Audio coupling capacitors

[k7mdo]

While working on my recently acquired Johnson Challenger it has become very apparent that I don't understand much about even the simplest circuit of an audio chain.

The transmitter uses a single 12ax7 providing drive for a 6AQ5 that modulates the finals.

The 12AX7 appears to provide two stages of amplification for the microphone. Stage one has a .001 mfd cap in series between the microphone and first stage grid..... then there is another .001 between the plate of stage one and the grid of stage two of the same tube... now comes the transition to the next tube (6AQ5) ... there is another .001 from stage two's plate followed immediately by a .001 to ground.... hmmmm didn't that next .001 just short my audio signal to ground?

I guess my question... "what is the audio signal path?".... I do see the 10 mfd to ground on the cathode of stage 2..... this is certainly a pathway of sorts but to where?

This is pretty novice stuff but I would like to understand it....

Thanks, Tom

[N2DTS]

Is that the stock setup?
I would think its a .01 between stages and a .001 to ground to limit the highs maybe.

The impedance the circuit runs at can change the value of capacitance, low impedance coupling caps can be 1000 uf (8 ohm) and .001 (2 meg ohms or more).

With screen modulation, its more important to limit the highs since there is no cheap iron to limit the range.

[w4bfs]

equal value capacitors form a voltage divider most likely to help prevent overdriving the next stage .... trace thru with your scope to verify

[k7mdo]

The values quoted are directly from the circuit diagram....

Tom

[WBear2GCR]

In the first stage, the impedance the 0.001 sees is 1 meg. So at this impedance, it's passing pretty much a "full range" audio signal. You can check with an online calculator to find out what the -3dB point is for that RC constant or find the equation and plug it into a calculator or pen & napkin.  At some "bass" freq it is rolling off.

The second case, where there is indeed a 0.001 and then a 0.001 to ground, it's not a simple voltage divider, in my view. At some frequency high enough, that looks like the HF is rolled off like a straight cap (6dB/oct) to ground. The load Z appears to be the 100kohm at the grid of the following tube. But it is in parallel with the cap to ground. So at some frequency where the reactance of the cap is greater than 100kohm (that being lower freq is higher Z) the effect of the cap is small(er). And, so it acts more or less like it is not there.

The plate sees the two caps in series as a 0.0005ufd.

You can empirically measure that effect by putting an audio generator on the input of the rig or the input of that stage, and looking at a scope placed on the grid of the next tube, or better still, its plate.

This also probably reduced "splatter" somewhat.            

Many of these rigs were set up to "compete" with the coming SSB "revolution", that being narrow passband. The impetus for this was that at that time there was so much AM activity that the bands were a mass of heterodynes. One of the ideas behind the SSB push was to get rid of the heterodynes and to make it possible for more QSOs to fit into the band allotments. So, they pinched off the HF response in both the SSB rigs and with the AM rigs (to "compete").

Today, we get the luxury of operating where there are few competing AM QSOs (if any) and a band that is really not terribly pressed for space, as it once was. In my view, today on-the-air activity is somewhat constant, but much much lower than in decades past.

[WB4AIO]

The standard formula for reasonable audio fidelity is that the coupling capacitor value in microfarads times the load resistance provided by the following stage in ohms should equal 25,000 or more.

So, if your load is 100,000 ohms, your coupling capacitor should be 0.25 microfarads -- or larger. If the load is a megohm, then 0.025 should be adequate.

If you want to purposely restrict your low-frequency response, of course, you wouldn't follow this formula.

If you want to pass pre-processed audio through the modulator (such as precisely peak-limited or clipped waveforms provided by an external compressor/peak limiter), though, the coupling capacitors should be even bigger. Bob Orban (an audio processing pioneer and inventor of the Optimod) said that to avoid serious waveform distortion of a processed signal, the AM modulator should have flat response down to 0.7 Hz (not a typo -- yes, less than 1 Hz). This would require multiplying the resultant capacitor values above by 20 or so.

Whenever you increase these capacitors, you do run the risk of instability if you don't also at the same time increase the values of the decoupling capacitors (the ones from the "far" end of the plate resistors to ground).

73 and all the best,


Kevin, WB4AIO.

[DMOD]

Audio response in the audio chain is a matter of preference.

With the present circuit, your audio response starts at about 200 Hz (due to C27, R14 and C29, R16 AND C32, R21)  and goes up to the point where the shunt caps (C33, C35) starts rolling off the higher frequencies. In other words, no mid or lower frequency response.

My preference is to let the audio response go down to 75 Hz, so C27 and C29 would be 0.0022 uF, AND C32 should be 0.05 uF.

I would recommend removing C33 AND C35. In place of C35, I would add a series 27k and a 470 pF "rolloff" circuit to ground. I.E., Replace C35 with the 470 pF cap in series with a 27k to ground.

I would also recommend replacing R16 with a 1 Meg potentiometer.

Phil - AC0OB

[k7mdo]

DMOD, I will study your recommendation but first will sort through the transmitter caps and resistors and replace as necessary, sticking to the original diagram.  Then start the "experimental" stages of mods.... the Challenger has developed a bad reputation from my reading but when I got it for nothing, I must try to bring it to life!

Thanks everyone for the comments, Tom

[N2DTS]

Cant say I ever saw a setup that way in anything else I ever looked at.
Seems very odd.

[DMOD]

Cant say I ever saw a setup that way in anything else I ever looked at.
Seems very odd.

The modulator is a Controlled Carrier "Shunt" modulator.

Phil - AC0OB

[N2DTS]

Was talking about the equal value caps.
Very odd.

[DMOD]

The coupling capacitors do not have to be the same value.

In fact, in this case, the last coupling capacitor, C32, should NOT have been a 0.001 uf. The manf. chose this value undoubtedly because of cost considerations.

If you are working into a next stage resistance that is 1/10 the resistance of the previous two stages, it makes no sense whatsoever to use a .001 as C32.

Hence my recommendation for using a 0.05 for C32 in order to get audio to pass down to at least 75Hz.

One could have simply scaled C32 to 0.022, but just in case a bit of 6AQ5 grid current is drawn on peaks that would reduce the input impedance, we chose a slightly higher cap value.

C33 is used to rolloff any high frequency crud that may have passed through the first stage, restricting the audio pass band. C35 is there to do the same thing for V6, further restricting the audio pass band.

In fact, C33 and C35 is there as an attempt to clean up the distortion and ultrasonic crud caused by the poor coupling cap values and the poor biasing of the previous stages.

Phil - AC0OB

[DMOD]

...I guess my question... "what is the audio signal path?".... I do see the 10 mfd to ground on the cathode of stage 2..... this is certainly a pathway of sorts but to where?
 
Thanks, Tom

Hi Tom,

The C30 electrolytic is there to stabilize the bias on speech amplifier number 2.

Without it, there would be a lot of negative feedback which would decrease the gain of stage 2.

With a bias on the stage 2 cathode of 0.8 volts, they must have been expecting an input voltage across R16 of approx. 1.6 volts P-P.

C36 is on the 6AQ5 cathode for the same purpose. With a bias on the 6AQ5 cathode of 13 volts, they must have been expecting an input voltage across R21 of at least 26 volts P-P.


Phil - AC0OB

[k7mdo]

DMOD, very helpful and it gives me more insight.  I actually now have the set working on 80 and 40 with crystals.  Haven't checked neutralization yet, somewhat due to the obvious danger of the set with the case off and upside down... so far, no modifications and have not got the REA and oscilloscope in to see what the modulation looks like. Mostly just pleased to have resurrected it from dead.

One odd result so far is that tuning the oscillator for peak current results in far more than is required.  This results in having to detune the oscillator to keep the excitation drive current to the finals from being too high.  Not sure what this does overall but the 6DQ6s don't want more than 5 mils on the grids between them. The measured output into my dummy load on cw is just over 60 watts, seems a little low. 

I am sure there will be some lessons yet to learn on this one!

Tom

[KD6VXI]

First thing to suspect on the 'my drive is too high,  my power output is too low'  would be metering resistors.

Perhaps your barely able to reach the required amount of drivedrive,  but a metering R out of tolerance is showing the opposite?

--Shane
KD6VXI

[k7mdo]

Thanks for the thought, will look into the metering situation. The meter itself is a vane type that reminds me of the meters that Heathkit used in sets like the AT-1.

The set is quite old and I haven't had time yet to verify all of the resistors... the few I checked were pretty close but the metering ones carry some current and may have seen some duress over time.

Tom

[DMOD]

One odd result so far is that tuning the oscillator for peak current results in far more than is required.  This results in having to detune the oscillator to keep the excitation drive current to the finals from being too high.  Not sure what this does overall but the 6DQ6s don't want more than 5 mils on the grids between them. The measured output into my dummy load on cw is just over 60 watts, seems a little low.  

I am sure there will be some lessons yet to learn on this one!

Tom

Hi again Tom, and have no fear.

The Challenger was designed with reserve (extra) RF voltage gain so that the RF drive levels would be more than sufficient when tube emission dropped.

Check out Page 3, Paragraphs B. 5, 6, of the Challenger Manual.

Yep, these classic sets are fun.


Phil - AC0OB