[Guest]

Problem with Philco 20: Please take the schematic at http://www.philcoradio.com/tech/images/20a.jpg as reference
I replaced inside of filter cap can with 20microF electrolytes (1-2, 1-3, 1-4). I get 307V at point D (it should be 250V), I get 346V at entrance to choke (pin 2 of filter can), I get -8V at point B, and 55V at points C. The 55V at the C's seems ok (current of the 71A's (2x18mA) on [1400 + 187 = pins 1-2 + 2-3 of the multiresistor 25]Ohm). So, B should be at 187/(1400 + 187) = 12% of 55V = 6.5V. The schematics at nostalgiaair.org, however, says the grids of the 71A's (meaning point B) should be at 50V !!! How can that be?
Questions: 1. what should I get at B and C? 2. why do I get way too much voltage at D? 3. could it be the too high values of the filter caps?

[AI2V]

I get -8V at point B, and 55V at points C. The 55V at the C's seems ok (current of the 71A's (2x18mA) on [1400 + 187 = pins 1-2 + 2-3 of the multiresistor 25]Ohm). So, B should be at 187/(1400 + 187) = 12% of 55V = 6.5V.

Chris-
I don't have a 20 to make references from, so I can't add much more than this:

The grid voltage is often measured with respect to the cathode and something around -55v would sound right for the grid of a 71-A with plate at 300v. 71-A's are not meant to have this plate voltage.

I suggest you back your filter caps back down to 8 or 10mfd per section. 20 is way too much and will raise your B+ too high; if you have excess hum later, you can experiment with bigger caps. In other words, make it play first.

[fabignoli]

Hello, sorry for my english, but in my humble opinion, a value of 20mf, for the first filter capacitor is too high. On mine, I've put capacitor of 2mf on 1-2. It's enough. because if you put sommething like 10mf and 20mf, the DC will be too high and it will be not good for the valve (the 80) because it will more work because of the charge and discharge of the first condenser.

After the choke (the second capacitor filter) you can put anything you want at point 1-4, 10mf or 20mf is OK. But not on point 1-2.

I've recently restart a philco model 20, I've written everything I've done here, but it's in french I'm sorry.


http://cfp.radio.free.fr/restaurations/p...lco20.html



best regards
Frederic ABIGNOLI