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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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21st Oct 2019, 5:30 pm | #41 |
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Re: Single-ended Transistor amps.
I remember a series on TV where some celebrities brought in a collection of ingredients and two celebrity chefs attempred to make appropriately impressive meals out of them.
Sometimes the ingredients were thought out for some intended objective, other times they were seemingly random and intended to really test the chefs. It can be fun to do electronics that way. One year at George Dobbs' QRP Christmas party, a bunch of us decided to just make a simple transceiver using what was lying around. We cheated with a sig gen as LO (Marconi TF166G that I'd rebuilt years earlier) A couple of schottky diodes would make a mixer, but we needed a transformer. Was there a 'surplus ferret core' lying around anywhere? "I've got something over here..." replied George. Digging behind the filing cabinet supporting the parish photocopier, he pulled out an animal stole. Animal species undetermined, but long and slender. Two of the little devils joined end to end, as if one had stopped too suddenly in a burrow and been tail-ended by the other. The composite hilariously had three pairs of legs. He explained it was left over after a parish jumble sale. I could understand why it hadn't sold! Anyway, we rolled it into a circle about 18" diameter and made a winding using it as a core. A computer speaker did the audio section. Yes, it received, with the coil resonated. We got a couple of QSOs into holland. Fun? Yes, lots! Did the PA station believe our description of what we were using? Objet-trouve radio! So why not audio? David
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22nd Oct 2019, 8:45 am | #42 | ||
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Re: Single-ended Transistor amps.
Quote:
EF98 plus OC16 is a pretty good combination... 20 years earlier of course you could get the same power gain and sensitivity from 6J7 plus 6V6 of course. But as the topic of this thread is single-ended transistor amps, I'd agree with you, this circuit topology is hard to beat for results and simplicity (I have a Philips 12V car radio which uses precisely this as its AF amplifier). |
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22nd Oct 2019, 9:35 am | #43 | |
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Re: Single-ended Transistor amps.
Quote:
It is a lot of fun making these amps. Probably an MPF102 would subs directly in for the EF98 (along with the 23:1 transformer) if one wasn't keen on the vacuum tube. But EF98's are pretty cool, working with the 12V anode voltage and if you throw in another EF98 as a pre-amp, the heaters just go in series. |
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22nd Oct 2019, 10:35 am | #44 |
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Re: Single-ended Transistor amps.
And if you do that, and operate from a supply around 13.5V, you can even use the heaters as part of the potential divider for biasing the transistor! Might have to take care during warm-up of course, when heater resistance is low - but a robust piece of Si or Ge ought to shrug it off.
Last edited by kalee20; 22nd Oct 2019 at 10:35 am. Reason: Fix quote |
22nd Oct 2019, 10:56 am | #45 |
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Re: Single-ended Transistor amps.
I had a Pye car radio back in the day that used a single OC16 in the output stage and the quality was excellent. It used a matching transformer for a 3 ohm speaker.
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22nd Oct 2019, 1:02 pm | #46 |
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Re: Single-ended Transistor amps.
That is an interesting idea, I had not thought of using the heaters like that. It could also make a mid supply voltage reference for some other circuitry too.
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9th Jan 2020, 12:37 pm | #47 | |
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Re: Single-ended Transistor amps.
Quote:
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9th Jan 2020, 2:37 pm | #48 |
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Re: Single-ended Transistor amps.
Many years ago my step dad gave me an old Philips car radio which I repaired and used for quite a few years in such cars as my Ford Fiesta mk1 and Escort mk1. It used an AD149 output transistor and did sound good driving a decent 3-4 ohm speaker.
We also came across single ended or class A sound output stages in some colour televisions, the Thorn/BRC 8000, 8500, and 8800 chassis and I think the later 9800 also used the same output stage. These used a single MJE340 transistor running from the main 180v HT line. They ran rather hot even though they were fitted to a heatsink. Sound quality was as good as it could be through the forward facing speaker.
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9th Jan 2020, 4:40 pm | #49 |
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Re: Single-ended Transistor amps.
Just as a followup (it was me who started the original thread) - I've got a single-ended amp going here: I used a NKT404 as the output-stage, driven by an OC72 which was driven by an unmarked OC71-black-painted-glass-envelope-style transistor I acquired ages ago in a 'bargain semiconductor pack'.
The output=transformer I wound on the core salvaged from a scrap "wall-wart" - I was able to separate the laminations, then I cut off all the existing windings and put on a new trifilar winding of 3x 60 turns of the wire salvaged from a dead Ethernet flylead - this gives me a range of transformation-ratios to experiment with. Biasing the transistor to pass about 0.8A from a 16V supply, the result into an old 4-Ohm 6x4 car-audio speaker is usably loud. There's noticeable background hiss which I guess probably comes from the unmarked OC71-alike in the front end (unmarked because it was noisy when tested?) but you don't notice it when the amp's being sensibly driven. |
10th Jan 2020, 12:09 am | #50 | |
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Re: Single-ended Transistor amps.
Quote:
I have attached the schematic so you can see the values I ended up with. You will notice the resistor in the cathode of the EF98, that gets shorted out by a 2N3643 transistor. This is just to equalize the volume on the short wave and MW bands and obviously doesn't need that in an amp, though when in circuit the NFB is helpful. The 23:1 transformer was a pre-made car radio part from Beacon Radio in NZ, and that is where you will find these 23:1 transformers, in old hybrid car radios. (Just in case anyone is wondering about the handful of MPF102's there, I wanted the SW band from 5.7 to 17 MHz. Unfortunately the triode section in the ECH83 is not up to that. So I based the L/O on the MPF102. If you ever do this don't forget the series diode & resistor for self bias, Fetrons require this too in an L/O or it malfunctions. Then it needed another MPF102 for a buffer as I like to have an ext L/O signal for a frequency counter with an IF offset. And then another MPF102 IF stage to improve the overall IF gain for SW use. The switches look odd, they are all Teledyne RF relays, switched by an industrial motor switch, a compact type made by Telemechanique, it feels so nice to switch you could do it all day). Last edited by Argus25; 10th Jan 2020 at 12:15 am. |
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