Resistor Substitute

[KJ7USA]

I have a Hammarlund HQ-180A that fried the 430 OHM 1W resistor that comes off the 6AQ5 (Audio Power Output tube). I was wondering if it would be okay to put a 470 OHM in place of this resistor since they are readily available at the store. If not, I'll order one online. Thanks for the help!

[w3jn]

Should be just fine, it's just the cathode self-bias resistor. 

[KA2DZT]

You can put a 4700ohm 1/2W  resistor in parallel with the 470ohm 1W resistor and that will bring it down to 427 ohms.  First you need to see why the resistor fried  to begin with.  A bad or shorted 6AQ5 could have fried the first resistor.

Fred

[KJ7USA]

Thanks for the replies. I tested the tube and it checked good. I think the cause was from me leaving the calibrator on too long without any speaker hooked up while I was aligning it.

[W3RSW]

I think there's a 22 ohm resistor in the secondary of the audio xfor. (unmodified circuit)  for just that sort of mistake.

Virtually every cathode resistor in the 6V6, 6AQ5, 50C5 single ended audio output tube  from ham rigs to the all American five, you name it,  runs hot in class A service.  All those cathode resistors were spec'd at only twice or less actual power dissipated.  e.g., Next to the 35W4 and 50C5, the cathode resistor and filter caps were most often replaced in AA5 service.  --Similar in all the 6AQ5, etc. Hammarlunds. (not counting for some 'strange' reason the 6C4 osc.   )

6AQ5 runs 56 to 47 ma constantly at 250 volts plate.   Say 0.046*250 = 11.5 watts with only 4.5 power output.  --One hot little bottle from plate power dissapated alone.  Add the dissipation of the screen and heat generated by the filament and the region around that tube socket can be quite toasty.

Now the grid in this service in a 6AQ5 specifically calls for -12.5 volts.  That's 12.5 * 0.046 = 0.575 watts to be dissipated in the cathode resistor.  A one watt resistor will almost always degrade over time from its own heat added to surrounding sub-chassis general heat.
  A two watt wire wound is actually a better choice.

[KJ7USA]

I'll go with the 2 W like you suggest. Good idea.

[KJ7USA]

I installed a 470 OHM 2 W resistor and the radio is working again. Is it normal for this resistor to get hot? I can only hold my finger on it for a few seconds.

[Pete, WA2CWA]

I changed out that 1 watt resistor for a 2 watt version years ago in the various Hammarlunds and other receivers that I had. Most of the original 1 watt resistors had degraded in value over the years.

[Pete, WA2CWA]

I installed a 470 OHM 2 W resistor and the radio is working again. Is it normal for this resistor to get hot? I can only hold my finger on it for about 10 seconds.

Make sure there is no positive voltage appearing on Pin 7 of the 6AQ5. It should be zero. If there is any positive voltage, you could have a leaky coupling cap to the grid causing the tube to draw more current then it should.

[KJ7USA]

There's no voltage on pin 7, but that resistor sure is hot!

[w4bfs]

need to measure cathode voltage to see what it actually is

[KA2DZT]

Is the cathode by-pass cap part of a multi-section filter capacitor?  Might be something leaking there.

Pull the 6AQ5 and see if the resistor still gets hot.  Measure the voltage across the resistor.

Just downloaded the whole manual.  C-82 is the power supply filter cap.  The 6AQ5 cathode by-pass cap is part of that multi-section cap.  You can disconnect the connection from this cap to the cathode and install a separate 40ufd 25v cap right at the 470ohm 2w resistor.

BTW, there should be about 13 volts at the cathode of the 6AQ5.  V*/R  13x13=169/470= about 0.4 watts.  The 2 watter should not feel hot,  maybe a little warm at most.

Fred

[KJ7USA]

Here are the voltage readings I got when tested per the manual.

Pin 2 - 16.7
Pin 3 - 6.1
Pin 5 - 281
Pin 6 - 278

After turning the receiver off for a while, the audio went away again when turned back on. I think I have an intermittent tube or something in the circuit caused it to go bad. Are the above voltages an indicator that something is discrepant?

[KA2DZT]

Most likely the tube.  With the tube out, measure for any voltage on the cathode (470 ohm) and for any voltage on the grid.  With the tube out, you should read no voltage on both of these two points.  Measuring 17.5 volts on the cathode is way too high.  V*/R  17.5x17.5 = 306/470 = 0.65 watts.  A 1 watter would feel hot and a 2 watter would feel warm to hot also.

Pull the tube and make those measurements, let us know.

Fred

[KA2DZT]

Here are the voltage readings I got when tested per the manual.

Pin 2 - 16.7
Pin 3 - 6.1
Pin 5 - 281
Pin 6 - 278

After turning the receiver off for a while, the audio went away again when turned back on. I think I have an intermittent tube or something in the circuit caused it to go bad. Are the above voltages an indicator that something is discrepant?

Only the cathode voltage is too high for some reason.  That's why I ask for the two measurements with the 6AQ5 out of the socket.  BTW the loss of audio can be any number of things other than the 6AQ5 circuit.

[KJ7USA]

With the tube removed the voltages were...

Pin 2 - 0
Pin 3 - 6.1
Pin 5 - 307
Pin 6 - 281

[KA2DZT]

You need to measure for any voltage on the grid (pins 1,7) with the tube out.

[KJ7USA]

Pins 1 and 7 have no voltage. I just left the receiver on with the new tube in there for an hour or so and it is much cooler than before.

[Pete, WA2CWA]

Pin 1 and pin7 are tied internally in the tube. Voltage on either pin with the tube in, should be zero or slightly negative.
Your V readings
Pin 2 - 16.7
Pin 3 - 6.1
Pin 5 - 281
Pin 6 - 278

Mfr Readings
Pin 2 - 13
Pin 3  - 6.3 AC
Pin 5 - 275
Pin 6 -250
Mfr readings are taken at 117 volts AC input. If your AC input is higher, your voltage readings will be higher.
From 6AQ5A specs:
Max for Pin 5 is 275 V
Max for Pin 6 is 275 V
Typical operating voltage for the Pin 5 is 275 V and for Pin 6 250 V. So, if your plate and screen voltages are running higher, the cathode voltage (Pin 2) will be higher and the resistor will pass a bit more current. The 2 watt resistor will be fine. Warm or hot shouldn't be a problem. Red and smokey; you have a problem.

[KA2DZT]

Ok FB,  no voltage on the grid or cathode with the tube out is a good sign.  Maybe your problem has been just a bad 6AQ5.  Let it run for awhile and see what happens.

I've had a good number of Hammarlund receivers over the past 55 years. They can be, at times, a PITA to troubleshoot.  The biggest problem with these sets are the IF cans going south.  Way back in the 60s you could order new IFs, those days are gone forever.  Now, you have to operate (fix) on the bad can.  That in itself can be difficult and the biggest problem is trying to figure which IF can is bad.

I still have a SP-200 (BC-1004) and a HQ-110.  The old sets have the large (bomb proof) IF cans, they never go bad.  Sets like the HQ-110 don't.

Hopefully you have your set fixed.

Fred

[KJ7USA]

Thanks everyone for all your help. I have learned a lot from you. She's been working without any problems after installing the new tube and resistor.