Author Topic: Amplifier Restoration for the common man.  (Read 2328 times)

AlexanderMartin

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Amplifier Restoration for the common man.
« on: July 11, 2015, 11:36:05 PM »
So I'd like to preface that this is a "beta" of this large article that I'm writing up, and I freely encourage critiquing of my work so it can be of the finest assistance for the people of Antique Radio Forum when I post it there.If you have any ideas what I should add to the original post, feel free to speak your mind.

So....What is an amplifier?
An amplifier, or specifically what we are explaining is vacuum tube audio amplification, is a piece of equipment that takes a weak audio signal from a phonograph cartridge or other sources and makes it bigger. The main affliction with amplifier design is to design a well powered amplifier with very low harmonic distortion. Harmonic distortion and many other factors can determine the sound quality of an amplifier. These amplifiers usually use tubes such as 6k6's, 6l6's, 6v6's, and a large pallette of preamp tubes and other main amp tubes.

Ok but.....why restore an amplifier, even if it works?? Isn't that a waste of money??
In my humble opinion, the philosophy of "if it aint broke dun fix it" does not apply with electronics. Electronics are not the most stable things in the world, even amplifiers from 1957 brand new had the possibility of breaking, hence why most companies had repair center fliers. The vintage amplifier has three main dispositions, failing tubes, failing capacitors, and failing resistors. These three elements will turn your amp into a very pretty paperweight.

Ok....well let's start with the first, what happens with tubes?? How do I fix them?
Tubes in audio applications most commonly turn weak, and that's simply with use. Tubes can also turn gassy, or even be dead shorted that can harm your transformers or even other (expensive) tubes. In order to check this, get a tube tester, preferably calibrated and restored. I use a Hickok 533A, and it is a heavenly tube tester and very, very precise. It is ESSENTIAL to check your tubes and make sure they are in good shape. If your tubes are weak or gassy, or even red plating, it can really harm your fidelity. Simply buy replacement tubes, I prefer getting NOS or tested strong. If you want to get picky be warned, the realm of high quality vacuum tubes is a pricey one, so heed that warning with great caution.

Ok, i'll get a tube tester.....but what's with capacitors? I hear about them all the time!
Capacitors are infamous to, well, frankly, turning into terribly pieces of shit that will effectively turn your tube amp into a brick. So, get yourself a proper soldering iron like a Weller 8100, pencil irons are absolutely terrible for this application unless you're working on circuit board equipment. Get good solder and if you're really determined get a good solder sucker like a Hakko FR300 (it works incredibly well for point-to-point wiring!!) I highly recommend getting capacitors from Mouser, particularly Panasonic ECQ series film caps, they will last forever and are only 30c a piece. For electrolytics, go with some good quality Nichicon PW series electrolytics. So wire them in where the old ones were, make sure your joints aren't cold, and you should be good.

Alright, ill look up soldering, but what about resistors? I never hear about them being bad?
Resistors are a mixed bag, they all depend on your standards. If you are incredibly picky and literally want the thing to be factory fresh, I say replace every single resistor in the amp. They in particular drift bad, way beyond their normal tolerance which if bad enough screw the circuit up real good. If you aren't too picky, you should be fine unless they go open (which rarely happens.) I get Vishay 2 watt film resistors, they are way overkill at 5% tolerance, they are way better then anything they ever had during that time. And at 10c a piece, you really can't go wrong. Either way, most people simply check them with a volt meter. If you want I recommend Fluke multimeters, they are fantastic for what they are. Some even check capacitance and diodes!

Ok, I have an amp that works, it just hums, what's the deal?
This is a sign of failing capacitors, either in the coupling section or in the electrolytics in the power supply. Usually it is the big electrolytics, which are large aluminum cylinders they mount on top of the chassis. They are quite bold and the values are usually imprinted on them. Do NOT half ass the values, the voltage is CRITICAL and needs to be either exact or over the original. Capacitance (uF) can be a bit higher (say a 50uF can be a 75uF) but never go beyond 25 or so. Mounting them is your choice, I mount terminal strips using screws and nuts already in the chassis if possible or rivet them in.

So here's a challenge to all of you, post common troubleshooting questions here and i will go as thorough as I can to respond and aid anyone, I want a good "one stop shop" solution for newbies so that we can all enjoy higher quality restoration. If you yourself feel like writing something up, please do and I will humbly include it and give credit respectively! Thank you and let's really turn this into a good hearted cooperative effort!

TC Chris

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Re: Amplifier Restoration for the common man.
« Reply #1 on: July 13, 2015, 08:35:10 PM »
Since this is aimed at newbies, and deals with tube voltages, include a warning about high voltage AC and DC and about using due caution when working around the equipment.  It's important not to frighten people, since for some electricity is a mysterious killer to start with, but making people respect high voltages is equally important.

And I don't recall seeing  a reference to polarity for electrolytic capacitors, either. 

You mentioned harmonic distortion so it might be helpful to have a brief discussion of what it is--adding multiples of the frequency you're trying to reproduce, so what comes out isn't faithful to what goes in ("fidelity").

How about explaining what  a preamp does, and what the power amp does that is different?

Chris Campbell

AlexanderMartin

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Re: Amplifier Restoration for the common man.
« Reply #2 on: July 13, 2015, 09:45:07 PM »
Excellent things chris, appreciate the help, I will tack these along appropriately:

Now, before you ever work on a tube amplifier or even electronics in general, be aware that there are lethal voltages contained within the unit itself. In order to keep your safety, never work on these units powered on and make sure to discharge the electrolytics by shorting out the positive and negative terminals on the capacitor.

But, is there a specific way to wire in capacitors?
Yes, and no. For regular capacitors that I mentioned (such as .047, .01, ect) they do not care how they are wired as modern capacitors are not polarized. However, the larger electrolytic capacitors are in fact polarized and need to be wired in correctly. A capacitors has a positive and negative end, for modern caps there is a stripe on the side of the caps that shows which side is negative. The lead with no stripe is the positive. Capacitors vary in wiring, most old caps refer to the negative as "COM" and have the negative go directly to the chassis ground.

You mentioned harmonic distortion, what is that?
Harmonic distortion tells  you how faithful the amplification of the audio signal is, say for instance, an amplifier with .005% harmonic distortion will sound more accurate then an amplifier with .5% distortion. Total Harmonic Distortion (or THD) is a good reference to the fidelity of an amplifier.

Ok, i hear about pre amp and main amp tubes, what do those do? how are they different?
Pre amp tubes are tubes that amplify the signal enough so that it is loud enough and useable for the main amplifiers to boost the signal. Common preamps are 12AU7's, 12AX7's, 6SN7, and a few other oddballs. Main amp tubes would be tubes like 6V6's, 6L6's, 6K6's, EL84'S, ect. Pre amp tubes are found to be the first step in amplification for the signal in any schematic for an amplifier. 

Thank you for the suggestions, Chris. Hope we can keep the suggestions coming.

624Magnificent

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Re: Amplifier Restoration for the common man.
« Reply #3 on: July 14, 2015, 08:16:11 AM »
good stuff!!
Tom

Consoleman

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Re: Amplifier Restoration for the common man.
« Reply #4 on: July 14, 2015, 08:33:26 AM »
On the safety subject, at times one needs to check voltages with the unit powered on. When doing this always have one hand in your back pocket. Having high voltage go into one hand with the other hand on the chassis can be lethal, unless someone has a defibrillator handy.
Mark

624Magnificent

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Re: Amplifier Restoration for the common man.
« Reply #5 on: July 14, 2015, 09:55:06 AM »
I worked summers in college at a paper mill. They taught every new employee the same thing. Don't touch anything electrical with two hands and if you throw a switch look away from the panel and keep one hand in your pocket. As it turned out my father, who worked at the same mill, one day turned to walk away from a panel next to his machine and the panel exploded. I can't remember if he had just pushed a button or not but he was lucky to not get seriously injured.
Tom

Duce67US

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Re: Amplifier Restoration for the common man.
« Reply #6 on: July 15, 2015, 07:09:03 AM »
I'm pretty sure you might want to mention Schematics / Service Documents / Datasheets.  What they are, where to get a copy, etc. 
Often there are some really helpful information included - outside of the actual schematic, like equivalent replacement parts, or component value modifications made by the factory through a particular series.   

While we are on the topic of documentation, if you read the RCA datasheet for the 5U4GB, states that the Filter-Input Capacitor should be 40uf.  There is a note about increasing the effective plate-supply impedance when a higher value is used. 
I only mention that because of your earlier post about choosing values of capacitance when replacing the Electrolytic Can. 

To add to the safety part,  it might be good to mention replacing the "death cap" with a modern equivalent,  also how to add a fuse to the circuit.

Motorola Minion

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Re: Amplifier Restoration for the common man.
« Reply #7 on: July 22, 2015, 03:50:48 PM »
We all want our restored tube (or transistor) equipment to last another 50 years even after its donated to a museum, etc. One item that costs a ton of money should it fail is the power transformer.
I don't know about your line voltage but mine is not 110, 115 or 120, it's 124!!!!
This is sort of like running and old car on new, ethanol-tainted gas. Its sure to shorten its life.

A good way to take it easy on our old "iron" and tubes is to add an inrush current limiter (aka thermistor) in series with the 120 volt mains along with a fuse/circuit breaker if not already so equipped.

A GE type CL-90 from Allied is $2 and has a resistance of 120 ohms when cold, decreasing over a few seconds to 1.18 ohms at 2 amps load. So when you switch your amp on, current increases gradually or "soft starts". ;)
Unless you have a very large power transformer, a 2 amp fuse is plenty. If one is not there, add one.

A type "Y2" safety capacitor is also very useful to add in our fine equipment.   See http://www.justradios.com/safetytips.html to step you through this.
Suffice to say, filtering out line noise and keeping the chassis from becoming "hot" with noise and hash will help your fidelity. Additionally, beware commercial type compact fluorescent and T8 lighting ballasts. The FCC requirements are much less stringent for generating line harmonics (as opposed to consumer/residential fluorescents) that will obliterate reception of your favourite FM station, not the ones you don't listen to. ::)
Tubes - Magical - Tubes

Dave

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Re: Amplifier Restoration for the common man.
« Reply #8 on: October 11, 2015, 07:46:29 PM »
For capacitor testing, I like to find an older capacitor tester that will test them at the rated working voltage. A multimeter will tell you the capacitance value, but won't reveal a leaky capacitor. The nice testers from HeathKit, etc can be found at swap meets for good prices and while they will need to be recapped, etc, they are well worth it.
Firebottles rule!!!

Imperial8

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Re: Amplifier Restoration for the common man.
« Reply #9 on: November 21, 2015, 01:43:24 PM »
*
Thank you to everyone who contributes to the information on this subject.  All of it will be very helpful to the Newbies!

This is a great forum for old high fidelity consoles......

G.

AlexanderMartin

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Re: Amplifier Restoration for the common man.
« Reply #10 on: November 27, 2015, 08:22:41 PM »
For capacitor testing, I like to find an older capacitor tester that will test them at the rated working voltage. A multimeter will tell you the capacitance value, but won't reveal a leaky capacitor. The nice testers from HeathKit, etc can be found at swap meets for good prices and while they will need to be recapped, etc, they are well worth it.
Agree totally, the biggest deal with those is the ability to test the parts at it's operational voltage. I'll wrap this thread up into a bundle sometime and post it as a complete PDF document or as an ARF thread. Thank you all for contributing.