Viking II Audio

[W7SOE]

I have finally dug into the VII to fix the audio.  I plan on starting with the K6AD "stage one" mods.  I have replaced the grid bypass cap on the first gain stage (6AU6) with a larger value.  I also re-biased the cathode to increase the plate voltage, it was 55V, now it is 135V.  K6AD saw it increase to 104V, hmmmmm.

I am seeing a sin wave on the output of the mod transformer, at 120Hz.  I see it on the plates of the 807s as well, though there is a lot more there as well.

120Hz?  Two times line hum f?  Is this a clue to something?

I added a 2.2k load resistor but is just attenuated it to 8V or so.  I added a de-coupling cap (.03uF) on the output to ground.  No joy.

Thanks

Rich

[Steve - WB3HUZ]

Full-wave rectifiers will produce 120 Hz hum, especially if not properly filtered.

[W7SOE]

OK, sounds like a good place to start.  Thank you.

[N3DRB The Derb]

I also re-biased the cathode to increase the plate voltage, it was 55V, now it is 135V.  K6AD saw it increase to 104V, hmmmmm.

huh?     bias concerns current flow and adjusting which class of service the tube is operating under, not voltage amounts as concerns the plate or screen. Wrong?

[W7SOE]

I also re-biased the cathode to increase the plate voltage, it was 55V, now it is 135V.  K6AD saw it increase to 104V, hmmmmm.

huh?     bias concerns current flow and adjusting which class of service the tube is operating under, not voltage amounts as concerns the plate or screen. Wrong?

At the risk of showing my ignorance..


Both the resistors into the plate and cathode were changed, I don't have the schematic here so I don't know the value change.   If the resistors feeding the plate are decreased and the cathode resistors are increased an equal amount then does the plate voltage not go up?  Would the gain remain the same...

Rich

[WA1GFZ]

Gain will drop off if values are reduced because the tube runs out of steam. Look at your low voltage supply. See what happens if you add a cap on the LV on the terminal strip near the audio driver tube. A 20 uf at 450 volts should be a good value. Changing resistor values will effect gain and frequency response.  Triode connected 6AQ5 driver is a great mod and easy to do. Use a 560 ohm at least 1 watt. I used a 2 watter cathode resistor and at least 15UF. across it. Screen tied directly to the plate.
Shunt feed the driver transformer if you use the stock V2 part. I used a 5 K 10 watt resistor between the plate/screen and LV coupled with a 4 uF 600 volt cap between the plate and transformer. The primary of the transformer connects to the coupling cap and other lead to ground.
The stock 6AU6 runs 10 or 12 ma while the 6AQ5 is closer to 40 ma.
Later I found a bigger transformer. This will drive the snot out of the 807s. I switched to AB2 and can drive the 807 grids 20 volts positive.

[WBear2GCR]

I prefer what is known in audio circles as the "Parafeed" method.

That means that rather than using a resistor to the plate of the driving tube (in this case before the little interstage transformer) you put a choke! The choke can be the primary of an output transformer - best that it be the primary of a transformer intended for SE use (gapped) or it can merely be a choke. If the inductance is high enough, it probably won't matter if it is gapped or not in practice.

The size of the choke usually will be on par with the transformer it is feeding, but larger is ok fine.

So you have B+ --> choke --> driver tube plate.
Then you take the output from the junction of the choke and driver tube plate.

The value of cap used and the inductance of the choke will have an effect on the freq response! You can "tune" that in a variety of ways. The usual method is to not try to tune it, but in some instances dorking the C value will vary the Q of the filter formed by the L & C and you can get some bass peaking before rolloff...

The advantage of this is that the inductor isn't as lossy as a resistor, and it tends to make the HF response much much better than a resistor will give you.

Of course you still get the DC off the primary of the interstage... which is good.

                  _-_-bear

[WBear2GCR]

I also re-biased the cathode to increase the plate voltage, it was 55V, now it is 135V.  K6AD saw it increase to 104V, hmmmmm.

huh?     bias concerns current flow and adjusting which class of service the tube is operating under, not voltage amounts as concerns the plate or screen. Wrong?

At the risk of showing my ignorance..


Both the resistors into the plate and cathode were changed, I don't have the schematic here so I don't know the value change.   If the resistors feeding the plate are decreased and the cathode resistors are increased an equal amount then does the plate voltage not go up?  Would the gain remain the same...

Rich

In general, as the plate Z goes up, so does the gain.
(thus the use of things like current sources instead of resistors on the plate in some circuits)
In general as the cathode resistor is increased, the gain goes down. (this is local 'degeneration' = feedback)

The case of equal plate and cathode resistances gives us the classic single tube phase inverter circuit.

Regardless of the values used on the plate or cathode, the tube still needs to biased to a usable/suitable amount of current WRT to the B+ present on the plate - which equals when combined simply wattage]/i].

Higher current implies a lower Plate Z... which  in some cases is required, in others, not.

No matter what a tube merely swings current WRT the voltage applied to the grid. That current is converted to a voltage by the drop taking place between the plate and the source, usually a resistor. So the tube works out to be sort of a variable resistor if you look at it in a simple way...

Of course, nothing flows from the plate to the cathode! 

              _-_-WBear2GCR

[WA1GFZ]

BAMA has the V2 manual if you need a schematic

[VE7 Kilohertz]

Hi Rich,

Check out my web page on how I mod'd my VII. It's so simple and sounds great. You just need one xfmr and a decent drive level of 0 - +4dBm.

http://www.qsl.net/ve7khz/VikingII.html

cheers

Paul

[AB2EZ]

My way of saying some of the things that the others have said:

A. Since these preamplifiers / amplifiers are designed to handle relatively low-level signals (both input and output) the focus is on: noise, bandwidth, and distortion, rather then on power. I.e., the audio output power levels that need to be produced are milliwatts or tens of milliwatts.

Tubes that are commonly used for these preamplifiers / amplifiers are designed to run with a very low resting current. If you look at the tube specification that is available from places like

http://datasheets.electron-tube.net/sheets/093/6/6AU6A.pdf

you will see that, for a 6AU6, in "typical, triode-connected operation" as a Class A amplifier (which is the class of operation you want), the resting plate current should be around 12.2 ma. Some of the other popular tubes that are used as preamplifiers / amplifiers in rigs like the Ranger ... e.g., the 12AX7... are designed to run at resting plate currents of around 1 ma. or 0.5 mA.

[If you increase the value of the cathode bias resistor to reduce the resting plate current (e.g., below the value recommended by the manufacturer of the tube), you can, for a triode operating in Class A, increase the gain of the stage somewhat (if you really need more gain)... because the plate resistance of the tube will increase somewhat, depending upon the details of where on its plate voltage v. plate current curves the tube ends up being biased... but you will not improve the desired performance of the stage in terms of linearity or bandwidth.]

If you want more voltage on the plate of the tube (e.g., 100 volts v. 50 volts), to bring the plate voltage up to the recommended value, then reduce the value of the resistor that is between the power supply and the plate of the tube. Then readjust the cathode resistor value (make it larger) to get back to the original target value of resting plate current.

B. When you look at the schematic of one of these preamplifiers, you will typically notice a large resistor in series with the plate power supply (e.g., 470k ohms in the Ranger preamplifier / amplifier). The real purpose of this resistor is to reduce the applied voltage on the plate of the tube... in order to reduce the plate dissipation that the tube experiences in actual operation, and to keep the plate voltage on the tube below the specified maximum plate voltage.  For example, if the resistor value is 470k ohms, and the resting current is 0.5 mA, then the resistor will drop the average plate voltage from whatever the power supply voltage is to a value which is 235 volts lower. This reduces the dissipation on the plate of tube, and keeps the plate voltage below the specified plate voltage for the tube.

C. If the cathode resistor is bypassed with a capacitor, then the voltage gain of the tube is equal to

    Gain= tube transconductance x (the total load resistance on the plate of the tube, including the tube's own "plate resistance").

    For example, if a 6AU6 is operated in triode-connected mode (i.e., the screen grid and the suppressor grid are connected directly to the plate), and biased per the manufacturer's specification (12.2 mA), it has a plate resistance of around 7,500 ohms. If there is a 15,000 ohm resistor between the plate of the 6AU6 and the power supply (effectively ground for audio frequencies of interest), and if all other load resistors from the plate to ground are much larger in value, then the total plate load resistance will be 5000 ohms (7500 ohms in parallel with 15,000 ohms) The transconductance of the tube is 4800 micromhos. Thus the voltage gain of the tube in this application is

Gain= .004800 mhos x 5000 ohms = 24

Note: One thing that is important is whether the peak audio output of the stage is a small fraction of the average plate voltage... so that the distortion is low. For ham radio applications, it is sufficient if the output voltage swing in each of the low-level stages is less than around 10% of the average plate voltage. So... if the average plate voltage on the tube in one of the low-level stages is 100 volts, the distortion will be low enough if the peak output swing of that stage is less than +/- 10 volts.

D. If a portion of the cathode resistance is not bypassed, and if the product of the transconductance of the tube and the un-bypassed portion of the cathode resistance is greater than 4, then the gain of the tube will be equal to:

Gain= total plate load resistance (including the plate resistance of the tube) / the un-bypassed portion of the cathode resistance.

For example, if the un-bypassed portion of the cathode resistance is 1000 ohms (which, by itself, would put much too much bias between the grid and the cathode... so you would need to compensate for this by biasing the grid appropriately), then the product of the transconductance of the 6AU6 (4800 micromhos) and the un-bypassed portion of the cathode resistance (1000 ohms) will be 4.8. Since this is greater than 4, the voltage gain of the stage will be

Gain= 5000/1000 = 5

This is, of course, less than the voltage gain (24) when the cathode resistor is totally bypassed, because the un-bypassed portion of the cathode resistor is producing negative feedback. What you get in return (for using negative feedback) is reduced distortion... but the distortion of the amplifier should already be low enough for this application... so it is not clear that there is much of an advantage of using negative feedback.

Bottom line:

i. Select the value of the series resistor between the plate of the tube and the power supply to produce the desired voltage on the plate of the tube when the desired average plate current is flowing. Example, if the power supply produces 300 volts, and if you want only 125 volts on the plate of the tube, then pick the series resistor to drop 175 volts when the desired average plate current is flowing. If the desired average plate current is 1 mA, then pick the resistor to have a value of 175k ohms. If the desired average plate current is ~12.2 mA, then pick the resistor value to be 15,000 ohms

ii. Select the cathode resistor to produce the proper bias between the cathode and the grid to cause the desired plate current to flow. If you need ~4 volts of bias between the grid and the cathode, and the associated average plate (and cathode) current is 12.2 mA, then you should use a cathode resistor whose value is 4 volts / 12.2mA = ~330 ohms.

iii. Bypass the cathode resistor with a capacitor whose reactance is less than or equal to 1/ [the transconductance of the tube] at the lowest frequency of interest. E.g., if the the transconductance of the tube is 4800 micromhos, then 1/[.004800 mhos]  has a value of ~208 ohms. If the lowest frequency of interest is 20 Hz, then the cathode bypass capacitor must have a value that is greater than or equal to 38.3 uF. [i.e., 1/[2 x pi x 20 Hz x 208 ohms] = 38.3 uF

Stu

[WA1GFZ]

BTW a triode connected 6AQ5 wants a 620 ohm cathode resistor with a solid state supply running about 325 volts. Convert to choke on the input and you will get about 250 volts. A 560 ohm will give you about 25 ma of plate current ( the value I used) I used a 40 ma driver transformer and got no performance gain isolating the DC so just dc connected it.

[W7SOE]

Thank you Stu, Frank and all,

Stu, a lot to absorb there.  I have attached a schematic of the first gain stage, both K6AD's mod'd version and stock.  You have given two ways of calculating the gain, since the modified circuit uses a non-bypassed cathode resistor which produces a product of that resistance and the trans-conductance is less than four (I get 2.26) then is the gain:

    Gain= tube transconductance x (the total load resistance on the plate of the tube, including the tube's own "plate resistance").

So here we have the ( (22k+220K in parallel with 470+2.7K) + plate resistance) * trans-conductance.

I cannot find a plate resistance for the 6AU6 in triode....

According the the ER article the mods raise the plate voltage (verified) and lower the gain.

Dumb question; why are the plate and cathode resistor added in parallel?

Rich

[AB2EZ]

Rich

I looked at the specification sheet and also the curves for the 6AU6 that I obtained from the web site shown in my previous post.

Looking at both schematics: the 6AU6 is connected as a pentode in both cases (i.e., the screen is not directly connected to the plate... so it is not "triode connected").

Therefore, the plate resistance is very high (as it always will be for a pentode).

Looking at the "characteristics in typical operation" for a pentode-connected configuration (on the specification sheet)... the plate resistance is around 1M ohm (much higher than the 7500 ohms I calculated for a triode-connected configuration of this same tube)

The equivalent plate load resistance is the plate resistance (assume 1M ohm) in parallel with all other un-bypassed resistors that connect between the plate and ground. Note: a resistor between the plate and the B+, if it is not bypassed, counts as a resistor in parallel with all of the others.

One does not include the cathode resistor (bypassed or not) in calculating the total equivalent plate load resistance

In the K6AD version of the schematic you have the following resistors in parallel, which comprise the equivalent plate load: 1M ohm (plate resistance of the pentode), 220k ohm (the un-bypassed resistor connected between the B+ and the plate, which connects to ground via the 1uF capacitor), and the input resistance of the "next stage". If I assume that the input resistance of the next stage is high enough to ignore, then the total equivalent plate load resistance is ~180,000 ohms.

Ignoring, for the moment, the effect of the 470 ohm resistor in the cathode, the "forward" voltage gain of this stage is

Forward voltage gain ~.004800 mhos (the transconductance of the pentode) x 180,000 ohms (the total equivalent plate resistance) = 864. [A very high number. I wonder if the "next stage" is placing an additional load on the plate of this tube... which could significantly reduce the forward gain from the value of 864]

Now, let's look at the effect of the 470 ohm un-bypassed cathode resistor.

The product of the transconductance of the tube and the resistance of the un-bypassed cathode resistance is ~.004800 mhos x 470 ohms = 2.2 (as per your calculation)

This means that the actual voltage gain of the stage, including the effect of the un-bypassed cathode resistance will be (using the standard formulas that apply to amplifiers with negative feedback):

Actual voltage gain = 864 (the forward voltage gain) / [1 + 2.2] = 270.



Another formula for the actual voltage gain = [total plate load resistance x transconductance]/ [1 + (un-bypassed cathode resistance x transconductance)]

Note, if the un-bypassed cathode resistance x the transconductance is large (e.g., greater than 4), then the above formula simplifies to: actual voltage gain ~ total plate load resistance / un-bypassed cathode resistance.

Best regards
Stu

[W7SOE]

Stu,
    That high gain number seems high, especially since one of the reasons for the mods is to lower the gain for the 1.6V peaks from a D-104 Mic.
Perhaps the answer is in the second stage, schematic attached.  Could the voltage divider (pot) simply be attenuating the gain from the first stage?

This is very educational (meaning I will have to re-read it several times), thank you.

I am confused as to why the bypassed resistors are omitted from the calculations, I have to assume this confusion stems from my thinking of this circuit only in terms of DC where the caps are irrelevant....

Rich

[AB2EZ]

Rich

The voltage divider created by the potentiometer can certainly attenuate the signal produced by the first stage. Since it is a 1M ohm potentiometer, it will not have a big effect on the total plate load on the 1st stage (180k => 132k, when you add the 1M load of the potentiometer, the forward gain goes from 864 => 633, and the actual gain, including the negative feedback associated with the 470 ohm cathode resistor, goes from 270=>200).

A resistor that is bypassed by a sufficiently large capacitor is invisible to audio signals. For example, the 2.7k ohm cathode resistor with at 50uF capacitor across it looks like only 159 ohms (impedance) at 20Hz, and only 15.9 ohms at 200 Hz.

The 1 uF capacitor between the 220k ohm resistor and ground has an impedance at 100 Hz of 1600 ohms. So, from the perspective of the 220k ohm resistor, that capacitor is essentially a short circuit to ground for audio frequencies of interest.

The 220k ohm resistor between the plate and the B+ implies that the average plate current is around 1 mA (i.e., 220k ohms x 1 mA = 220 volts of voltage drop between the B+ and the plate). If the average plate current (which is the same as the average cathode current) is 1 mA, then the cathode-to-grid bias is: (2700 ohms + 470 ohms) x 1 mA = 3.17 volts. Looking at the curves in the specification sheet, this is consistent with a plate voltage of 375 volts - 220volts = 175 volts, and a screen voltage of around 75 volts. If the screen current is 0.2 mA, then that current flowing through the 470k ohm resistor would produce a screen voltage of 175 volts - (475k ohms x 0.2mA) =  80 volts... so that is self-consistent.

If you look at the screen of the tube shown in the K6AD version, the 0.33uF capacitor represents a 4800 ohm impedance to ground at 100 Hz. At 100 Hz, if the peak audio frequency component of the screen current is no greater than 0.2 mA, then the screen voltage change caused by that 100 Hz audio frequency screen current will be less than 0.2mA x 4800 ohms ~ 1 volt. Meanwhile, the DC screen voltage is much greater than that... so the screen is essentially at a fixed voltage.

Note that the 6AU6 is being operated at a plate current of ~ 1mA, which is much lower than the 5-10 mA assumed in the data sheets for a "typical application" .


Looking at the curve on page 5 of the data sheet... and assuming that the screen voltage is around 75 volts... and assuming that the resting plate current is around 1 mA... the curves imply that the transconductance will be 1666 micromhos (1 mA change in plate current for a .6 volt change in grid-to-cathode voltage).

Using the transconductance of .00166 mhos, derived from the curves for low current (~1mA) operation, instead of the value of .0048 mhos given in the specification sheet for "typical operation", we obtain:

Forward gain = 132k ohms x .00166 mhos = 219

Actual gain (including the effects of the un-bypassed cathode resistor = 219 / [1 + 0.78] = 123

Stu 

   

[WBear2GCR]

From a practical point of view, the design of the first stage of these transmitters have what appears to be 'extra gain' because of the wide range of microphones that might be connected to them.

While the D-104 has a very high output, many mics have a fraction of that. IF the peak output of the D-104 is 1.6volts peak, consider what happens if you connect a mic with only 160mv peak or 16mv peak?

The design idea is to provide gain at the front end that can then be "knocked down" using that mic gain control if there is a "hot mic" - while designing the first stage so that it can still swing a reasonably high percentage of its plate voltage... conversely, if you put a lower "sensitivity" (output) mic onto the rig then you can turn the mic gain control up and still fully modulate.

Those who modify a rig for use with an external "chain" usually will make a provision to "inject" the audio later in the chain where the signal level is already  at "line level". There are a variety of means by which this can be accomplished.

              _-_-WBear2GCR

[AB2EZ]

I agree 100% with Bear's observations...

Building an input audio chain for these rigs... with the constraints imposed by the available technologies and a very price-sensitive market... was a real challenge.

The approach of using a simple, single-tube input preamplifier, which could work ("plug and play") with a wide range of microphone output levels and microphone output impedances, inevitably led to compromises in performance. For example, susceptibility to hum.

In today's context, if one is willing to move away from the "stock" design, it is better to feed the microphone into an audio preamplifier that is designed to work with that microphone (e.g., a special high impedance preamplifier for microphones like a D-104, and a different microphone preamplifier for dynamic microphones)... and to feed the "line level" output of the preamplifier / audio processing chain into the boat anchor transmitter's audio chain at the input of the stage which comes after the rig's 1st preamplifier stage.

In the case of a D-104, it is important that the connection between the microphone element of the D-104 and the preamplifier be well-shielded. If the preamplifier is placed inside the base/stand of the D-104, and if the base/stand of the D-104 is connected, internally, to the ground side of the preamplifier, and if the preamplifier is powered by a battery inside the base/stand, one can be reasonably assured that neither hum nor rf interference (rf getting into the microphone) will be a problem.

With my D-104, I use an external, well-engineered, commercial, 10M ohm input impedance preamplifier ("type 85 direct box") that is sold by Countryman Associates Inc. It connects to the D-104 via a well-shielded cable... with the shield grounded to the base/stand of the D-104. Even with those precautions, it is a good idea not to run the shielded cable too close to any transformers or AC power lines. The output of the Countryman box is a line level, balanced, audio source (XLR). The Countryman box can be powered either with an internal battery or with 48 volt phantom powering. I use phantom powering from the mixer that the Countryman box plugs into.

Stu