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 Post subject: Interstage Transformer Needed for 30's National Dobro Amp
PostPosted: Oct Mon 01, 2018 12:25 am 
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Posts: 18
I’m working on an amplifier from the early 30’s and need advice on choosing a transformer replacement. The amp is an early National Dobro, No 220, no known schematic. The signal goes through a volume pot into one half of a 6A6, through a cap into the other half of the 6A6, from the plate into an interstage transformer with center tap outputs to a pair of 42 output tubes.

One of the interstage outputs has opened. DC resistances are 940 ohms on the input, 1560 ohms on one of the secondary’s, and open on the other but assume it to be 1.5K.

Dimensions: 2” tall, 1-5/8” wide, about 2.5” long or 3-1/2” long with the mounting tabs.

Any ideas for a good replacement transformer?

Regards,
Jim


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 3:33 am 
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We need more details for an accurate answer. Could you draw a schematic to show us?
It sounds like the 6A6 is wired as a two-stage preamp.
Is this a 4 tube circuit?
Tubes and More might have the transformer you needs. I would browse the supplier's list.

Just an idea - If you can't find a transformer, you could add a tube socket and rewire the amp so that another duo-triode tube, 6N7 or the 6A6, provides the 2 preamp stages.
Then use a 6N7 or 6A6 as the phase inverter for the 42 tubes, tube replacing the center tapped transformer.
Another way is using an added single triode tube as a cathodyne inverter.
There are Fender schematics using modern tubes that can be copied.

The 6A6 tube dates from its debut in August 1934, FYI, so 1935 would be a reasonable date Good luck searching and fixing.

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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 4:36 am 
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It has 4 tubes, the rectifier is a #80, a 6A6, and two 42 outputs. Correct, the 6A6 is wired as a two-stage preamp. Adding another tube for the PI is a great idea and would be easy to do. However, my goal is to restore the amp close to its original design.

Although I don’t have a schematic for it, just some hand drawn notes, the Natl Dobro Model 6107 is very similar. Replace the 79 and 56 input tubes with the dual triode 6A6, replace the 2A3's with 45's, replace the 5Z3 with an 80, and move the volume pot to the input jack and you're looking at my amp.


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 11:09 am 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 545
Location: 18424 PA
Don't modify the amp as suggested. Interstage transformers aren't hard to find. I have a few here, let me dig them out and see what i have. Do you have any dimensions or pictures to see the size?

I have a prewar Supro (national dobro) that the tubes don't match any schematic but found one close enough.


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 11:51 am 
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https://www.tubesandmore.com/products/t ... Interstage

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


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 4:10 pm 
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N3, thank you for offering to see if you have one laying around. It would be nice to find one close to original. If not, I'll take a look at the ones at Antique Electronic Supply. Which raises the question, they sell 4 - which one would you use? Some of the old reference materials like Hammond Transformers catalogs lists a type 834, S or PP plates to PP Grids, Primary Impedance 5000/20000, Secondary Impedance 10000/40000. Why are two numbers listed for each?

Dimensions: 2” tall, 1-5/8” wide, about 2.5” long or 3-1/2” long with the mounting tabs. I'll send a photo after work.


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 4:51 pm 
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Transformers are specified by impedance ratio. The example you cite is 1:2 (step-up).
They could have given any 2 numbers....eg 7,500:15,000. Is still 1:2

EDIT---above I used impedance ratios. What is more common is to specify the turns ratio**. The transformer in question is 1:1.4, which is actually a bit lower than what we typically see.


**impedance ratio is the square of the turns ratio.

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


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 01, 2018 11:17 pm 
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Location: NJ 08520
DCResistances indicate near a 1:3 overall step-up ratio. So, in push-pull, close to a step-up of 1:1.4 each half. We can get the amp working again, with the 1/2 open winding, but with reduced output. Using the good secondary winding, wire in two, matched 100K Ohm, 1/2 watt resistors, in series, across the good winding. The junction of the two resistors becomes the new center tap of the secondary, that point going to ground (or to the negative supply in a fixed bias amp). Then, each outer "good coil" connection goes to either grid of the opt tube...

That old radio trick works and can work very well, especially with Class A step-up applications (like this National triode amp), when "power drive" is not needed. Give it a try...the old-time "Hints and Kinks" derived from resourcefulness still can save the day...


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Tue 02, 2018 2:10 am 
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Joined: Sep Tue 15, 2015 1:16 am
Posts: 545
Location: 18424 PA
In many Sams schematics, written as impedance,often spec out the dc resistance and often list what tubes it is used for.

The ratio means nothing if it can't handle the current. Size matters. We all know tubes can get along with many different output transformers and will live a long life.

TVAVT, i have 2 here right now. One is a silver regular transformer looking one which is listed as 25k-13k. about 2 inches square 2.5" mounting holes. 12at7 driving a pair of 12ab5's. Have schematic.

Second one is a old RCA out of a PA. amplifier, early 40's? 2"wx 2.5"d x 3" high. Stancor A-4405, Thordar'n T20A17. I have the schematic for this also. Sams says 6j5 matched to PP grids (6v6 quad) 1.9k primary, 800ohm ct sec (dc resistance). I'm sure this one can handle twice the current of the smaller one easy.

Can send pictures, email me at the house , my call sign @ptd.net

Just realized these are both step down in dc resistance, you appear to need a step up ratio? I guess it depends on the wire size so dc resistance can be fooling. I may have 1 step up but it's small, stancor? and has written 10ma interstage on it. A53c 7k- 15k ratio 1:3 pri 10 ma max. That may work as 6a6 is showing me 7ma plate current? Only about 1.75" sq.

Will look up the catalog on that bunker of doom site, may be able to get the ratio or i can throw them on the signal generator and figure it out.


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Tue 02, 2018 4:52 am 
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Joined: Jan Sun 17, 2010 2:51 am
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Here are the dimensions and a photo of the transformer.


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DobroAmo_021resized201.jpg
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IMG_3249.JPG
IMG_3249.JPG [ 63.43 KiB | Viewed 1418 times ]
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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Tue 02, 2018 9:32 pm 
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Location: Ohio 45177
Have you considered taking it out of the amp for closer examination, to see if there is something like a broken winding wire near the terminal or such?

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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Fri 05, 2018 6:59 am 
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Joined: Jan Sun 17, 2010 2:51 am
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Yes. I've removed & disassembled the transformer, and peeled back the tape from the bobbin. There are no visible breaks.

N3UVT looked through his parts and nothing quite matched up - thank you for checking.

Back on the hunt!


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Sun 07, 2018 4:01 pm 
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At the risk of being redundant, the DC resistance is not relevant in determining what transformer to use.
If you really want to know the turns ratio of the existing one, you could wire it up to an audio oscillator......or you you could tear it apart and count the turns......hint: there's lots...:)

I would think the P-T156 at antique radio supply would work fine.

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


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Mon 08, 2018 7:07 am 
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Joined: Jan Sun 17, 2010 2:51 am
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Good advice, thanks. I'm in the middle of packing house for moving, the first area to get packed was my shop. Instead of digging out test equipment or manually counting the turns, I think I'll order the P-T156 and call it good!


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 Post subject: Re: Interstage Transformer Needed for 30's National Dobro Am
PostPosted: Oct Wed 10, 2018 7:11 pm 
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I just am working on a radio with push pull output and found opens in both the driver and output transformers. Fairly low power amp though. I dug my junkbox and found a Stancor 4 watt universal output that I can use as a functional replacement and one of those AES transformers to drive with. Will wait to see how that works out as the repairs are pretty extensive at this point. Still got parts "in the mail".

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