[bfrohwein]

Howdy all!

My mother sent me her PT6 to clean/replace the volume control cause it's crunchy at lower volumes and she said the pilot lamp was blowing out every 20 to 30 power cycles.

So I got to thinking about it. I came across an article a while back on the ARF. what do you all think about adding two 6.8 volt 5 watt zeners in series to stop the surge when it's first powered up?

as for the surge.. Would that be because I used 33 uf caps for 22A and 22B? if I remember correctly (it's been a few years since I redid this set) I used 33 because I was getting some hum using 22uf. when it's warmed up the voltage @ the pilot lamp is 5.7 volts.

the placement on my drawing was just random, btw..

[Image: http://i1303.photobucket.com/albums/ag14...7e4040.jpg]

[Image: http://i1303.photobucket.com/albums/ag14...a73194.jpg]

Thanks for your input!

Bret

[morzh]

1. Not sure what you are trying to acomplish. What surge? How do you know it is there?
2. The way you show it a) it is not clear how the complete hookup looks, b) if you keep what you have and add zeners you steal 6.8V from the filaments, why?
3. How does your filter caps cause any surge through filaments (if this is what you mean?).

[Mondial]

If you want to stop the voltage surge on the pilot light, connect the two zener diodes (in series) across the pilot light itself. This will limit the maximum peak voltage across the bulb to 6.8 v. (4.8 V RMS which may make the bulb a bit dim). I would suggest using 9.1 V zeners which will allow the full 6.3V RMS ac volts across the bulb, but not allow it to go any higher.

[bfrohwein]

I wanted to stop the surge. I Thought using too high of a value causes higher voltages..

thanks mondial.

[Mondial]

The voltage surge across the pilot light is not related to the filter caps. A larger value cap will increase the DC voltage from the power supply but not the filament voltage.

The reason the pilot lamp surges is because it is connected in series with the tube filaments. On first turn on, the resistance of the cold tube filaments is lower than when at operating temperature, so there is a surge of current as they warm up. Because the pilot light filament heats much faster than the tube filaments it flares brightly, and is subjected to overvoltage until the tube filaments are fully heated.

[Arran]

.It looks like this was one of the earlier Philco AC/DC models where they insisted on using a 35Z3 rather then a 35Z5. The 35Z5 has a heater tap for the pilot lamp that has 6.3 volts across it, this one has the pilot lamp parallel with a resistor and that resistor is in series with the tube filaments. No the new filter caps have nothing to do with the pilot lamp burning out, that could be caused by using the wrong bulb type or by the resistor it's paralleled with being defective neither of which has anything to do with the B+ supply. One thing you could do to stop the surge is use a current limiter, Philco used an early version of one of these in the early 1950s calling it a tube saver.
Regards
Arran

[bfrohwein]

It's a 1941 PT2 in a wooden case.

I was going to use a CL90 to begin with. but I was thinking about the heat and probably a little bored and wanted something exotic. my 53-701 has one of those tube savers in there. If I remember correctly it said 680 @ room temperature and 100 when hot. Although on that set they use the heater tap on the 35W4.

[bfrohwein]

success! works like a charm...

I ended up using the 6.8 volt 5 watt. the 9.1 clamped @ 9.2 but the spike from turning it off and back on quickly was still blowing the bulb. that was my second concern.

just out of curiosity, for the rapid off-on test. I tried a modern 47 (AML from pinball life) and an NOS Tung-Sol 47. The AML blew @ 9.2 and the Tung-sol did not.

I did suspect part of the problem here were the cheap, modern bulbs.

Also.. No 35Z3's were destroyed in this test...

I did test voltages on the tubes as well and all filament voltages are where they should be.

Thanks Again, Mondial

[bfrohwein]

I suppose I should let this thread go away... turned out to be the bulbs.. I switched to Eiko brand and they're not burning out on repeated hot power cycles..