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 Converting a Radiogram Amp like the STC 8471 to Stereo?
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 Return to top of page · Post #: 16 · Written at 7:08:59 AM on 31 October 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1251

Yes, Ian is right, the trannys that worked with 6M5's will be fine, the music will come out trust me!
Just use 2 speakers that are close in size and type, don't worry about the ohms too much the music will still come out!

Of course if you were trying to get maximum output then of course you would try and get the impedance matching as correct as possible but for just listening to the music just wire the bits together and see how it sounds.


People fuss too much about valve speaker matching, but you are dealing with inductive devices whose impedances vary all over the shop with frequency and power level and in the end you test and try and get a best mean matching level even when working at lab level.


There is a big difference between Hi-Fi performance and listening to records via a production turntable.


Cheers, Fred.


 
 Return to top of page · Post #: 17 · Written at 9:14:39 AM on 31 October 2018.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

JJ and Ian , how silly of me you can see my experience and knowledge is lacking, Impedance vs Resistance, need to get my text book out.

I will see what I can find out about these Output Transformers, I know the model of radios from the two donor junk chasis I have.

Ian , Yes I would like some base response, so you think the 8in radio speakers I have might not provide this, I have a few woofers, could we include them into the circuit, the cabinet design is still fluid, it is you circuit modification that can help dictate this.


 
 Return to top of page · Post #: 18 · Written at 10:20:09 AM on 31 October 2018.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 544

The easiest way for you to get Bass, is to build a vented enclosure or at least a large baffle board.
And the largest diameter speaker you can manage.
JJ


 
 Return to top of page · Post #: 19 · Written at 10:28:56 AM on 31 October 2018.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Second attempt at this response Fred, seem to have lost my last post.
Thanks for the pragmatic perspective and I am certainly not trying to get HI Fi Performance out of this project .
I just want to be able to play many of the low quality second hand records I have collected over the years 60s and 70s jazz and instrumentals on a nice sounding valve system. As many of these records are stereo and I have one of the best stereo ceramic cartridges of the time I though it would be nice to give this a ago.
I also did not want to play these poor quality records on my best turntable/cartridge set ups.

What about bass response do I need to consider larger speakers or introduce a woofer if that is possible?

Or wait till I have the circuit mod from Ian built and then take a leaf out of you book and on the bench just keep trying speakers until I get the sound I want. The good old "trial and error design approach"


 
 Return to top of page · Post #: 20 · Written at 2:54:38 PM on 31 October 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1251

To get "good" bass response the WHOLE system from the groove in the record to the air being pushed about by the speaker cone has to be able to respond to say 30Hz to 3000Hz. In the chain anything that restricts that response like the output tranny or small speaker will be the limiting factor. Johnny is right in that the speaker needs to be able to respond to the bass and be mounted in a baffled or ported box.
For your average used 78 or 45 record a well baffled average 10" or 12" should have some bass response through your average transformer and at least you will just hear Bass or Organ notes but not well. Anymore than that you are getting into Hi-Fi and looking at 2 or 3 way speakers crossovers, big output transformers and so on and so on.
Just hookup your best trannys, your best speakers in a baffled boxes and see what that sound like to start off with.
Fred.


 
 Return to top of page · Post #: 21 · Written at 1:41:51 PM on 23 December 2018.
Aurther Dungar's Gravatar
 Location: Brisbane, QLD
 Member since 8 July 2018
 Member #: 2263
 Postcount: 11

Duplicating the driver and output stages of your existing amplifier is the best idea,
How ever I have some concerns, the main one being the power supply,
You may get away with the extra load on the main B+,
how ever the extra filament loads will be too much
No transformer will have that much headroom.
But transformers are not hard to find.
Good luck !


 
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