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Old 20th May 2010, 01:38 AM   #1
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Question Eico HF-81 modification opinion

Hi! I'm very new to the forum... in fact every forum... so please don't be to severe.
I just bought (for peanuts) an HF-81. It's my first tube amp.
Unit is in operating condition. I plugged it to my turntable and on a small speakers. Sound was OK but there is Hum in preamp stage (Phono Mic Tape). None at all in other stages( Aux1, Aux2...).

I consider to change :
1-All Caps and resistors in the preamp stage and PSupply
2- Supply filaments with DC (bridge + 12000mF + diodes for 6.3V
3- Disconnect neutral from case ground and fit 3 prongs cord
4- Shielding back of power trans

Any opinion or advice

Thanks... Robert
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Old 20th May 2010, 04:24 AM   #2
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Sounds like a good start, but I would think the caps and resistors in the main amp should also be replaced, not just preamp and PS.

Definitely disconnect neutral from case and fit proper 3-core power lead.

You might like to make the above changes and hear the results first, before going to the trouble of supplying the preamp heaters with DC -- who knows, you might not need to.
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Old 20th May 2010, 04:33 AM   #3
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Hi Robert --

Welcome to the group! Regarding your HF-81:

1. I certainly hope the neutral side of the line is not connected to the chassis of your amplifier! Possibly you're referring to the AC line cap to ground that Eico used? It originally provided a 50/50 chance at best as to which side of the line the chassis got referenced to -- but it is a safety hazzard in any event, and at the very least, it should be replaced with a safety rated unit. If the neutral side of the line is grounded in your unit, its a huge invitation to ground loops with other pieces of equipment. Using a 3 wire power cord will eliminate all safety concerns, although ground loops may still be a problem.

2. All Eicos use AC on the preamp heaters, which makes for numerous comments about hum and noise in these circuits. Certainly try adjusting the hum balance control first. If you do convert the preamp heaters to DC, that will surely quiet things down, but you will want to think that through very carefully. The power transformer in the HF-81 already runs quite warm with the original design. If you are planning to use the original 6.3 volt winding for the source of your DC supply, that will pull even more current from the transformer, which couldn't be good. You will probably need to consider an additional small transformer to provide the power you need for this conversion.

3. Also, pay attention to the cathode bypass caps in the phono preamp section. The RIAA eq is provided by NFB around the second stage only of that design (plate to grid FB), precisely so that the cathodes can be AC grounded. If the caps are bad, they will lift the cathodes from ground AC wise, and allow hum to creep into the circuit from the AC heaters. I would check these first before committing to a full blown DC conversion.

Good luck with your amp!

Dave
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Old 20th May 2010, 04:46 AM   #4
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Robert,

You can't rectify the filament winding. When a cap. I/P filter is employed, the RMS current rating of the winding needs to be 2X the DC draw. What you can do is to install Sovtek 12AX7LPSs in the phono position. The 'LPS is a genuine 7025 equivalent and has a hum bucking, spiral wound, heater. Between hum bucking heaters and correct use of the hum adjustment pot. (R5), you may be able to avoid fabricating a DC heater supply, for the phono section. Another noise reducing technique you can try is to bias the phono section heaters to about +70 VDC, by placing a resistive voltage divider across B+ and ground. R5's wiper is grounded in EICO's schematic. You would connect the wiper to the divider's tap and restore the AC connection to ground by connecting an electrolytic cap. between the wiper and ground.

Definitely replace every electrolytic cap. in the unit.

Safety requires removal of the cap. (C48) to ground associated with the on/off switch. "Vintage" units frequently have that bugger, which goes by the disparaging name of death cap(acitor).
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Old 20th May 2010, 03:43 PM   #5
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Thank's everybody for those comments.
I will surely change other caps and resistors if these first steps are successful.

Concerning neutral, the central tap of transfo is direct to chassis and one side of line trough cap. Connecting chassis to ground provoque sparks - that I don't like - when the lead touch ground... So disconnecting every neutral from chassis and grounding chassis seem appropriate and safer.

I tried adjusting hum balance but it's not enough. I should say to that filament winding of V2 is not runned as in construction diagram and may be to close from grid.
I did'nt think rectifying filament would be a big problem, at least less trouble than fitting a new tube.
There is 2 filament windings in the tranfo. One for diodes V11, V12 (about 2 A), and one for others tubes (4,7 A according to tubes specs). Adjusting current to these specs (with lower voltage if needed) should'nt affect transformer power consumption.. Am I wrong?
Should I consider running these 2 windigs together for all tubes?

One more question to Dave: what do you mean by NFB and FB? Bias resistor?
(Happy to know there is RIAA, it's not mentioned in specs and I was unable to "see" it in schematic)

I should finally say that I don't intend, for the moment, to make big expenses on that. Not until I'm convinced it worth it.

Thank you everybody once more and excuse these questions from a newbie.

Robert
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Old 20th May 2010, 05:48 PM   #6
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Robert if I understand you correctly you are referring to the transformer center tap which is actually on the high voltage secondary and needs to be connected to chassis ground in order for the amplifier to work.

The cap C48 absolutely needs to be removed and it is a good idea to install a 3 wire power cord. If you have a ground loop issue you may be able to use a ground loop breaker which is basically a pair anti-parallel diodes in parallel with a 5.1 ohm 10W resistor. Diodes should be something like the 1N5408 (1Kpiv/3A). The best place for this is between the main chassis safety ground and the amplifier internal ground scheme. This placement however will not work if everything is randomly grounded to the chassis which I believe may be the case. In that case you may place the ground loop breaker in series with the safety ground - note however that this is not nearly as safe or reliable as a direct chassis connection, but is much better than no safety ground at all. You do this at your own risk, and make sure you use components that can handle the power required in the event of a failure until the breaker in your panel trips off.

You could of course completely rebuild using the best modern practices and have both a safe and hum free amplifier, but it is really a lot of work. (I rarely do this.)

Eli's suggestion to use Sovtek 12AX7LPS is a great one, not only do they sound good but the spiral wound filament cancels magnetic fields around the filament and should help with the hum issue.
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Last edited by kevinkr; 20th May 2010 at 06:02 PM.
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Old 20th May 2010, 06:53 PM   #7
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Robert --

NFB is just a short way to refer to "Negative Feedback". Likewise, FB also refers to "Feedback", although without any designation as to whether it is negative or positive. However, once a discussion has established which type of feedback is being discussed, it is then common to just refer to feedback through out that particular discussion as simply FB.

Equalization for the RIAA response is typically created in one of two ways in almost all audio equipment of the Eico's era: Either passively, where the signal is passed through various resistive and capacitive networks before going on to the next stage of amplification, or actively, where the signal at some latter point in the circuit is taken and again altered by resistors and capacitors, but then reinserted back into an earlier portion of the amplifier circuits to alter the amplification characteristics and create the desired response. The second approach is what Eico used in the HF-81. In either case, the network that the signal passes through to create the desired equalization is called the equalization network, and in the HF-81 , it is used to form a negative feedback network (NFB) around the second stage of the phono preamplifiers to achieve the desired RIAA response. In the HF-81's case, the response is created by taking a portion of the signal from the output of the second stage, putting it through the proper EQ network, and then reinserting it into the grid of that same stage. Therefore, the feedback becomes negative since the signal at the plate of any tube is always opposite in phase to that at the grid of that tube. The EQ network components are usually located on the selector switch, as is the case with the HF-81. It is sometimes difficult to know which of these components are used and how they are connected to create the RIAA response, since many of them are often used to help create other response curves offered as well. In the HF-81, the RIAA response is created by the 2.2meg/2700pf and 90K/850pf networks. The feedback signal from the output of the second stage is first sent through the former network, then through the latter network, and then finally reinserted back into the grid of that same stage to create the appropriate response.

Dave
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Old 21st May 2010, 03:31 PM   #8
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Thank you
1) Sorry, I just understood Eli's message to replace 12AX7 in preamp with Solvtek... I first thought to add these in phono. That's why I reply it's big trouble to fit a new tube. As all tubes are actually originals Eico by Mullard, I would like to keep them, as far as possible.

2) Going back to my books, I understood the loss of power with DC conversion. So options are :
a) Keep AC filament and displace V2 wiring (not preffered)
b) DC heating with old transfo (fear of over load)
c) DC heating with new separate transf (Not enough place)
d) DC Heating, adding a small transfo parallel to old one

3) Concerning ground : As I can see there is 6 points where some elements is connected to chassis. Not much trouble to isolate them and connect them together with big wire. I'll remove C48, connect ground to chassis and install ground loop breaker between ground and neutral (center tap of transfo). Put switch on Hot lin too.

Geat advices from every one. Thank's to Dave for RIAA explanation, to Kevin for ground discussion and to Eli.

It's time to work on it. I will try to keep you informed..

Robert
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