I have been away from this site for a while as I have been away from my workshop with other priorities but I hope to get back into my passion again.

I have had an STC 8471 Chasis for sometime and was inspired by Fred's project to build a Radiogram and I recently picked up a Garrard SL65 turntable with a lovely Sonotone 9TA stereo Ceramic Cartridge. I do like playing mono 78s and have a few turntables for playing these. But as this Turntable and Cartridge sounds so good, I thought I would begin a project of brining them together in my own cabinet yet to be designed and see if I could play stereo records.

I was wondering if any one out there has attempted to convert one of these old chasis like the STC8471 so that it could take both channels seperatly from the cartridge to drive two separate speakers in stereo.

If anyone has any thoughts on this I would be keen to hear from you, or point me to where I might get some insights how to attempts this but also please be blunt and tell me if it is a crazy idea.

Thanks George

It to me is plausible, albeit that it is a mod. If you stay with Valves one could duplicate the 1st AF & power OP but the transformer has to withstand it or be added too.

If did not want to do that there is plenty of space in a radiogram for a separate AMP. Like one out of an old record player & some switching.

Marc

Hi Marc,
Yes I was also concerned about the Tranny, but if you are using just the amplifier PU input and not the radio at all, does the tranny not have some spare capacity to withstand the current drain of a duplicate AMP??

As I am designing the cabinet, as. I only have the chasis for the STC 8741 , I suppose I could design it to house another old radio chassis valve amp but would it not need to be matched somehow with the STC 8741.
When you suggest "some switching". Could you explain what you mean?
Thanks

Can't find that circuit. What is the valve lineup? If the audio amp is something like a 6BD7 and 6M5 (i.e. 9 pin noval) you could keep the radio and build two amps based on, say, 6DX8s, 6BM8s or similar triode-pentodes without exceeding the B+ current budget. Just bias them back a little if using 6BM8s. That way you won't have to add more sockets.

Biggest problem will be the Radio- Phono changeover switch, you'll need to scrounge an extra pole on the switch from somewhere.

Hi Ian,
Nice to hear from you, I have sent you the circuit for the STC 8471 and would love to hear more about your idea.
If anyone else needs that circuit let me know.
George

Hi George

That's a push-pull amplifier. There's enough resources there to turn it into two stereo amps without worrying about the power supply loading. You can use the existing valves without having to add or change anything much.....

You'd probably want to retain the bass and treble tone controls. That will mean replacing the volume, bass and treble pots with dual ganged pots of the same value. You'll need to find a pair of suitable single ended class A output transformers, 5k ohms to whatever speaker impedance you have.

Absolutely possible without changing the authenticity or appearance of the thing too much. Looks like a very worthwhile project - and I don't feel too bad about modifying a relatively new chassis (1957) this way.

I'll draw up the circuit for you in Altium when I get a spare hour or so at work. Might take a couple of weeks. I'll try and make it so you can leave a lot of the connections as they are. Meanwhile if you can source the transformers and the pots...

Ian,
I was hoping to hear that and I thank you so much for taking the time to have a look at the schematic. I will start the search for the pots and transformers, I have quite a few speaker that might be suitable. I will also start designing the cabinet.
I will make this a project to document and put it into the project section once I have completed this so other may benefit and get ideas for there own conversions.
I am looking forward to starting this now that it is plausible.
Cheers George

Hi George, yes follow Ian's ideas and put the project up in the special projects section its getting quite lonely there!
Fred.

Nice to hear from you Fred, hope you are well and getting the right rest.

I will attempt to write it up as I go and take some pics and post in the projects section once I am done, though you have set a high standard and as I said earlier I was inspired with what you did with your STC 8471 though if I recall you did not go Stereo?

I started looking for the output Tramsformers for each of the single ended class A amps but what is out there on the web varies in price considerably and I am not sure what why.

Could these output transformer not be ratted from some old Radio chasis, how critical is the spec, if they are about 5K to 4-8ohm would that not be ok, and do they have to be an exact matched pair?

George

George possibly you may find a junk chassis with a stereo set up with outputs like 6BM8's.
That will automatically have 2 output transformers and a bunch of dual pots for the volume and tone.
I would not sweat about the matching ohms too much you can have twice or half the proper ohms and the music will still come out.


I left the STC as a mono, it never occurred to me to make it stereo...………...lack of imagination!

Take lots of photos as you plod along and by viewing those you can join a story up with words.
I've got fairly slick at it as my background was always preparing reports on things, doing quotations and briefs.
I do a lot of magazine coin articles and photos are a must to put the words into pictures if you see what I mean!

Fred.

Now somewhere between NBN dropping out. I did point out in another post that there is a 6BM8 circuit, on its data sheet, that actually does not use OP transformers.

Yeah, but you need a rare high impedance speaker, circa 800 ohms, or you get a power output in the order of milliwatts.

Well I dug around my collection of speakers to find suitable candidates for this project.
I have a nice 10in Ampillion with the output tranny mounted measures about 3-3.5 Kohms but I only have the one. I also found a very old Octanganal shaped speaker 8 in dia which measure about 350 ohms.


Is it at all possible to use the existing 10in with the out put Tranny for one channel and use the 350 ohm without an output tranny? Most likely a dumb suggestion but as a qualified Cheapskate I had to ask.

So the nearest pair of speakers that are from older style radios that I have are 8 in and 8 ohms

so I am still on the the hunt for two output trannies, I do not have a chassis with stereo set up as Fred suggested though I do have two junked 50s chassis with output trannies to suit the 6M5 but I gather they will not be suitable as the primary is only 400ohms.

I think you will find that output transformers for 6M5 are around 7K.
Ideal for your project.
JJ

Yes, DC resistance is not impedance. The DC resistance gives you absolutely no clue as to the primary impedance of a transformer.

The ones for the 6M5s will be close enough, depending on your speaker impedance. If they are very old they may be intended for 2 ohm speakers, a bit later, maybe 3.5 ohms. Do the markings on them give you any clues?

Physical size matters too. You do want some bass response I assume?