Vintage audio pages

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2. To tube or not to tube!A bit of history
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My audio pages!

Tips and how-to section:

In this section I have included some tips on how to troubleshoot and fix this old tube hifi equipment.

I'm no engineer or anything, just an average guy with a life long interrest in tube electronics. I

hope you find this page helpfull!

howto

howto

Problems I have encountered over the years

These are some of the things that I have frequently had to replace in my components. Some may seem

obvious while others may seem a bit odd. Anyway, it is always a new experience everytime you open

up a new treasure (or bag of worms!)

One of the most important things to remember when working on vacuum tube audio is that

the voltages that tubes run at are very lethal. If you touch certain things you will not have a second chance!

Tube amp inspection and cleaning

The first thing I do with any tube amp I purchase is to remove the covers and inspect it. What I look for are obvious signs of

component failure like badly burned resistors and capacitors that are leaking powder or fluids. I also look to see if wax or tar is oozing out of any of the transformers. Next, I will thouroughly clean the chassis by brushing and vacuuming it. I will remove all the tubes, making sure I know where they go so I don't end up with a pile of tubes and no legend telling me which one came from what socket. This is especially important in recievers since there are many different types of tubes in a reciever yet they will all tend to use the same type of tube socket. Nothing is more irritating then to realize that half the tubes have their numbers wore off, or to find out that the tube placement sticker is missing and the reciever doesn't have the tube numbers stamped in the chassis by the socket. Next I will flush all the controls and switches with contact cleaner followed by a spray of DeOxit. This procedure will clean the crud out of the component, and also leave a protective deoxidizing coating on the contacts in the component. I will also do this to the tube sockets. I like to wipe up any excess spray that ends up on the chassis with a rag and q-tips. While I have the tubes out, I will clean the chassis, and polish it with Brasso or polishing compound. This just makes the amp or reciever look nice. It is not necessary for the amp or reciever to actually function.

Initial Power up

this can be the most stressful time in the restoration process. What the heck is going to happen when

I put the juice to this baby? Well, to help avoid catastrophe, I have a few suggestions.

First, what I always do is connect an ohm meter across the amps power cord, turn the power switch on and see how many ohms I get. If it is above 2 ohms, things are cool and I will assume that the power transformer is not shorted. Next I remove the

tubes and plug the unit into my Variac and bring the power up while monitoring the AC voltages coming out of the

power transformer. I check the filament, bias and high voltage values to make sure everything is there and within specification.

If the amp passes this test I will install all the tubes and again connect it to the Variac. I will start off at about 40 volts on the variac and just let the amp run till things start to glow, and I begin to get high voltage in the power supply. At this point, I look for any unusual voltage drops in the power supply circuit. Such voltage drops could indicate a shorted capacitor or an open resistor. Next I will crank the variac to around 55 volts. At this point, I will usually be able to hear a little hiss and hum in the speaker, and the amp will also, if everything is working, pass a signal. It may not sound too great, but it should be there. It was at this stage that my first Scott 299C exhibited the leaky capacitor phenomena. When I pushed the variac to 80 volts, the B+ did not increase and the first filter capacitor began to get noticeably hot. This amp actually worked as long as I kept it's line voltage below 60 volts! Anyway, by using the variac, I had the time to safely ascertain that I did have a leaky capacitor. You can make a makeshift variac of sorts by wiring a light socket in series with the amp's power cord and the wall socket. You can use different wattage bulbs to change the voltage seen at the amp's power cord. If you are really into collecting and restoring antique radios and vintage audio you owe it to yourself to purchase a variac. Used, I have seen them as cheap as $20.00 and new they range from $80.00 - over $200. depending on their size. They are reliable too, so don't be afraid of buying an old one. My variac is from the 40's! Yea, it may look really retro a la Frankenstein, but it works great!

Now, after the amp has either successfully run at 60 - 80 volts for a while, or you have replaced whatever components

were causing it not to work at 60 - 80 volts it is time to crank the variac up to 100 volts and plug in a turntable and tuner. Now, switch the amp to tuner and see if both channels work and sound clear using the tuner as a source. Let her rip for a while paying special attention to any abnormal distortions of the sound, or any unusual noises like squeals, pops and hisses. Now turn the volume control down and see if there is any hum. Sometimes an amp will be quiet for the first several minutes of operation, then it will start to hum. This can be indicative of a power supply capacitor problem, weak or gassy tubes, and improperly balanced and biased tubes. Some amps have a hum balance control that can cause the problem as well. Next, give the phono section a check out by selecting the turntable and playing an LP. If everything is all right, you should hear a nice clean crisp signal coming from both speakers. I don't care what anyone says, tube amps from the 'golden era' of hi-fi (1955-1965) have the best sounding phono preamp sections. So, if you have been listening to those old LP's on a solid state amp (that is if you hadn't become so disgusted with the sound you tossed them and the turntable!) you won't believe how good they will sound through a tube amp.

When I finally got my first 299C going, I plugged a Techniques SL3300 turntable into it, plugged it into a pair of Mirage 190is speakers and felt my jaw drop at the sound. I couldn't believe it! This thing sounded better playing an LP then new solid state gear does playing CD's. You must bear in mind that my LP's are, for the most part, in quite good shape. If you have LP's that have been played extensively on inexpensive 'close-n-play' quality turntables then they are probably trashed and nothing is going to make them sound great.

Well, at this point, you are ready to just plug the amp into the wall socket. It is fully functional, but not guaranteed reliable.

Any amp that is 40+ years old can and will have certain components that are all but guaranteed to fail. In the sections that follow I outline what components I have seen fail frequently. I also make suggestions for component replacement so as to make the vintage hi-fi amp or receiver a reliable and enjoyable piece of equipment to own.

There is just something satisfying about restoring and being able to use an elegant piece of true hi-fi history.

Vintage hi-fi is not for everyone, but you owe it to yourself to at least give it a listen. Many people become converts after listening to a friends system. Everyone that hears one of my Scott systems, or my Dyna and Fisher systems is amazed. They just cannot believe that such great sound can come out of equipment that is so old. Yep, like the guitarist trying to play rock 'n roll on a solid state combo amp. He just doesn't get it. The solid state amp just doesn't have the 'feel', the 'touch'.

Power supply capacitors

or the big bang theory of electronics

These are usually in the form of a cylindrical can and usually house 2 or more capacitors

that utilize a common ground. Anytime an amp or tuner exhibits a higher

then normal amount of AC hum, these babies are suspect. I find that the first one to go is usually

the one that is connected to the rectifier tube, or the first filter stage. Also, any capacitor near a source

of heat will tend to die early. A capacitance checker is not always a reliable way to find out if

a capacitor is leaking since many do not check at as high a voltage as the amp runs.

In one of my Scott 299C's, this capacitor checked fine on a capacitance meter, and showed no short

when using an ohm meter. When I powered up the amp using a variac. I noticed that the capacitor

would allow the voltage to rise to 265 volts, then, when I cranked the variac up a little higher, the

B+ voltage stopped climbing and the 5AR4 rectifier started to glow red! Another thing,

the capacitor got noticeably hot. Well, I changed the capacitor and got a B+ of 406v.

Much better! I figured that if one of the power supply capacitors had failed, they would all soon

follow suit so I changed them all. This amp is almost 40 years old so I figured, why take

chances? Coupling capacitors can also pose a problem in vintage amps and receivers.

These are the capacitors that couple the plate of the preceding stage's tube to the grid of

the next stage's tube. The most common problem associated with these capacitors is DC voltage

leak. What this does is change the bias of the tube. If a grid coupling capacitor

connecting the phase splitter to the grid of an output tube is leaking DC, it will cause the

output tube to run hot, and fail. In many cases, the tube's plate will start to glow red, the amp will sound distorted,

loose power and eventually blow it's fuse. The only problem is that this all usually occurs

kind of fast, and if you do not catch it quickly, your output tubes will be history.

For reliability sake, I like to replace all the coupling capacitors in a vintage amp.

I prefer metalized polypropylene. They are relatively inexpensive and sound much better

then run-of-the-mill paper capacitors.

Another capacitor that one should look at is the cathode bypass capacitor. If your amp

is a cathode bias type, the capacitor that is connected from the tube's cathode to ground

can either open or short. If it opens, chances are you will hardly notice, but if it shorts, you have a

big problem. What will happen is that you will create a bias condition where the tube is

running very hot. This will react in exactly the same way as when a fixed bias amp has a shorted

coupling capacitor. Needless to say, replace the cathode bias capacitor and save yourself future headaches.

Resistors

You would not believe how many tubes I see replaced needlessly because a do it

yourselfer thought that those pops and crackles in his amp were the result of

a faulty tube. In many cases it is actually the plate resistor that is at fault.

These resistors will probably set you back a whopping $.25 - .50 each!

A heck of a lot cheaper then any tube you might want to replace. In my Dyna PAS2,

I always had this hissing and crackling sound emanating from the left channel. As

a lot of people do, I went off to the store to pick up a couple 12AX7's and proceeded to

replace the tubes in the left channel. Well, it seemed better, but the objectionable sounds were still there.

When the preamp was on for a while, it quieted down, but even then, you heard the occasional

pop and snap. Anyway, I noticed, while using my meter to test the plate voltages on the 12AX7's,

that one of the plates had voltage that would erratically go up and down. I looked at the circuit diagram and decided that the plate resistor was the most obvious suspect. I changed the resistor and voila! no more problem!

I find that carbon resistors do a couple nasty things. First, they like to change value as they age.

You can check this by using an ohm meter and comparing the reading to the stated value of the resistor. My

rule of thumb is that if the resistor drifts more then 5% of it's original value, it should be replaced.

Another problem is that carbon resistors can develop thermal irregularities. By this I mean

that a resistor may change value with temperature, and may develop instability as well. It

may work fine till it is warm then completely open! Needless to say, carbon film resistors can

create some real headaches with respect to noises and the like in amps and receivers.

One thing more. Some circuits require a high degree of balance in order to function properly.

Such a circuit is the phase splitter. I try to match all the resistors in the phase splitter circuit.

A lot of great amps can end up sounding awful simply because their phase splitter is not balanced.

Also, the resistors connecting the bias voltage to the output tubes must be matched. Uneven

biasing of output tubes can also make an amp sound like crap.

A word of caution.

Resistors can and do get hot! especially the large power resistors. This is normal

so be careful not to get burned!

Tubes

There are essentially three types of tubes being sold today. NOS (new old stock), New manufacture, and good tested used.

Nos are the most expensive and in my opinion, the best. New Manufacture are good as well, but I find that the quality of a lot, but not all, new manufacture tubes isn't as good as most of the NOS. Also, I have experienced longer tube life with NOS. Another option is good used tubes. But, be careful here, Most people who sell used good tubes just plop them

in a tube checker, and if the checker says the tube is good, they sell it as good used. The only foolproof way to determine if an audio tube will perform good is to use it in an actual amp circuit. You would not believe how many tubes have checked good on my tube checker but sounded like crap in an amp. A tube checker cannot tell you if the tube is noisy or has bad microphonics.It also cannot tell you if the tube is going to sound lifeless in your particular amp.The only tubes I buy as good used are ones like 7591's which are very expensive and can only be purchased as NOS.

The great thing about a stereo amp or receiver is that if you suspect a tube problem, you can take the tubes and swap them from channel to channel to see if the problem stays in the same channel, or switches channels when you swap the tubes. This technique has saved me a lot of unnecessary tube changes.

Anyway, There are several ways a tube can deteriorate and still function. But suffice it to say that output tubes

will need to be changed once and a while. You can prolong their life by properly biasing them or even biasing them a little cooler then normal (higher negative grid bias). But, be careful, if you run them too cool, the amp may not sound too good.

In my Scott 299C, the actual voltage on the grids is -20.3 volts when they are biased correctly. I find that I can

tweek the bias up to -22 volts and the amp still sounds great. When I do this, the transformers all run a lot cooler, and

the 7591 output tubes last forever.

Another thing I see a lot of is this 'Triode Connecting' of amps designed to run in pentode mode, or ultralinear.

In a 1957 hi-fi annual I have, H.A. Hartley puts the whole thing into perspective. I quote:

One real advance in output tube design is found in the Dutch Phillips-Mullard EL34 and the American

Tung-Sol 6550 pentodes. These are intended for distributed load or "Ultra-linear" operation, but I am quite sure it won't be very long before some writer comes out with an amplifier design using these very same pentodes as triodes.

H.A. Hartley

Please do not get me wrong, I have listened to several triode amps and they sound great,

but, to use the EL34 as a triode is not my idea of a practical use of such a fine tube. I converted my ST70 to triode mode using the EL34's and the classic removing the ultralinear taps and connect the grid via a resistor mod and the amp just didn't

sound any better if as good as it did stock.. Also, I lost about 50% of the amp's power. Now, for kicks,

I plopped in a quartet of 6B4 RCA tubes I have, did the cathode bias thing with the filament center taps, rewired the tube sockets, and ended up with a fairly good sounding amp. My feelings about push pull amps like the ST70 and

ones like my Scotts and the Fisher 500C is that they are a nice compromise between the sonic purity of the SE triode amp and the sonic harshness of a good transistor amp. A push pull amp has the power to drive real world speakers while retaining the

sound tubes have become famous for.

My personal favorite audio output tubes are the EL34 and EL84. To me, these two tubes have it all. They seem to have crisp highs, a nice airy midrange, and tight bass. You must remember that a lousy circuit as well as poor quality components can make any amp sound like crud, but most amps that I have heard which employ either of these tubes sound quite good. The two amps I own using these tubes are the ST70 (EL34) and the Scott 299 (7189 derivative of the EL84). To me, these are the best sounding amps I own. Although my Fisher 500C and Scott 299C's and 340B sound great, I feel that all those amps and

receivers would probably sound better using an EL34 as opposed to the 7591. The only reason I do not convert them is that the EL34 would not fit in the case (too tall), the filament current is a lot higher and the power transformer would not be up to the task of feeding those filaments.

Transformers

These guys can be the most misunderstood of all. First off, they do have DC resistance like a resistor,

but basic DC ohms law doesn't necessarily hold true for them because they work in an AC environment and their

Inductive Reactance is frequency dependent. With out going into the dreaded formulas suffice it to say that

the actual current seen across a 60 Hz 115 volt line when connected to a power transformer

that has a DC primary resistance of 3 ohms is not going to be 38 amperes!

When testing transformers what you are looking for is continuity, but no direct shorts.

Primaries measure from 2 ohms to around 20 ohms on most hifi amps, and the secondaries for the

high voltage will usually read between 40 and 200 ohms. The only one that will seem shorted

is the filament winding or windings. Since these are usually 12 volt or lower, they will

measure 1 ohm or less. The best way to test a transformer after doing a resistance check

it to remove all the tubes and test each winding with about 20 volts to the primary, you should

have about 1/6th of the rated voltage AC at each winding. So around 1 volt on the

5 and 6 volt windings, and around 40 - 80 volts on the high voltage winding (depends on the actual

rating of the transformer).

A power transformer that is leaking tar or oil is not necessarily bad! But, it has been overheated.

This is a good indication that something is out of wack in the amp, and you should troubleshoot

an over current problem in the amp (like improper bias setting, leaky coupling capacitor, shorted

power supply capacitor, etc.)

Most hi-fi amps of the 50's and early 60's were biased "Hot". By this I mean that they draw almost full current even

at idle like a class A amp. Although this is great for keeping a constant load on the power supply,

it is not good for the longevity of the transformers, both power and output. On my ST-70's I bias them around

84 ma as opposed to the factory setting of 100 ma per pair of output tubes. My amps sound the same

but run a lot cooler. I have done this to all my amps and they all sound the same, but run cooler.

OUTPUT TRANSFORMERS

These are very special transformers designed to couple the high impedance output tubes to

the low impedance voice coil of the speaker. The most common killers of these transformers are

high current conditions caused by a shorted tube, defective bias circuit, improperly set bias, or shorted

coupling capacitors. It is imperative that your tubes be in reasonably good shape, and that you bias

the amp in such a way that it does not exceed the power rating of the output transformer or the

plate dissipation rating of the output tubes! More amps are needlessly damaged because their owners

do not take the above precautions. Transformers are hard to find and expensive. It is well worth your time and

a couple dollars to replace the coupling capacitors, bias rectifier, and to make sure the output tubes

are good and to properly set the bias and check it regularly. Any time you see an output tube plate glow red

TURN THE AMP OFF! and determine why this condition exists. Failure to do so could cost you

a transformer or two and a lot of money.

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