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 Post subject: PM speaker audio output
PostPosted: Dec Fri 16, 2016 11:32 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 87
Location: Jamison PA
I'm working on my Sentinel 63B radio. I discovered that the input of the audio transformer is open. The secondary (cut wires in pic) show continuity. I unbolted the transformer from the speaker and cut the secondary wires to remove from the speaker frame.

I figured I'd grind the rivets off and carefully unwrap it to find the break. If I'm not successful, could someone recommend a audio transformer replacement?. The speaker has no ohms on it only part number. Same for audio output. It is a 6 volt vibrator farm radio. I did a full recap but have found this problem, and certainly am not going to hear anything soon. The speaker has good magnetism, and would crackle if I ran a piece of metal across the two plug posts (it only has two) when b+ was applied.

Thank you

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Fri 16, 2016 11:55 pm 
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A 41 tube should operate into 7500 - 9000 ohm impedance. The speaker is most likely rated 8 ohms impedance. This isn't the same as resistance read on an ohm meter. Look for a transformer rated around 8000 ohms primary and 8 ohm secondary. Exact specifications aren't critical.

AES, www.tubesandmore.com and others sell universal output transformers. With this type of transformer a person can select the best tap.

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 Post subject: Re: PM speaker audio output
PostPosted: Dec Sat 17, 2016 12:04 am 
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Location: Jamison PA
Thanks Norm!!

Much appreciated!!

I'm hoping the break is on the surface, but who knows?? :?

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Sat 17, 2016 12:21 am 
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Joined: Jan Wed 07, 2015 10:39 pm
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Location: Jamison PA
This transformer appears to be a good replacement. I found that a mouse ate the fine winding...

Which primary wires would be used in this. That's not clear to me as I have only 2 inputs on the plug.

Thank you

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Sat 17, 2016 1:13 am 
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Location: Santee Calif. 92071
Blue and Brown. Red wire for push pull. Yours is single ended. Red wire not used.


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Sat 17, 2016 1:31 am 
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Actually that is the wrong transformer for you. Y want the one with only two primary leads and the multi tap output, rated for 3.4 watts or more.

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 Post subject: Re: PM speaker audio output
PostPosted: Dec Sat 17, 2016 4:58 am 
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westcoastjohn wrote:
Actually that is the wrong transformer for you. Y want the one with only two primary leads and the multi tap output, rated for 3.4 watts or more.

Right, but you don't need to handle 3.4 watts. Looking at the schematic, the cathode of the 41 tube has 9 volts across 750 ohms which works out to 12mA bias. The plate voltage is given at 125 volts. Allowing for a maximum plate swing of twice that or 250 volts and zero current under maximum signal conditions gives a load impedance of 125v/12mA = 10417 ohms (10K would be close enough).

Looking at the data sheet for the 41, http://www.mif.pg.gda.pl/homepages/frank/sheets/093/4/41.pdf, there is a listing for a class A amp with 100v on both the plate and screen, using a 12K load, and having 0.35W output. I think the Hammond 125ASE would work: https://www.hammfg.com/files/parts/pdf/125ASE.pdf using the Green and Black secondary wires for an 8 ohm speaker or the Orange and Black secondary wires for an 4 ohm speaker.

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John

EDIT: Yes, I know. I didn't account for the screen current in my calculation. This would place the plate load impedance closer to the 12K listed on the 41 data sheet, but the Hammond part doesn't have that option and you'd never be able to tell the difference using a 10K load instead.

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 Post subject: Re: PM speaker audio output update
PostPosted: Nov Sat 25, 2017 5:38 pm 
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Been a while :roll:
I've had it sorted out on the bench fixing small issues over time. I typically play with these in the winter months. It's pretty much done now, just need to apply speaker cloth and reinstall.

So I used the transformer I referenced in my earlier post as I had purchased it before my posting. Any strong station sounds more distorted the louder I go.. Low volume sounds fine. I would have expected a clear sound turning it up.

I wired it for 8, 8000. Am I "overdriving" it?. Should I get the Hammond instead?

Thank you

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Nov Sun 26, 2017 4:59 am 
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Measure the resistance of the speaker. An 8 ohm speaker will read around 6 ohms on a DMM.

Does this circuit have AVC? Could be something wrong in that area.

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 Post subject: Re: PM speaker audio output
PostPosted: Nov Sun 26, 2017 6:50 pm 
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Location: Jamison PA
Well I had forgotten to check the avc and the bias cell.

Its dead, and I assume doing nothing. Plate voltage on the 75 tube is a little high, 65 vs 40. Other pins are reading as the diagram says.

How would you replace this cell?. Watch battery, or ??

Thank you!!

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Nov Sun 26, 2017 6:57 pm 
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Mike

Put a jumper across the bias cell. The radio will work without the cell but not with it dead (open). 500K resistor in series would be better off as 4.7 or 10 meg. Later radios didn't use a bias cell.

http://www.nostalgiaair.org/pagesbymode ... 018254.pdf

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 Post subject: Re: PM speaker audio output
PostPosted: Nov Sun 26, 2017 8:12 pm 
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Joined: Jan Wed 07, 2015 10:39 pm
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Location: Jamison PA
Thank you Norm!!

I take it I should cut the upper wire in pic going to ground and use a 4.7 Meg resistor from that to the other side of the existing 500K dogbone shown in the picture?. I think the schematic number is 6984, but very hard to read.

The radio plays, but as you turn up the volume, the speech and clarity of the sound become distorted. I double checked and do not see any voltage on the terminals of the battery cell.

MIke


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 Post subject: Re: PM speaker audio output
PostPosted: Nov Mon 27, 2017 1:22 am 
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Mike

Schematic shows 1.25 volts for the bias cell. Later radios didn't use bias cells but increased grid resistor values. Using 4.7 to 10 meg would be ok instead of the bias cell. If you do this short across the cell.

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 Post subject: Re: PM speaker audio output
PostPosted: Dec Tue 05, 2017 1:36 am 
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Joined: Jan Wed 07, 2015 10:39 pm
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Location: Jamison PA
My eyes were off reading the post
I actually just used a 1 meg resistor across the bias cell
I had used jumper wires and tried it with and without the resistor
It sounds better with the resistor and seems to pull in stations a little better I've been listening to it for nearly an hour.
Why would it improve the sound?
Should I order the right resistor??

Thank you


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Tue 05, 2017 2:21 am 
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Location: Long Island
The bias cell is used to set the grid voltage of the first audio tube (the 75) correctly. The circuit has to be completed by something, which is why it works better with a resistor across the cell. But it would probably sound better yet if you replaced the cell with a new one, restoring the original bias conditions. A watch battery should do the trick. There are plastic coin cell holders available online and their use is highly recommended as soldering to watch batteries usually kills them.

The other thing you've got working against you is, you used a push-pull output transformer but you've got a single ended output tube. Due to the fact that the DC currents in a push-pull circuit are balanced, the output transformers are smaller and lighter compared to single-ended transformers. With less steel in them, push-pull transformers saturate easily when used in single-ended output circuits. This is probably why the radio plays okay at low volume but distorts when you turn the volume up.

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 Post subject: Re: PM speaker audio output
PostPosted: Dec Tue 05, 2017 4:32 am 
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Joined: Jan Wed 07, 2015 10:39 pm
Posts: 87
Location: Jamison PA
Thank you
I did swap the transformer for the Hammond as suggested before jumping the bias cell

I realize the bias cell is the original design, but would the 10 meg resistor actually be better than sticking to the cell or ??

I don't know so figured I'd get opinions please

Thank you again :mrgreen:

Mike


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 Post subject: Re: PM speaker audio output
PostPosted: Dec Tue 05, 2017 4:59 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7216
Location: Long Island
10 meg is awfully high for that circuit. It would probably work better with a smaller value resistor, perhaps half a meg. Bear in mind that there is already a half meg resistor in series with the battery. But the battery provides a stable grid bias voltage so you'd get the least distortion with that.

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