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  #31  
Old 02-21-2015, 06:09 PM
dieseljeep dieseljeep is offline
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Originally Posted by TUD1 View Post
It took all morning to get the chassis out if the cabinet. By the time we go it apart, it was time to go. One of the older gentlemen there said I could replace the 6X5 with a 5Y3. If I remember correctly.
The pinout is totally different on the 5Y3 VS the 6X5. Plus the heater voltage.
The last chassis like that, I replaced to PT on, I replaced the 6X5, with a 5Y4, as the socket didn't have all the contacts for the more common 5Y3.
If the transformer has a separate five volt winding for a rectifier tube, I prefer to use a five volt rectifier tube, fully knowing that the six volt rectifier tube isn't that reliable.
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  #32  
Old 02-21-2015, 06:30 PM
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Originally Posted by Kevin Kuehn View Post
The 5Y3 requires a 5V filament supply versus the 6X5 uses 6.3V. Also the current draw may be different? I was not aware that there is a known failure problem using the 6X5GT. Thought that was only with the original 6X5.
The NOS one that went bad on me was a 6X5GT....
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  #33  
Old 02-21-2015, 07:03 PM
dieseljeep dieseljeep is offline
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Originally Posted by Electronic M View Post
The NOS one that went bad on me was a 6X5GT....
There was a large difference. The 6X5 only draws 600ma, heater current@ 6.3 volts. I was tempted to use a 6AX5, but the heater current is 1.2 amps@ 6.3 amps. The Zenith power transformers were undersized, as it was, without adding to the load.
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  #34  
Old 02-21-2015, 09:01 PM
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Quote:
Originally Posted by dieseljeep View Post
My Zenith chairside has the same chassis. That chassis uses the direct coupled triode driver and 6AC5 triode output. Why????? most of the other Zeniths used penthodes.
Mine came with, probably the last 6X5GT's made. It has the small internal structure like the 6X4. I'll leave it!
I wondered this as well and looked at a book I just got "understanding hi-fi circuits" by Norman Crowhurst. What I took away was this:
The triode output versus the pentode has lower harmonic distortion but the plate efficiency is also much lower, reducing output power. Additionally, the handling of varying plate load, due to output transformer and speaker reactance across the frequency spectrum is improved with the triode. It is likely a tradeoff that Zenith accepted in exchange for better fidelity.

This odd feature of the Zeniths is motivation enough to recap (and replace the tranny) and give it a serious listen.
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  #35  
Old 02-21-2015, 09:33 PM
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Tube data sheets show the 6AC5 capable of 3-4 watts, with around 250 or so plate volts...-in SE..which your chassis is.
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  #36  
Old 02-28-2015, 05:23 PM
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I went to the workshop this morning and started early on my Zenith. I replaced a few small capacitors, and a 16 MFD 475 volt big capacitor. I checked almost all the tubes and put on a new power cord. I then proceed to "smoke test." So I yell "smoke test!" I powered it up slowly on an isolation transformer, and when I got up to about 50 volts, the current jumped way up. We concluded that the circuit is short.
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  #37  
Old 02-28-2015, 09:10 PM
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ok...at this point...you need ot see if the power tranny is shorted or not.

With the 6X5 rect out of the ckt and NO diodes in to sub it....( IOW--and OPEN ckt on the HV winding of the tranny), and ALL tubes out of the set...do another variac test. THIS time...you should NOT get much draw--no more than maybe 25 watts or so--likely less--with an open ckt on the transformer. If you DO--then disconnect ALL leads from it-other than the primary--and try again. If it STILL drews over 1 amp or higher--with no laod--the HV winding is shorted. if it then drops to a very low draw...there is SOMETHING still shorted i the ckt, likely a cap somewhere.
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  #38  
Old 02-28-2015, 10:46 PM
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I won't be able to work on this radio again for two weeks, the radio club will all be at the BirmingHAMfest on March 7, and so will I. So I should take all the tubes out and try it on the variac? I kind of suspected it was a capacitor, those darn things love to go short.

I got this ARRL The Radio Amateur's Handbook today while working on my Zenith. It goes into extreme detail on electrical theory of all types. There's some pretty brutal math in there.
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Last edited by TUD1; 08-07-2016 at 10:21 PM.
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  #39  
Old 03-04-2015, 09:32 PM
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In the absence of my Zenith, my 1941 Philco is doing an A1 job. It sounds amazing and hasn't skipped a beat.
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Last edited by TUD1; 08-07-2016 at 10:21 PM.
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  #40  
Old 03-04-2015, 09:33 PM
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The inside. It gets a little warm back there after a while.
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Last edited by TUD1; 08-07-2016 at 10:21 PM.
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