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Old 28th Jul 2021, 9:37 am   #21
Radio Wrangler
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Default Re: Valve amplifier design 1965

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Originally Posted by Electronpusher0 View Post
A bit more research on the Radford revival site shows that Radford did indeed produce an amplifier using crossover feedback, the TT100!
And the schematic attached show some backsliding! The MJ340 long tailed pair isn't cascoded, so the high frequency imbalance due to Miller and the asymmetric impednces at the bases is back. Bailey's improvement to the triode-triode phase splitter is also appropriate with transistors.

What's more transistors have some added problems with miller-related effects. Ccb is a diode junction and the capacitance is that of a varactor diode, so the HF roll-off pumps with strong signals... hello intermods! then there's the Early effect.

David
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Old 28th Jul 2021, 10:16 am   #22
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Default Re: Valve amplifier design 1965

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... that little balun transformer looks to be pretty easy. A transmission line transformer on a ferrite core might even be a possibility. The low inductance and the low number of turns and the low impedance mean the spurious resonances ought to be fairly easy to push well above the unity loop gain frequency ...
I know O about transformer design, I'm afraid, but might a good-quality commercial audio isolation transformer be up to the job ? I see that 200ohm ones are available for use with microphones, and something like the Neutrik NTM1 https://www.neutrik.com/en/product/ntm1 looks like a nice piece of work (fully shielded, for example).

Then again, perhaps the trade-offs that have had to be made to get decent bass capability, which we wouldn't need, make these transformers sub-optimal for us. I note that the NTM1 has a secondary winding resistance of more than 100ohms for example.

Cheers,

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Old 28th Jul 2021, 11:25 am   #23
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Default Re: Valve amplifier design 1965

Most dynamic microphone transformers are voltage step-up jobs to suit more typical input impedances of amplifying devices.

The lack of need for bass response is a great freedom in the design of this transformer. Remember that you want its behaviour to be good far above the audio range, to and beyond the unity loop gain point which has to be as far above the audio range as the amount of feedback you want to be still there at the top of the audio range.

So the feedback balun has to go a lot higher in frequency than the output transformer. But it doesn't have to go so low. So the number of octaves it needs to work well over might not be quite so arduous. BUT the reduced impedance is a gift made in heaven. Transformers at low-Z are an awful lot easier to design.

A long time ago, I had to develop a transformer-based impedance measuring bridge. The wanted frequency range was 60kHz to 18.6MHz. It had to be to full accuracy spec over this range, so the transformers had to be good well over an octave beyond each end.

There was one design freedom, it was used between 75 Ohm source and detector, but the detector was ludicrously sensitive, so I could afford a lot of insertion loss. So I used it. A resistive pad reduced that 75 Ohm Zo as far as I needed, and in the new low Z regime, I didn't need quite so much primary inductance. Transmission line techniques handled the top end (Ruthroff: "Some transmission line transformers")

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Old 28th Jul 2021, 2:58 pm   #24
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Default Re: Valve amplifier design 1965

This thread is fascinating and I am sure it has a long way to go yet.
Mods, perhaps it should be moved from "wanted" to vintage audio.

Peter
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Old 28th Jul 2021, 4:07 pm   #25
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Default Re: Valve amplifier design 1965

Well, the feedback from the output transformer primary starts rolling off at 6.77kHz (the series 47kΩ and 500pF).

As David points out, the little balun transformer is loaded by 68Ω - the dominant load. So allowing for the transformer itself introducing negligible LF phase-shift, we'd like the reactance of the half-winding to be at least 680Ω at this 6.77kHz.

That then puts the inductance at 15.9mH.

I tend to think that saturation will not be an issue - the voltage across a half-winding will be well under a volt (47kΩ and 68Ω forming a 690:1 voltage divider from the voltage on the output transformer primary); the frequency is high, so flux swing will be small. There's the DC cathode current flowing through it which will tend to polarise the transformer core, but a possible refinement might be to put a 1μF capacitor in the line from the transformer to the junction of the 68Ω and 680Ω resistors, which will eliminate that.

16mH is easily achievable, with a high-permeability ferrite core. Ed's suggestion of nickel-iron would probably be better still - inductance achieved with fewer turns, which would mean less problems with parasitic capacitance and leakage inductance at the HF end.
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Old 28th Jul 2021, 4:25 pm   #26
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Default Re: Valve amplifier design 1965

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Originally Posted by Electronpusher0 View Post
This thread is fascinating and I am sure it has a long way to go yet.
Mods, perhaps it should be moved from "wanted" to vintage audio.

Peter
Good idea!

Presto! Changeo!

Done!

David
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Old 28th Jul 2021, 5:28 pm   #27
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Default Re: Valve amplifier design 1965

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Well, the feedback from the output transformer primary starts rolling off at 6.77kHz (the series 47kΩ and 500pF).
I have just plotted the crossover points of the HF and LF feedback for both the Bailey EE design and the TT100 using LT spice

The Bailey EE design crosses at 6.4kHz and -3.26dB
The TT100 crosses at 1.446kHz and -2.7dB

Peter
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Old 30th Jul 2021, 10:55 am   #28
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Default Re: Valve amplifier design 1965

With reference to the inverting transformer in the feedback, Kalee has observed

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There's the DC cathode current flowing through it which will tend to polarise the transformer core.
Because the transformer is connected directly across the 68 ohm resistor and at dc carries the Cathode current of the Cascode stage.

I have been comparing the Bailey / Radford circuits that used the ECC88 Cascode front end, the Bailey design we are discussing here, the STA100 and the STA25

The Anode load resistors are different in each case and the Anode currents are consequently different.

Circuit Anode resistor Anode current (approx)
STA25 22k 3.5mA
STA100 47k 2.5mA
Bailey 100k 1.5mA

Perhaps Bailey chose a lower value of Anode current (and hence Cathode current) to minimise this polarisation.

Peter
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Old 30th Jul 2021, 11:22 am   #29
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Default Re: Valve amplifier design 1965

The need for only low winding inductance plays nicely into tolerating some direct current.

It needs watching and checking, but shouldn't be too difficult. I'm wondering about mumetal dust or sen dust toroid cores?

David
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Old 30th Jul 2021, 12:05 pm   #30
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Default Re: Valve amplifier design 1965

Bobdger and I are experimenting with ferrite transformers, I can try checking the bandwidth at 1V ac both with some dc and without.

Peter
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Old 30th Jul 2021, 1:33 pm   #31
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Default Re: Valve amplifier design 1965

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The need for only low winding inductance plays nicely into tolerating some direct current.
It does indeed, but it would be nicer still if there were no DC.

A toroid, of mumetal or amorphous alloy, permeability approaching 100,000 would allow the inductance to be achieved with the fewest turns - and few turns gives the lowest self-capacitance (remembering that this is the transformer which handles the HF stuff). Polarising the core then becomes more of an issue.

It could well be that the numbers indicate it's a non-problem, and even a larger, lower-permeability core which is saturation-proof with 1.5mA, still has capacitances in the low tens of pF. All the same, if it's a choice of DC or no DC, I know which I'd prefer!
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Old 31st Jul 2021, 6:55 pm   #32
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Default Re: Valve amplifier design 1965

I have never heard of a KT90, Are these new developments or do they go back to the early days, ie late 40’s and 50’s?
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