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 Post subject: Schematic error?
PostPosted: Sep Sat 18, 2021 12:23 am 
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Joined: Dec Sat 14, 2013 2:50 pm
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Location: 60062
I am working on the Detrola 146 set. There is a cap between the 75 tube plate and 42 tube grid which should be .01uF as per schematic. The old paper one installed on the chassis is .001uF. The radio works. Should I recap it following schematic or keep the .001 value? Thanks.

http://www.nostalgiaair.org/pagesbymode ... 003886.pdf


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 Post subject: Re: Schematic error?
PostPosted: Sep Sat 18, 2021 1:48 am 
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Location: Mountains of Mourne. Ireland.
This is more legible.
Attached below is a schematic with alignment, chassis layout, trimmer locations and parts list...
for Detrola model 146E
from Gernsback's Official Radio Service Manual, Volume seven. 1937
/download/file.php?id=359033

Greg.


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 Post subject: Re: Schematic error?
PostPosted: Sep Sat 18, 2021 1:51 am 
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Location: Stone Mountain, GA
Hard to read the schematic. Can't make out the grid resister on the 42 but may be 500k (M).

At 500k meg .01 would give low freq response down to almost subharmonic.
0.001 would start to drop off at below 300hz.

If it sounds good leave it.

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 Post subject: Re: Schematic error?
PostPosted: Sep Sun 19, 2021 2:22 am 
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Location: Near Brandon, Iowa
This would be the coupling capacitor that links the plate circuit of the first audio amp triode to the signal grid of the audio output tube. 0.01 uF to 0.05 uF is the customary value. 0.001 uF may have been intentionally installed by a service tech in order to reduce residual hum (by reducing the amp's low-frequency response).


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 Post subject: Re: Schematic error?
PostPosted: Sep Sun 19, 2021 2:31 am 
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Location: Detroit, MI USA
You may or may not be able to hear the difference in this case, but I'd go with what the schematic calls for.

It's entirely possible that the .001 could have been put in on the assembly line for a couple of reasons. One, Detrola was notorious for using what they had on hand, to keep the production lines running so if they were out of .01's and had lots of '001's, a decision could have been made by a supervisor to use those instead for several hours or days. Second, it could have easily been a mistake by an assembler not paying attention and grabbing a capacitor out of the wrong bin, or the parts within the bins could accidentally have been mixed.

Having met and/or interviewed by phone a lot of former Detrola employees including many of the women who worked on the assembly lines, all of those possibilities would not have been unlikely to take place on any given day.

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 Post subject: Re: Schematic error?
PostPosted: Sep Sun 19, 2021 4:03 am 
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Joined: Dec Sat 14, 2013 2:50 pm
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Location: 60062
Thanks guys,
Just finished recap and used .01uF as per schematic. Works and sounds just fine.


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 Post subject: Re: Schematic error?
PostPosted: Sep Sun 19, 2021 9:07 am 
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There was a reason why coupling capacitors in vintage tube radios in the audio stages were reduced in value so as to roll off the low frequency audio response to a value higher than one might expect.

Oddly, although the reason for this was patently obvious, it appears few people know about the reason it was done, selecting lower coupling capacitance values in the audio stages.

One of the classic problems in radios was that acoustic vibrations from the speaker would vibrate the vanes of the tuning capacitor, creating a feedback loop. The main solution was to put the variable capacitor on rubber mounts. This helped, sometimes not enough, so the next move was to lower the LF response in the audio.


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