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 Post subject: Solid state transformer
PostPosted: Aug Wed 08, 2018 7:07 pm 
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pixellany wrote:
I think it's time for some specificity......I have seen many audio filters that would be very happy with DC. What is an example of one that would not?

I think you need to explain what you are doing.


Here's the circuit in question for a R-390 receiver. The first schematic is the circuit to the right of the second schematic.

Attachment:
R-390 1.png
R-390 1.png [ 193.99 KiB | Viewed 4725 times ]


Attachment:
R-390 2.jpg
R-390 2.jpg [ 162.91 KiB | Viewed 4725 times ]


Now would it be possible to just make the 12AU7 stage a cathode follower and have the same max output voltage that is present on the secondary of the transformer or would the impedance be higher than 600 ohms doing it that way?

I would also need to duplicate the treble cut (if any is present) provided by the 1500pF cap unless that is just there to keep the amp from breaking into ultrasonic oscillation.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Wed 08, 2018 10:24 pm 
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Your circuit snips don't show what the transformer is driving----the implication is 600 ohms.

The gain of v601 is roughly the net plate impedance divided by the 680 ohm cathode resistor.

The net plate impedance is the parallel combo of the 12AU7 plate resistance and the reflected impedance at the input of the transformer.

once you know the gain of V601 you can get the net gain to the output of the transformer. If the net gain is 1, then the cathode follower approach should work. Let's see some numbers.....

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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 11:43 am 
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The transformer shown in the first photo drives the filters as seen in the second photo.

Here's a simplified diagram.

Attachment:
Transformer.png
Transformer.png [ 5.33 KiB | Viewed 4680 times ]


So basically I needs to measure the input to the 12AU7 then measure the output of the transformer and see what the voltages are then do Av=Vout/Vin to find the overall gain of the circuit.

If necessary I do have an extra 12AT7 triode section that could be used as a cathode follower since I take the output from the local level control and fed an external push pull 45 amp with the local audio amp disconnected completely.

I tried once messing around with a cathode follower, but it didn't work good, although I later learned while messing with a service grade RF generator that a cathode follower must be properly biased in order for it to work properly.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 1:58 pm 
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In no particular order:

Any tube or transistor circuit needs to have the correct bias---it is not a unique feature of a cathode follower.

I assume that the filters are passive, and are designed for an impedance of 600 ohms. You expressed a concern earlier about DC going to the filters---if they are passive and don't couple DC to the output, then no problem.

Yes, you need to total gain of the existing circuitry in order to design a replacement.

From the other thread, I had assumed you wanted a S/S circuit---but tubes will work also

Here's a nice article on the design of cathode followers:
http://www.valvewizard.co.uk/accf.html

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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 3:20 pm 
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Solid state would work as well, but if the spare tube section can be made to work I'll go that route.

Whatever is the easiest to implement.

I do have the area where the regulator tubes went where I can install a capacitor so it should be easy enough to fit the proper film cap.

That said I cannot use DC coupling as there's two relays (break in, squelch) that short the audio line to ground before the filters.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 3:27 pm 
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One can design an DC-coupled circuit that does not mind being shorted to ground, but it sounds like you don't need to.

After reading the link on designing cathode followers, let us know of any more questions.

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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 4:08 pm 
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The transformer primary can be replaced by a 10K resistor, right?


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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 5:46 pm 
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Tube Radio wrote:
The transformer primary can be replaced by a 10K resistor, right?

I can't relate that question to what we have been talking about.....
I thought you wanted to replace the transformer with circuitry that drives the 600-ohm impedance of the filters. That means the amplifier stage needs to have an output impedance of 600 ohms. We were talking about a cathode follower, but you can also get a low output impedance from a common-cathode stage.

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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 7:15 pm 
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The idea was to use the 12AU7 stage as a voltage amp then follow it with a cathode follower, but if the gain from the grid of the 12AU7 to the secondary of the transformer is 1 or less than 1 then I can just make a cathode follower out of the 12AU7 stage.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 7:51 pm 
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So what is your next step?

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 Post subject: Re: Solid state transformer
PostPosted: Aug Thu 09, 2018 9:22 pm 
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The results are in.

400Hz fed to my part 15 transmitter located in the same room.

At the 12AU7 grid I get 1.233 Vrms
At the transformer secondary I get 1.619 Vrms

For a net gain of 1.3

So that pretty much rules out making the 12AU7 a cathode follower as I gotta keep the same output given I use the line audio out to drive a pair of headphones at times and need its level to remain proper plus I'm not sure how a level reduction will affect the audio filters.

Now I'll go look over that site more closely and design a cathode follower with the 12AT7 section.

Looks like I can use this circuit

https://www.ampbooks.com/mobile/classic ... -follower/

Will be easiest to implement as it only requires two resistors and a capacitor.

The idea is to get the audio frequency response of the R-390 down to around 40-50Hz with the limiting factor then being the driver transformer on the 45 amp which is pretty much flat to 60Hz as I recall then starts to roll off some. Also there is a slight roll off due to the cap coupling the signal to the 12AU7 grid, but that is in the IF chassis so it will be left alone

Now if I need to reduce the output level any would it be best to alter Rl or RK1 or divide the value of RK2 between two resistors to form a voltage divider?

The transformer of course will be left in so that it can be easily put back to stock configuration if so desired at a later date.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Fri 10, 2018 4:20 am 
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Well I tried two cathode follower configurations.

The one I linked to I got it close enough to the original output voltage, but it was heavily distorted when I played music through my transmitter. Now without the 600 ohm load connected I got a good bit more output, but I just couldn't get it with the load connected.

Now had I thought more about my modding the cathode follower in a Stark LSG-10 service grade RF generator I would have known the cathode follower might not work as I thought it would.

I then tried the cathode follower that is cap coupled to the voltage amp tube and uses two equal resistors to set bias. That didn't work as the voltage at the center of two 10 meg resistors was something like 35Vdc and it should have been half the B+ which is at 174Vdc after the choke that feeds the audio circuitry which would make the grid at 87Vdc.

So I am thinking that I will order an Edcor 10K to 600 ohm transformer and go that route.

I'll put the circuit back stock so I can see how much voltage is on the primary as that will tell me if I need the 1/2 watt one or the two watt one. Plus it will also give me the B+ current drawn by the tube so I can either figure out or ask them how much DC current the transformer can handle before frequency response is affected.

I would have a plate current of 6.5mA. The signal on the plate is 20Vrms so I would need the 2 watt transformer.

Ship time is 8 weeks as it is built to order.

I'll ask in the classifieds and see what is available.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Fri 10, 2018 1:35 pm 
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For future reference, it might be good to figure out how to make the circuit work. At low frequencies, these things are pretty deterministic.

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 Post subject: Re: Solid state transformer
PostPosted: Aug Fri 10, 2018 2:16 pm 
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I tried for around 3-4 hours last night to make it work. Tried several resistor values for all the resistors. I could get the proper voltage at the lower modulation level, but at the highest modulation level the sinewave was very distorted and I could get it to where the sinewave was not distorted at the higher modulation level, but I could not get enough gain to produced the same voltage level at the lower modulation level without introducing distortion.

The problem is the 600 ohm impedance.

I could get much more output from the cathode follower when the load was not conencted. I tried a resistor in series with the 600 ohm load and it worked, but the voltage changed a bit much as I switched from wide to the first filter so I knew that would not be ok given it didn't change with the transformer in use.

I then decided why dick around with a circuit when I know a 10K to 600 ohm transformer will work and can get one.

I think some of my limit is the 180Vdec B+ plus the tubes used.


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 Post subject: Re: Solid state transformer
PostPosted: Aug Fri 10, 2018 10:23 pm 
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Quote:
.............when I know a 10K to 600 ohm transformer will work and can get one.

Indeed!!
IOW---what problem were you trying to solve?

There are many points of circuit design that could help you, but i wonder if you are aware of one principle. If something has an output impedance of 600 ohms, and you attach a 600-ohm load, the voltage will drop in 1/2. that needs to be accounted for in the design.

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 Post subject: Re: Solid state transformer
PostPosted: Aug Fri 10, 2018 10:58 pm 
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I was trying to use parts on hand versus buying a transformer.

Yes i know about the impedance.

I would have needed a lower impedance from the cathode follower and i just could not get it.

But what I don't get is with the two 10 meg resistors why was I not getting 1/2 B+ voltage on the grid?


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 Post subject: Re: Solid state transformer
PostPosted: Aug Sat 11, 2018 2:43 pm 
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Tube Radio wrote:
... But what I don't get is with the two 10 meg resistors why was I not getting 1/2 B+ voltage on the grid?

The simple answer is non-zero grid current. However, without seeing the actual circuit, it's hard to tell. And, keep in mind that the actual circuit may be different from what you intended.
John

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 Post subject: Re: Solid state transformer
PostPosted: Aug Sat 11, 2018 5:17 pm 
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I Was thinking of " unauthorized grid current", but my (limited) experience is that it it make the grid go positive, thus driving the tube on harder.

The circuit in question comes from the site I linked earlier. They show a very large cathode resistor which...in their words....mitigates the effects of the high value grid divider.

BUT--the OP's application does not require those high megohm resistors.

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 Post subject: Re: Solid state transformer
PostPosted: Aug Sun 12, 2018 1:27 am 
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So could the resistance I measured just be the resistance of the grid to the cathode as I did measure it with the receiver on, but the external 180 volt regulated B+ supply off.

I got an ad in the classifieds for a transformer.

I also ordered an Edcor XSM series 10K to 600 ohm transformer. 8 week shipping time so that will give 8 weeks for my ad to be responded to with a transformer that works.

Don't know how pushing nearly 9mA through the edcor transformer will affect the frequency response, but it only needs to be good to 40-50Hz.

The receiver sounds great as is, but is missing a slight bit of bass response.

Replacing the output transformer of the push pull 45 amp which is just a power transformer I had laying around will help as well. It feeds the speaker through a Hammond 600 to 4-8 ohm transformer.

For the output this will work.

https://www.edcorusa.com/gxpp10-5k


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 Post subject: Re: Solid state transformer
PostPosted: Aug Sun 12, 2018 1:07 pm 
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Quote:
So could the resistance I measured just be the resistance of the grid to the cathode as I did measure it with the receiver on, but the external 180 volt regulated B+ supply off.

What resistance were you measuring??

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