In addition to boatanchor receivers and transmitters, I occasionally enjoy working on tube audio amps. I have admired the build-quality of Eico power amps such as the HF- series. This HF-87 is definitely of boatanchor weight at over 30 pounds. The amp uses two pairs of push-pull EL-34 / 6CA7 power output tubes for 35 watts per channel. A pair of 6SN7 and a 12AX7/ ECC83 round out the rest of the tube complement. It has the power of two Eico HF-35 amps on a single chassis. Like the HF-35, the HF-87 uses cathode bias. Unlike the HF-35, the HF-87 uses silicon rectifiers for B+ and stacked electrolytic capacitors. A "surgistor" is wired at the input of the power supply to limit the initial power-up surge and protect the B+ diodes. The HF-87 uses a single triode (half the 12AX7) for input for each channel. An input of 0.38 volts is required for full output for each channel. By comparison, the HF-35 uses a pentode for input.
Repairs
When purchased one of the EL-34 output tubes was broken, likely from being moved. I replaced the EL-34. I reformed the electrolytics slowly, using an external supply and also cleaned the controls. The amp worked after this treatment, but I replaced the coupling caps with excellent tested orange drop caps because even a little leakage can affect audio quality. Greater leakage can stress or damage the output transformers and tubes.
Smoke from the surgistor
When first powered, the surgistor emitted a puff of smoke. Scary to be sure, but it simply burned off a bit of dust from its exposed heating element.
One of these amps that has been dormant for some time may indeed do this the first time it is powered, but there should not be a repeat performance. I suppose the surgistor could easily be replaced by a modern NTC surge-limit thermistor which, when cold, exhibits a high initial resistance but after heating up drops to a very low resistance.
Free manual and schematic
A partial schematic can be found at this source.
The full manual for the amp can be found at Abeltronics in the UK along with a variety of other audio manuals.
7-16-09
A Heathkit IM-13 and other VTVMs were the previous items on the bench.