13.0 × 10.4 inches (185 × 330 × 264 mm) and weighs approximately 15.2 lb (6.9 kg).
Owners sourcing replacement tubes or setting bias after a retube should consult a qualified amplifier technician, particularly when working inside a tube amplifier chassis where lethal voltages are present even after power-off.
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