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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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Old 21st Oct 2019, 5:30 pm   #41
Radio Wrangler
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Default 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?

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Old 22nd Oct 2019, 8:45 am   #42
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Default Re: Single-ended Transistor amps.

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Originally Posted by Argus25 View Post
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To be fair, you can get an equivalent power gain from just ONE device, a power MOSFET operating in Class A... which you can use a 22MΩ gate-tie resistor with.
Yes, but to be very fair, both the EF98 and the OC16 are pre-1960 technology.

And, not only that, your power mosfet has an input capacitance many orders of magnitude higher than the grid capacitance of an EF98.... The G1 of the EF98 just looks like your 10M resistor, the gate of the power mosfet looks like the 10M plus a parallel 500 to 5000pF capacitor in parallel, depending on the power mosfet type.
True on both counts, though in terms of power gain the input capacitances of the MOSFET have no effect as the resulting input current is 'wattless.' I did consider it before I posted!

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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Old 22nd Oct 2019, 9:35 am   #43
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Default Re: Single-ended Transistor amps.

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....though in terms of power gain the input capacitances of the MOSFET have no effect as the resulting input current is 'wattless.' I did consider it before I posted!
I agree and its nice with the EF98 that for practical purposes its input with the 10M resistor behaves as an "R" and not a "Z", so you can drive that from practically any audio source without loading it differently over the frequency spectrum.

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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Old 22nd Oct 2019, 10:35 am   #44
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Default Re: Single-ended Transistor amps.

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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.
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.

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Old 22nd Oct 2019, 10:56 am   #45
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Default 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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Old 22nd Oct 2019, 1:02 pm   #46
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Default Re: Single-ended Transistor amps.

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Originally Posted by kalee20 View Post

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!
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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Old 9th Jan 2020, 12:37 pm   #47
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Thumbs up Re: Single-ended Transistor amps.

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I have attached a copy of Mullard's original document supporting the OC16. Not much of a power transistor by today's standards, but then it was revolutionary and I still like it.

This is the circuit I used in the radio I posted a link to in post 7.

There is a lot than can be learnt from this circuit that can be generalized to higher power versions.

In this case the quiescent emitter current is 400mA. This results in a 0.88V drop at the emitter, which is only about 6% of the supply rail...so its not really needed to have an elaborate bias arrangement to eliminate this. The AC gain is reduced a little by the NFB resistor Rf, and the stage produces a very faithful sine wave <5% distortion, which I cannot hear. You can tame the higher range frequency response with a capacitor in parallel with Rf, or make a tone control there.

As stated it requires a heatsink of 5.5 degC/W or less. ( in my radio I increased Re to 3.3R increasing the emitter drop to 1.38V, used an 820R for Rf and R2 turned out to be 68R for 0.42A emitter current). The RMS collector voltage still reaches 7V before clipping.

There are two key points though,

The Thevenin resistance of the bias circuit is less than 10R and is about 8.7R. This is essential because of the germanium transistor and its leakage properties.

Secondly, with the driver transformer and an appropriated driver device you can easily generate the appropriate drive power which is not insignificant, as you can see from the drive requirement specs.

I used a 22:8 R impedance ratio choke as the output transformer.

When the EF98 is used as the driver (handy as it runs off 12V) the required transformer has a turn's ratio of 23:1, the DCR of the secondary should be less than 1R and the primary less than 200R.

It is quite surprising how pleasant these class A stages sound, a lot like a valve radio...but devoid of hum, especially running off a 12V auto battery.

With a pair of OC16's in push pull you can get 17 to 20W with a 14V supply, which is very impressive for the day though you need a watt of drive power, which of course you could get from another OC16 in class A.

Also, one other interesting thing, using the EF98, it normally has a 10Meg Ohm G1 resistor. So you end up with an amplifier with two active devices EF98/OC16 & two inductive devices transformer/choke. With an input impedance of 10M, and output impedance of what your speaker is say 4R. It takes about 1 to 2V peak at the EF98 grid to get full power. That has to be one of the most spectacular power and impedance ratio transformations off all time, for just the two devices.
Hello ! Can You send me the schematics of these amp or that amp what is used in your radio, with ef98 and oc16? Thank You!!!! btw I gor some oc16 laying around me, and I want make an nice little amp for my vintage speakers!
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Old 9th Jan 2020, 2:37 pm   #48
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Default 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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Old 9th Jan 2020, 4:40 pm   #49
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Default 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.
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Old 10th Jan 2020, 12:09 am   #50
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Default Re: Single-ended Transistor amps.

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Originally Posted by robertnagy1961 View Post
Can You send me the schematics of these amp or that amp what is used in your radio, with ef98 and oc16? Thank You!!!! btw I got some oc16 laying around me, and I want make an nice little amp for my vintage speakers!
Hi,
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).
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Last edited by Argus25; 10th Jan 2020 at 12:15 am.
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