Eico HF-81 with Pre Out?

[L8 Start]

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?

 

[JonSnell Electronic]

Move the feed from pre to post tone control.

 

[jmsent]

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.

 

[Eli Duttman]

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.

 

[jmsent]

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.

 

[L8 Start]

..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!

[IMGDEAD]https://i.imgsafe.org/cee0c10.png[/IMGDEAD]

 

[BinaryMike]

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?

 

[Eli Duttman]

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.

 

[L8 Start]

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!

 

[anatech]

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

 

[ericj]

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.

Easy. Look for a unity-gain-stable part with jfet input and a good strong output, low noise, etc. High dollar parts would include the opa2107, ad8620, opa627, probably others.

On a tighter budget a pair of opa604 chips might be just right.

If you have 12v or better yet 12vct heaters in this amp (I am not familiar and didn't look) you could run the buffer's supply off of that. It will use almost no power compared to a single filament. It doesn't need to be regulated, just well filtered. Does need to be a dual supply if you want to avoid capacitor-coupling the output buffer as well. you can use a tle2426 or another cheap opamp to create a 'virtual' ground between the power rails.

The output from the amp will be capacitor coupled but there might still be a turn-on thump. strapping a couple zeners across the inputs might take care of transients for you.

From there it's just a 500k resistor from non-inverting input to ground, tie output to inverting input, and apply power. The opa604 has a facility to trim output offset (since it's not a precision part) so you might add the trimpots for that.

 

[Eli Duttman]

Chris,

What is the real current capability of the HF81's B+ PSU? Each 6CA4/EZ81 is good for 150 mA. However, 300 mA. seems a lot for an EL84 integrated amp.

If "stealing" a few mA. from the B+ supply is OK, high voltage MOSFETs could be used inside the HF81 to buffer "special" O/Ps.

 

[anatech]

Hi Eli,
To be honest with you, I don't normally check that. Eico rates this as a 14 watt per channel amplifier, and I'll believe them on that. That figure is for 1% THD in the mid-range, so they are being optimistic just like everyone else. Those EZ-81 tubes are good for about 150 mA per section at that voltage (340 V), and can accept a 50 uF input filter capacitor as well. Eico is using 30 uF in that position.

I have to guess that they are betting on a long service life for the rectifiers as the B+ is good for 1/2 an ampere (just over) if you only look at the rectifiers. That's crazy power, but I don't think the power transformer can deliver that current.

Very impressive numbers, but my job ends when the unit is restored and meets or exceeds it's specifications. Have you ever pushed one just to see what it would do? How about just the power supply?

-Chris

 

[kstagger]

If I remember correctly, the Eico HF-85 preamp tape outs were also connected to the linestage, but bypassed the tone controls. I imagine the preamp stages are similar, down to the AC for the filaments.

 

[anatech]

I have an HF-85 also. I biased the AC heaters up to about 35 VDC. No hum now.

I've never compared the preamp sections between the two, so I can't say.

-Chris