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 Post subject: Solid State Transformer Substitute: Update
PostPosted: Jun Tue 26, 2018 4:37 am 
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Several of us have been experimenting with solid state replacements that can be hidden in defective audio transformers of 1920's radios.

This is some information on my latest version which works quite well in my test setup and in radio tests. The circuit operates the 201A or similar tube at a constant plate voltage of 45 volts. The tube operates as a current source and the transformer circuit converts the current to a voltage and then amplifies the voltage 3.5 or 5 times depending on some component values. The circuit works well for both the detector and second stage applications.

The two main advantages over transformer coupling are:
1. The frequency response is flat from 20Hz to 20KHz. (My best transformer is 350Hz to 6KHz. Most are much worse.)
2. The tubes should have better life when operated at lower voltage and plate current. Tube gain is as good or better than operation with a transformer at 90V.

Circuit:
Attachment:
SSTS CKT 25JUN18.jpg
SSTS CKT 25JUN18.jpg [ 31.74 KiB | Viewed 5900 times ]


Parts layout on 0.1" perf board, top view:
Attachment:
SSTS_TOP_25JUN18.jpg
SSTS_TOP_25JUN18.jpg [ 76 KiB | Viewed 5900 times ]


Parts layout on 0.1" perf board, bottom view:
Attachment:
SSTS_BACK_25JUN18.jpg
SSTS_BACK_25JUN18.jpg [ 74.44 KiB | Viewed 5900 times ]


Circuit on test fixture:
Attachment:
SSTS 1 23JUN18.jpg
SSTS 1 23JUN18.jpg [ 104.82 KiB | Viewed 5900 times ]


Close up of circuit:
Attachment:
SSTS 2 23JUN18.jpg
SSTS 2 23JUN18.jpg [ 131.58 KiB | Viewed 5900 times ]


If you use one of these on a radio with a C- supply, be aware that each solid state transformer uses about 1mA of current.
Most C- supplies should be OK but the voltage may need to be adjusted with the radio in operation.

Detector applications can operate directly from a 90V supply since the circuit reduces the tube plate voltage to 45 volts.

Hope you find this useful,
Jay

PC boards are available here: https://dirtypcbs.com/store/designer/details/24497/6085/solid-state-transformer-substitute
With slow shipping, the cost is about $14 for 27 to 36 boards. Quantity depends on how many have defects.


Last edited by JnTX on Jul Sat 07, 2018 7:59 pm, edited 1 time in total.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jun Tue 26, 2018 9:48 pm 
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COOL!

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jun Tue 26, 2018 11:50 pm 
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Great work.
This is a work-around within the abilities of those who might not be able to wind a transformer or have a transformer housing that's too small to install a modern replacement.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 16, 2018 1:18 pm 
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Would this work for instance with a radio that uses a higher B+ like the AK-40 with a 27 as the driver and a 71A output?


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sat 28, 2018 1:46 am 
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Sorry to be slow answering. Right now the design is for a maximum B+ of 90V. I probably will not work in a higher voltage design until I need one myself.
Jay


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sat 28, 2018 7:02 am 
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Oh ok.


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sat 28, 2018 7:52 am 
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Question to Poster: why are Darlington transistors used? Is a gain of 10,000 really needed?

Tube Radio's question of a higher voltage rating got me looking, but the higher voltage Darlingtons have much lower gain.


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sat 28, 2018 9:38 am 
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Johnnysan wrote:
Question to Poster: why are Darlington transistors used? Is a gain of 10,000 really needed?

Tube Radio's question of a higher voltage rating got me looking, but the higher voltage Darlingtons have much lower gain.

I would have guessed that the constraint is impedance. If that's true, then I think MOSFETs would be an option.

Higher B+: maybe just put a zener in series?

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sun 29, 2018 12:54 am 
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Not sure about the zener


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Sun 29, 2018 10:57 am 
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Tube Radio wrote:
Not sure about the zener

Note sure about what??---a zener in series with the B+ will drop the voltage. Not an elegant solution but works fine in a situation like this with very low current draw.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 3:44 am 
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pixellany wrote:
Tube Radio wrote:
Not sure about the zener

Note sure about what??---a zener in series with the B+ will drop the voltage. Not an elegant solution but works fine in a situation like this with very low current draw.


It also drops the plate voltage to the tube which for tubes designed for higher voltages such as the 27 it may make the tube have lower gain or lower maximum output voltage or both. All which would affect the circuit negatively.

Suppose it would be easy to adapt the circuit with higher voltage transistors and get similar results.

Perhaps the ZTX458 would work but using four to duplicate the darlington configuration.

That said I'd reduce the value of the .1uF caps to start rolling off t he bass around 40-50Hz as most if not all antique speakers cannot reproduce much if any real bass except the one speaker which I think was a large square that did have decent bass response. The reason is because the low power output many of these early radios have and if using an antique speaker there's no need in passing bass signals the speaker cannot reproduce as that would mean the radio won't play quite as loud. My knowledge of that comes from a Halicrafters S-41W where I tried to make it reproduce bass only to find it could no longer play as loud before distorting.


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 4:54 am 
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I think this circuit (a modification of the OP's circuit) would fill the bill for B+ values from 45v to 180v:
Attachment:
SS-TNX-BJT-MOSFET 45-180V Corrected.jpg
SS-TNX-BJT-MOSFET 45-180V Corrected.jpg [ 49.2 KiB | Viewed 5284 times ]

The input transistor has a 400v VCEO and the MOSFET a VDS of 240v and both are in the "E-Line" through hole package. The response is from about 36Hz to 13.5KHz at the -3dB points. The 1K/100pF in the gate of the MOSFET and the 220pF across the drain resistor serve to limit the high end. Without them the upper -3dB frequency is north of 100KHz.
John

EDIT: Corrected parts designators on schematic.

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Last edited by OldWireBender on Jul Mon 30, 2018 6:23 pm, edited 2 times in total.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 11:38 am 
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Awesome.

I might have to try that.

What's the gain of the circuit?


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 1:37 pm 
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Tube Radio wrote:

What's the gain of the circuit?

First stage:
I'm working on it....

Second stage: Aprrox. 56 / 10


To finish the earlier discussion about the zener:
You would only add the zener when needed

EDIT: fixed an error

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 2:35 pm 
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I think the 1st stage gain is simply Gm times 12K, where Gm is for the driving tube.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 3:02 pm 
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Provided that circuit is a 1:3 ratio it would work good for AK TRF radios.


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 3:12 pm 
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Tube Radio wrote:
Provided that circuit is a 1:3 ratio it would work good for AK TRF radios.

Many sets had interstage transformers that were more than 1:3, but it is not unusual to see people recommend 1:3 for everything.

Keep in mind thst the circuit here includes gain that would have been in the stage preceding the interstage.

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 5:40 pm 
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Yep some do have more than a 1:3 ratio.

I'm very tempted to breadboard that circuit and see how it works in my AK-40.


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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 6:34 pm 
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Tube Radio wrote:
Awesome.

I might have to try that.

What's the gain of the circuit?

The gain of the stage from the junction of R3 and C1 to the output at "G" is about 5. For a gain of 3, change R7 to 43K and R8 to 12.7K (a standard 1% value).

However, the overall gain is actually a transimpedance since it converts a current from the plate of the driving tube to a voltage at the output. In effect the overall transimpedance is the voltage gain of the MOSFET stage times the value of R3 or 36K for a MOSFET stage gain of 3 and 60K for a MOSFET stage gain of 5. So for for the values shown in the schematic a 100uA RMS signal at the plate of the driving tube would generate a 6 volt RMS output.

Note that it is also possible to adjust the transimpedance by changing the value of R3.

On another note, if so desired a surface mount version could be made by subbing SMD parts for the resistors and capacitors and using a FMMT458 for Q1 and a DMN24H3D5L for Q2.

Keep in mind, too, that the voltage rating of the caps should be higher than the expected "plate" voltage at "B".

John

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 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Jul Mon 30, 2018 6:47 pm 
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Oh ok. When I get paid again I might play around with it.


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