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Old 15th June 2013, 02:33 AM   #1
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Default New to Tube amps, heathkit question

OK I am brand new here and just starting out so I will apologize if I am in the wrong area for this post. I just received an old Heathkit amplifier from a friend of mine that scraps metal. I told him I was looking for and old tube amp to tinker with and 3 months later he drops a heathkit ea-2 on me for free. It is missing a tube but other wise looks pretty good. I want to get started on getting the amp working and read a few articles about replacing electrolytic caps and coupling caps. I have NOT attempted to plug the unit in and figure it would not work anyway being the EZ81 tube is missing. I have never worked on a tube amp before and have repaired some solid state amps before. I have limited knowledge of electronics. I know what a resistor, capacitor, transistor, etc look like and have some knowledge of what they do but lack full understanding of what makes the entire circuit work. I got a schematic online and believe I have identified all the caps the need replacing. While I had all the tubes out I decided to run a few quick tests and already think I have a problem. The resistance between the Blue/yellow and Green/Yellow measures 83.2 ohms and the resistance between the Green and Blue measures 129.1. From what I see these are connected to pins 7 and 9 of each EL84 tube in a push-pull. I would assume the the resistance should be the same between each because if not a sine wave would be unbalanced leaving the top half of the wave with less or more amplitude then the bottom half. So I have a few questions. 1) is my assumption correct that the output transformer needs to be replaced? 2) if so where would I purchase a suitable output transformer? last question and probably the most important. Is this amp worth the time and effort? I don't mind the time and expense as I am learning but would rather spend the time and effort on something that will give me years of listening pleasure would this be classified as a good HIFI amp especially if I can get another one and run them for stereo. I see a lot of people talk about the Heathkit Williamson and open ended amps. There is a lot going on for a newbie like me any help suggestions would be appreciated
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Old 15th June 2013, 03:19 AM   #2
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I don't have the heathkit schematic, but in most 6BQ5/EL84 circuits the plates (pin 7) each go to an output transformer winding, and the center tap of the primary of the output transformer goes to one of the high voltage pins of the big B+ capacitor. You may have to lift the wire off the capacitor to get a good resistance reading, as these old capacitors are often shorted, or measure partially shorted until "reformed". The first step of "reforming" an electrolytic capacitor is put the plus of the ohms scale on the pin, the minus on the chassis, and run it until the resistance is infinity.
a good source of transformers for EL84/6BQ5 is the ST35 replacement output transformer sold by triodeelectronics.com in Illinois. Or maybe SCA35, whatever. The old dynaco 6BQ5 tube power amp.
Pins 9 of the El84/6BQ5 should go through a resistor (each tube) to a lower voltage tap on the B+ capacitor, going by the Hammond AO39 schematic anyway.
any metalwork will get you started on a tube amp. Heathkits had a little cheaper input connectors and volume pots than a dynaco, but those probably have to be replaced anyway due to corrosion. Unless your input pot is a centralab or CDE, then start by just cleaning it.
heathkit output transformers weren't as good as dynaco's either IMHO, but then, if you're going to replace it, go for the 1960's standard, right off. Dynaco bought premium plate coupler caps, most of mine are still good. I don't know what grade coupler cap heathkit used but suspect they were cheaper. Guitar amp people always replace all the paper dielectric plate coupler caps on their old amps, those tended to be really cheap. Measure your resistors before powering up, too, a lot of high value ones (over 3 k) might have gotten water soaked and increased in value. I use metal film resistors these days, 2 and 3 watt ones for the voltage rating.
EL84 amps are about 13 W/channel, so to really enjoy it you'll need a very efficient speaker. My SP2-XT's are 101 db@1 W @1 m, which is about as sensitive as they come.
With point to point wiring, and a drill to mount a bigger power transformer and punch 7/8" holes for the octal sockets, you could eventually upgrade to EL34/6CA7' s some day and go for 35 W/ch. classicvalve.ca has got some driver boards you could mount on standoffs instead of punching a dyna sized hole.
But making this thing work is great training. Don't use two hands to measure tube voltages, current across your heart over 24 v can stop it. Use a clip lead on the meter negative and one hand. don't work tubes alone. Don't work tubes distracted by television or a cell phone. use a lot of light. Read the safety sticky at the top about measuring discharging electrolytic capacitors to a safe voltage before touching any metal under the deck.
Have fun.
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Last edited by indianajo; 15th June 2013 at 03:32 AM.
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Old 15th June 2013, 03:38 AM   #3
Enzo is offline Enzo  United States
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First off, the voltages in a tube amp can KILL YOU, so please be very careful, and never assume a cap is discharged.

Thee transformer works on turns ratios, not resistance. There are two ways to wind a center tapped transformer. One is to start with wire, and wrap half the turns around the form, then attach a tap wire, then wind the remaining half of the turns. This results in one side of the transformer wound around th small core, while the other side of the winding is wrapped around the first half. The result of that is that the second half wire has to go farther to make one turn than the inner windings. Wire has resistance by the foot, so if there is 50% more wire for a number of turns, the resistance will also be 50% higher. (I made up the percents.)

The other way is to use two pieces of wire - one for each half - and wind them at the same time. When done, we join two of the ends to make a center tap. That results in both halves making the same path around the core, and the winding resistance will be even.

Both methods work, because as I said, transformers work on turns ratios. The first method is cheaper to make.

SO, no, I don't think your transformer is bad, and I think you do not have to replace it.

Generally, when transformers fail, it is by shorting a couple turns together or shorting to frame. And of course they can also arc. If you have two adjacent turns short together, your meter would never detect it. A short to frame you can measure. An arc only occurs at high voltage, so your meter cannot detect that either. What doesn't happen is transformers wholesale changing their wire resistance. I am sure there are exceptions to any rule of thumb, but like the wise man says, "When you see hoof prints in the dirt, think horses, not zebras."

When the circuit is not powered, the center tap is really the only part connected to anything. The ends of the winding connect to tube plates, which are just pieces of metal sitting in a vacuum. So to measure the primary winding resistance and resistance from ends to center tap, you really don't need to disconnect anything.

Worth? Well, the thing cost you nothing, right? SO if you screw it up, at least you didn't spend a couple hundred bucks on a kit. I doubt your transformers are an issue. If I wind up wrong, well, sorry, but I don't think so. That means you are buying caps and resistors and maybe tubes. Mostly inexpensive stuff. The experience translates, amps are amps. Larger tubes and higher voltages and currents don't change the basic underlying circuit structure. YOu can learn a lot about amps working on this thing. And they are nice enough little amps.
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Old 15th June 2013, 03:42 AM   #4
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Welcome to the board with all of us crazies. What you have is a nice little audio amplifier that's good for about 10 to 12 watts and a good place to start learning. I will say that the output transformer is most likely OK. What you are measuring is DC resistance of the primary that gets physically larger as more wire is added. This means that it requires a longer length of wire to make the same number of turns as the "coil build" gets larger. This is normal.

Now it could be that the transformer is still defective, but you will not really know this until you get it going or make further tests. Having continuity between the five primary wires is good. There should be no measurable resistance between the primary and secondary and/or the steel core. That would mean a short.

Many on this forum will say the replace all the filter capacitors as a matter of course. (I have a slightly different opinion regarding this, but after fifty plus years of repairing electronics what the h*ll do I know.) This includes all electrolytic capacitors in general. In your amp these are C1-2 & 3 (the big metal can) C4, C32 (if there) & perhaps even C30. C16 & 19 should be checked for leakage once you get it up and running.

Is restoring this amp worth it? It's really up to you. If the amp is clean and you like it, then yea sure because it was free.
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Last edited by HollowState; 15th June 2013 at 03:52 AM.
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Old 15th June 2013, 08:59 AM   #5
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That the 6CA4/EZ81 rectifier is missing is not a surprise. The type is highly desirable. The schematic H/S uploaded is interesting, as it shows a 6BW4 being suitable. Unless you source a 2nd EA-2 specimen, investing the minimum amount of money in a learner project seems correct. A look at Jim McShane's site shows a new 6CA4/EZ81 costing $19.50 and a new 6BW4 costing $5.50. Buy a 6BW4!

While it may be possible to reform the "ancient" electrolytic caps., it is a fact that modern 'lytics are superior. Also, 'lytics literally dry out over time. The "knee jerk" recommendation to replace all 'lytics is made (IMO) for good reason.
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Old 15th June 2013, 08:13 PM   #6
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Thank you all for the great information. I do not have any measurement between any primaries to any secondary. I also did not have any readings from any of the taps to ground so and I do have different readings between all the wires and center tap. I do understand the voltages present and will discharge all with a resistor jumped between each before working on the amp. I have 4 children and do plan to stay alive. I also heard that jumping a 1k resistor from ground to the pin 1 on the 12AX7 socket for about 2 minutes wold thoroughly bleed all Caps. With that said I think I am going to dive in. I just ordered another EA-2 from ebay. I paid 87 for it plus shipping for a total investment of 103.00 for 2 amps to start with. I downloaded a few book from tubebooks.org to start learning how the circuits work. It will take a while but I will get there. I have about 30 years of soldering experience as my father and I would build healthkit stuff when I was younger. Together we built a 5 inch black and white TV, a solid State stereo and a few resistor and capacitor substitution boxes. Currently I repair all kinds of stuff for family. I have more of a mechanical aptitude than electronic but have tons of pulled stuff from old PC power supplies, printer power supplies etc. I gut everything. I have a Fluke meter with capacitance ability. I am thinking of gutting both amps. Checking all resistors for proper values recapping with some sprague orange drops for coupling and substituting sprauge atoms for the electrolytics. I do have a quick qustion. the 4 capacities listed on the can are 450 at 30mf, 400 at 20mf, 350 at 20 and 300 at 20. Can I replace all with 450 33mf? Does the voltage rating change anything in the circuit? I would think that you could always substitute a higher voltage without any issue but that has always worked with low voltage DC circuitry. Also i read that is is OK to go higher on the MFD rating as long as you don't exceed an 80% rise of the recommended value. Are these safe assumptions. Also I see some talk abut the russian 40-y PIO caps for coupling. Any thoughts as to recap with sprague orange drops or the russian PIO
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Old 15th June 2013, 09:29 PM   #7
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With vacuum rectifiers, there are published limits on the value of the 1st filter capacitor. Exceed the published limit and you risk arcing at turn on. Turn on arcing can destroy a vacuum rectifier. The 6CA4/EZ81 data sheet is here and the 6BW4 data sheet is there.

Acquiring a 2nd specimen means you are deadly serious about tubed stereo music reproduction. IMO, Heath's circuitry has serious flaws. For instance, the tuner I/P and the, currently useless, piezoelectric cart. I/P are substantially attenuated before being fed into the 2nd gain block of the mag. phono preamp. Are you planning on a simple overhaul or is upgrading of circuitry under consideration? Is there somebody local who can mentor you, along with this forum's members?

Both 716P series "Orange Drops" and Soviet surplus PIO parts are quite satisfactory. Mixing the 2 "flavors" up could easily yield the best end result.
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Old 15th June 2013, 10:15 PM   #8
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Quote:
Originally Posted by uncledude View Post
I do have a quick qustion. the 4 capacities listed on the can are 450 at 30mf, 400 at 20mf, 350 at 20 and 300 at 20. Can I replace all with 450 33mf?
Yes you can.

Quote:
Does the voltage rating change anything in the circuit?
In this case no, not at all. Decending voltage ratings in electrolytic capacitors back in the day was more of a size/cost issue. Today that's almost moot.

Quote:
Also i read that is is OK to go higher on the MFD rating as long as you don't exceed an 80% rise of the recommended value. Are these safe assumptions.
The capacitor directly off the rectifier cathode is the critical one concerning value size. Leave this value close to what it is to prevent start up stress on the rectifier tube. Other values down the line, so to speak, can be doubled or more.

Quote:
Also I see some talk abut the russian 40-y PIO caps for coupling. Any thoughts as to recap with sprague orange drops or the russian PIO
PIO (paper-in-oil) capacitors use some type of paper as the insulating dielectric that seperates the conductive material inside. Paper is a relatively poor dielectric and produces a capacitor that is "slower" then the more modern plastic dielectrics like polyester, polycarbonate, polypropylene & teflon. This means that they are less able to pass high frequencies easily. The result is a sound characteristic that is slightly different from the newer plastics. One could perhaps call it "tube sound" or a more mellow or vintage sound. It is also subjective to the individual.

Replacing PIO's with plastics in the un-equalized audio stages usually does no harm, and can even be an improvement sonically. (again subjectively) But care must be exercised when replacing them in the equalized phone stage. For you that's C6, C7 & C8. Try to leave these alone if possible because changes here can upset the playback RIAA curve. The values chosen here could be based on the dielectrics used at the time of manufacture. This is a fine point in audio, and Heath may not have done their design work that carefully in this area. Still, try not to mess with them.
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Old 16th June 2013, 12:24 AM   #9
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Sorry about the bad info on the unbalanced output transformer turns resistance. The ones I have measured have had equal resistance, but they weren't heathkit.
I replaced .20 uf paper caps with .22 polyester caps in my 1961 built PAS2 preamp, and ended up with a circuit that was too trebly; I have to turn the treble cut control down. So maybe paper caps would be a better choice. Maybe a .018 uf cap parallel to a .0022 cap would solve the problem,too but it is not my highest priority project now.
Modern metal film resistors (over 100kohm) did reduce the hiss in the PAS2 some. None of my 1961 resistors were out of tolerance. the cap that was causing my channel inbalance, had a big burn in the wax case carefully hidden on the bottom where I didn't see it until I replaced the tenth capacitor.
Note unwashed flux on kit builds can cause pops and shorts in the 60th year. I washed & washed, finally had to look at the circuit board in the dark to see the pop. It was shorting where I didn't work, ie the kit builder in 1961 left flux on the board. I replaced two 12ax7 tubes for popping in 1971, probably flux was the real problem. DI water is the solution, even 60 years later.
That is a laugh about nearby mentors. Maybe in Philadelphia, certainly not anywhere I have lived. You could join the ham radio club and listen to people gas endlessly about CQ's and 2 meter and DF's and all those stories of the congenitally motor mouthed ham fraternity, or you could work alone. Just the sound of ham radios offended my ears; my Father's Foley's FM radio actually was the first radio that had a bit of decent sound. I got a bit of education from the dynaco build manuals, a few tips from the MacIntosh salesman at their "clinic" at a hifi store. The RCA Radiotron handbook at the library was pathologically (radiologically?) interested in radio, oscillators all that stuff. Hurrah, the internet.
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Last edited by indianajo; 16th June 2013 at 12:37 AM.
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Old 17th June 2013, 12:05 AM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by uncledude View Post
OK I am brand new here and just starting out so I will apologize if I am in the wrong area for this post. I just received an old Heathkit amplifier from a friend of mine that scraps metal. I told him I was looking for and old tube amp to tinker with and 3 months later he drops a heathkit ea-2 on me for free. It is missing a tube but other wise looks pretty good. I want to get started on getting the amp working and read a few articles about replacing electrolytic caps and coupling caps. I have NOT attempted to plug the unit in and figure it would not work anyway being the EZ81 tube ....

You can replace the EZ81 with a pair of 10 cent 1n4007 diodes. Solder them across the tube socket. This will raise the voltage up a little but this is mostly a good thing, especilly if you replace al the caps

Don't worry about the unbalanced resistance in the transformer. It is not broken it is just a low-cost transformer. That is the way it's made.

You should not expect low 0.1% distortion form this thing. But it might be under 10% and not sound bad

Build a "dim bulb tester". That is just a light bulb base in series with the AC mains. The bulb is a resisoer that limits current. Run the amp like that at forest here for more detail: Powering Up Your Radio Safely With a Dim-bulb Tester
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