A new project fell into my lap...

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I fully intend to keep up the fight until this thing sings once again.  I just can't get anything done during the week while I'm teaching.  I'm sure I will be back at it this weekend after grading papers and coaching a cross-country meet. 

 

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OK, got some new parts in and spent part of my weekend basically rebuilding the whole system.  The only items that I did not replace at this point is the stylus/cartrigde, the output transformer and the switch/potentiometer.

My voltages are much stronger, but still not up to spec and we're still not actually amplifying.  It does seem to pass whatever input it gets to the speaker, though.

Here is the schematic with voltages labeled.  All voltages are DC unless noted with VAC.  I have to think that the only remaining culprit would be the output transformer.

Here it is:

 

[2wo]

Per your posted voltages, it looks like it is working fine. Keep in mind that the tube has to drive the speaker as well as provide gain. I am too lazy to look up the tube. Try driving it with a high output line stage, like a Foreplay 2 and see how that works...John 

 

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Unfortunately, it isn't really working.  Paul Joppa mentioned that I should have closer to 135v on pin 7 and I'm a bit shy on that.  I even hooked up my signal generator to the input and go no amplification whatsoever.  It basically sounded the same if I hooked the generator up to the inputs or just straight to the speaker.  It is passing but not amplifying the signal.

I will probably swap out the output xformer with a speco 7010 (old style) when I get a chance.  May be a few days.

 

[Paul Birkeland]

I wouldn't bother replacing the output transformer, you can generate 60Hz and measure the input vs. output voltage (AC) to see if the ratio still looks good.

The 25C5 is drawing a pretty healthy amount of current, and it looks like you have ~254 Ohms of resistance on the primary of the output transformer, which is generally a good sign. 

Looking at the 25C4 datasheet, there's an operating point that is 110V plate, 110V screen, 7.5V bias, 4ma screen current, 50ma plate current. 

This isn't horribly off from your datapoint, the decreased screen voltage will raise the gain slightly.  With the book operating point, you get 1.9 watts output, which would be 3.9V into 8 Ohms (2.75V into 4 Ohms), but with more voltage needed at the grid of the 25C5 to get you there (the output transformer step-down isn't helping).

You could have a bit of a dead 25C5, did you try another?  If you dropped the screen voltage even lower, I would expect gain to go up bit.  Also, a ceramic phono cartridge can have in excess of 1V of output, which would not require much voltage gain from the 25C5.  (Frankly, if the ceramic cartridge was removed and replaced by a moving magnet cartridge, you'll really spin your wheels trying to get the player to work properly)

 

[2wo]

You did replace the 30u & 50u caps?

 

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Yes, every cap and resistor was replaced.  I have three tubes.  The original plus two NOS tubes recently purchased.  Could be the ones I bought were not all that good.  I will do some tube swapping Friday night.

 

[2wo]

The Selenium rectifier as well?

 

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Rectifier got replaced with the caps.  The only remaining original parts are the socket, switch/pot, and the cartridge/stylus.

Last night I connected the cartridge directly to a rat shack "Archer" 9v amp and got a little gain but not a lot and it was pretty noisy since it was not properly grounded.  This at least tells me the cartridge is putting out some signal.  Just do not know if it is up to spec or not.

I have had one online vendor tell me I would not be able to find a new replacement cartridge.  Buggers!

 

[Grainger49]

If it is set up for a ceramic cartridge you can feed it one leg of your CD player's output to test it. 

 

[Paul Birkeland]

You could try this circuit with the TL-431.  You won't need a transformer, but you will need the volume control. I'd set this up to drive the 25C5, gain will no longer be an issue.