[Steve Davis]

Just noticed a model 70 chassis on ebay, item number 370232120717 (sorry, I don't know how to link. I'm kind of dumb with computers). Any how this guy has this thing running on the bench with no speaker connected! The screen grid of the 47 output is glowing bright orange

Steve

[kruc]

The seller doesn't appear to be an electronics tech. In his description of a Philco 46-350 that sold last month he states "UNTESTED BECAUSE CORD IS FRAYED." Fortunately for us the radio gods have saved many radios by causing the cords to self destruct, protecting many of these vintage treasures from their knuckle dragging owners. All these pictures prove is that the heavy filament winding is intact. As the seller advises "SOLD AS IS FOR PARTS OR RESTORATION", I'd follow the auction if I were looking for a spare 70 chassis.
Dave

[Arran]

That Canadian Philco Model 706A (U.S model 46-1201) that I am working on was saved in much that manner, the cord was intact but was insulated with old crumbling rubber so I don't think that it was plugged in for some time. As a matter of fact I doubt whether anyone has taken the chassis out since the set was built, it had all of the original condensers and resistors as far as I can tell. But if you saw how many wires have to be unsoldered to remove that chassis it would have dawned on any weekend amateur repairman that getting at the tubes was the easy part and maybe this set should be put aside for later.
Best Regards
Arran

[Ron Ramirez]

http://cgi.ebay.com/370232120717

I've seen the glowing screen grid in many 70 sets (with speaker connected, that is), and here is my opinion on why this happens.

Philco ran a higher B+ on the screen grid than on the plate of the pentode audio output tubes of most of its radios in the 1930s. You take that, combined with the fact that the B+ applied to those 47 tubes was already at design maximum level, and then add in today's higher line voltages, and voila - orange screen grid because the voltage is now too high!

I've started dropping the screen voltage on these sets by adding a 5.1K resistor in series with the 47 screen grid. This lowers the screen voltage to a level below design maximum. It makes the tube much happier. Yes, it's a mod, but it is completely and easily reversible.

[Arran]

I can't tell from that picture whether the screen grid is glowing or not. A lot of directly heated output tubes tend to look like that but upon closer examination it's simply a reflection from the heater. If the screen is glowing then Ron's explanation makes sense if that's what's happening, I had the idea that it might be caused by powering up the radio without a speaker. Without the speaker field connected the power supply is almost running with no load aside from the power output stage, I can't remember whether the output transformer is speaker or chassis mounted on a model 70. If the output transformer is on the speaker then the power supply would be running flat out with no load, and the screen would be getting 400-500 volts or so, that will make it glow!
Best Regards
Arran

[Ron Ramirez]

Yes, Arran, with the speaker disconnected, the plate of the 47 tube is not getting any voltage, but the screen grid is!

The output transformer is mounted to the speaker on the Model 70, by the way, as it was in all Philco home AC sets through the 1938 model year. In the 1939 season they began putting the output transformer under the chassis in most models. These tend not to survive while the speaker-mounted outputs are usually still good.

[Steve Davis]

Philco probably saved about a penny on each set by connecting the screen grid directly to B+.

Steve

[kruc]

Ron, does that 5.1k resistor mod apply to a Model 71 with it's 42 output tube also? I noticed that the 71's screen grid and plate voltages are the same.
Dave

[Ron Ramirez]

Yes, it does. I did the same thing to my "Super 71" when I built it, because Philco was running a higher B+ on the screen than on the plate.

[kruc]

I'm interested in your Super 71, Ron. I'll read the archives to find details. I just got a 71 in original smoke covered, curtain burning condition from a trinket seller on ePay. I've got a boatload of caps coming from AES for it. It's next in line after my 3rd 20, luckily I just located a work-around on the main site for the 20's interstage open primary. BTW, are 400v caps kosher for the 20's filter, or should I go higher? I've never measured voltage across the 1uf and 1.5uf caps on my earlier rebuilds that have 400v units, but I suspect there isn't much of a margin.
Dave

[Music in a bottle]

http://cgi.ebay.com/ws/ebayISAPI.dll?Vie...0232120717

I've seen the glowing screen grid in many 70 sets (with speaker connected, that is), and here is my opinion on why this happens.

Philco ran a higher B+ on the screen grid than on the plate of the pentode audio output tubes of most of its radios in the 1930s. You take that, combined with the fact that the B+ applied to those 47 tubes was already at design maximum level, and then add in today's higher line voltages, and voila - orange screen grid because the voltage is now too high!

I've started dropping the screen voltage on these sets by adding a 5.1K resistor in series with the 47 screen grid. This lowers the screen voltage to a level below design maximum. It makes the tube much happier. Yes, it's a mod, but it is completely and easily reversible.

Ron, would it be advisable to run the model 70 sets with a Variac to get the correct operating line voltage? What line voltage were these sets designed around?

[codefox1]

I'd always go with the 630 VDC caps, they are only a couple of pennies more, and certainly size is not a problem. Solder once, listen many times. Expect all voltages to be a little larger with today's line voltage. And if you replace filter caps with a little larger size, you will get a few more volts too. So sometimes you have to tame the B+ a little to provide long term reliability. Those tubes aren't getting any younger. N.B. - also applies to series string sets, and double careful on "portables" with their delicate filaments.

You don't want to starve the filaments though, because that can create problems of it's own, like getters not getting hot enough to do their job of cleaning up the rarified atmosphere. It's certainly is a juggling act, isn't it?

[kruc]

Codefox, I'm keeping the same capacitance the sets were designed for, I just wondered if my 400v caps that were at their upper voltage limit. I don't have an issue with the slightly higher price of the 630volt caps, I just happened to get some 400v units at the time. I'm trying to stay away from electrolytics whenever possible when I rebuild a unit, that's why I'm using plastic caps whenever possible. FWIW, I found 600v plastic caps at AES. B+ shouldn't be an issue after replacing the filter with the same capacitance.
Music, my 20 lists a primary of 105-125volts. However at 125 the secondaries will put out a higher voltage than the tube filaments and plates were designed for. Recent threads on ARF discussed using a bucking transformer or a resistor in series with the primary. The obvious issues are finding space for a transformer or adequate area for heatsinking a wirewound resistor. I have a Japanese superhet with a 100v primary I run off a variac as I haven't gotten around to reducing the input voltage permanently.
Dave

[Ron Ramirez]

Ron, would it be advisable to run the model 70 sets with a Variac to get the correct operating line voltage? What line voltage were these sets designed around?

You could use a Variac, but a lot of guys use a transformer setup to "buck" the line voltage to a lower level. Maybe Ed Locker or someone else who has done this will chime in here with details. (I haven't built one, but I have one that Ed built.)

At the time the Model 70 was built, 110 volts was apparently the norm, with 115 being the upper limit. Now, 125 is the norm, and mine has run as high as 128. Old radios do not like this higher line voltage.

[Music in a bottle]

Thanks for the heads up on the voltage. After my set gets restored this winter (maybe) I'm going to employ my Variac for controlling that excess voltage. If you were to utilize an inline fuse, what would you recommend?