A Little Help For Eico's HF-81

[Dandy]

If nothing has changed in your Eico, except the tube swap in and out, then it sounds like your ears prefer the Fisher. Are you using the same source and speakers?

 

[ssouci]

If nothing has changed in your Eico, except the tube swap in and out, then it sounds like your ears prefer the Fisher. Are you using the same source and speakers?

yes: using same source, speakers, and wires

I don't know what else could have changed, other than my getting used to the Fisher... but even then, I'm pretty sensitive to wanting decent high frequencies, so I always pay attention to that with any amp, so I would have been a bit disappointed with it before

the Eico does have Russian PIO coupling caps... any chance that I just happened to reach some weird critical burn-in point?... I've heard rumors that sometimes caps can sound good initially, then bad for a bit, then suddenly better (and their best) later on

thanks

 

[ssouci]

which specific components or physical aspects in an HF-81 would have the greatest impact on high frequencies, especially the lack thereof?

certain tubes or caps?

(obviously there are no adjustments other than the treble control)

 

[Dandy]

the Eico does have Russian PIO coupling caps... any chance that I just happened to reach some weird critical burn-in point?... I've heard rumors that sometimes caps can sound good initially, then bad for a bit, then suddenly better (and their best) later on

thanks

I think subjective factors are more likely to be in play here: memories, emotions, tiredness, etc. The good thing is those HF81s are in demand, so if it doesn't start sounding right to you, can always sell it on.

It would be quite easy to try some other caps in the place of those PIO caps.

Re the question in your next post, the tone control components are in the PECs, so again not a part that would have been changed in your rebuild. The circuit follows the Baxandall design, I think. See http://www.learnabout-electronics.org/Amplifiers/amplifiers42.php. But you need input from people who actually own an HF81. I don't, sadly. Dave may well provide a pinpoint answer.

 

[Patrice B]

I run mine tone flat with big Electrovoices and I have a lot of treble. It easily compare to my Fisher X-100-2. Mind you, I use the original ceramic coupling caps with Russian tubes like you.

 

[gbwine]

Reducing Output Stage Sensitivity to Noise

In the last post, the effort was about reducing power supply noise supplied to the amplifier from the get-go. This time, the effort is to make the output stage less sensitive to the noise to begin with -- this because the output stage receives the least amount of filtered B+ voltage in the design. This effort can have additional benefits as well.

From a theory standpoint, the output stage will be least sensitive to power supply noise if the output tubes are perfectly balanced with respect to current flow. The B+ voltages are applied to the output stage in a "common mode" fashion, so that both output tubes of the push-pull circuit receive the same power supply noise that rides on top of the B+ voltage. Since both sides of the push-pull stage receive the exact same noise signal then, the noise component will cancel out in the OPT if the output tubes are perfectly balanced with respect to DC current flow.

In the original design, Eico counted heavily on this basic principle of common mode cancellation through balanced operation. They did not go so far as to specify that the output tubes be matched pairs, as this was first and foremost a kit unit with compromises made to maintain economy. But there is no doubt however that the use of matched output tubes will certainly help towards achieving that end.

To minimize output stage sensitivity to power supply noise then, the tubes must operate with a balanced current flow through them. But balancing the quiescent current flow in the output stage has two other benefits as well, one of which is quite significant.

For starters, when the DC current flow is balance through both halves of the output transformer, not only is is very immune to common mode power supply noise, but it also makes the OPT less susceptible to magnetically induced noise as well.

In the HF-81, the power transformer is quite large in comparison to the OPTs, so there is plenty of magnetic noise impressed upon them. The closest OPT is turned 90 degrees to help minimize any coupling effects. But with so large a power transformer, some coupling can still take place -- and it does. Just give a listen close up to the speaker of Channel 2 when the unit is first turned on. Before the tubes warm up, you will hear the hum of magnetic coupling taking place between the power transformer, and this closest OPT. In the subject factory built HF-81, this OPT was mounted well off of perpendicular to the power transformer, which allowed the OPT to then pick up more noise than an optimum mounting would. For this OPT then, proper orientation is important. Channel 1's OPT is mounted with the same magnetic orientation as the power transformer, so the only help it receives is from being mounted at some distance from the power transformer.

In both cases however, having the DC current flow balanced through each half of the OPT primary winding biases the winding to a higher energy level than that of the available magnetic noise from the power transformer, making the winding rather immune to the magnetic noise impressed upon it. How well balanced the current flow is determines how immune the winding is to the external magnetic noise impressed upon it.

From an audible standpoint however, providing a balanced quiescent current flow through the OPT always enhances the low frequency performance of the OPT. With balance current flow, LF distortion drops to a minimum, and maximum LF power transfer is obtained. When the currents are not balanced, the magnetizing effects in each half of the primary winding don't effectively cancel each other out. Depending on how great the mismatch is, this can cause the core of the transformer to saturate earlier than it would with an otherwise balanced condition. Therefore, balancing the quiescent current flow through the OPTs not only makes them have greater immunity to common mode injected and magnetically coupled noise, but also allows them to achieve optimum LF performance as well. The OPTs of the HF-81 have a bigger core than many EL84 based amplifiers, but are hardly the biggest by a long shot. Balancing the quiescent current flow in them then can have a significant impact on their LF capabilities.

But there are other benefits to balanced operation also. Invariably, in push-pull output stages, one tube begins to become a current hog, drawing more of the current, which in common cathode resistor cathode biased designs, inherently causes the other tube to draw less current. This causes the hog tube to wear out faster, and even potentially over dissipate itself which can lead to its demise even faster. As a result, one popular modification is to raise the value of the cathode bias resistor to lower the overall dissipation levels in the output stage -- not only to account for today's higher line voltages -- but further to the point that even if one tube becomes a hog, it won't go chernobyl in the process.

Raising the cathode bias resistor slightly to account for higher line voltages is fine, but raising it further to account for the "what ifs" only serves to reduce power output and increase distortion. Therefore, maintaining a balanced current flow through the output stage also serves to allow maximum performance from the output stage, while achieving maximum tube life from it as well.

So while balancing output stage quiescent currents produces many benefits, Eico didn't provide any means to accomplish this other than through the use of matched output tubes -- which are rarely "identically" matched when new, or if they are, rarely stay so over time. Therefore, some means of accomplishing balanced output stage operation would be a very worthwhile improvement for the HF-81, with positive impacts on hum, LF performance, and tube life.

A suitable DC balancing circuit was then devised, along the same lines as those used by Williamson in his original amplifier. I modified his basic design with component values as appropriate for EL84 type tubes, and improved it by including matched 10 ohm 1/4 watt resistors in each cathode leg, which allows a simple voltmeter to be used to adjust for a 0.00 volt difference between two test points (the two cathode terminals), indicating perfect quiescent balance. This is similar to the same approach Heath adopted in the W-5M, but only requires one cathode bypass cap. The 1/4 watt resistors not only allow for an easy DC balance adjustment to be made, but also provide protection for the OPTs should a tube decide to become socially unacceptable.

A terminal block was mounted atop the rear of the chassis to effect the test point connections, allowing for easy balance adjustments to be checked or made, without removal of the bottom cover. Channel 2's balance control occupies the former position of the hum balance control (which is why it was moved), while a new mirror image hole was drilled to accommodate the balance control for channel 1.

Pics include:

1. The output stages modified to include DC balance capabilities in each channel. The stock value cathode bias resistors were maintained, while additional resistive elements of the balance circuits effectively raise this value to about 185 ohms. This causes the tubes to operate at a conservative 80% of the Design Center dissipation rating for the tubes.

2. Top side, the two DC balance controls are readily apparent, as is the terminal block at the rear of the chassis to allow for easy meter connections to make the adjustments. The shielded cable to the panel lamp is also visible in this pic.

Next up, quieting down the line/tone amp stages.

Dave

Hey Dave, I'm new to the site .......... if you can remember back to the work you did here. I recently picked up an 81 and plan to do a little work on it. I'm a hobbyist though. Between your pics and the schematic you provided I think I understand what you did, but I also see another resistor running from pin 9 to ground on valve 9. What's that for? Thanks for sharing this btw. I did a little testing on my unit and quickly concluded that anything close to balanced output is pot luck. Wouldn't have a clue how to approach it w/o your expertise. George.

 

[tubeactive]

Hi gbwine, Welcome to AK. There is a tremendous amount of appreciation and circuit analyses of the HF-81 within this and other audio forums. The search function in AK is very worthy. Also, when you cut and pasted the dcgillespie commentary, including the schematic could be helpful as it is now a few pages away. There are many helpful soundmen onboard at different times of day. You might consider contacting our extremely helpful guru dcg directly with a pm "Conversation."

 

[gbwine]

Hi gbwine, Welcome to AK. There is a tremendous amount of appreciation and circuit analyses of the HF-81 within this and other audio forums. The search function in AK is very worthy. Also, when you cut and pasted the dcgillespie commentary, including the schematic could be helpful as it is now a few pages away. There are many helpful soundmen onboard at different times of day. You might consider contacting our extremely helpful guru dcg directly with a pm "Conversation."

Got it, thanks.

 

[Ken Mitchell]

I had heard of these Eico amps because a member of my KEF Enthusiasts Group on Facebook has some other Eico models that he treasures. I have been seeing an Eico HF-81 on Craigslist for a few months, but I really don't feel it's worth the asking price...or anywhere near it. What's the going rate?

https://boston.craigslist.org/sob/ele/d/eico-hf-81-stereo-tube/6676053711.html

 

[tubeactive]

These amps come in a variety of conditions. They can be "as-is" or with tarnished face and even without tubes and with unknown condition internally. They do run hot after an hour. They can need the power tranny replaced, about $100. plus shipping for a modern pwr tranny from Heyboer Trans. There is a "Dollar and Sense" thread here in AK about values, but $700. for a good condition, working unit was/is current. Again, certain factors will increase or decrease desirability.

Genuine Mullard tubes, even marked "Eico by Mullard" run about $35. per tube these days. The earliest units included long plate, Mullard made 12AX7/ECC83s which now trade at over $100. each. Your Boston Craig's List link has currently expired, so there is no knowledge of condition for us to imagine. Restored or retro-mod, upgraded units have been seen with asking prices over $1K. Research and listening tests are in order as these will probably not get any lower in price. Since that Stereophile rave review, more than a few years ago, prices did steadily rise.

The preamp section is very similar with the model HF-85 Eico preamp. Phono stage is identical to the HF-85. The amp section is a sweet sounder with very good output tranny iron. I have completely retro-modded mine, converted the Mic and Tape (Head) inputs to use as more turntable inputs, one with variable phono EQ and would not think of selling it easily....

 

[Ken Mitchell]

These amps come in a variety of conditions. They can be "as-is" or with tarnished face and even without tubes and with unknown condition internally. They do run hot after an hour. They can need the power tranny replaced, about $100. plus shipping for a modern pwr tranny from Heyboer Trans. There is a "Dollar and Sense" thread here in AK about values, but $700. for a good condition, working unit was/is current. Again, certain factors will increase or decrease desirability.

Genuine Mullard tubes, even marked "Eico by Mullard" run about $35. per tube these days. The earliest units included long plate, Mullard made 12AX7/ECC83s which now trade at over $100. each. Your Boston Craig's List link has currently expired, so there is no knowledge of condition for us to imagine. Restored or retro-mod, upgraded units have been seen with asking prices over $1K. Research and listening tests are in order as these will probably not get any lower in price. Since that Stereophile rave review, more than a few years ago, prices did steadily rise.

The preamp section is very similar with the model HF-85 Eico preamp. Phono stage is identical to the HF-85. The amp section is a sweet sounder with very good output tranny iron. I have completely retro-modded mine, converted the Mic and Tape (Head) inputs to use as more turntable inputs, one with variable phono EQ and would not think of selling it easily....

It's been posted again. https://boston.craigslist.org/sob/ele/d/eico-hf-81-stereo-tube/6708930259.html

When that one expires (because I don't think anyone would pay that much for one you can't test), it'll go away for a few weeks and get posted again. That's my theory.

 

[tubeactive]

Well, if you keep waiting, it might be gone sooner than you gamble. No offense, but the price is not crazy if you can check the trannies before offering some cash. Seller mentions he has the cage/cover. He also mentioned there is static in one channel, indicating the amp might actually work. Getting rid of noises is not that difficult...could be a dirty/oxidized connection, switch or tube socket.

Maybe, you should go see it and bring the missing signal tubes. Maybe, the seller is open to hooking it up so you can give a listen ? There are still what I call "survivor" units out there with all original parts inside. The brass face looks better than most, from an original status. Since it has been for sale for a while, if you go over and offer some cash, in hand, it could probably be yours. Best Regards and Good Luck with your audio journeys...

 

[Ken Mitchell]

Well, if you keep waiting, it might be gone sooner than you gamble. No offense, but the price is not crazy if you can check the trannies before offering some cash. Seller mentions he has the cage/cover. He also mentioned there is static in one channel, indicating the amp might actually work. Getting rid of noises is not that difficult...could be a dirty/oxidized connection, switch or tube socket.

Maybe, you should go see it and bring the missing signal tubes. Maybe, the seller is open to hooking it up so you can give a listen ? There are still what I call "survivor" units out there with all original parts inside. The brass face looks better than most, from an original status. Since it has been for sale for a while, if you go over and offer some cash, in hand, it could probably be yours. Best Regards and Good Luck with your audio journeys...

I have ZERO interest in owning it. I just thought I would share and offer to pick up and ship it if someone like you might want it.

I wouldn't know the first thing about getting the right tubes it needs and bringing them to test.

 

[Blackdog61]

Dave, I hope it’s ok to keep this going. Your dedication to getting the most out of classic tube gear really amazes me.
With the exception of a PAS that I have used for a long, long time, all of the tube gear in the house runs the stock circuit or very close to it. I simply don’t have the understanding or desire to deal with modifications.

My 81 sounds really good but it has some noise in the left channel, probably 120hz. The can is original but measures OK, about 20-30% high on all sections. Could that be a contributing factor?
Also, I have read that the rectifier tubes should be fairly well matched. Mine are old RCAs which probably need to go.

We’re enjoying the sound of this thing and want it to be as silent and healthy as it can be without trying to make it something it’s not.

Thanks

Edit: The noise in the left channel sounds a bit like tube rush but I switched all the tubes from L to R.

 

[Patrice B]

I'm not Dave but yes, you should replace the electrolytics. However, bad PS caps would probably makes hum in the two channels. What do you think Mr Gillespie?

As a sidenote, mine hums a little more on one channel. Related to power transfo and layout as I've understood.

 

[Patrice B]

Could also be a mismatched or worn quad of 6BQ5's.

 

[Blackdog61]

They are surely worn but test good and are matched. I swapped them L-R so that’s probably not it.

 

[hereforever]

I bought a replacement transformer from Heyboer and added a copper flux band around the outside of the core. My amplifier is very quiet now, I use to have 8 mV of noise on the output and it was very noticeable on my speakers. This new transformer with the Flux Band is much quieter, and is about 1 mV of noise at the output. I must mention that the replacement transformer has a few more lams and has a slightly higher current rating. Other than that it is an EXACT replica. I needed to use a CL22 thermistor to bring the voltages in the normal range. I highly recommend doing this mod if you own one of these units. I think even adding a flux band to the original unit would really improve things.

Reviving an old post. I'm interested in trying this on the stock power transformer in my HF-81. Does the copper go all the way around and should it be grounded or left floating? I'm trying to visualize how it should be done...

 

[Hi-FiGuy]

The band goes outside the core and should be grounded. If you do a search for transformer flux band, you can see photos.

 

[hereforever]

Got it thanks. Any recommended thickness for the copper for it to be most effective? I already have some conductive shielding tape backed with adhesive that is .00125"