Why the high voltage capacitor in an AA5?

The practice of putting a fuse in the cathode of an output tube is a good idea. Just remember to make it accessible.

Well, it was surprising to me too. The caps were only marked with the value and voltage rating-no manufacturers name, were very small and light weight. Keeping in mind that caps and resistors have gotten smaller and generally better than they used to be those caps seemed just too small for what they were marked. Maybe it was a bad batch. It was some time ago-not recently. I never did email them back about it. The rest of the caps I got were fine. 

I was buying electrolytics from MCM Electronics. You could choose the brand and style you wanted from them at reasonable prices. They also had a large stock of Japanese transistors and ICs. One time I ordered some 211 tubes from them at $25 a piece on a close out sale. 

MCM got bought out by Newark a while ago. I've looked at what they now call "Newark 14" for various items and supplies but haven't ordered anything yet. MCM type items can still be found there. 

Lately, I've been getting some caps from Radio Daze and others from eBay sellers-some directly from China.

Whenever possible, I buy my 'lytics from BND Enterprises. https://www.bdent.com/
 Yeah, they are the single ended ones, but they back 100% of what they sell.
 I am surprised about the 'lytics from Canada. I always had good luck from them.

Frank,
Well, I can't argue with that. I do like checking out caps on the Eico equipment I have here now. I used to use a Sprague TO-6 cap checker that was good for checking leakage. I may start checking out the new film caps now-it would be fun and only take a few minutes. I do check out older used and NOS film caps I have on hand here and the occasional mica for replacement use. Some equipment I will put all new film caps in so as to match. Some time back I put in an order with the cap supplier up in Canada (I think it was Just Radios). The service was fast but in that particular case the generic (no markings at all and very small) 10uF 450V electrolytics they sent were not good at higher voltages. They got really flaky in regards to leakage current above 400VDC. Good electrolytics are made to run continuously at the rated voltage.

Rod,
 When I was servicing full time I always checked every part possible before I installed it.
 There were so many counterfeit parts and poor quality control on OEM parts I made it standard practice.
 On capacitors, I always tested them at full rated voltage whenever possible. I found many "new" ones that wouldn't hold up to their supposed ratings.
  I started this because I lost over $200.00 on one TV repair because I didn't check the "new" parts the supplier sent. Plus a P.O.'d customer.

1. Yes. A low amperage fuse could be installed in the cathode lead of the higher output amplifiers. .1 to .2 Amp  fuses would be good. 

2. I've seen a very few reports about yellow film caps being bad out of the package but that hasn't been the case here. I do check electrolytics out before installing but usually don't do that with the film caps.

73, Rod  WB6FBF

Glad to get verification of my suspicions.
I always replaced them with the original voltage (or higher) capacitor, when I did repairs.
 I still wonder about the quality of the imported "banana caps" though.

Could you install a fuse in the cathode line?

I myself have had problem radio receivers with blown audio output transformers. I got in an otherwise nice short-wave equipped Silvertone console with a bad, open push-pull output transformer. I replaced it with another and restored operation enjoying the results. Well, it wasn't long before it went bad again with another burned out output xfmr and that puzzled me. Can't be sure now but I may not have put back in the primary capacitor which I have since found has at least (3) beneficial functions. I didn't think they were important in those days. I used to cut out those capacitors that went directly from the plate to cathode or to chassis ground (B-) in an effort to eliminate shorts that could take out the transformer directly. 

I never got around to looking into the Silvertone radio problem again. But in working with a couple of Heathkit DX-100 ham radio transmitters I realized the problem cropped up when an audio output stage is overdriven to cutoff and beyond to the point that conduction in the tube stops (in other words the tube (tubes) open up-no plate current can flow) and the energy in the primary of the transformer has to go somewhere. Receivers do have their speaker present on the output transformer secondary that should act as a  discharge path for surges but such failures can and do occur anyway. 

In the particular case of an AM Phone transmitter, when the Class C final amplifier plate current is also cutoff due to excessive negative modulation, there will be no load present on secondary side of the Modulation transformer. This is where the shunt capacitor comes in and why it is so important. It provides a discharge path for the currents and energy stored in the transformer during modulation. If you happen to be blasting into your microphone under these conditions you may get big trouble. 

In the restoration of the first of my (2) DX-100 transmitters I had cut out the shunt capacitor on the modulation transformer as usual. It looked bad. It the case of modulation transformers, the cap can be installed across the push-pull primary side or across the secondary winding. The DX-100 called for a .01uF cap across the primary. I thought that excessively large a value (might roll off the high frequencies-not true in practice though) but in fact the choice depends on the circuit impedances involved and lower impedance circuits call for larger capacitors to be employed. The plate voltage on the modulators and final run about 750VDC. My guess is the plate to plate load on the modulators is about 6000-8000 Ohms. The secondary impedance is around 3000 Ohms-fairly low. So a .01 cap is not a bad choice for the job. 

Well, I got a lot of trouble with the 1625 modulators in the testing and initial operating of the rig. Surges were present and 1625 tubes were being ruined as a result-not the mod transformer at least. It was always in the same position 1625 tube socket that would go bad. The tubes would open up internally. I started to run out of 1625s and had to ask around for spares. What was happening was that the internal cathode connection ribbon of metal in the tubes would open up just like a fuse going open. You could see it with an inspection of the tubes. After this happening a few times, I was convinced and forced to put in a new .01 cap across the transformer primary. I don't remember the voltage rating of the cap but would guess it would have to be at least twice the plate voltage on the tubes-so I am guessing it would be rated 1500-2000VDC. That solved the problem and no more blown 1625 tubes.

FrankB wrote:

I have been told "The new type capacitors at 630V work fine in that position". [You must be registered and logged in to see this image.]

In a word "NO", but who cares?

The issue with plain banana yellows is the metalizing cannot propagate the high peak pulse from the collapse of the magnetic field in the audio output transformer.

In fact this similar same pulse, appears in vibrator power supplies, strobe trigger circuits and to a lesser degree the AC power-line. In 'scoping the pulse it is comprised of a number of frequencies when combined look like a mount Everest.

What happens is the metalizing cannot support the current flow as the dielectric (normally) polarizes, net result the metalizing breaks away into isolated islands. There is no "self-healing burnout". After the pulse event the capacity is reduced, still not leaking, however, depending upon the B+ these islands can micro-arc and make electrical noise. This is not an arc between "plates". This occurs in the common "yellows" when used in the plate output circuit, buffer and pulse networks (not really a radio application). There is no breakdown of the plate in a foil capacitor, simply leakage in the dielectric if paper. OEM would specify a higher voltage in a foil/paper unit for the plate circuit. For a buffer the internal design with a foil/paper is a bit different to account for the far greater energy in the pulse, generally, it is a type of non-inductive winding.

How to specify the "correct" type of capacitor for the plate circuit? Most cap manufacturer still have foil capacitors with a variety of dielectrics. Those that are suitable will have a published rating called dv/dt, that is delta voltage over delta time. That in of itself is a clue to the robustness of the cap. In of itself this is like a speedometer for the cap, "how fast can the cap go"... The rating is locked to the value and voltage rating of the cap. So cap selection is for the microfarad and the voltage as specified by the OEM. As for some of the OEM selections I have seen at 200 volts for the plate bypass, well it must be one of those $9.95 disposable radios...

So, simply choose the ratings say .01/630 but from the offering of caps with a dv/dt rating. That should be safe enough. If the OEM does specify 1kv or higher go with that voltage rating . I would suspect that higher voltage if there was a high plate resistance tube in the last audio like some of the pentodes. If the value is odd like tone tweaking and as a plate bypass tone will be modified. Opt to use two or more identical caps series or parallel. The dv/dt rating will be much higher in other words shorter time over greater voltage. These will combine for better resistance to a destructive pulse.

This technique is applied to critical values of buffer caps where two caps of half the desired value are used because the dv/dt for a single cap is insufficient for the rise time of the pulse.

It is practical to always choose a a "buffer" rated cap for that service, however, a cap with dv/dt ratings can be pressed into service.

For the plate circuits(s) I plural because often, this includes some sort tone network as those too as they are expose to the same pulse. Often there is a rheostat or potentiometer involved and too often the control is at the B+. Having experienced tone control failures, a re-configuration of the tone network to take the control out of the high voltage is done. Simply halve the value in mf and place that value each side of the control if need be.

Note that caps built with the a published dv/dt rating do not become "safety" caps, those that do not fail shorted. Even though there can be some spikes on the line, safety caps should still be used and substituting a dv/dt rated cap in not a good idea.

Note there are metalized dv/dt rated caps those are a viable substitute for a rated foil/film cap. It have been my finding that these are not popularly made.

There is an extensive WIKI on caps that may help "confuse" one more.

My experience with metalized caps comes with bird dogging failures in pulse circuits where the pulse is the nature of the signal. Actually seeing a metalized cap change value progressively after three hits of a pulse that was normal operation for the circuit. Having taken dipped metalized and "cracking" them open to find the broken metalized areas. Disassembling audio transformers to find the break, seeing the carbonized area and the ends of the winding balled and tiny copper balls surrounding the area. Clear signs of a high energy pulse that did not get absorbed by a shunting capacitor.

In short, using an antique radio in an electrical storm is not only annoying but some risk to the radio.

At the end of the day folks will repair their radios or customers radios in any fashion desired. Even the best knowledge cannot prevent poor component choices. I know I have made my share...

O.T.

IMHO (speculation at best) audio transformer failures in 20's radios are partly responsible to use during an electrical storm...

Yes, I have seen transformers simply corroded, apparently from corrosive contaminates during manufacture. Someone brought their fish & chips to the winding room...

So, raising the voltage rating of a capacitor in a particular application is no solution for failure, it is the internal construction that counts...

I am still reluctant to populate my Scott Philharmonic with yellows, but I won't use caps "rolled on thighs of virgins" either... "Specs, man, just give me the specs."

As always, YMMV

Chas

I have often been asked why the capacitor across the primary of the audio output transformer is often rated at 1200, 1600, 1800, or 2KV.
 It has to do with the inductive "kick" in that position.
 I have been told "The new type capacitors at 630V work fine in that position".
 I am aware that the newer type caps have great self healing abilities, but am quite skeptical as just how "great: they actually are.
 
This "kick" is also why buffer capacitors in car radios  have such a high voltage rating.