[Brad Winder]

New toy in the collection to play with- 3650 tombstone! Very heavy!

Chassis is clean, but one of the electrolytic cans has blown it's top, and the 80 rectifier tests open filament, so...may find bad things when I take the chassis out. 

[MrFixr55]

Nice looking set! Hope the power transformer secondary for HV did not open. Blown 80s are why I put diodes in series with the xformer leads to the rectifier plates If there is a plate to cathode short, the existence of the solid state diode prevents a filament to plate short from blowing the transformer. That Idea may have come from the Phil's Old Radios website, I am not sure, but I greatly thank the person who thought it up.

[Brad Winder]

Oh really? I will check to see if that information is up on that site...

[MrFixr55]

The "Diode" mod that I describe is useful for radios that use the 80, 5Y3, 6Y4, 5Z3, 5U4 etc. full wave rectifiers, where the filament is the cathode.  if the filament opens from overheating, etc. or in the old 80s, just from age, and touches the plate, the tube is shorted from plate to cathode.  this puts AC across the electrolytic filter caps, shorting and possibly blowing them up.  In addition, for 1/2 cycle, there will be a dead short across the transformer secondary.  Since these sets usually are not fused, the secondary will burn out, sometimes with fire.  This actually happened to me and I lost an early post-war Philco console, my parents and sister lost hair and I was forced to put the radio to the curb.  Mom would not let me restore it and Dad knew where his bread was buttered. 

The actual procedure is pretty easy.  You can use 1N4007 (1A, 1000PIV) diodes.  I use 3A 1000PRV diodes.  Simply:
-Unsolder the 2 HV "legs" from the Plate pins of the 80, 5Y3, etc.
-Solder the cathode (band) end of a diode to each plate pin of the rectifier socket.
-Slip a piece of heat shrink tubing over each of the 2 transformer HV secondary lead, as far down the lead as possible.
-Solder each of the 2 HV leads to the anode of each of the 2 diode installed.
-Slide the heat shrink tubing over the diodes, covering all exposed leads and the solder "joint between each diode and the
  transformer secondary lead.  Shrink the tubing.
-Add a fuse to the transformer primary between the power cord and power switch.

See the next post for a Scematic showing the "Mod".

[MrFixr55]

Since a picture is worth 1,000 words (or 10,000 of my words, attached below is a partial schematic of a Philco Model 20 Power supply section with the diode mod and fuse added:
   

It is not necessary to do this mod on sets with 5AR4, 84, 6X5, 7Y4 etc. rectifiers, as these tubes have a "heater/ sleeve" cathode arrangement, which are not likely to short.

I cannot remember where I learned of this mod, I did not think it up myself. I believe that it was from "Phil's Old Radios"; I am not sure. However, I gratefully give credit to whomever thought this up and first published it on the internet!

[GarySP]

I never saw that modification. Ounce of prevention never hurts! Take care and BE HEALTHY! Gary

[Arran]

Quote: "It is not necessary to do this mod on sets with 5AR4, 84, 6X5, 7Y4 etc. rectifiers, as these tubes have a "heater/ sleeve" cathode arrangement, which are not likely to short." Actually it is probably more necessary, because one of the failure modes in a 6X5 is a heater to cathode short, the isolation can break down if the tube is overloaded. I've never had any problems with an #80 itself, in fact it's no unusual for me to run into a 1930s era AC set that still has it's original #80 rectifier. More often then not it's a short, or low resistance across the H.V line that cause the rectifier to fail, usually the input filter cap. I've also had HV windings develop shorts between the turns, particularly on early 1940s set with small transformers, presumably they were damaged in some way.
Regards
Arran

[MrFixr55]

Hi Arran,

It happened to me twice, once with an old school 80, the other with a 5Y3. The one that all but caught fire was due to a filament to plate short on a 5Y3 which was a much stronger construction than an 80. This was a working post war Philco console radio that was about 25 years old at the time. I do run old 80s, especially globes in my Philco 20, Radiola 17, 60 and my GE S-22 (RCA R7A), but I have recapped these, performed the diode mod and added AGC fuses for slightly above the power rating.

The diode mod will not protect from a heater to cathode short on a 6X5. Yeah, that failure is a disaster that can be only protected by a fuse (or not, depending on the rating used). The 6X5 and its cousins are not prone to plate-filament shorts.

Yes, I did have 2 radios that had cooked power transformers when I received them. My RCA 5T7 (ca 1936) had a transformer that would heat up with no load. When i opened the transformer all the insulation paper was black. The other was a GE G53, ca 1938. (GE was building their own sets by then; it was not built by RCA and did not have the usual "This set designed to use RCA Radiotrons". There was a sticker advertising GE Metal tubes.) The tube complement was the same except that the GE used a 6K6 instead of a 6F6. The transformer in this set had actually caught fire.

[morzh]

might as well toss the 80 and save 10 Watts of power. Of course, the benefit of slow voltage rise will be gone.
On the other hands, the limit on the filter capacitance will also be gone.

[Brad Winder]

Lol! I believe that I will put another 80 in. I'm not a fan of mods unless they are truly necessary. I've honestly never had a problem with an 80 rectifier causing issues, which I cannot say for later rectifier tubes.

[MrFixr55]

I hear U. Fortunately, the mod can be rather well disguised with spaghetti, heat shrink, etc. (Heck, if one has modern caps and resistors exposed, this is nothing. For authenticity, correct B+ voltage and slow B+ rise, I leave the 80 / 5Y3/etc. rectifier in.

Let us know how you make out after the resto. Looks like a neat radio, what with the 42s in Triode mode, AB2 I suppose. Really nice looking chassis and cabinet!

[Radioroslyn]

The thing I'm interested in is on something like the high tube count table top sets and the current consumption thru the 80. Like the 16B or 116B RCA sez the max current output is 125ma. I'm thinking these sets gotta be pushing the 80 close to it's limits. It must be putting a beatdown on the 80 and I'm a little surprised that Philco didn't opt to use the 5Z3  in the table top sets. It's good for 250 ma but had a 3a filament vs a 2a in the 80. Not just the 16's but the ones that use the super class A output stage.

I've got a 16B up on it side on the bench. I should hook a meter to it and see what the current is. I've been fooling with car stuff lately.

[markg123]

Hey MrFixr55 - Unless I missed it, you didn't give a recommendation for the fuse on the transformer primary between the power cord and switch. What do you/would you use there?

[radiorich]

Hello mark,
Far as your Answer to Line fuse you could easily mount one under the chassis here is the type I use .
Also I see your plate on the back of the radio says it draws 110 watts so that means its pulling just under 1 amp but you will want to go over that and I would use a slow blow .
Sincerely Richard

[MrFixr55]

Hi Mark,

Sorry, juggling chainsaws. Rich answered your question well. My preference for fast fuses with current ratings about 1.4X the advertised current is based on the fact that tube style rectifiers will provide a soft start to the power supply, coupled with the relatively small filter caps used in vintage radios. On the other hand, cold tube heaters draw a high amount of current. I will defer to others who have restored more radios than I have.

Solid state radios or tube amps with solid state rectifiers and large caps have a high inrush current and often, a slow fuse is specified. The issue is how long a part such as a rectifier or transformer winding can take an excess load. Solid state amps usually have speaker fuses. Replace them with exactly what the manufacturer recommends. These are usually fast fuses. However, nothing blows faster than an output transistor. It will short, thereby blowing the fuse. I repaired many an amp when I was a kid because the users would put about four 8 Ohm speakers in parallel and blast the thing or be sloppy on the speaker connections.