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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.  As 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. 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  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.”
 Today 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. 
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.
Results
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.
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