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Post by Richard on Feb 13, 2021 12:45:01 GMT 12
I've always thought the Radio Corporation of NZ manufactured models Columbus and Courtenay 75 and 90 were very well designed receivers, especially when they were using negative feedback in the audio stages to flatten the frequency response back in 1939 and combined with a wide I.F response, producing some pretty good audio on the AM band. The negative feedback amplifier was invented by Harold Stephen Black at Bell Laboratories in 1927, and granted a patent in 1937, so RCNZ were onto it very quickly, especially, if you look at American "Zenith" and "Philco" Radios produced around this time and those manufacturers weren't using it. In fact, looking into the 1940's schematics, they still weren't ! It shows that a NZ manufacturer was producing some very good products for its time and the audio quality far superior to American Radio's.   |
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Steve
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Post by Steve on Feb 14, 2021 12:05:15 GMT 12
I might be a little biased, but I think the model 90 is my favourite sounding valve radio of all time, I still remember the first time I turned one on after repairing a blown transformer and several caps - I hadn't even replaced those lovely oil-filled 0.1uF metal-body caps that are soldered into cups all over the place (and which leak said oil all over the place as well) at that point. The volume, tone, bass response... everything just came together and I was actually shocked that something so old could sound so good - particularly given it only had one 10" EM speaker that was 70-odd years old.
Slight aside: I'm looking forward to restoring my model 88 to see how that compares to my 90, with its fancy 3-position IF and silentode.
Radio Corp did a lot of shopping in the US, both for parts and chief technical staff - so its possible the negative feedback technique came with an engineer who'd been exposed to it before leaving the States. There is a letter amongst board members from 1937 (during what I'll call Spiraldialgate) where one of the board members suggests they should ask Jensen, who was done with his speaker development process at Radio Corp and was heading back to the States, to keep an eye out for a 'decent' factory manager on his return.
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Post by Richard on Feb 14, 2021 16:17:57 GMT 12
Having a quick look at "Radio Ltd" schematics, they appear to use neg feedback around 1945, maybe earlier?
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Post by DHockey on Feb 15, 2021 7:52:13 GMT 12
I had the same experience as Steve when I restored my first Columbus 90. At first it sounded pretty average, with the tone control in the 1 position. However I was astounded at how good it sounded when I rotated the control to positions 5 and 6! It was as crisp and clear as an FM radio. The other Columbus set which sounds nearly as good is the model 44, but I don’t think it uses negative feedback. I’m surprised that more manufacturers didn’t use negative feedback, it’s a relatively straightforward concept that makes a huge difference to how good a radio sounds, more so than things like push – pull audio. Maybe it was the case that negative feedback was not quite as easy to market as a feature than things like push – pull audio output and high valve counts.
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Steve
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Post by Steve on Feb 15, 2021 9:38:24 GMT 12
Good to know about the 44 - I have a nice tidy one in my queue that I bought cheap in Dunedin a few years back, its the one on the site (with awful photos I had planned to change because I was going to restore it that weekend but never did) - I like some of the other nifty design ideas Radio Corp put in to that one (tuned power supply etc) and I'm quite fond of the look of it. I might be wrong (it happens more than I care to admit) but the 44's schematic shows a resistive divider across the output transformer feeding back into the grid of the 6V6 - is this not negative feedback? Not being an electronics engineer I'm not sure what actually constitutes official 'negative feedback', and the reading I've done on it left me thinking either its something specific nobody really understands or its almost any instance of the signal being fed back at 180 degrees out from the signal its feeding in to. In terms of advertising: I guess people think more is better... so push pull is more power AND more valves... negative feedback is one more wire, maybe a resistor or two... I've never heard of an advert hyping the benefits of 'more wires'  Although manufacturers were good at coming up with fancy buzzwords for things like this - 'Pentonic' Tone Control for example - from models like the Ultimate RBC. I note with interest that the model 90 doesn't talk about using negative feedback in advertising that I've seen (or even the booklet they produced on it), but it does advertise the great speaker design and a special new output transformer to enhance the bass capabilities of the set.
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Post by Richard on Feb 15, 2021 10:45:30 GMT 12
Your Model 44 does indeed have neg feedback, you are correct with the divider across the OP transformer back to the plate of the preceding tube/valve 6SJ7. I think this is called "Plate to Plate" feedback ? Obviously not as good as getting feedback from the secondary of the OP transformer, so that it includes distortions in the transformer, but still an improvement in Audio quality.
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Post by DHockey on Feb 15, 2021 10:59:01 GMT 12
Thanks Richard, yes now that I actually go and look at the schematic rather than relying on my memory I can see the 44 does use feedback, which would be one of the reasons it sounds so good.
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Steve
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Post by Steve on Feb 16, 2021 8:18:09 GMT 12
I was just reminded of the Clipper 5M4's I have worked on... The later variants of the Junior chassis appear to use feedback from the speaker... I noted the physical differences between the early and late model chassis' at the time, but I didn't draw out the changes - I must go back and take a look again... at first glance it looks like it could be positive feedback(?!) here because it looks almost like it feeds on to the volume control center tap... I'll need to take a closer look.
Clipper 5M4 Junior chassis - early version in foreground
I know its not Radio Corp, but it just popped into my mind so I thought I'd add it to the mix of information. This set was from 1954 but I believe ran for a few years.
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Post by Richard on Feb 16, 2021 11:28:40 GMT 12
If it was positive feedback , then your radio would be an oscillator. |
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Post by davidballantynezl1tfg on Feb 17, 2021 9:34:16 GMT 12
When you replace a speaker transformer in a set with Negative feedback you have a 50% chance of having positive feedback. Lots of squeals which may vary with the volume control. Swap over the transformer leads. David
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Although manufacturers were good at coming up with fancy buzzwords for things like this - 'Pentonic' Tone Control for example - from models like the Ultimate RBC. I note with interest that the model 90 doesn't talk about using negative feedback in advertising that I've seen (or even the booklet they produced on it), but it does advertise the great speaker design and a special new output transformer to enhance the bass capabilities of the set.
