Eico HF-89 Power Amplifier

Eico’s HF-89 is a high performance power amplifier and a great value when it was introduced. It is now a classic, and somewhat rare. The HF-89 features excellent output transformers, giving it a wide and flat frequency response with 50 watts/channel capability into 8 ohms.

Approximately 60 years old, the original HF-89 had some features that could benefit from modernization including:

  • Input and output jacks and terminals. The original inputs were located toward the front of the amplifier, which are somewhat inconvenient for patch cables. The pair of right and left RCA input jacks are too close together to accommodate modern cables. The rear speaker terminals were crowded screw-anchored types that don’t readily handle modern speaker cables. The 32 ohm outputs are an artifact of the era and are now rarely used – if ever.
  • There is no front panel power indicator, so the user has to rely upon the glow from the tube filaments to know that the amp has been powered.
  • The lack of screen resistors on each of the power tubes increases the likelihood of a significant – if not catastrophic – failure if something were to go wrong with the output stage.
  • All of the electrolytic capacitors must be replaced for reliability and proper operation, along with any noisy or out-of-spec carbon compensation resistors.
  • The original power cord could be inserted into the mains outlet in two ways, one of which is optimal.
  • The scheme for biasing the output tubes can be improved.
  • The original metal feet did not offer much vibration isolation and could scratch the surface.
  • A heated bi-metallic switch provided a time delay, but these become unreliable over time.
  • Fresh paint on the transformers would improve the appearance.

The photos below show a “before” and “after” rebuild and improvement by Yaeger Sound Works (a previous owner added unsightly 1/4” jacks to the rear panel).

Before:

Original Front View

Figure 1: Front view of original amplifier

Original Top View
Top View

Figure 2: Top view

Original Rear
Original Rear

Figure 3. Rear view

Original Wiring
Original Wiring

Figure 4. Chassis wiring

After Upgrade & Modifications:

Figure 5: Front view

Figure 6: Top view

Figure 7: Rear view

Figure 8: Rear view with new cage

Modifications and upgrade list:

  • Dave Gillespie’s EFB™ (Enhanced Fixed Bias) circuit installed. The proprietary circuit increases output power, reduces distortion, and gives longer output tube life.
  • Separate chokes added to the output transformer B+ supplies.
  • New convenient input jacks and speaker terminals. A custom back panel was fabricated to include cut-outs for Pomona™ three-way speaker terminals and Neutrik™ RCA input jacks. Provisions are made for 4 & 8 ohm speakers only.
  • A front LED indicates power on.
  • 100 ohm screen resistors add output stage protection.
  • All of the electrolytic capacitors have been replaced, and the value of the power supply filter capacitors were increased. Old resistors were replaced with precision metal film types; polypropylene coupling and mica bypass caps replaced the original mylar and ceramic caps.
  • An IEC power connector replaces the two wire power cord.
  • The bias point probe sockets are now clustered together and easier to use.
  • Rubber metal feet replace the original metal types.
  • An NTC thermistor limits inrush current.
  • All transformers were repainted, and rusty hardware on the chassis was replaced.
  • A custom fabricated cage offers protection.

For more information, please contact us at email here.