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 Post subject: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 6:31 am 
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I got a output transformer on a 1930 Sparton 400... that's sending noise back to my audio tubes (71A) from the primary side, Ive check both 71A tubes and they both have this issue from the output transformer, signal is very clean going into the audio tube, but hear trash coming back from the transformer.. I have tried swapping out the transformers I had a 124a and P-T31which didn't match the speaker field or voice coil.. in the pictures I have Ohmed out the stock parts to give you an idea... I'm trying to find a transformer which will match this

link to diagram
http://www.nostalgiaair.org/pagesbymode ... 029725.pdf


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 7:05 am 
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Location: Powell River BC Canada
Did you try new 71-A tubes ? Also sure that red and blue wires to the speaker
haven't got exchanged.

I know the 71-A circuits and that weak tubes can make the sound distorted.

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 7:10 am 
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Hi.... thank you for your input... interesting..... the 71A tubes have tested out to the Fair state to good but not new...just looked at the diagram,,, switch the black wires wire blue run to the set...and the other side to the tap will Ill give that a shot


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 3:36 pm 
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This may sound like a nitpick, but an output transformer would not typically send noise back into the output tubes. The only way i can imagine this happening is an intermittent connection--on either the primary or secondary side.

Ideally, the OPT transforms the speaker impedance into a load on the tube which is the speaker impedance times the square of the OPT turns ratio.
The tube responds to the voltage input on the grid and to the plate load. Ideally, the plate load is constant and resistive, but in the real world it has reactive components and points of resonance. This contributes to the overall distortion but does not create noise.

Other random comments:
--you can't spec a replacement OPT with DC resistance readings---you need the turns ratio
--the field coil resistance is not relevant
--If the existing OPT has rational DC resistances (not intermittent), ti is almost certainly OK.

Ideally, look at the output tube plate voltage with a scope.

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 5:36 pm 
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Your resistance measurements don't make any sense. Measure resistance, white to white. What is it?
Now measure resistance from each white to black. Resistance is ______ and ______.

You should have added this to the other thread instead of starting a new topic.
An answer to my question over there also would have been helpful.
Jeff

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 6:48 pm 
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Sorry for the confusion from the other tread.. was trying to load pictures and the website was kicking them back....here is what I got...


I have the measurements requested

pin2 to pin2 (without tubes) came in at 497 ohms
pin2 (1) to tap was at 234 ohms
pin2 (2) to tap was at 259 ohms

however the 71a filaments to ground came in at 132 Vac
71a filament across pin1 and 4 came in at 4.8 Vac
71a grid bias to ground was at 1 vac


Can this problem be the input transformer? since both 71a grid (1vt) goes into it and not giving me the 60vts that is needed?


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 10:36 pm 
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The schematic linked does not specifically cover model 400---does yours match up to the model 410?

in the 410 schematic, you will see that the filament winding for the output tubes has a center tap--grounded through a 1250 ohm resistor. is that in yours?

With the resistor in place, there should be ~ 2.5VAC to ground on either leg of the filament circuit.

In the 410 and 420, that resistor sets the bias. The grids are at ground, and the cathode should be positive. (V = Total plate current for both tubes X 1250ohms)

If in doubt about the input (interstage) transformer, check the DC resistances

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 10:57 pm 
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OK, binary, now measure the transformer like I said above. If the results are the same then I say the transformer is OK.

I think the grid bias voltage is a DC measurement, but I am not sure how to place the leads.

You have 71 tubes and the schematic says 183 tubes. I wonder if that makes a difference.
Jeff

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 11:21 pm 
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71A takes twice the filament current

71A has a maximum plate voltage of 180, and the 183 shows data at 250 (they don't state a maximum). The bias is higher, but, if you reduce the 183 plate voltage to 180, they may be similar.
https://frank.pocnet.net/sheets/127/7/71A.pdf
https://frank.pocnet.net/sheets/127/1/183.pdf

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 11:41 pm 
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I guess my question is, would running the 71 at 230 plate volts cause the low volume/ noise problem the OP describes?
Jeff

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Mon 17, 2019 11:48 pm 
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jrehkopf wrote:
I guess my question is, would running the 71 at 230 plate volts cause the low volume/ noise problem the OP describes?
Jeff

Probably not...

Assuming that the model 400 has the same circuit as the 410 and 420, the output tubes use cathode biasing (AKA "self-bias")
Once the tube is set at the desired operating point, anything that would tend to increase plate current will increase the bias, thus offsetting or "dampening" the effect.
I'd be more concerned about the stated maximum plate voltage---but: keep in mind that the specs are givenin reference to the cathode. With the cathode at +50, a plate voltage of 230 is right at the stated maximum.

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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Tue 18, 2019 3:42 am 
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Thanks guys for all the input... I will try to answer all in this block...

The overall problem.. is low volume (even when the dial is all the way up which lead me over to the audio stage of this unit... both output tubes are 71A.

Mark...Yes, this is 410 with the 1250 ohm resistor as a tap... With the resistor in place, there should be ~ 2.5VAC to ground on either leg of the filament circuit. I will relook at this point again, did a measurement on those legs and did get 5. VAC , However filament points on pin 1 and 4 of the 71A socket to ground is giving me 132 VAC, when checking 1 and 4 together I get 4.8 VAC.. So this 1250 ohm resistor my be an issue.

Jeff... got the same reading and the output transformer is good.. The grid bias on both the 71A are at 1VDC (which is not right) which is linked to input (interstage) transformer, (which also my be an issue) Does any know what reading I should be getting on this?

I being told that this set started life with 483 which was equal to 183 to a 71A..again I can pull in the main station on hear it via the speaker but its just very low


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Tue 18, 2019 5:10 am 
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To measure the AC component of a voltage with both AC and DC components may require an Output meter. Or use a capacitor in series with your AC voltmeter to block the DC.

Many multimeters on their AC range respond to the DC component in an unpredictable way.

Ted


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 Post subject: Re: Sparton Output transformer Blues
PostPosted: Jun Tue 18, 2019 6:07 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The 183 is made for a grid bias of -60 volts with a plate current of 30 mA per tube.
The two tubes return to ground via the 1250 ohm resistor in the center tap of the 2.5 volt
filament circuit.
Attachment:
183.jpg
183.jpg [ 156.92 KiB | Viewed 481 times ]


Two tubes will drawing 60 mA, producing 75 volts bias which is ballpark, considering :
71-A tubes draw only 20 mA per tube and want a bias of -40.5 volts. For one 71-A the resistor is
2150 ohms, so for two it should be 1075 but : the 71-A is rated at 0.79 watts per tube,
while the 183 is rated at 1.8 watts.
Attachment:
Check to see if shorted to ground.JPG
Check to see if shorted to ground.JPG [ 48.15 KiB | Viewed 481 times ]

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