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Philco 87 grid voltages - Printable Version

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Philco 87 grid voltages - HooiserScootrer - 04-17-2019

Good morning,

I'm new to the Phorum but expect to visit often, hopefully more to improve my knowledge than plea for help.

I completed the electronic restoration of a model 87 and it plays wonderfully.  The only thing that has me scratching my head are the grid voltages on the #26 RF tubes and the 45 output tubes.  Rider's calls for 6 volts on the RF tube grids and 45 on the 45 tube grids. I believe they are all mislabeled as positive rather than negative voltages but that's not the issue. My issue is that I measure 0 volts on all the grids.  I'm measuring from chassis ground. The plate voltages are within range of the schematic.

The radio performs perfectly so I don't think I have a problem but why am I not getting negative voltages at the grids?  The only issue this radio had was an open field coil which I had rewound by the gentlemen at the Alabama Historical Radio Society. They do great work. Oh, it also had sprung tuning cables. That was a joy to fix. Finally decided to give up on the cables and use fishing leaders. 

Thanks in advance for any help on my voltage conundrum.

Scott

RE: Philco 87 grid voltages - rfeenstra - 04-17-2019

Welcome!  Are the cathodes running at a positive voltage giving a net negative voltage on the grids? Would have to look at the schematic to see how they are biased.

RE: Philco 87 grid voltages - Radioroslyn - 04-17-2019

Hi Scott and welcome,
Some of those early set w/the directly heated tubes can be a little confusing. It's also not too uncommon for the diagram not to notate + or - voltages, just voltage and not polarity. To get a better idea whether to grid is + or - measure from the filament to grid rather that the chassis. Funny thing is that RCA sez the 45's grids should be - 50-60v not 6v. >Oh, it also had sprung tuning cables. That was a joy to fix. Yes they are! [Image: https://philcoradio.com/phorum/images/smilies/icon_evil.gif] There was a thread abt it here a week or two ago. A lot of these early sets have open primaries on the rf coils and the audio transformers. No big deal to fix or replace.
Have had a few 87's over the yrs great players for '29 technology.

GL

RE: Philco 87 grid voltages - HooiserScootrer - 04-17-2019

Gents,

Thanks for pointing me in the right direction. I should have realized that the negative point of reference for the grids was the filaments, measuring from there shows that all is well with the grid voltages.

I am impressed by the sensitivity of this radio. With just a 6 foot wire in my basement I'm picking up stations 300 miles away (at high noon). It should be a hot performer upstairs with a proper antenna and ground.

Cheers,

Scott