Change EL84 tubes on Eico HF 81

Hi San jose. Hey, Merry Christmas.

Is your HF-81 circuit still stock?

It is always advisable to replace the el84's with closely matched pairs/channel with any self biasing type amplifier.
Randomly substituting output tubes in an amplifier employing a fixed biasing circuit is just asking for trouble, particulary with a 60 year old amplifier.
It would be in your best interest to check the value of the cathode bias resistors in your unit before swapping out the old tubes.

After close to 60 years,the original resistors have more than likely drifted away considerably from their original value.

The original cathode resistors were 165 ohms [rated at 5 watts] and should meaure within a 5% tolerance.

Furthermore: When substituting old stock el84's for the modern Russian equivalents. I would highly recommended increasing the cathode resistance to 180 ohms [rated for 5 watts]

Cheers...david

David. Thanks for your infor on substituting old stock el84's for the modern Russian equivalents. How can we check the cathode resistance to 180 ohms? I will buy brand new quad EL84 Russian Mullard tubes. Can you give me detail on measure on EL84 tubes?

Regards and Merry Christmas

TH

Vintage tube amplifiers like your Eico (and Heathkits, Scott’s, Fisher’s, GE’s, Bogen, etc. too) were designed to operate on a line voltage lower (typically 115-117 VAC) than you likely have in your home. The EL84s were originally run at their absolute limit just to get that last watt. I would definitely recommend using a set of 7189 tubes instead of standard EL84s. The 7189 is rated for a plate voltage of 400 volts whereas the EL84 has a 300 volt rating. Avoid the 7189A as it has a different pinout  than the EL84 and 7189. I see Tung-Sol has new Russian production 7189s available in matched sets from Antique Audio Supply, Tube Depot and other tube vendors. Alternately, you could build a bucking transformer to install externally in line with the AC cord to lower the AC input voltage. Plans are available online (it’s a very simple and inexpensive device to build, but you are working with AC line voltages) but I don’t know of anyone offering an assembled one for sale. All vintage tube power amps should be using one of these. BTW, I have been using a bucking transformer on my Heathkit AA-151 EL84 integrated amplifier for 8+ hours a day for over 5 years now and the output tubes (NOS 7189) still test (and sound!) as new. It drives B&W 805 D3’s better than it has any right to :).