Germanium Amplifier

[Nick]

Just a couple of thought, you are never going to be able to use much in the way of negative feedback in that amp (at least by comparison with a modern direct coupled circuit. That interstage is going to add so much group delay and general phase shifting nastyness that the most you can manage is PP valve amp levels of feedback. But I wonder if its worth trying a NPN for Q1 and taking the feedback back to Q1's base.

The fact that you are getting problems with sharing the power supply is pointing to generally phase shifting issues. I am reminded of the joke about asking for directions in Ireland, and the answer being "If I was you, I wouldn't start from here".

[Mike H]

PNP oc35s ( never heard of them) I always thought oc28s were the swanky ones!

I'd like to hear them.

And these are ex-RAF military ones, so even swankier.

[Mike H]

Just a couple of thought, you are never going to be able to use much in the way of negative feedback in that amp (at least by comparison with a modern direct coupled circuit. That interstage is going to add so much group delay and general phase shifting nastyness that the most you can manage is PP valve amp levels of feedback. But I wonder if its worth trying a NPN for Q1 and taking the feedback back to Q1's base.

The fact that you are getting problems with sharing the power supply is pointing to generally phase shifting issues. I am reminded of the joke about asking for directions in Ireland, and the answer being "If I was you, I wouldn't start from here".

Yes, yes, and yes. And yes. And .....

It's another one of those out of a library book in the 1970's. "most you can manage is PP valve amp levels of feedback", I came to the same conclusion, hence reducing the gain. But I'd be satisfied with that, pending what it sounds like.

"I wonder if its worth trying a NPN for Q1 and taking the feedback back to Q1's base" - I tried converting to NPN, but couldn't make it work - it went very complicated very quickly...

Just for reference, it's pretty much based on this - the 'Burns Orbit', which was a guitar amp apparently - I think -
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burns_orbit_3 (2).jpg (130.91 KiB) Viewed 1619 times

[Mike H]

Warmer weather is on the way Mike, soon be working on your geranium amplifier...

As usual, the last few days I've been down a different rabbit hole...

[Mike H]

This is cobblers and do this better is my kind of self-encouragement!

I was part-paid (aka "take them please before I dump them on eBay") for a repair job a while back in a few OC35s! Matched pairs apparently back in the 60s or 70s and stored in these polystyrene boxes.

Shiny shiny!

Coolio! Think mine came from Langrex ... were in individual white carboard boxes, each stuck into polystyrene block, plus a mica insulator 'gasket', and official looking RAF type lettering and codes on the outside, including, 'Tested April 1977' stamps.

[Mike H]

Ah found a picture ...
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cv7084_box.jpg (47.95 KiB) Viewed 1614 times

[Mike H]

But I wonder if its worth trying a NPN for Q1 and taking the feedback back to Q1's base.

Oh wait, sorry I misunderstood - forgot where 'Q1' was - I know what you mean now - I shall have a ponder..... back shortly ...

"I have a cunning plan my lord"

The fact that you are getting problems with sharing the power supply is pointing to generally phase shifting issues. I am reminded of the joke about asking for directions in Ireland, and the answer being "If I was you, I wouldn't start from here".

The 'frontend' already existed as a stand-alone SE MOSFET amp - the same applied there - without any extra o/p stage added on.

[Nick]

Just for reference, it's pretty much based on this - the 'Burns Orbit', which was a guitar amp apparently - I think -

What I love about that circuit is how much you can see valve thinking. Its basically a valve amp circuit with lower voltages and transistors.

[Mike H]

Definitely got that going on hasn't it? This is a bit too new, let's go with what we know ...

[Mike H]

Alright, so then I found this fabulous thing - it's another one from the school exercise book... this is Fig 10.38 from the original library book...
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Fig1028.jpg (152.53 KiB) Viewed 1541 times

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They are taking the NFB back to the driver stage emitter, so I tried that... first I did what you suggested, take it back to base, that did the trick, but was a bit awkward to drive - not impossible, just awkward. So I went with this...
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Germanium amp 2024-C4.png (108.05 KiB) Viewed 1541 times

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It seems completely stable, such that all the extra paraphernalia that was added to try keep it under control, could be entirely removed! Can't be bad.

So now, the first stage makes do with local emitter feedback, with a bit of Voltage gain to bring the output V up to 9 - 10X the input V. Also it does DC biasing of the 2nd stage. Sweeet!

[Mike H]

Frequency sweep, checks out OK...
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Germanium amp 2024-C4 f-sweep.png (9.09 KiB) Viewed 1540 times

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Noise plot, it is what it is...
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Germanium amp 2024-C4-noise.png (9.99 KiB) Viewed 1540 times

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Clipping with 2V peak input. No unexpected behaviour...
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Germanium amp 2024-C4-clip 2V.png (12.23 KiB) Viewed 1540 times

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[Mike H]

First 30 seconds of a WAV file - Life Is A Long Song by Jethro Tull.....
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Germanium amp 2024-C4-wav.png (18.95 KiB) Viewed 1538 times

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I had the EP of this when it first came out, so 1971-ish, ad often used it as a 'test signal' for whatever I was building at the time.

Not an ideal choice to be honest, and caused a bit of head-scratching sometimes, until the penny finally dropped that, like a lot of Tull's stuff, the tonal balance can be quite weird, usually weak bass and a reedy sounding treble - leading one to chase a 'problem' in one's project that didn't actually exist - no, it really does sound like that...

Anyhow thought it would be amusing to simulate it, just for old time's sake

[Mike H]

With improved output transistor biasing - originally thermistors were added for safety, they basically went across the transformer secondaries, because the DC resistances of same formed part of the resistor divider that biases each o/p transistor, as per the Burns-Orbit. The original thermistors are 100Ω, not ideal values, and not ideally placed, because if they go low resistance they will load the transformer. Hence, reworked to remove the secondaries from the bias chain, and so the low impedance end of it is a resistor and thermistor in parallel, 10Ω for each seems to be it.


Latest schematic -
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Germanium amp 2024-C5.png (110.84 KiB) Viewed 1500 times

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Step temperature plots, 20 - 60°C in 5° steps - top to bottom starting with green - this is the current thru either of the OC35 emitter resistors -
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Germanium amp 2024-C5-step temp 20-60.png (12.93 KiB) Viewed 1500 times

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Footnote - it didn't help the instability issues that the transformer is/was lightly loaded. Not shown on the schematic, but each secondary has 100Ω in parallel. This was also what the 100Ω thermistors did, but now they're removed. That seems to be the MO.

Onwards...