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Dyna=20 Amp

Our Famous Original Dyna St-70 Rebuild=20 Plans

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From Audio Basics, July, 1982

Hello again, I am glad that the feedback I am getting from many of = you=20 indicates that you are happy with the information and ideas I have been=20 describing. Some of you have complained that you want more "meat" - = technical=20 information on audio equipment. So, this month we will get a bit more = technical=20 and go through the operation of a vacuum tube amplifier, what it does, = what it=20 cannot do, and what you can do about it. The data will pertain to vacuum = tube=20 amplifiers in general, and the Dyna St-70 specifically, as we get so = many=20 requests for information on how to modify them.

VACUUM TUBE AMPLIFIERS

Any audio amplifier can essentially be modeled as a two stage device; = a=20 voltage amplifier (which amplifies the amplitude of the signal) followed = by a=20 current amplifier (which supplies the drive current to drive the now = large=20 amplitude signal into a low impedance load - the speaker system). = Although many=20 tube circuits will also have voltage gain in the current amplifier = section, that=20 is not important to the discussion that follows. The closed loop gain of = the=20 amplifier is determined by a voltage divider which sends a portion of = the output=20 signal back to the input of the voltage amplifier out of phase. The = amplifier=20 then does not amplify the input signal, it amplifies a signal being the=20 difference between input and feedback signal.

If the voltage amplifier and current amplifier were perfectly linear, = the=20 difference between the input and feedback signal would be only a small = version=20 of the input signal and the results would be "perfect." In theory, any=20 difference between input and output is supposed to be eliminated by = feedback.=20 The feedback signal, which is a duplicate of the output signal, when = subtracted=20 from the input signal, should create a "difference signal" which is=20 "pre-distorted" by the amount of distortion in the circuits, but out of = phase=20 with the circuit's distortion. The "pre-distortion" in the "difference" = signal,=20 when added out of phase to the actual distortion in the circuits, = cancels=20 exactly, giving a "perfect" output signal. That is the way it is = supposed to=20 work.

By the way, there ain't no such thing as a no feedback amplifier! = There are=20 many different feedback schemes. One can choose to use lots of local = feedback in=20 each stage and little overall loop feedback, but each device in itself = (whether=20 it is transistor or tube) has internal feedback. If one attempts to make = an=20 amplifier with "no feedback" except for that inherent in the devices = themselves=20 then the design becomes absolutely dependent of the characteristics of = each=20 independent device. No two tubes or transistors, even of the same type, = are=20 identical. Even if you painstakingly select and bias each individual = device one=20 at a time, its characteristics will change with variations in = temperature,=20 current, voltage, and age. It will be impossible to make any two = channels the=20 same and to keep them the same. The main characteristics of a so called = "no=20 feedback" amplifier are: very high cost (each unit is essentially a one = off),=20 very hot running as the devices have to be biased on very hard, unstable = operation as the unit changes characteristics with age and temperature, = no two=20 samples will sound the same as they are device dependent, and lots of=20 "blow-ups." Obviously, repairs will be expensive as a repair is = essentially a=20 re-engineering of that unit with new devices and re-biasing of each = device. I=20 can live without it. The "wonderful" sound of a "no feedback" amplifier = is the=20 wonderful sound of lots of instabilities and underdamped oscillations. = You may=20 like it, I don't, it isn't music.

Thus, those that choose to build stable, repeatable, and rationally = priced=20 amplifiers will use some feedback. The catch is in knowing what the = feedback can=20 and cannot do in the real world, and to use the feedback properly, so = that the=20 unit does not only measure well, but actually performs well under = transient=20 conditions in the real world.

Now, back to that vacuum tube amplifier. Remember we mentioned that = if the=20 voltage amplifier and the current amplifier were perfect, everything = would be=20 just fine. Sorry folks, the internal circuits are not perfect and that = is where=20 the troubles start.

Inasmuch as the feedback is supposed to compensate for any = non-linearities=20 between input and output, it is nice to know what non-linearities = exist.

First of all each individual active device is nonlinear. Its transfer = characteristics are exponential, not linear, be it tube, FET, or = transistor.=20 Refer to the sketches below. If the device was perfect, its transfer = function=20 would be a straight line, and the slope would remain the same at all=20 frequencies. The actual characteristics are shown in the second and = third=20 sketches. Note that the characteristics are actually exponential. The = device is=20 only very linear near the center-line of its operation, and the harder = it is=20 "worked", the less linear it becomes, finally becoming 100% nonlinear = when its=20 absolute limitations are reached. In addition, the slope of the transfer = function becomes less at higher frequencies as the gain of the device = reduces.=20 Thus if one attempts to get the same output from any given device at = higher=20 frequency, one will drive it into gross non-linearity sooner, as its = headroom is=20 less. In a similar fashion, at very low frequencies (approaching DC) the = device=20 becomes more nonlinear and its gain drops. In addition, because the = slope=20 changes with frequency, a kind of phase distortion is introduced which = is not=20 measured in standard IHF distortion tests, which measure only single = frequency=20 performance.

3D"Performance

Obviously, to optimize internal linearity, it is very desirable to = operate=20 each device within as narrow a bandwidth and as limited an amplitude = range as=20 possible while still covering the audio frequency range of interest.

In a vacuum tube amplifier another major non-linearity is the output=20 transformer. The output transformer's primary coil is just that-a very = large=20 coil (inductor) in series with the output tubes. Obviously the coil = becomes very=20 resistive as the frequency goes up, and in an audio amplifier this = happens well=20 within the audio range, rolling off the high frequency output. At very = low=20 frequencies the core of the transformer saturates giving very nonlinear = bass=20 performance. If one wants good high frequency performance then one must = have=20 very small output transformers so that the coil inductance is low. If = one wants=20 good low frequency performance one must have very large transformers so = the core=20 does not saturate. These requirements are mutually self exclusive. These = requirements become more difficult to meet as the power rating goes up. = If one=20 attempts to "get around" this by designing a tube amplifier without = output=20 transformers, then one is faced with the problem that output tubes have = very=20 high output impedance and will not drive normal loudspeaker loads (8 s = nominal)=20 without severe non-linearities.

Since the output transformer is a very narrow band and nonlinear = device, it=20 is obviously necessary to not feed into the transformer a signal that it = cannot=20 handle-the bandwidth of the amplifier must be limited to within the = bandwidth of=20 the output transformer. You cannot stuff 10 pounds in a 1 pound = sack.

The power supply is another source of distortion. The power supply = can be=20 considered to be in series with and part of the output circuit. All = current that=20 flows through the output circuit and speaker load first must flow = through the=20 power supply. The frequency limits of a power supply are real. It can be = modeled=20 as an inductor in series with a capacitor. Obviously at DC the = capacitor's=20 impedance is infinitely high, and at high frequencies the inductance is=20 infinitely high.

Thus, at very low and very high frequencies, the power supply is not = capable=20 at all. Unless great care is used in the power supply design, it may = have=20 multiple resonances, and actually be high impedance at many frequencies = within=20 the audio range. Consider also that since the power supply is part of = the output=20 circuit, if somebody offers a "wonderful" vacuum tube amplifier with a=20 "wonderful" solid state power supply, you no longer have a vacuum tube=20 amplifier, but a solid state amplifier, so how can it be a "wonderful" = tube=20 amplifier?

Of course the power supply is also attached to the voltage amplifier = section.=20 Consider that all current draw by the output section causes a signal to = show up=20 on the power supply feeds. Any given device or circuit will work best = when its=20 supply is absolutely stable. Circuits are designed to reject power = supply=20 variations, but the supply rejection isn't absolute. Thus the more = signal that=20 shows up on the power supply feed to the voltage amplifier, the more = distortion=20 and instabilities there will be, as this is a signal injected into the = circuit=20 at the wrong place. Since we already know that the power supply is less=20 effective at very high and very low frequencies, obviously the power = supply=20 related distortions will be greater at very high and very low = frequencies.=20 Again, a very good reason to bandwidth limit the amplifier to within the = capability of its power supply.

Understand of course that we are considering basic ground rules in = general.=20 There are many different kinds of voltage amplifier and current = amplifier=20 configurations that work fairly well, some simple, some complex. The = important=20 thing to know is that unless the circuits are executed to obey the = guidelines=20 established above the distortion will be very high under real world = conditions,=20 no matter what the linearity of each section is and no matter how high a = quality=20 of parts are used.

Another common problem with vacuum tube amplifiers is the value = chosen for=20 the interstage coupling capacitors. In the case of the Dynaco St-70 for = example=20 (see attached schematic) coupling capacitors C10 and C11 are 0.1 uF. = This=20 introduces another large low frequency roll-off within the feedback = loop. Since=20 the amplifier actually amplifies the difference between input and = feedback, and=20 since the feedback is the difference between input and feedback, and = since the=20 feedback is taken off the output of the amplifier, at low frequencies = the=20 difference signal becomes very large partially due to the roll-off = caused by the=20 0.1 uF coupling capacitors. Now remember that we have shown that any = circuit=20 becomes less linear with increasing amplitude and at the frequency = extremes. The=20 roll-off caused by the small value interstage coupler makes the front = end work=20 very hard to generate a large low frequency correction signal. This = causes the=20 front end to run in a very nonlinear mode at low frequencies. You hear = it as=20 "muddy bass." The "cure" is quite simple, make the interstage capacitor = large=20 enough in value so that the loop roll-off is minimized, thus reducing = the=20 correction required, and letting the front end run in a more linear = mode. The=20 low frequency correction signal is easy to see on an oscilloscope. Using = a low=20 frequency square wave as a source (20 Hz is fine) look at the signal on = the=20 output side of the interstage coupler. Note that it looks much like the = input=20 signal. Now look at the signal on the input side of the coupler. You = will find=20 the circuit is generating a signal with a large bass boost! (This is = true in=20 most tube preamps too!) What is happening is that the "flat" input = signal is=20 rolled off by the interstage coupling capacitor. Then the rolled off = signal is=20 fed back to the input and a correction signal is generated with a large = bass=20 boost to make up for the roll-off. The boosted signal is then rolled off = again=20 by the coupling capacitor and its output looks just fine. But the = "monkey=20 motion" has ruined the voltage amplifier's linearity at low = frequencies.

Now let's look at what is wrong with the original Dyna St-70 in = detail. Refer=20 to the audio channel schematic again, keeping in mind that the "dashed" = section=20 is our addition, the original has the input connected to V2 directly = with a=20 piece of wire.

What we have is a typical vacuum tube amplifier with unlimited = bandwidth=20 input acceptance (DC coupled) but with limited bandwidth output = transformers and=20 small interstage coupling capacitors. The power supply is also limited=20 bandwidth, being pretty feeble at both low and high frequencies.

A very low frequency signal is rolled off by the interstage coupling=20 capacitors, turned into a "lump" as the output transformer core = saturates, and=20 is further distorted as the power supply runs out of steam. The feedback = signal,=20 being taken off after all the disasters occur is very different from the = input=20 signal. This generates an enormous "difference" signal which drives the = front=20 end into 100% distortion trying to make an impossible correction. = Inasmuch as=20 the circuits are underdamped too, the "blob" makes the amp ring for a = few cycles=20 attempting to digest the mess. Some people call these distortions and = ringing,=20 which extends up into the mid-range, "concert hall sound." Sorry, it = isn't=20 concert hall sound, it is distortion. If you like it you have bad = taste.

At high frequencies the compensation in the voltage amplifier rolls = off the=20 signal, the active devices roll it off further, and the output = transformers=20 attenuate the highs further yet. This generates another huge correction = signal=20 at high frequencies, again more than the headroom of the front end, = clipping the=20 correction signal once again. Of course the high impedance of the supply = has=20 further compounded the problems. The amp is driven into hard slew = limiting and=20 all signal entering the amp while any internal device is slewing is = erased. Gobs=20 of high frequency distortion are added and part of the music is forever = lost. It=20 is very strange to think that some people use the St-70 to drive = tweeters when=20 it doesn't "tweet" at all-it does kind of "squeak."

Obviously the power supply of the St-70 must be much improved. NO! = Not=20 necessarily! Think a minute. Consider that the power bandwidth of the = power=20 supply must be greater than the bandwidth of the audio circuit. There = are two=20 ways to get this ratio in proper order. The expensive (and stupid) way = is to=20 build a huge power supply-and if the amp has DC coupled inputs you can = never=20 make it big enough. The easy and smart way is to limit the bandwidth of = the=20 circuit to within the capabilities of the existing power supply, = especially if=20 it is absolutely necessary to bandwidth limit the inputs anyway to make = the=20 input bandwidth within the capabilities of the output transformers.

As mentioned earlier, the interstage coupling capacitors, C10 and C11 = are too=20 small. Note that as long as the input is DC coupled, it is not possible = to make=20 C10 and C11 big enough, as even a very large capacitor will have an = inside the=20 loop roll off when compared to DC input acceptance.

To install the input bandwidth limited filter on the St-70 you will = need 8=20 parts: (2) 10,000 resistors, (2) 470,000 resistors (5% carbon film 1/4 = watt=20 parts from Radio Shack are just fine, and it would be better if you = could use a=20 meter and "pair" them, so they are matched within 1%.) You will also = need (2)=20 1000 pF capacitors (mica, polystyrene, or mylar are O.K., of about 100 = volt=20 rating - the capacitors used should be physically small) and (2) 0.02 uF = capacitors (film) 100 volt rating, again physically as small as = possible. Again,=20 Radio Shack will have adequate parts and if you can match them on a = precision=20 capacitance meter it will be helpful. The capacitor values suggested are = not=20 absolute. Anything from about 0.02 to 0.033 uF is O.K. for the larger = cap, and=20 800 to 1200 pF for the smaller capacitor.

The new 6 dB per octave low pass and high pass filter is installed on = the=20 input jacks on the bottom inside of the chassis. We suggest that the = mono-stereo=20 switch wiring be eliminated as the performance is poorer when bridged = mono=20 because of the difference between the two channels (no two output tubes = or=20 output transformers are identical).

If you decide to eliminate the mono-stereo = switch,=20 then do the following:

1. Remove all the wires from the input jack and mono-stereo switch = except for=20 the two wires going directly from the input jack ground lugs to the PC = card=20 (these are actually extensions of the leads of two 10 resistors mounted = on the=20 card). These remain. Also remove the two original 470,000 resistors from = the=20 jack and switch.

2. Remove the two wires running from the hot lugs of the input jack = to=20 eyelets 7 and 17 on the PC card.

3. Connect a 10,000 resistor in series with a 0.02 uF capacitor and = connect=20 the capacitor end of the series set to the left channel hot input jack = and the=20 resistor end to eyelet 7 on the PC card.

4. Connect another 10,000 resistor in series with a 0.02 uF capacitor = and=20 connect the capacitor end to the right channel hot input jack and the = resistor=20 end to eyelet 17 on the PC card.

5. Connect a 1000 pF capacitor in parallel with a 470,000 resistor = and=20 install the resistor between the left channel ground lug and eyelet 7 on = the PC=20 card.

6. Connect another 1000 pF capacitor in parallel with a 470,000 = resistor and=20 connect the resistor from the right channel ground lug on the input jack = to=20 eyelet 17 on the PC card.

If you must keep the mono-stereo switch = option, do the=20 following instead of the last set of instructions:

1. Remove the two wires running from the left and right channel hot = lugs on=20 the input jacks to eyelets 7 and 17 on the PC card.

2. Connect a 10,000 resistor in series with a 0.02 uF capacitor and = connect=20 the capacitor end to the left channel hot input lug and the resistor end = to=20 eyelet 7 on the PC card.

3. Connect another 10,000 resistor in series with a 0.02 uF capacitor = and=20 connect the capacitor end to the right channel hot input lug and the = resistor=20 end to eyelet 17 on the PC card.

4. Install a 1000 pF capacitor in parallel with each of the two = existing=20 470,000 resistors on the input jack and mono-stereo switch.

For mono operation, the amp is switched to mono, only one input jack = is used=20 (either left or right, but not both). Connect a jumper wire from the = left output=20 ground to the right output ground terminal. Connect a jumper wire from = the left=20 16 tap to the right 16 tap (for 8 speakers). Take the output from the = same=20 channel that you have the input jack connected to, using the 16 and = ground=20 terminals (for 8 speakers). To use 4 speakers connect the jumper from = the left 8=20 output tap to the right 8 output jack and connect the load from ground = to 8 on=20 the channel used. This arrangement parallels the two channels for = somewhat=20 higher power, but lower definition performance.

Now that the input bandwidth is set to a rational, finite limit, it = is=20 possible to make the interstage coupling capacitors "big enough." You = will need=20 to acquire four 1 uF at 400 volt film capacitors (mylar, polypropylene, = or=20 whatever). Again Radio Shack will have adequate parts.

Locate and remove the four large identical black tubular 0.1 uF at = 400 volt=20 capacitors from the PC card. They are positioned parallel with the front = of the=20 chassis, one at each corner of the PC card.

Replace each with a 1.0 uF at 400 volt capacitor. The exact value of = the=20 replacements is not critical. They can be anything between 0.8 uF to 2.0 = uF at=20 400 volts or higher. It is important that all four new capacitors be = exactly the=20 same.

Further detail improvements can be made to the St-70. The bias supply = capacitors in old St-70 amps should be replaced. We suggest that the two = original 50 uF capacitors (C3 and C4, located on the 7 lug terminal = strip under=20 the chassis) be replaced with new 100 uF at 80 volt electrolytic = capacitors=20 (again, available at Radio Shack). Note that the positive end of each = cap is=20 connected to ground. Do not use a larger capacitor in this application = or the=20 supply will come up too slowly, over-biasing the output tubes at turn = on.

Although the original power supply is now adequate, further = reductions in hum=20 and noise can be made by installing an additional 100 uF at 500 volt=20 electrolytic capacitor (a 450 volt rated cap with a 500+ volt surge = rating is=20 adequate unless you have high line voltage) from pin 8 of the power = supply tube=20 (V1-5AR4) to chassis ground at the ground lug near the quad filter. The = positive=20 end of the cap goes to the tube socket, the negative end to ground.

Inasmuch as the perceived "image" and "depth" of an audio system is = dependent=20 upon both channels having exactly the same gain and phase response, and = because=20 the resistors in the St-70 (and other tube amps) may have drifted out of = specification over the years, it is helpful to replace all of the = resistors with=20 new tight tolerance parts. The gain determining resistors especially = should be=20 matched to each other within 1%. The RN60D and RL42S metal film = resistors shown=20 on the attached St-70 parts list are a good choice. However using 1/2 = watt=20 carbon film resistors for the RN60s and 2 watt carbon film resistors for = the=20 RL42S types is just fine, except you will have to sort more of them to = get a=20 tight channel-to-channel match.

Because selenium rectifiers (the small little finned block located in = the=20 bottom middle of the chassis) become resistive with age, you may be able = to=20 increase the voltage to your bias supply by substituting a lN4004 = silicon diode=20 for this part (Dl). Because the negative voltage to the bias supply will = now be=20 higher than stock, it probably will also be necessary to change the = value of Rl=20 (10,000 2 watt resistor) to 18,000 2 watt to allow the amp to bias = adjust at=20 1.56 volts DC across R20 in the center of rotation of P1 and P2.

The St-70 and other tube amplifiers run very hot. This tends to make = solder=20 joints deteriorate with time. Re-solder all solder connections in the = amplifier,=20 including all parts, leads, and the tube sockets on the PC card. Clean = the input=20 jacks, output terminals, the bias pots, and all the tube sockets with = DeoxIT-D5=20 (we have 5 oz. spray cans available for $20). Usually lightly "crimping" = the hot=20 (inner) terminals of the input jacks will eliminate patch cable = dropouts.

In the St-70 the noise characteristics, gain, power, and slew rate = are=20 dependent upon having high quality tubes in the unit. We have SovtEC = EL34G+=20 output tubes available at $80 per set of four plus $6 shipping and a = SovtEC 12X4=20 rectifier tube available for $15 plus $6 shipping. We do not have 7199 = tubes=20 available.

Refer to the attached schematic and parts list for other service and=20 adjustment notes on the St-70.

I assume you have noticed we have not spent much time on the inner = details of=20 the circuit topography of the St-70. There may, or may not be "better" = input,=20 phase inverter, and output circuits available. The point is that almost = all tube=20 amps are mistakenly DC coupled and whatever the internal circuits are, = they are=20 driven into gross nonlinearities. The important concept is that any tube = amp in=20 which the input is limited to within the internal capabilities of the = circuit=20 will outperform any tube amp that can be driven into internal overload, = no=20 matter how expensive or complex the circuits may be. And the final = limitations=20 of a tube amplifier are the output transformers. Lots of money spent = trying to=20 achieve a "better" drive circuit probably is of little value, because = the output=20 transformers still are the limits of performance.

THINGS YOU SHOULD NOT DO TO YOUR VACUUM TUBE AMPLIFIER (AND=20 WHY).

DO NOT install a solid state diode bridge to replace the vacuum tube=20 rectifier. The supply is operating at 500 volts with line surges over = 1000=20 volts! There are no reliable diodes available to operate at this = voltage. You=20 will be in great danger of blowing the diode bridge and damaging your = power=20 transformer and filter capacitor. In addition, the solid state supply = will "turn=20 on" instantly, and the full B+ voltage will be fed to the tubes before = the=20 heaters have warmed up and turned the tubes on. This will tend to over = voltage=20 the quad filter capacitor and capacitors downstream, which may damage = them. The=20 output tubes will run hotter than normal and have a short service life. = There=20 are no useful redeeming advantages to a solid state diode bridge.

DO NOT install solid state regulators. The "aftermarket" circuits we = have=20 seen use transistors with inadequate voltage ratings (operating in the = "blow-up"=20 mode) and have severe slew rate limitations. Remember, the bandwidth of = your=20 power supply must be greater than the audio circuits, and a series = bipolar=20 regulator is bog slow! It will change the sound, it makes it much = worse!

DO NOT add external power capacitors. The long hookup wires will have = lots of=20 inductance and impair the high frequency performance.

DO NOT rewire the amplifier internally with "magic wire." The chances = are you=20 will screw up the lead routings, add longer lead runs than the original = and=20 increase stray inductances. The probabilities of internal short circuits = and bad=20 connections increase as the wires are too large for reliable = termination.

DO NOT replace your capacitors with high priced and physically large = "wonder=20 caps." The larger the physical size of a given value capacitor, the = greater its=20 inductance will be, and the more trash it will dump into the circuit. = Magic=20 "wonder caps" do change the sound, they make it worse!

DO NOT use polystyrene capacitors near heat generating components. = They=20 change value with temperature, and near an output tube they may even = melt.

DO NOT ship your vacuum tube amplifier to us to fix if you screw it = up unless=20 you CALL US FIRST at 651 330-9871. Output tubes don't survive shipping, = tube=20 amps are heavy and expensive to ship, and their performance is limited. = One of=20 the "joys" of owning a vacuum tube amplifier is learning how to fix it = yourself.=20 If you don't want to do this, you shouldn't own a vacuum tube = amplifier.

2005 Note:

We can get much better results now with our complete $449=20 Ultimate=20 70 Rebuild Kit. It includes a new motherboard = with all=20 new audio circuits (using the information contained = herein=20 and much more), a new B+ supply built of modern=20 PC-card-mounted high efficiency, high reliability = capacitors,=20 and a new bias supply. It also includes new tubes to = replace=20 the now obsolete 7199 tubes. A new Input/Output jack = set kit=20 is available too for=20 $65=20 with gold plated input jacks and high quality Pomona = 5-way=20 binding post output jacks. No cutting or drilling is = required=20 and step-by-step instructions, schematics, parts = lists, and=20 wiring diagrams are provided. The kit instruction = manual and=20 bare PC card are available for $149=20 if you want to provide your own parts. The price of = the=20 manual and PC card can be applied to the later = purchase of=20 the rest of the kit as long as they are current. The = sonic=20 quality is really special.

Formula to Solve 6 dB/Octave High & Low = Pass Filter=20 Such as Recommended for the St-70

3D"filter

Service Notes

Examining the 1.56 volt biaset test point on the St-70 can tell you = much=20 about the condition of the amplifier. If the output tubes are old, it = may be=20 impossible to adjust the bias pots to bring the voltage up high enough. = Replace=20 the output tubes. A shorted output tube may cause the bias reading to = run away=20 high. 7199 tubes are best selected by examining the output of the amp on = a=20 scope. A low gain or noisy 7199 will show excess output hum and/or not = make full=20 power. Fuse blowing can be caused by two problems. A hard blow soon = after=20 turn-on indicates a power supply short, either a defective 5AR4 tube or = a=20 shorted quad filter cap. A fuse that blows soft or after a few minutes = of=20 operation indicates a problem with the audio circuits - probably a bad = 6CA7=20 tube. Note that the bias setting will vary with AC line voltage so the = value=20 isn't an absolute. It is possible to swap tubes channel to channel = (except the=20 5AR4) one at a time to locate a defective tube.

The new input filter circuit provides -3 dB poles at 16 Hz and 17 KHz = which=20 keeps the audio circuit working within the limits of the output = transformers.=20 The larger C10 and C11 effectively takes them out of circuit for AC = signal=20 purposes after the input filters have been installed. It is much more = important=20 to install the new input filter circuits than to replace all of the = resistors=20 and capacitors. Match R and C values channel to channel for a good gain = match=20 between the channels.

Dyna St-70 Amplifier Suggested Modifications Parts List

C1 0.02 uF 1000V disc P1 10 k bias trimpot

C2 0.02 uF 1000V disc P2 10 k bias trimpot

C3 100 uF 80V electrolytic R1 10 k 2W

C4 100 uF 80V electrolytic R2 10 k 2W

C5A 30 uF 525V electrolytic R3 6.8 k 2W

C5B 20 uF 525V electrolytic R4 22 k 2W

C5C 20 uF 525V electrolytic R5 10 k 0.5W

C5D 20 uF 525V electrolytic R6 475 k 0.5W

C6 0.02 uF 50V film R7 10 ohm 0.5W

C7 1000 pF 50V film R8 330 k 2W

C8 0.05 uF 400V film R9 1.5 M 0.5W

C8 0.05 uF 400V film R10 270 k 2W

C10 1 uF 400V film R11 620 0.5W

C11 1 uF 400V film R12 47 0.5W

C12 390 pF 500V mica R13 18 k 0.5W

D1 1N4003 or 1N4004 silicon = diode R14 47 k 2W matched within 1% of = =20 R15

V1 5AR4 rectifier tube R15 47 k 2W matched with 1% of = R14

V2 7199 pentode, triode tube R16 1 k 2W

V3 6CA7/EL34 pentode tube R17 270 k 0.5W matched within 1% = of=20 R18

V4 6CA7/EL34 pentode tube R18 270 k 0.5W matched within 1% = of=20 R17

L1 choke, Dynaco C-354 R19 1 k 0.5W

S1 power switch, SPST R20 15.6 1W

F1 fuse, 3 ampere slo-blo 3AG R21 1 k 0.5W

T1 power transformer, Dynaco = PA-060 T2 output transformer, Dynaco=20 A-470

Schematic

Note: Input circuit (C6, R5, C7, and R6) is new and replaces the = original=20 direct coupled input circuit. All wiring is removed from the input jacks = except=20 R7 leads and then the new input circuit is installed. The stereo-mono = switch=20 should be eliminated. C3, C4, C10, and C11 are revised values. Adjust = bias (P1=20 & P2) for 1.56 volts across R20. L1 can be replaced with a 30 10 = watt=20 resistor to make amplifier operational if L1 is defective. This input = filter=20 circuit is also helpful for the Dyna MKIII and MKIV amplifiers.

=09

Dyna Hafler Rebuilds

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