Photo of Eico HF-20 amplifier

Easy Way to Turn
the Eico HF-20
into a Monoblock

Drive the tape output to use the HF-20 as a great monoblock.

Copyright © 2010 by Stephen H. Lafferty. All rights reserved. Rev. 0a

It’s hard not to notice that Eico HF-20 integrated amplifiers have been plentiful on the vintage market. Eico sold them as far back as 1955, and they became a hit in the DIY hifi movement which flourished in postwar America. HF20 adAs you can see from the 1957 ad at right, the per-
formance of the HF-20 is quite good, due in part to its excellent output transformer.

It’s only natural that today’s vacuum tube enthusiasts would want to use these workhorse amps. You need two for stereo, of course. The problem is that it’s an integrated amp. If you try to use a pair as such, you have difficulty maintaining balance between the channels, while you adjust the volume with two controls.

An easy solution for this is to add a preamp to the system and use the HF-20’s as basic monoblocks.Photo of stereo system with Eico ST-84 preamp, two HF-20 amplifiers I watched my big brother Bill, do just that back around 1961. In recent years, I have reproduced the system he built, finding and restoring each of the pieces. You can see the result at right, with the Eico ST-84 preamp driving the HF-20’s. That worked well except for the fact that it cascades the tone controls of the preamp with those of the HF-20’s. That leads to frequency response errors.

Using the Tape Output as an Input

In the early days of hifi, designers at Eico (and other companies) were just learning about the best ways to configure integrated hifi amplifiers. Unfortunately for the
HF-20 design, they ended-up making a bad choice in the placement of the Tape Thumbnail image of Eico HF-20 schematic Output. (Click on the thumbnail at right to see the schematic) They put it after the Level and Tone controls instead of before them, as is the standard today. I can just hear them debating the merits: “But this way, you can use the tone controls to correct the frequency response for recordings...” “Yeah, but you would be forever screwing-up recordings due to having the tone controls set to compensate for speakers and such.”

Photo of HF20 tape outputToday though, we can turn that oddity into an advantage: The tape output jack (at left) provides almost a direct connection to the input of the power amplifier (PA) portion of the HF-20. If the Loudness control is set to max, there will not be any processing of a signal driven into the tape output, before it is passed to the PA. However, to utilize this properly, we must do something about the internal amp (V2), which is already driving that point. The problem is not just the loading of the plate resistance of V2; there is also negative feedback present which lowers the impedance at the output of V2, substantially. Photo of 12AX7 pins with pin-6 insulated

One could simply pull V2, but I didn’t want to leave my HF-20 with an empty socket on top. Another option would be to disconnect either C7 or V2-6 under the chassis. I would like to keep these restored HF-20’s in stock condition, though. My solution is to use a piece of narrow diameter heatshrink to insulate pin-6 of V2, disconnecting the plate, as shown at right. This works well and leaves the amp nearly pristine.


As expected, driving the tape output from the preamp delivers excellent audio reproduction. Bypassing the preceding two stages reduces noise, distortion and improves frequency response. Sensitivity of the new input seems just right for the
ST-84 preamp. On the HF-20’s, it’s best to keep Loudness at max, though I use a slight decrease on one side to compensate for a minor imbalance. Positions of the Level, Bass and Treble controls do not matter much but it would be good practice to keep level at minimum and tone controls centered. Set that way, the input impedance should be about 77kohms.


Reader Comments

Posted by Steve L. June 25, 2015 - 02:41 pm
Hi Keith, You are quite right to be concerned with the effect of removing tubes on power supply voltages but the low level V2 stage in question only draws about 1mA total plate current. I just measured voltages on all four sections of filter cap C27, with and without V2 present. That cap has sections rated at 350V and 450V. The biggest swing without/with the tube was from 102V to 148V and the highest affected voltage went from 174V to 190V. (One section stayed ~253V.) Thus, there is no risk that C27 will be stressed and no need for a load to ground. By the way, the voltages in this unit seem modestly low but even allowing a generous factor for that still leaves C27 far below its ratings. I also tested with the recommended mod of insulating only V2-6 and as expected, voltages were even lower than without V2. Thank you for your comments.

Posted by Keith June 24, 2015 - 12:57 pm
There is something else you need to be concerned with... the low level stages of the HF-20 run at lower voltages than the driver and output stages, and receive extra filtering. These lower voltages are created by passing the main B+ voltage through an additional series resistor, and to an extra capacitor section (actually two stages of this). The voltage that you have at these points is determined by the values of the series resistors and the current drawn by the tubes connected at those points. If you disconnect the plates of those tubes, the current draw is gone, and the voltage will drift up to the value at the other end of the resistor. Since the different sections of the capacitors are NOT all rated for the same voltage, this may cause you to exceed the voltage ratings on the lower voltage sections. If this happens you will blow the capacitors. To hold that voltage to the intended levels, you NEED to provide some sort of load to ground at those points (to replace the load originally provided by the tube).

Posted by Steve L. August 09, 2012 - 06:46 am
Hi Bktheking, Sorry that I don't really have much advice on tube brands. However, from our page on testing 7591 tubes, I can generally recommend any good brand of true NOS tubes. Of course, the problem there is being sure that they are new. If it must be a new-manufacture tube, the Electro-Harmonix brand generally did okay.

Posted by bktheking August 08, 2012 - 11:56 pm
I've now got three units and have a matching pair of outputs so one unit is going to an ak'r. What is the best output tube to run in these?

Posted by Steve L. July 29, 2012 - 07:20 am
Hi Brad, Actually, the temperature of the transformer will decrease, as the load is reduced. That is because the reduced current dissipates less power in the internal resistances of the transformer. Here is an application note, which discusses the subject:
Thank you for your comments.

Posted by bktheking July 28, 2012 - 11:33 pm
It was me that made the comment. My understanding is: If a transformer is designed to put out a certain amperage to run a load albeit heater voltage, removing a tube would increase temperature of the transformer due to the fact that the tube isn't present and the amperage that was "supposed" to be used it missing. A user answered the post buy saying a missing tube would decrease the temp of the transformer instead of increase. I've now got a second hf-20 on the way and will try removing the tubes V2 and running the tape out as an input once it's rebuilt.

Cheers- Brad

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