need help with HF-81 hummer

[Tom Mobley]

Well, I've replaced the rectifier tubes with NOS Sylvania EZ81's, the 12AU7's with NOS Mullard CV4003 12AU7 ECC82, the 12AX7's with a real decent set of Telefunken, not NOS but excellent, and the EL84's with a real nice set of NOS Raytheon's. The stock Pyramid 30mfd PS caps went out for a set of 40mfd Spraugue Atoms. there is a hum that starts as soon as powered up, both sides, unaffected by adjustment of any knob or the hum pot. This hum hasn't changed since the first day I had it with the bad EZ81's. Any bright ideas?

It's really drowned out by any music at all, but it bugs me.

Also, sometimes I feel the PS (I think) humming and/or vibrating. I thought this would be fixed by the new EZ81 or the Sprague's, but noooooooo. It did start running cooler when I replaced the stock caps and for some reason the whole amp seemed to run a bunch cooler after the Tele 12AX7's were installed. They came in last. I've not hardly had the volume knob up to 2, so I don't think it's a "hard to drive speakers at high volume" type deal.

Aside from that and a tendency to drop out channel 1 at very low volumes this thing is just terrific on my DIY LS's. Absolutely kicks ***. (my asterisks )

Tom

[mobile homeless]

What kind of hum is it? How low? IT could be your main metal can cap on top of the chassis but I have to say this usually does not go bad, although NOS's did. But you really dont see these go as often as the two main PS caps you replaced - they are pretty tough.

It sounds like you might have some sort of grounding problem which Pat reports as being problematic with his unit before he sold it to Tim. In my two units, I have not had any hum problems at all; in fact, it has been a very quiet amp, more so than many. But, I have heard a few others such as Roger Stevens and then PAt talk about the grounding problem.

It' so hard to trouble shoot from afar. Besides the caps, and some grounding short, or a poor ground, I dont know. MY bet is one of the above suggestions. Ground difficulties / ground short / Metal Can Cap. Bet it's something with grounding.

[Tom Mobley]

Thanks Kelly, for taking time to respond to this.

I'm figuring it's a ground issue also. on the bottom of the chassis there are a couple of solder lugs, look like copper eyelet crimp terminals, attached with bolt and nut to the chassis, couple ground wires attached to each. soldered, not crimped, I think I'll pull these, clean them, add a starlock washer between the lug and the chassis to ensure good contact.

On another front, it had a brand new power cord attached when it showed up. This is a two conductor cord, no ground. Typical hardware store item, has a polarized plug. What does current conventional wisdom recommend here? install three wire cord, attach ground conductor to chassis?

Also, I've discovered there's a another one of those black Pyramid caps on the power lead where it comes off the convenience outlet. Looks like it's connected from hot lead to ground. it's buried, can't read the value of it but it's physically smaller than the .1mfds on the tubes. Schematic shows a cap there, looks like it's labeled as a .03, does that make sense? Could it be the problem?

any ideas/suggestions welcomed.

Tom

[Pland]

Hi Tom,

Clean all the grounds,and by all means use a star washer when re-connecting.All of the electrolytics,including the above chassis can on my HF81 were shot.Flip the amp over and attach a similar value cap to the can leads,observing polarity.If the noise goes away,bingo.Replace the can with a clamp type ,and the problem is gone for the next 40 years.

How efficient are your speakers,and how loud is the hum?If you're describing a miniscule sound heard 1 ft.away with no signal with 100+db eff. speaks, this may be normal.Otherwise I'd bet on a ground or the big can type cap.

I replaced the cap you're describing on my amp,and the noise floor remained the same.Do the above first,and I bet your problem vanishes.

Best,

Pat

[NOSValves]

I had the same hum problem with the fire trap I had last year and replaced lytics, regrounded, added grounding you name it and the hum just kept on going. That's why I parted it out !! I have heard that the HF-81 is prone to some hum and why I feel its not all that great a amp. All my other amps 8 total even the cheapo heathkit AA-151 have absolutely zero hum dead silent at full volume on all inputs except phono which really isn't a hum. This is on 104db speakers. Is this hum what everyone expereinces with these amps to some degree or is it just a few bad lemons ??

Craig

[mobile homeless]

Mdeneen got smart. I should follow suit. It's like beating your head against a wall and coming back to see if it feels the same.

See two posts above. My two units have no hum.

ps- Craig, you didnt part your HF-81 out; you sold it to another forum member sans tubes.

[No Disc]

Tom -

Try Pats advice, he knows what he's talking about. The unit I got from him has NO hum whatsoever.

Congrats on the Eico BTW.

- tb

[NOSValves]

Mobile,

Kiss my A$$ ! If Mdeneen got tired of banging his head against the wall it was from Idiots like you. In my mind I did part it out. I kept all the tubes and sold the chassis. I was going to part it out further but he ask for it he got it.

Once again all I did was state what I had heard and read from a link you gave me a long time ago don't have it anymore but the person had spent hours trying to tame the hum of the HF-81 !! This is why I asked if others have this same problem or if it is just some. You say yours don't hum ?? I seen you write that one of your units was humming not long ago !!

I'm not trying to put it down just repeating rumors I have read !! Pland post would lead me to believe that some hum is normal for this amp wouldn't it you ??

Quit being so defensive about the EICO man !!!Your the one that starts these problem being the cry baby you are !!

[Pland]

Hi NOS,

Actually after I was finished,the Eico was as quiet as my 299's.I just wanted make sure Tom wasn't 1 inch from a driver and was hearing slight noise.I re-read his post, however,and would bet on the above chassis can,particularly if he can hear Power tranny "buzz".Eicos' tend to run hot,and if you lose filtering in the PS,the power transformer will actually make mechanical noise when hot .You could hear mine from about ten

feet.

Tom,also try reversing the plug in the wall socket ,and make sure you're not creating a loop via house grounding or other component interaction.Also,after the amp is warm(30 minutes or so)feel the top of the can capacitor.If it's warm or very warm,there's your culprit.

As far as the volume dropping on one channel at very low settings,you've got a stereo volume control mis-tracking left to right.Try cleaning it again but consider an upgrade to Alps or a resistor type pot(see DIY cable).If Eico went a little cheapo in areas,the controls are probably the worst.

Best,

Pat

[NOSValves]

Pland,

I hear what your saying and I myself seriously didn't think that all HF-81's have a hum. I was just trying to find out if there are some out there that are prone to hum or if they all hum slightly ?? I need to look up the link where I read about someone installing a ground loop because of what he thought was a design flaw. I believe he thought it was in the Main Power transformer somehow bleeding into the signal. Mind you I'm recalling this from almost a year ago ! No big deal seems like anytime I try to discuss anything to do with the HF-81 or ask cry baby a simple question he gets his panties in a ruffle !!

Craig

[Pland]

Hi NOS,

Actually,buzzing transformers are common on HF81's until you get all the uf and voltages under control.Most reviews and owner questions bring this up.Optimally,Eico ran everything real hot(150+ degrees for transformers according to the owners manual)which tends to scare everyone anyway.Also,I don't see where Tom replaced the coupling caps,which is a must.If he loses one of the original ceramics(aaargh)he can have an output tube run away and take out a transformer.Tom,do this ASAP.

As far as some amps being noisy other than the above,I don't see any reason other than maybe some of the kits being poorly built,with bad soldering and wire dressing.Mine was a kit,but whoever built it put the factory to shame,as the workmanship was first rate.

When I got mine,the first thing I did was to pull the bottom cover,check all the resistors and voltages,and replaced all the Pyramid caps(which are the coupling caps in a 299). with new Auricaps.Since I was too stupid to pay actual attention to the shematics,it never occured to me that the coupling caps are ceramics,and that I had REPLACED ALL THE WRONG CAPS.I had to sheepishly send Tim Babb a set of Auricaps in the correct values after I realized what I had done.To his benefit he now has high zoot caps pretty much everywhere,so I suspect his amp is a real performer.It helps to activate your brain when doing a rebuild.

Best,

Pat

[Tom Mobley]

yeah, those little .025uf ceramic disk caps on the bottom of the tubes, sort of peach colored? there's six of them, I've got new Auricaps for those but just haven't had time to put them in yet. Turns out that AES place is not too far from where I live, about 20 miles across town, I picked them up there the other day. They printed out the Sam's photofact deal for me too. It's pretty nice, gives a parts list, schematic similar to the one Kelly posted, ident's all the caps and resistors on photos of the chassis top and bottom. five big legal size pages altogether. good for de-mystifying the whole thing.

that can cap shows as a 20-40-40, think I'll look around for one.

Pat, Craig, Kelly et al, thanks for taking time to respond.

Tom

[Pland]

Hi Tom,

Surplus Sales of Nebraska has some 20/40/40/450v can caps.May require a bracket but no big deal.$24.00(www.surplussales.com)

Best,

Pat

[Pland]

Tom,

You can also leave the can in place for a stock appearance,and use Sprague atoms under chassis.Short the can cap sections to ground with an insulated screwdriver before working,and simply wire a cap to replace each individual section,observing polarity and symbol for values(they'll be printed on the can with a dome,pyramid,etc. shape)and use the common can ground for the three individual caps.You could finish your amp this weekend without having to scour the country for a 35 year old replacement FP type can.

Best,

Pat

[mobile homeless]

I was going to mention the turn of the power cord orientation as well. This has cured some hum problems with my gear. Also, pay attention to where you have your gear plugged in. Are you using a power strip? Plug the EICO into its OWN outlet in the wall and not into a power strip. Once you do this, try different power strategies with your gear.

Perhaps it is the can cap on top as Pat suggests; I just have not had problem with this can in either of my units nor have I had many people write me with this problem from other owners checking out the page I did. I get a five to ten mail a month from EICO people having just purchased a unit. But, obviously this does happen as evidenced by Pat etc. (Regarding the caps, as I remember, I think I wrote Pat about the output coupling caps not being done; Pat, I dont think you put caps in unnecessary places as much as just doing the phono stage and the input coupling caps - if you see Tim's page with a pic, you can see what was done. He just needed the four remaining output couplings done). You obviously did a great job with the unit and put some nice stuff within.

You did put two different caps in the PS section filtering but obviously of the same spec. If there is one thing I wanted to experiement with it was putting in something like two ELNA Cerafines in the PS section but it would have to be above chassis probably due to size. They would actually fit above where the tone controls stick out with a shield already in place between the rec tubes.

One thing that really amazed me about the EICO, was how well this amp did with even the little stock ceramic discs. I hate those things but was really shocked at how transparent and open the unit was WITH these pieces. As I have said in the past, this was one of the first pieces of gear that really made me quetion the roll of parts vs the roll of a well-engineered piece of gear with excellent transformers. I have still yet to figure out exactly what makes this integrated sound so good. The all triode front end and great transformers surely play a large part.

As an aside, after thinking some more on the parts, I am more inclined to recommend replacing caps with very near the same value (besides an up in the voltage).

One other note on this and every other amp; DO NOT EVER used the plugs in the back of ANY of your gear to power another component (especialyl a CD player or preamp). Although the EICO (and a host of others) have the outlet in the back, it's best to disable it (hell, intstall a femail IEC if you like to give you cable flexibility). Powering ANYTHING from these rear outlets on preamps and amps is a definite NO GO and sonically a disappointment. This holds for ALL modern gear as well. I have seen so many used the plugs on the back of their AV, preamp, and receivers to power other units. DONT. It is hurting (sonically ) both pieces of gear.

Enough prattle. I dont mind any discussion of any of these integrateds as long as sweeping generalizations dont occur.

[NOSValves]

Tom Mobley,

Could I possibly get you to send me a full copy of that phot fact Please !! It might come in handy !!

Pat,

The hum I'm speaking of from what I remember wasn't a tranformer Buzz it was hum coming thru the speakers. I believe the guy thought it was a grounding or shielding issue from the power transformer or maybe outputs trannies. I really have to dig up that sight up it had lots of mods for HF-81's the guy really seemed to know his stuff.

The funny thing is about Tims amp is I put those caps you sent to him in LOL !!! We were both at a Chicago horn club meeting and he needed it done so the story goes on !!

Craig

[NOSValves]

Okay I took the time to find where I got the Idea that HF-81's had a inherent hum problem and low and behold its from a link on Mobiles page LOL !! Anyway here is a copy of what steve went thru to get rid of hum on his HF-81. Hope he doesn't mind me posting it here.

August 30, 1999

Revisions since June 26, 1999

I think I've finally solved the mysterious hum problem, or at least mitigated it as much as probably the amplifier layout and component age allows.

1. First, I down-sized those coupling caps described below in the June 26 update from .1 to .047mfd@400vdc 716P Orange Drops. I happened to remember that the originals were .025 ceramics, so I think .1's are a bit of overkill, and I was trying to reduce the hum anyway. Fortunately, I like the .047's better anyway.

2. Next, I had experienced a newer, stranger thing prior to the above change--not just residual hum in one channel that I originally attributed to an AC balance problem in one of the driver stages, but real hum when I connected associated equipment. I can't remember ever hearing it before, so I put a ground lift (a 3-pin to 2-pin AC adapter) on the power cord and it went away. But the amp seemed to be vibrating strangely a LOT more than normal. So after a few days of getting more and more worried, I took it back downstairs to the shop and opened it up and saw wax that had run out of the power tranny! Yikes! NOW I know what was causing it--I had a switching power supply-based nicad battery charger on the same power strip that is notoriously famous for generating hash and RFI (it's a Yaesu NC-29 I think), and it was feeding noise back onto the AC line and when I pulled the ground, the remaining line bypass filter caps inside the HF-81 from hot-to-ground and from neutral-to-ground--now floating at the common point due to the ground lift--were apparently giving the power tranny heartburn to a major extent. So I decided to do several things:

3. First up was remove the two AC line bypass caps--I did this on the PAS-3X when it developed hum when used with the Stereo 70, which I think is more of a Stereo 70 leaky power transformer-induced phenomenon, but the power tranny in the PAS could be leaky, too. It's seriously undersized and old--let's face it. But I left the green wire of the three-wire power cord connected to ground instead of applying it to a SPST slide switch like I did in the PAS-3X. Instead, I put an DPDT slide switch in the position formerly occupied by the accessory AC socket on the HF-81, and wired it up with both sections in parallel and used it to switch a .047mfd@600vdc 716P Orange Drop from one side of the power transformer primary winding to the other, as in any common guitar amp--a typical "ground switch", if you will.

4.Then I bit the bullet and COMPLETELY rewired the 12AX7 driver stage--every plate resistor, every cathode resistor, and every small value compensation cap. The original caps were 225pf ceramic jobs--I used 180pf silver micas, both because I had four of them, and because the HF-86 uses 125pf caps in the same circuit but with 82K splitter resistors instead of the 100K's in the HF-81, and bottom line they sound pretty nice. The original parts weren't THAT out of tolerance--the worst were a couple of the compensation caps--but I had to do it. I think I left the driver stage grid resistors alone, but I also replaced all four of the output stage 330K grid resistors as well.

5. I also rearranged the grounding of the amp. In a former abortive attempt to eliminate hum, I had tied all chassis grounds together with a ground wire "string" (which could also be looked at as a "loop", and we all know what a ground loop can do). So with the driver stage rebuild I separated each channel's grounding circuits from each other and from the power supply, leaving all of that to the chassis.

6. I also redesigned the output stage cathode circuit. I changed the value of the bias resistors from 165 ohms to 180 ohms--the amp was running too hot--and rearranged the cathode circuit grounding scheme, eliminating all tube socket mounting ring grounds in doing so. Now both cathode resistors and bypass caps (470mfd@35vdc electrolytics) connect to a common chassis ground point between the output stages to which no other grounds are attached.

7. Last, but not necessarily in order--I'm recalling this from memory--I redesigned the filament wiring circuit so that each channel has its own dedicated branch running off the hum balance pot, instead as in the original, where one channel's output tube filaments were on the filament circuit of the other channel. It's all one circuit with one hum balance pot, but I decided that I'd divide and (hopefully) conquer the hum in any way I could.

That's about it. I could probably do more--I've thought of putting .022mfd@400vdc 716P Orange Drops from the hum balance pot center tap to either side as in the Pilot 602M I just rebuilt, but haven't tried that one yet. Bottom line again is that the amp is stable, doesn't hum when other components are attached, allows for selecting a position of the new "ground switch" that produces the least amount of noise--particluarly noticeable on an AM tuner--and runs cooler with no noticeable hum in the speaker. If you look at the scope, the right side STILL has about 20 millivolts of 120Hz residual AC hum, but the power tranny isn't buzzing anymore or leaking wax, and the amp sounds terrific. So fuggit, I say.

Oh, yeah--the late but sometimes great Ei 12AU7's sound terrific in the HF-81, as long as you can get ones that aren't microphonic. Mullards are just too thin. Telefunkens are, of course, the best.

[Pland]

Hi,

Small world,isn't it?At least Mobile and Tim were discrete enough to not call me a complete moron when I kept insisting the amp had new coupling caps!If you got to listen to Tim's HF81,then you can confirm that it's very quiet.When I got it,it had 60hz hum through the speakers ,AND the tranny buzzing I described.I wasn't sure it wasn't going to burst into flames at any moment.But again,replacing the filtering caps and confirming the grounds completely corrected the problem.

I've read the post you included,and I think the guy missed something basic,or maybe had a home wiring issue.Although my experience is limited to one HF81,it responded immediately to the "usual suspects" that create hum,without any exotic circuit changes.

How did you like Tim's amp compared to a 299?

Best,

Pat

[NOSValves]

Pat,

actually the situation we were in listening to it was strange and it left me wanting a better setup to audition it in. The setup was with a Biwired set of altec speakers and a electronic crossover and they used a SS amp for the bottom end and the HF-81 for the top. Nothing was dialed in and it just wasn't fair to compare and Tim had to leave. Later that day we hooked my 222C with all its bass mods to the bottom and my 299 to the top end and some other gentelman tweaked the speaker placement and crossover settings and the 3 of us that were left there thought it was the best the system had sounded all day. But again I think the entire setup was not Optimal for any of the other amps that day and there was so much noise from all the people there it isn't a fair comparison. Like mobile has said these meeting are never a great place to audition anything. Just to much russle and bussle.

I have a HF-81 here now and its in need of a new Power transformer. Other than that its in great shape the wiring looks great compared to the last one I had and cosmetically the thing is almost perfect. I have one forum member that offered me a donor cosmetically damaged HF-81 for free but I'm not sure if he's sending it and when so I'm playing the waiting game there. I have a spare trans out of a heathkit 6BQ5 amp with the same basic setup I may use or see if I can get a Hammond or something if the Donor Unit doesn't pan out. I hate waiting but you can't beat the price !!

Craig

[Tom Mobley]

Pat,

Thanks for clarifying that about the can cap. I was wondering what it actually was, basically just 3 big caps stacked up. I'm at work now till 7:30pm MST tonight, but I'll look at the underchassis space to see if there's room for the upgrade. It's kinda tight under there, and that 40 would be the same size as the 40's I already put in.

Would those disc caps on the multi-ganged function switch be considered to be "in the signal path"?

Tom

Jeez, I started this about 9:00am MST, just got it sent now. been a little busy today....Sheesh. What happened to my nice quiet Saturdays with an OC-6 net connection?