Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Wed 27, 2021 5:46 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 29 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Tue 07, 2018 12:50 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21413
Location: Warner Robins, GA
Wonder if a circuit like this could be used to replace a 10K to 600 ohm transformer with a B+ of 180 Vdc?


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Tue 07, 2018 7:32 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30042
Location: Annapolis, MD
Tube Radio wrote:
Wonder if a circuit like this could be used to replace a 10K to 600 ohm transformer with a B+ of 180 Vdc?

Yes.

If there is anything magic about transformers, it is the ability to do isolation and impedance matching with minimal parts. But they do take up space.

First, we need complete requirements: For example, the transformer in your example would have a voltage step-down of ~ 4:1. The equivalent S/S circuit could be simply a MOSFET source follower with a divider on the input (or output). Add some caps for DC isolation.

Again---define the requirements first.

_________________
-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Wed 08, 2018 12:25 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21413
Location: Warner Robins, GA
I'm debating on that or using a proper transformer with better frequency response.

The problem then becomes the coupling cap which for a 600 ohm load should be 20uF.

For a film cap that isn't necessarily small.


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Wed 08, 2018 3:43 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30042
Location: Annapolis, MD
Tube Radio wrote:
I'm debating on that or using a proper transformer with better frequency response.
The problem then becomes the coupling cap which for a 600 ohm load should be 20uF.

So make it DC-coupled.....

You can almost always design a circuit that outperforms a transformer----it might just take a few more parts and some quality time with a program like Spice......

_________________
-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Wed 08, 2018 4:21 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21413
Location: Warner Robins, GA
Doubt the audio filters would like dc.


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Wed 08, 2018 6:27 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30042
Location: Annapolis, MD
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.

_________________
-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Aug Wed 08, 2018 7:08 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21413
Location: Warner Robins, GA
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.


Decided to start my own topic.

viewtopic.php?f=12&t=344651


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Sep Mon 13, 2021 6:29 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3831
Location: Boston, Massachusetts
Is anyone making these pre-assembled?

_________________
Andrew


Top
 Profile  
 
 Post subject: Re: Solid State Transformer Substitute: Update
PostPosted: Sep Mon 13, 2021 11:18 am 
Member

Joined: Jan Thu 16, 2020 12:29 am
Posts: 1573
I think the idea is basically sound. However you need to look at ALL the functions performed by the vintage audio inter-stage transformer in the typical 1920's radio. These are:

1) A DC resistance on the primary that is much lower than the AC impedance, this holds the anode voltage of the plate of the driving tube, fairly close to the HT rail. This is possible, because the primary magnetization current stores energy in the core of the transformer. This means that the primary side voltage swing, generally is above and below the HT rail.

Unfortunately this function cannot be provided by a circuit without energy storage, or a higher voltage source than the existing B+. And this function, provided by the transformer, acts to increase the dynamic range of signal handling prior to distortion. As you can see, from your circuit the plate voltage of your driving tube is held at 1/2 of the B+, this is not nearly as satisfactory as a transformer situation. It is interesting, in the case of transformers operating in class A, how transformer core energy storage and the benefit of it gets forgotten by designers in the quest to eliminate expensive and heavy transformers from the design.

2) DC isolation between stages.

3) Voltage magnification (gain)

4) Bandpass frequency filtering, in that , most radios relied on the fact that radio frequencies were heavily attenuated by the bandpass response of the transformer being only audio capable.


You have succeeded with point 2 & 3, however points 1 and 4 I think are problematic.

Still, with regard to point 1 if the signal levels are low, you may not detect an issue and with regard to point 4 you could fix this up with base to collector capacitors to roll off the frequency response above 10kHz to 15Khz.

So my assessment of this idea, is that:

In lieu of a real transformer, it is a good solution, to getting a radio up and running again. But, since it is inferior to a well designed transformer, and it is for a vintage radio, my preferred method is to rewind the transformer, if it has failed.

The frequency response of the typical 1920's inter-stage transformer has remained an enigma for many. One reason is that existing computer transformer modelling software was not designed to model them. It was designed to model the transfer of power and impedance matching considerations. The vintage class A audio inter-stage transformer is not an impedance matching device (unless its for a class B output stage)

The vintage audio inter-stage transformer behaves as a coupled resonant circuit, where the damping of it, and the audio band-pass response depends very much on the plate resistance of the tube driving the transformer primary and the transformer's parameters.

I have performed an analysis of this in the past and the equation for the frequency response, for those interested, is in this article pages 11 to 30:

https://www.worldphaco.com/uploads/THE_GREBE_MU-1.pdf


Top
 Profile  
 
Post New Topic Post Reply  [ 29 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 10 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB