Emerson BA199 circuit operation

[radiosoul]

Hello,

I have finished initial work on an Emerson BA199 (photo attached). The usual restoration process has been completed, capacitors replaced, resistors checked and some replaced, DC checks done, valves checked. I powered it up on a Variac on an isolating transformer and it burst into life. Wasn't expecting that! A brief alignment at 1400kHz done. One of the trimmers (C5) is way out (minimum capacitance) and I can’t improve on that. The only difference between the as-built set and the circuit diagram (attached) is that the 6C6 anode capacitor C14 goes straight to chassis.

The audio is reasonably good at high volume but distorted at low volume. Admittedly, the aerial is a couple of metres of cable strung up in the workshop so a better aerial might improve matters. Input voltage is 115VAC.

I'm using this very basic receiver as a bit of a personal "teach-in" on valve operation.

Attached is the circuit diagram of the set. The 6D6 is the RF amp and 6C6 is the detector. The 6C6 has been replaced with a 77 valve which has much the same operating characteristics other than a somewhat wobbly graph at low anode voltages for the anode current vs grid voltage graph on the last page of the data sheet (attached).

Question: My reading of the circuit is that the 77 is an anode bend detector. Looking at its characteristic, if you believe the anode and screen voltages in the small table below the circuit diagram are correct and then look at the 77 characteristic, anode voltage = 30 Volts and grid voltage = 0V, then we are on the bend of the graph. Do I have the right end of the stick, (anode)?

Actual working voltages are: Anode: 57V, Grid-2 (screen): 43.7V, Cathode: 2.1V (directly connected to the suppressor grid G3.). These figures contradict those in the table and as far as I can see when comparing the characteristics of the two valves, the difference appears to be down to the anode current drawn by the 77, which is about 4.5 mA (Anode: 57V, Grid: 0V) whereas the 6C6 draws about 6.8mA under the same conditions. However, when I measure the voltage drop across R6 (500kOhm) I get 42 Volts. Calculating the current through it, which I would expect to be the 6C6 anode current, I get 0.1mA! So either something in my methodology is wrong or the valve is really drawing very little anode current. Comments welcome.

Looking at the front end, the volume control (R1) lifts the cathode of the 6D6 from about 2.0V (full volume) to 21V lowest volume. My take is that when R1 is turned down to low volume the cathode is biased more and more positive which reduces electron flow in the 6D6 and which reduces volume. Can’t quite see why R1 is connected to the primary of the 1st RF tuned circuit coil (T1) – other than having a DC path to chassis, unless it serves a dual purpose of biasing off the 6D6 and shunting the RF signal to chassis. Again, suggestions/corrections welcome.

[radiosoul]

Hi again!

Here is a revised circuit diagram of the Emerson BA199.

Radiosoul

[ms660]

The 77 valve's Ia-Va characteristics were plotted with a g2 (screen) voltage of 100 Volts, the operating voltage of g2 in the receiver will be substantially less than that, hence your observation, your measured 2.1 Volts at the cathode calculates to 0.1mA cathode current, the service sheet also quotes 2.1 Volts but that was measured using 1,000 Ohm per Volt meter, with that in mind I would say that the cathode voltage in the receiver should be a bit higher when measured with a modern high impedance meter.

The best characteristic curve for seeing the anode bend is the transfer characteristic curve (Ia-Vg) but there's non shown in the data sheets I've found so far, however you can draw one that will give a reasonable curve by using the data from the Ia-Va curves.

The volume control circuit is designed to reduce the RF signal to g1 as well as reducing the mutual conductance of the valve, it's a common set up for those types of receivers.

Lawrence.

[G6Tanuki]

Yes the potentiometer arrangement connected to both the cathode of the RF amp and the antenna coil is a cunning approach, at the highest gain setting the bit of a pot between the antenna and ground does very little but as you reduce the gain there is both an increase in the bias on the RF amp and increased attenuation of the input signal by the pot.

This helps deal with the situation where the increased bias on the valve when you wanted less gain would otherwise make it more susceptible to crosdmodulation effects - just what you don't want when dealing with strong signals!!

[radiosoul]

Hi to you both.

I never fail to be amazed by the fact that the minute I sit down to do restoration and do a post I end up with my day business getting in the way. One day I'll stop that and get on with things I really enjoy!

Thank you both for your contributions here. Much appreciated.

Lawrence, I rechecked the voltage across the cathode resistor with a Keysight 34461A DVM (10 MOhm input resistance min) and measured 2.8 Volts. I should have taken notice when you mentioned "calculated" anode current since 2.8V divided by R4 (25kOhm) (which I missed doing earlier) gives about the same figure of 0.1mA.

I'm still surprised at the low current in the 6C6 valve because as far as I can see 0.1mA anode current is pretty much off the 1a/Va graph. In the Ia/Va graph I've taken Ec1 as the control grid voltage but I haven't yet seen the definitions of all of the terms. I may be reading "Ec1" as cathode voltage and not control grid voltage? Anyway, looking at the graph of Ia/Va, with Va measured at 57 Volts, even if g2 (their Ec2) was lower than the 100V I would have expected Ia to be somewhere in the region of 1.5 - 2.0 mA. This isn't the case so I'm still not clear on that. Maybe interpreting the graph symbols incorrectly.

I assume my assumption of anode bend detection is correct?


G6Tanuki, as for the comments from Lawrence, I wasn't too far off the mark with the function of the volume control. Thank you.

Still fiddling with this set before I put it back in its cabinet.


radiosoul

[AlanC]

I'm sure I read somewhere that distortion at low signal levels is a disadvantage of anode bend detectors... it makes sense if you consider that the valve is biassed almost to cut-off, and is therefore (as you say) on a rather non-linear part of it's curve.

Found it: https://www.vintage-radio.net/forum/...ad.php?t=23528
Post #2

So yours might be behaving as it should, even if not as you want!

[radiosoul]

Hi AlanC,

You definitely have it. I did read some old texts on anode bend detection before posting and one of its "patented hazards" is distortion when there is a high degree of modulation of the carrier signal due to the anode current vs grid voltage curve. (not that most stations broadcast at 100% modulation of course). Couple that with your observation on biasing then, I should be grateful that the radio is performing well (Radio 5 Live) at half-volume!

Also, because of the curve, the detector becomes a square-law detector which produces sum and difference frequencies at the valve anode.

Still need to satisfy myself about why the 6C6 detector is taking so little anode current.