Supersonic Conquest radio's PR80 and PR80C

[Mike Watterson]

It's a nice looking board

hFe readings on Germaniums generally are very low as compared to current silicon. Modern meters often don't cope with them either. I think it's probably their very high leakage, numbers of microamps, as compared to hFe's often of twenty or less. Power transistors can be even worse.

I have an old analogue meter which tests them Ok but when I tried some with a modern digital meter I had I got no sensible readings.

All true. Also if I'm building rather than restoring I'd often use Silicon and change the bias. PNP keeps the supply rail and electrolytics the same, but NPN is no problem, same bias as for PNP silicon.

A huge issue with a germanium power stage is the thermal runaway, so can need an NTC thermistor or something, on the heatsink. Typical example. A silicon based amp can still have thermal runaway, but it's rarer.

[Podbuilder]

My board is about populated and ready for transformers.  No vintage radios were or have been destroyed in the process.  In fact I quite like my old radio that picks up the local AM station that only ever seems to play oldie songs..perfect!!..

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Here's my handy work so far.



I have the input transformer worked out.   DCR 200ohms : 100ohms CT.  50ohms each side of the CT.  2:1 wind ratio.  With the size of my bobbin I'll go with 40AWG wire and just roll out 200ohms in length.  Secondary ohms might be a little higher then 100ohms as Im rolling it over the top at a 2:1 ratio.  

The primary transformer is still a bit of a mystery.  If anyone has one of these old 7 pin transformers and can measure the DCR across / between all pins and check for continuity I'd be extremely greatful.  Thanks for all the input to date.

[Podbuilder]

First transformer worked out ok.  Didnt quite hit the specs 200 ohms : 100ish ohms DCR on the driver transformer.  I have another transformer half made the exactly hit 200ohms for the primary side.  I'll finish it when more wire arrives.  I knew the trimmed legs of the capacitors / resistors would come in handy at some point.    Winding ratio was 2:1.

[ppppenguin]

The resistance shouldn't be important within reason. I wouldn't worry about trying to hit an exact value. The inductance needs to be high enough and the ratio correct.

[Mike Watterson]

I agree, the inductance, turns ratio, core coupling and properties are far more important than resistance values on a small audio transformer.

[Podbuilder]

Using a 11.35v AC power supply on the primary gives 6.06v across secondary and 3.6v & 2.98v to CT.  I checked my turns ratio again and noticed I set it incorrectly.  Set to 1.76 not 2.  I shouldnt be winding transformers late at night..  anyway.   Not too bad for a first attempt. 

Im still after any info on the audio output transformer if anyone reading this owns one.

[Podbuilder]

I just tested the inductance.  Here's what Mr AI thought.

Analysis of Measured Transformer Inductance
Summary of Results
Winding Measured Inductance (H)
Primary 0.863
Secondary 0.266
CT–Leg1 0.066
CT–Leg2 0.063
Interpretation

1. Inductance Ratio and Turns Ratio
Inductance is proportional to the square of the number of turns

2. For your transformer, the measured inductance of the primary is notably higher than that of the secondary, consistent with the primary having more turns and likely experiencing marginally better coupling to the core.

The ratio of primary to secondary inductance (0.863 H : 0.266 H ≈ 3.25:1) is roughly in line with the square of the turns ratio suggesting measurement consistency and good transformer construction.

2. Full Secondary vs. Center Tap Inductance
The two measurements (CT–Leg1: 0.066 H, CT–Leg2: 0.063 H) are very close, indicating the secondary is well center-tapped and the winding distribution is relatively uniform.

The sum of the CT–Leg1 and CT–Leg2 inductances (≈0.129 H) is lower than the measured full secondary inductance (0.266 H), reflecting mutual magnetic coupling effects: when measuring across the entire secondary, both halves support the magnetic field, and the measured inductance is more than the sum of the parts due to constructive coupling.

3. Implications
Good core coupling: The measured inductance values are reasonably high for your winding and core structure, supporting efficient transformer operation and minimal unnecessary core magnetizing current.

Transformer symmetry: The near match in CT–Leg inductances shows careful winding and symmetry, important for balanced dual-voltage or full-wave center-tapped applications.

Absence of significant defects: No major imbalance or unexpectedly low inductance values were detected, suggesting the windings are intact, core is assembled correctly, and there are no shorted turns.

4. What These Values Reveal
Primary and secondary inductances are consistent with turns ratio and expected core behavior.

Center tap measurements confirm good winding and splitting accuracy.

The transformer should perform well for intended AC voltage transformation, and the core is providing effective magnetic coupling.

Key Takeaways
The transformer’s measured inductance values align with theoretical expectations for your winding and core.

Both symmetry and high-enough inductance confirm good design and practical function for power transfer and voltage stepping.

Inductance measured at the center taps supports proper full-wave rectification or dual-rail operation with minimal imbalance.

If you need further interpretation or investigation into other winding properties, let me know!

[Podbuilder]

Second attempt was better 2:1 ratio this time which was reflected in voltage and CT voltages.  11.36v Primary : 5.31v secondary CT 2.62v and 2.62v.   From what I've read about this audio circuit ,  it was used in a few different radios.  Some with 35ohm speakers and some with 8 ohm speakers.  All the documents people show (me included) show a 35ohm speaker, but that actual Deacy Amp has two 16 ohm speakers in parallel giving 8 ohms.   So we can safely assume dont wind a 35 ohm transformer coil for the speaker unless you want smoke.  I'll wind my audio output transformer soon and see how it goes.   Not sure how the newer germanium transistors will go. 

[Mike Watterson]

Note that unless a pair of windings are bifilar to make a centre tap, that the same turns ratio (needed for true centre-tap) on a normal core results in different resistances, because one winding is on top of the other, and thus needs more wire for the same number of turns.

Also for audio, the actual inductance isn't the most important aspect, but that at the lowest frequency the core doesn't saturate. With too many turns you have issues with the self capacitance and too much inductance for the high end of the frequency response. The thickness and material of core laminations and air spacing etc affects linearity and saturation. A core with DC saturates more easily. In this respect the driver for a push-pull has more issues with DC than the output transformer, because if the o/p is wound correctly the DC magnetisation currents cancel and the core only needs to cope with AC.

[Podbuilder]

Thanks for the info.  Im new to transformer winding and  I wrote the program Im using to wind the coils.  It spits out the G-code for my CNC machine.  It takes into account the increasing diameter of the coil, length of wire and increased DC resistance with each consective layer.  But thanks for the advice.  I was placing the CT at the middle of the wire length, not the middle turns ratio.  I'll make some adjustment to my program now.     That's an easy fix.