• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Eico HF-81 rebuild: follow-up

Patrice B

Patrice B

Super Member
Hi

A couple of days ago: http://www.audiokarma.org/forums/showthread.php?t=294761

Anyway, yesterday, I did the following:

- replacing all filter caps (30ufx2, 40ufx2, 20uf) with single electrolytics: 33ufx2, 47ufx2 and 22uf;
- replaced the four .1uf caps in the line in stage with four german NOS caps (I know, I know, I'm supposed to put "new" caps but I wanna try some vintage NIB caps once...);
- added 210 ohms of resistor network (2x330 ohms paralleled with 47 ohms in serie, total around 211 ohms) just after rectifiers to get the wanted 340v);
- replaced the two 50uf bypass caps with two 47uf;
- added a slide switch to power on the EICO, just where the AC outlet was originally. I'm not sure I intend to repair/fix the treble-switch pot as the tone function works well, only the switch side is dead;
- tested the old 6BQ5's with my Jackson 648: all tests just under "good" (around 65-70%) so I've put a spare mix of 6BQ5's I had lying around. They're far to be matched but they test good. Two RCA's, a Fleetwood and a Rogers...;
- measured current draw on the 165ohms@5w resistors and it was too high so I put 180ohms instead. I get 35ma on the right side and 33ma on left now (was 37 and 35). Is it too high?

For the first time, I've decided to let the other parts in place as there's only ceramics left (or almost). I know some says to put something better here but I like the sound the way it is now with the ceramics. I won't recap the phono section neither as I won't use it...

Some remaining quirks:

- the "focus level" (why Eico didn't used the word loudness or volume?) mistrack at low level, meaning right comes first then left wake up suddenly. It's ok however at mid level;
- when volume is completely off, there's a slight hiss on left and slight hum on right. You have to put your ear on the speakers to hear it however. Probably a tube and/or an old carbon comp. resistor somewhere...;
- there's sometimes a "thump" of the power tranny when I start the amp and I tried to put some thermistor to cure it. It seemed to work... but I don't know the voltage rating of the thermistor I bought (some spare parts unidentified, all they had here) so I do not want to risk a fire under chassis. Could you tell me if this thermistor will handle 120 volts?

IMGP1451.jpg


Did I say it sounds absolutely wonderful...!

Here are some pics (unfinished, there are some cosmetic to work on... Someone painted white lines on the faceplate and knobs). I also plan to polish the faceplate and add an headphone circuit, using the two 1/4 inch jack holes on the side):

IMGP1446.jpg


IMGP1447.jpg


IMGP1448.jpg


IMGP1449.jpg


IMGP1450.jpg


Thanks.

Patrice
 
Last edited:
Register to hide this ad
Patrice --

Since you established the correct B+ level with the extra PS resistance you added, you might consider leaving the original cathode resistors in place depending on what your goals are.

If your goal is driven by tube life, then the higher values will help a little, but hardly so much as to add a significant amount of life to the tubes. Ditto for heat build up in the cabinet. The tubes will run a little cooler, but hardly so much to make any significant change in overall operating temps.

From a performance standpoint however, the extra cathode resistance will reduce power output and increase distortion as well. Your figures with the original value resistors indicate that the tubes were operating at an average of <75% of design max dissipation levels, so there is little need to adjust the current draw for any concern of that issue. The best performance will be had using the original value cathode resistors.

Finally, since the HF-81 is a Class AB1 amplifier, you might also consider replacing your B+ dropping resistor stack with an appropriate zener diode string to get the drop you need. This could be made up of a series of such diodes that would add up to the voltage needed for the required drop, as well as a safe wattage rating needed for cool operation. You could also add a small resistor (say around 22 ohms) of appropriate wattage in with the string to add some surge protection if you like, but since the rectifiers are of the slow warm up type, this will hardly be necessary.

The benefit of this approach is that you will still obtain the voltage drop necessary to let you achieve the target B+ level, but you will add very little if any impedance to the power supply in the process. That will let the supply's regulation remain as designed which is always important for a Class AB1 amplifier. Using the resistance stack adds significant impedance and reduces regulation notably. The effect on performance is not insignificant.

Finally, if your B+ levels are so high as to require that much adjustment in the first place, it would be wise to also check your heater voltage as well. If this is approaching a 10% over-voltage then it would be best to use some method to adjust the AC voltage powering the Eico to begin with. You might just find that in using that approach, no B+ adjustment will even be required. A variac or appropriate buck transformer will work well if required.

Dave
 
Ok... So from a sonic standpoint, I removed the new bias res. and put the old ones.

From the same perspective, am I better to remove the resistor stack added?

I had 367 volts instead of 340. Is it such a big variation (around 8% more)?

Thanks.

Patrice
 
As I mentioned previously, I would check the voltage at your heaters as well. If they are running 8% high as well, that coupled with the elevated B+ would be significant -- not only on the output tubes, but the power transformer as well. If that is the case, I would first consider lowering the overall AC voltage powering the unit, as that would take care of the heaters, and likely the B+ issue also.

If the heaters are within say 5% of nominal voltage (6.30 vac), yet the B+ is running at the level you indicate, then an adjustment of the B+ would still be worthwhile. I would again recommend using the zener drop method to minimize any increase in power supply impedance.

Dave
 
Heaters are at 6.5 vac... so I guess it's ok.

Also stated on the schematic:

"Nominal tolerance on components values makes possible a variation of 15% in voltage and resistance readings."

367 vs 340 is 8% over, so I'll live with that.

AC line is now 121.5 and measurements for the Eico were taken at 117.

Patrice
 
I would probably put a CL-80 in each leg of the AC coming in.

Your heaters may go to say 6.3v ac (a fine value) you'd have dropped the inrush
current and the incoming AC down a tad, should make the power tranny happier. :)
(your B+ should drop a tad as well from the CL-80's)
 
Job almost done!

Hi,

I finally entered the aesthetic state of restoration. Some Brasso on the faceplate... and unfortunately on the cage!!!

I wasn't aware that the cage was only painted... Obviously, the Brass removed some paint so I had to find something similar. As you would see in the pics, the match is not perfect as the cage is more yellow-ish.

As for the knobs, I used some white labels that I cut to replace the long gone white paint for the little pointers.

Now, all is remaining is to do the headphone circuit inside...

For those wondering how it sounds with the Minimus's: incredibly good even if they're not tube efficient.

IMGP1452.jpg


IMGP1453.jpg


IMGP1454.jpg


IMGP1455.jpg


BTW, I've got a replacement 250k audio pot yesterday but the shaft is 1 inches instead of the needed 1 and a half inches. I'll surely find a way to work on this sooner or later.

As for B+ and bias: old 165 ohms in place and 220 ohms removed.

Patrice
 
Last edited:
Oh... I also got two NOS 12AU7's branded Sylvania instead of mismatched ones already there. In fact, there was a Mullard (Eico) on right and someone put a 12AX7 (Amperex Bugle Boy) on the left.

I was scratching my head why it sounded distorted until I realized that the wrong tube type was used on one side. Probably to give a "crunch" to the guitar sound!:guitar:

Only one problem, one of the new Sylvania exhibit a little "purring" (low level however) but it's not audible unless the ear is right at the speaker. Maybe it'll tame down with some use.
 
Update:

After my usual AC test at the speakers terminals (with a 440hZ test tone), I had to balance things out with a 90k resistor from wiper to ground on right channel.

This way, I have almost perfect balance through 3 to 5 on the volume scale (enough loud for me...). Like I said, at low level (2 and under), balance is in favor of right channel (1 dB difference at level 2).

The "focus" (balance) pot shows: 750k on one side and 810k on the other so it is out of tolerance.

This little misbalance could also be related to the fact that all resistors are original and that they have drifted. An example, the 300k measure from 352 to 362. Same goes for the 100K (97k to 104k).

The reason why I did this workaround is that I've heard that replacing the carbon comp. with metal change the sound in a bad way. Is it true?
 
Last edited:
Tasty!!! I just tried it on my Electrovoice's for the first time and, sure it sounded really good with the Minimus's but now, it reached another level.

Now, about the carbon comp., I read a couple of things on the web and it appears there are some voodoo implied.

My hiss on left (and hum on right) could be fixed with the use of the resistors I bought for replacement...

Some 100k (old carbon comp) measure 65k when the amp was run! Ouch!

I guess it could also cure (or help) the imbalance thing.

Patrice

**That's the hottest of all my amps. It gets incredibly hot after half an hour, particularly the power tranny.
 
I tend to use Carbon Films in most places, with just a few carbon comps in place.
(But I replace all "important" carbon comp resistors as they all just plain drift)

I find Carbon Film to be quiet, hold tolerance an sound better then metal resistors.
 
Kegger said:
I tend to use Carbon Films in most places, with just a few carbon comps in place.
(But I replace all "important" carbon comp resistors as they all just plain drift)

I find Carbon Film to be quiet, hold tolerance an sound better then metal resistors.
Click to expand...

I buy cheapies I find in the electronic supply store here in Montréal area. I guess they are metal film or metal oxide...

Sure, the hiss on left is not disturbing when playing music but I like my amps to be as quietest as possible when idle.
 
To reduce heat...

To reduce heat, I removed the cover and installed a small 80mm computer fan behind running on a 12v adapter.

I'm quite amazed each time I do this little improvement (fan) at how much it reduces heat. It's really night and day...
 
Another update...

Well,

It looked as the hiss was more pronounced so I attacked at the source: the resistors.

My plan was to begin near the faceplate, meaning going from the 12AU7's then goes up (toward EL84's) until I reached the problematic one.

Half an hour later, bingo! One of the culprit was a 1.8k or a 150k... As per Sam's schematic, R20, 21, 49 or 50...

**Ouch... I have to go to work again! I've put 6.8k instead of 1.8K...

I'll report back.

http://home.earthlink.net/~eico_hf81/schematic/eico_hf81_schematic_full.gif
 
Last edited:
Much better. Now, it's 1.8k like supposed... and the hiss is not returned anyway.

It helps sometimes to write in this forum as I could had missed this one...

BTW, I also removed the death cap as I understand it wasn't needed as someone put a three-prong cord on this one.

Patrice
 
Ok...

All resistors in the main signal path replaced: noise floor obviously decreased.

There's a little amount of hiss (white noise) remaining, a little more on left. Maybe the tubes, maybe something else (old ceramic caps). Anyway, these things aren't perfect isn't it?

Once in there, I also replaced the 68k, 22k@1w and 4.7k@2w power supply resistors.

The only things I haven't touched so far are the components in the phono stage.

Bias is at around 36ma per tubes now. Amazingly, one of the 165 ohms resistors that measured 169 ohms a few days ago is now 176 ohms...

I was afraid that removing the carbon comp. resistors could change the sound in a bad way but it's not the case.
 
Hi again,

What (or where) should I investigate you think to get rid of the little hiss?

I guess it has nothing to do with the phono stage as it hisses on all inputs as soon the amp is on.

I read on the Eico group about shielding the grids wires. Is it a possible cure?

I already swapped tubes without any changes. Mind you, it's not that a big thing and the hiss is now lower than before since all resistors were replaced.

Unfortunately, I'm a perfectionnist and if there's a way to improve things, I'll try it. My Fisher X-100A is a reference in this domain as there's the most black background I've heard from a tube amp: no hiss, no hum at all, even cranked to max volume!

My others tube amps hiss or hum (really slightly) in varying degrees:

- Scott LK-72: little 120hZ equal on both channels;
- Heatkit AA-100: a little hum, more pronounced on left (and some crackles and pops at high level but some resistors are originals. I have to replace them);
- Heathkit AA-32: little hum, equal on both channels;
- Clairtone S-601: a little buzz on right and little hum on left;
- Harman Kardon A500: hiss on both channels and a little hum on right;

As you can see, I can live with little imperfections... sometimes.

All these amps have been serviced from A to Z so it's not related to bad components.

Also important to note that these noises couldn't be heard from the listening position, only with the ears right on the speakers. These noise levels are also really low...

Thanks.

Patrice

BTW, I had to maintain the 90k resistor from right wiper to ground on the volume pot to keep balance equal.
 
Update again...

Hello,

I've finally replaced the two original 165 ohms cathode resistors with a pair of 330 ohms: exactly 166.2 ohms on each side.

37.3 ma average per tubes on right
39.4 ma average per tubes on left

I also reinstalled the cage but this time, with a little fan pushing air just behind. I used one of the screws of the cage to fix it.

As for the faceplate, after one week there was already some signs of tarnish around the knobs. I've put some lacquer on it for protection.

Patrice

**I plan to contact Heyboer to get a power transfo replacement as a backup. This way, if this one die, I won't be screwed... 371 volts readings yesterday evening on the first measurement point (supposed to be 340v).
 
Today: I made a bucking transformer using a 120v/12v@2A tranny.

There's a center tap so I was able to wire it to get minus 6 volts on the AC line. I'm now around 115-116v and I measure 342v instead of 371v after rectifiers.

Do you think 2A is enough (the fuse on the Eico is rated 2A)?

Tomorrow, I'm supposed to get a variac. I was planning to use it instead of the bucking tranny but now that it works, I,ll keep the variac for diagnosing purposes.

Patrice
 
is it possible to physically *remove* the power switch from the back of the treble control assembly of an HF-81?

and, can I remove it while the control is in place? (hopefully!)

my treble control will only rotate up to the 3 o'clock position (for one channel), and I suspect that the power switch is broken inside and causing this

(I have bypassed this switch, so would be happy to have it gone)

thanks!
 
You must log in or register to reply here.
Back
Top Bottom