Cathode Follower outputs??

I'm restoring a Maggy tuner/receiver as part of the concert grand unit I recently acquired. Working on the tuner I had the output connected to a Sony stereo thru the aux inputs. I was noticing a very noisy static/crackling sound almost overpowering the FM station or other signal I was inputing to the Maggy coming out of the "outputs" on the Maggy. I couldn't find anything really wrong with the Maggy. Tubes all okay, etc., but when I, instead, connected the Maggy to another simple solid state amplifier I did not have the noise at all. In looking at the schematic for the Maggy I noticed that on the main outputs it says "cathode follower". Is this a normal type of output or something peculiar to these units?? And, if so, why would I have this noise on one amp, but not on another? Otherwise, both amplifiers work just fine with other inputs to them. Also, if I take the output from the Maggy from the "tape recording output" I do not have this problem. The Maggy schematic does show the main outputs being taken from the cathodes of the final AF amp stages, whereas the tape outputs come from one stage prior to this and, of course, are not affected by the volume and tone controls. Anybody have any ideas on this?

Harry, the machine probably has leaky capacitors which are putting DC at the outputs.

The amp --must-- be rebuilt for safe use.

Take a voltmeter and measure for this. There is nothing exotic about cathode followers. They are simply low impedance outputs that allow a longer cable between the output and whatever device is connected externally.

They are called cathode followers because they take the signal from a tube's cathode instead of the plate. The signal at a cathode is typically slightly lower than at it's grid so there is a gain penalty imposed here so either the previous stages will need to have sufficient extra gain or another intermediate stage must be added before the cathode follower tube. Because of this and the benefits of the lower impedance output, a cathode follower was considered a luxury in consumer equipment.

Rob/NYC

Thanks for reply. I'll check for DC on the outputs. I've already rebuilt the amps and they are working just fine and have been reinstalled in the cabinet. I'm just aligning and testing the tuner on the bench and have been using another convenient amp to run the tests. Yes, I kind of remember them talking about these 'cathode follower' outputs allowing long runs between outputs and inputs, of course, in this instance the 'long run' between tuner and amps is only about two feet at the most.

Just to get things clear, you are saying that the cathode follower output from the -tuner- has noise, but the earlier stage from the detector (tape out) does not?

Look also for a noisy resistor(s) in that stage as well as a gassy or microphonic tube. Have you checked the electrolytic cap that filters the DC for that stage? Usually you can just temporarily parallel a similar or greater cap to test.

Rob

Yes, that is correct. Prior stage has no noise, only the final stage which is cathode follower, and IT works just fine with the Maggy amps and the other bench test amp I use. The only time I notice the noise is with the Sony receiver auxiliary input. Must have something to do with this receiver, rather than the Maggy, although it works just fine with other input sources such as a CD player, ceramic phono cartridge, etc.

Now I seem to be having trying to align the FM section of the tuner, although it did seem to be working so-so before trying to align. For some reason, following the instructions in the SAMS photofact, I do not seem to be getting the readings I should be. Maybe my signal generator is not working correctly, OR maybe I must have a VTVM vs. a digital VOM? Maybe it is loading the circuit so that I don't see the voltages I should be seeing. I'm rechecking the instructions and making sure I've got the connection points for the alignment correct. Of course, there is also the possibility that the SAMS is incorrect. Wouldn't be the first time!

Alignment can be tricky -especially on FM sections.

I've done hundreds of them since the late 1960's . For AM I simply do peaking if the circuit is basically Ok. If I don't feel the performance is typical for that design I use alignment as a troubleshooting tool and use an injection oscillator @455khz and diode probe.

For FM it really depends on whether the circuit is essentially performing well and then just a touch of of the limiter/discriminator or ratio detector may do.

With a set as old as yours a thorough rebuild is indicated and some things to look out for are:

Broken windings on the i.F. trans. These can be heat related on tube sets and unfortunately can happen during alignment when the tran core becomes fused to the bobbin and causes it to turn. yep, I've had that...once is enough.

ANother tricky heat or moisture problem occurs on old-style I.F. trans when the mica caps are integrated into the base of the tran. This can be a bitch to deal with. When I've suspected this I set up for a normal sweep alignment, then while watching the display on the scope I slightly wiggle the tran itself and then the solder connections (use a plastic stick or toy pliers). If the dip changes markedly you probably have a defective resonant mica in the base.

Thankfully this is rare on the later trans as they used actual mica caps inside the can next to the coil. I've never had a problem with these. If you have older, cans approx 3/4in square or more you may have the integrated mica's.

What symptoms are you experiencing?

Rob

Thanks for your continuing help on this. First, of course, I'm going to re-check my connections for the alignment to make sure I'm hooking up to the correct points. I do have a scope, but am unsure of it's use, so am trying to use the alignment section which uses just a signal gen. and VTVM. I am using a Heathkit RF signal generator set to produce the 10.7 mc (un-modulated) injected to the grid of the FM mixer, pin #2 (6EA8). To monitor this, according to SAMS, I have the vm connected to the minus side of an output network from the ratio detector (6AL5). This point is actually the minus side of a 4mfd electrolytic mounted on the pcb of the FM tuner section. The meter and rf gen commons are both connected to the chassis as ground, which it is. Setting my meter on the lowest setting which is .25 volts dc I am supposed to watch as I adjust the various coils and set for the maximum deflection of meter needle. The meter barely moves! Actually, since the meter is connected between the minus side of the electrolytic and the chassis ground I would think the reading I would be getting would be minus rather than plus?? If I switch to a digital meter I do, in fact, see a tiny minus reading of maybe 170 millivolts so I tweaked until the reading was the greatest minus value.
After doing this, I could still receive some fair signals from the FM stations on the low end of the band (89.1 mc) but signals at the higher end seemed to get much worse. At the beginning of this alignment I was instructed to set the tuning dial at a point of non interference near 90MC which I did. I do believe the RF gen is okay, but then I don't have anyway of checking it for accuracy. I did try and hook it up to the scope to check it and wasn't exactly sure of how to set the scope or read what I was seeing on the screen. Maybe I should never have started this procedure? Don't fix it if it ain't broke!

You need an accurate 10.7 signal for this procedure, a crystal controlled osc is needed here -a variable is not accurate enough.

What I think has happened is that you misaligned the circuit and the inherent lower sensitivity at higher frequencies in old R.F. amps is making things worse.

Assuming you have accurate voltages and good tubes, you will need to get an accurate 10.7 and work backwards from the last stage which is the discriminator tran towards the first I.F.

If you don't have a crystal marker gen available you may be able to get a decently accurate signal from the variable gen by loosely coupling it to a good working radio and tuning the gen till it blanks out a station. Don't make a direct connection just place a little wire on top of the good set and see if you can get enough signal from the gen to do the trick. Make sure the gen is at/near 10.7 mhz NOT any other freq!

We can go into other items if necessary later, right now good voltages and tubes are priority.

BTW: Have you tried asking about this on the Antique Radios section?

Rob

All this confusion is why I won't recommend novices/tinkerers messing around with alignment of such things.
"The Golden Screwdriver" sometimes talked about.
With no background in electronics and RF understanding, things usually turn ugly.

The voltage on the cathode follower output may exceed the voltage rating on the Sony input capacitors. The output capacitor may have no pull-down resistor, expecting this to be in the amp.

Record-changer wrote:The voltage on the cathode follower output may exceed the voltage rating on the Sony input capacitors. The output capacitor may have no pull-down resistor, expecting this to be in the amp.

The 61 series radio chassis has no output coupling capacitors.
The 8 volt bias at the outputs can be shorted by incorrect connection to another type of amplifier other than the one intended in the set.
This can result in distortion, and possible damage to the tube involved.