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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, 5:19 pm   #41
Diabolical Artificer
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Default Re: Single-ended Transistor amps.

i have few traniie driver tfmr's if you need any for your amp build, look forward to seeing how you get on. apologies fore bad typing can't use right hand, whacked this morning shifting stage gear..

andy.
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Old 21st Oct 2019, 5:30 pm   #42
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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?

David
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Old 22nd Oct 2019, 8:45 am   #43
kalee20
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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   #44
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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   #45
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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.

Last edited by kalee20; 22nd Oct 2019 at 10:35 am. Reason: Fix quote
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Old 22nd Oct 2019, 10:56 am   #46
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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.

Peter
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Old 22nd Oct 2019, 1:02 pm   #47
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Default Re: Single-ended Transistor amps.

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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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