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Eico HF-81 with Pre Out?
Eico HF-81 with Pre Out?
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Old 9th May 2016, 07:58 PM   #1
L8 Start is offline L8 Start
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Default Eico HF-81 with Pre Out?

I'm a newbie restoring my first amp, and it happens to be an EICO HF81.

As you know, the "Tape Out" section is basically a "Line Out" without any volume control or any of that sweet treble/bass tone control. I have zero use for a Tape Out, and I'm guessing *most* people don't use a Tape Out anymore.

I've always wondered how to go about tapping the circuit where I could effectively make a "Pre Out" instead- one that maintains the active tone stack and volume control functionalities.

Has anyone done this with an Eico HF81?
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Old 9th May 2016, 08:53 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Move the feed from pre to post tone control.
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Old 9th May 2016, 09:12 PM   #3
jmsent is offline jmsent  United States
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Quote:
Originally Posted by L8 Start View Post
I'm a newbie restoring my first amp, and it happens to be an EICO HF81.

As you know, the "Tape Out" section is basically a "Line Out" without any volume control or any of that sweet treble/bass tone control. I have zero use for a Tape Out, and I'm guessing *most* people don't use a Tape Out anymore.

I've always wondered how to go about tapping the circuit where I could effectively make a "Pre Out" instead- one that maintains the active tone stack and volume control functionalities.

Has anyone done this with an Eico HF81?
Did you actually try it out? The schematic shows the tape out jacks as being wired after the volume and tone controls. In fact, they're in parallel with the input to the power amp section.
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Old 9th May 2016, 09:57 PM   #4
Eli Duttman is offline Eli Duttman  United States
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Yes, the schematic (uploaded) shows preamp out, not proper recording, jacks.

However, I'd be cautious about using them, unless they are buffered. Notice that those jacks are driven by 12AU7 section plates. The I/P impedance of many modern (particularly SS) devices in parallel with the HF81's power O/P section will load those common cathode sections down.

Like many units from that time frame, the HF81 has tape head I/Ps, which are (IMO/IME) irrelevant in today's environment. I will repeat a suggestion I've made numerous times. Hard wire a single set of I/Ps to the low level mag. preamp section, which is locked to RIAA. 50 plus year old, corroded, slide switches in mV. level signal circuitry constitute a prescription for trouble. All other I/Ps should be line level. An inexpensive Lorlin brand rotary switch can be purchased from Mouser to modernize the source selection capability.
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Last edited by Eli Duttman; 9th May 2016 at 09:59 PM. Reason: grammar
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Old 9th May 2016, 10:18 PM   #5
jmsent is offline jmsent  United States
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Quote:
Originally Posted by Eli Duttman View Post
Yes, the schematic (uploaded) shows preamp out, not proper recording, jacks.

However, I'd be cautious about using them, unless they are buffered. Notice that those jacks are driven by 12AU7 section plates. The I/P impedance of many modern (particularly SS) devices in parallel with the HF81's power O/P section will load those common cathode sections down.

Like many units from that time frame, the HF81 has tape head I/Ps, which are (IMO/IME) irrelevant in today's environment. I will repeat a suggestion I've made numerous times. Hard wire a single set of I/Ps to the low level mag. preamp section, which is locked to RIAA. 50 plus year old, corroded, slide switches in mV. level signal circuitry constitute a prescription for trouble. All other I/Ps should be line level. An inexpensive Lorlin brand rotary switch can be purchased from Mouser to modernize the source selection capability.
I think you could get away with a tube amplifier of e.g. 500k input impedance without much trouble, but not much more. But then there's the issue of switching off the internal power amp, which should be done if not being used...or loaded at the output. As to the other suggestions, good ones, but probably not suitable for beginners. The HF81 is one of the more collectible integrateds out there amongst the vintage tube guys, so the effect of major modification on value should also be considered.
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Old 9th May 2016, 10:28 PM   #6
L8 Start is offline L8 Start
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..and just like that, I learned something.

Eli Duttman- Thank you. I plan to keep a single low level mag input as suggested, and move the phono section off the switch.

Aux 1 and 2 will be line level inputs.

Speaker taps will be 4 and 8 ohms only (not enough space for 16).

jmsent- I see the tape out wires once tied to the mono/stereo switch (now bypassed), so I guess it really did maintain volume and tone control…weird set-up …..but now that I know it's an unbuffered output- I'll just leave it be.


This project is teaching me a lot…In hindsight, I might have been better off stripping the whole amp down and building it back from scratch. That idea intimidated me at first, but now that I seem to be doing so many things one step at a time, I would have been better served to start with a fresh chassis.

So far, I've put in a replacement power transformer (heyboer)
replaced all the coupling caps, bias resistors, power supply caps, dropping resistors, an IEC jack, power switch, volume pot, main fuse, and a dozen other little things.

And now, it's the RCA's and binding posts…it's a mess right now. I can't wait to get it back together again. Each time I work on it, I miss it terribly!

Click the image to open in full size.
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Old 9th May 2016, 11:42 PM   #7
BinaryMike is offline BinaryMike  United States
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Eico HF-81 with Pre Out?
It seems clear that the Sams schematic has the preamp outputs mislabeled as tape outputs. The Eico schematic probably is correct. And those preamp outputs might be fairly robust because there's a feedback loop around the driving stage. Perhaps the Eico manual would offer a preamp output impedance spec?
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Old 10th May 2016, 12:07 AM   #8
Eli Duttman is offline Eli Duttman  United States
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I should have spotted that the low level mag. preamp section uses AC heaters, sooner. If a DC heater supply for those 2 tubes is not going to be undertaken, use the Sovtek 12AX7LPS in those sockets. The 'LPS is genuine 7025 equivalent, which contains spiral wound, hum bucking, heater wire.

BTW, constructing a buffer is not that big of a deal. Even opamps configured for unity gain will be quite transparent, provided they never clip. A suitable bipolar PSU can be constructed from an inexpensive AnTek power trafo, 4X Schottky diodes, some caps., and 7815/7915 3 terminal regulator "chips". Pick an opamp with JFET I/Ps whose reputation is good. Such a buffer would be a good idea too, if the preamp out jacks were altered to be true recording O/Ps. "Ripping" vinyl to computer resident digital files would be a recording O/P use.
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Old 10th May 2016, 12:52 AM   #9
L8 Start is offline L8 Start
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Quote:
Originally Posted by Eli Duttman View Post
BTW, constructing a buffer is not that big of a deal. Even opamps configured for unity gain will be quite transparent, provided they never clip. A suitable bipolar PSU can be constructed from an inexpensive AnTek power trafo, 4X Schottky diodes, some caps., and 7815/7915 3 terminal regulator "chips". Pick an opamp with JFET I/Ps whose reputation is good. Such a buffer would be a good idea too, if the preamp out jacks were altered to be true recording O/Ps. "Ripping" vinyl to computer resident digital files would be a recording O/P use.
The only buffer I have experience with was an Aikido Cathode Follower (ACF) tube buffer I built a couple of years ago. It required a dedicated chassis, of course. My current "knowledge" of opamp based buffers is nonexistent…but I really like your idea. This Hf81 came to me as a console mounted version with a dilapidated wood cabinet. A new walnut cabinet is in the works, and I'm betting I could house a little buffer PCB (and a tiny project chassis) discretely on the back.

I need to digest this idea more, but thanks!
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Old 10th May 2016, 02:03 AM   #10
anatech is offline anatech  Canada
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Eico HF-81 with Pre Out?
Hi L8 Start,
If you use an op amp buffer, put some protection from positive peaks from the inputs to the B+ line to the op amps. Depending on the dv/dt on turn-on and off, you could get peaks above the supply voltage for the op amp. They don't like that very much at all! You should probably protect for the negative polarity as well.

This is a very nice integrated amplifier. I have my own to do on top of work for other folks. Try to keep it original best you can. I will be wiring the phono input direct, and also the feedback for the RIAA eq as was suggested earlier. Always a very good idea for these older products. And yes, sometimes those selector switches can be too noisy for phono circuits.

Enjoy yours!

-Chris
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