Can this bulb be subbed for diodes in this (NOT Germanium) pre amp ?

[Rob Flinn]

I am working on a pair of Ludwig Jahnke germanium pre amps (see attached circuit). There is a filament bulb (12v 120mA) between the output pair of transistors (part 81). On one of the preamps I am dealing with this bulb has blown.

EDIT. I originally thought these were germanium because of the negative supply, but looking up the transistor numbers all transistors are in fact Silicon

Questions.
1. I am guessing the bulb is to keep some sort of bias on transistor part 78. Is this the case, & does the voltage across the bulb change during operation ?
2. Can I sub a diode in or does this circuit need to have a bulb there ?

Thanks

Rob

 

[doorunrun]

I'm not familiar with the preamp. My guess is it's acting as a fuse for the final transistors.
Lamps can be used as a "ballast" for troubleshooting vintage equipment when inserted in series with the power cord.
I think the principle is the same here.

Maybe there's a "pico" fuse that can be substituted for the bulb.

Good luck!

 

[zamproject]

Yep, look like variable current limiting ? soft knee security limiter ?
Not sure about pico fuse, is it standard (but small) fuse?... but maybe a polyfuse can do the trick ?

How the bulb behave in the working unit ? current ? voltage across ? producing light ?
RAFI still have a bulb catalogue, maybe have a look for direct replacement.
ref 1.90.060.003/0000 is given for 12V 0.12A in BA9 base.

 

[JohnRoberts]

The parts list calls it a 12v, 12A lamp (rafi?). I'd start there. [edit 0,12A which apparently means 0.12A /edit]

JR

 

[doorunrun]

A quick check at Mouser...
A small cylindrical fuse rated at 125mA:
https://www.mouser.com/ProductDetail/Bel-Fuse/MRF-125?qs=MvPYbBWWJyuZyn%2BUGTNHjg==
Incandescent lamp with bi-pin base:
https://www.mouser.com/ProductDetail/Bel-Fuse/MRF-125?qs=MvPYbBWWJyuZyn%2BUGTNHjg==
Just some quick ideas.

 

[Voyager10]

A bulb behaves like a resistor, except that as it passes more current, it heats up, and the resistance increases (often dramatically).

In the circuit its role is to set the current through transistors 78 & 79, but the non-linear behaviour helps to limit thermal runaway (see Push-Pull Class B and Class AB Amplifiers [Analog Devices Wiki]).

By my calculation there is about 1.5V between Q78 and Q79 bases, so 0.2-0.3V across the bulb. That's way lower than the 12V/120mA bulb rating, so it's hard to predict what the actual current through Q78/Q79 is. (I wouldn't exceed 20mA here, the transistors will get uncomfortably hot otherwise).

A diode won't do the trick (it has exactly the wrong type of V-I characteristic compared to a bulb). Most other push-pull outputs just have a resistor here, but perhaps the bulb is part of the mojo?

 

[Voyager10]

Just re--read the original post. Do you have a second preamp with a working bulb in it?

If so, measure the 'cold' resistance (just with a multimeter, with the power off) of the working bulb and pick a 12V substitute bulb with a similar measured value. The cold resistance will be very close to the value at the operating point of 0.2-0.3V.

 

[JohnRoberts]

If you know what the old one looks like, that could help identify a suitable replacement.

JR

 

[Rob Flinn]

Thanks for the responses everyone. I do have a second working pre amp here & it says 12v 0.12A like it says in the parts list. I did measure the cold resistance earlier & I think (from memory) it was 46ohms The bulb is a similar size to an MES but with flying wires & it solders into a holder.

 

[gyraf]

Yes, its probably a simplified PTC thermistor, like also used in wien-bridge oscillators (although different reason)

 

[Wilhelm]

I am working on a pair of Ludwig Jahnke germanium pre amps (see attached circuit). There is a filament bulb (12v 120mA) between the output pair of transistors (part 81). On one of the preamps I am dealing with this bulb has blown.

Questions.
1. I am guessing the bulb is to keep some sort of bias on transistor part 78. Is this the case, & does the voltage across the bulb change during operation ?
2. Can I sub a diode in or does this circuit need to have a bulb there ?

Thanks

Rob

The purpose would have been to thermally stabilize the output pair; the resistivity of the bulb increases with temperature. Germanium transistors are much more affected by temperature than silicon - and they did'nt have PTC resistors back then - so a bulb was used instead ( Thermal run-away . Beware ! ) Perhaps you can substitute it with modern-day temperature-dependent resistor (positive-temp-dependency), a PTC resistor. The other -working- unit should be good for measuring the relevant voltage-drops across it's bulb ( when the circuit is turned on, of course), so you can calculate it's idle-current-resistance (appr. corresponding to the PTC's room-temp, nominal resistance-value). It's hard to tell exactly which characteristics the substituting resistor(s) should have, but if you're willing to experiment, it should be safe starting with a a PTC that has a strong increase in resistance with temp, first - and then from there, work your way down to a type that keeps the sonic merits as good as the unit with the bulb still installed - without risking thermal run-away

 

[Rob Flinn]

The purpose would have been to thermally stabilize the output pair; the resistivity of the bulb increases with temperature. Germanium transistors are much more affected by temperature than silicon - and they did'nt have PTC resistors back then - so a bulb was used instead ( Thermal run-away . Beware ! ) Perhaps you can substitute it with modern-day temperature-dependent resistor (positive-temp-dependency), a PTC resistor. The other -working- unit should be good for measuring the relevant voltage-drops across it's bulb ( when the circuit is turned on, of course), so you can calculate it's idle-current-resistance (appr. corresponding to the PTC's room-temp, nominal resistance-value). It's hard to tell exactly which characteristics the substituting resistor(s) should have, but if you're willing to experiment, it should be safe starting with a a PTC that has a strong increase in resistance with temp, first - and then from there, work your way down to a type that keeps the sonic merits as good as the unit with the bulb still installed - without risking thermal run-away

Having looked up the transistor numbers they are infact all silicon. I stupidly made the assumption that they were silicon because of the neative supply.

 

[Mr. Moscode]

How much current does goes through the bulb with no signal? You can figure out the power dissipation in the xstors. You will have to put in a 1 ohm or less resistor to measure quiescent current or run it through a ma meter.

As far as I can see, as someone said I think, the bulb is there to current limit the opt stage. If it get's hot the R value goes up and it current limits.
Normally you would find a diode or 2 between the bases of the output devices mechanically thermally attached to them and emitter resistors between the 2 transistors. If the transistors heat up the bias drops to limit current.

No the diode is not the right thing to put in there. It will have the opposite effect, voltage drop goes down as the diode heats up.

Simplest is to find another light bulb or 2 and check the ckt. You want to know why the bulb blew out. Also check the output xsters. Bulbs do die on their own.

 

[JohnRoberts]

Bulb filaments can fail randomly.

KISS replace with a similarly rated incandescent bulb.

JR

 

[Rob Flinn]

Bulb filaments can fail randomly.

KISS replace with a similarly rated incandescent bulb.

JR

That is exactly what I plan to do but not so simple to find. Only place I have seen a 12v 120mA is on ebay from Germany so far.

 

[Voyager10]

Just for the sake of science I found the closest thing I had handy - a 12V 1.2W bulb (i.e. 100mA) and measured the V/I curve at low voltages:

Voltage V	Current I	Equivalent resistor (V/I, ohms)
50mV	3.6mA	13.9
100mV	6.9mA	14.5
150mV	9.2mA	16.3
200mV	11.0mA	18.2
250mV	12.5mA	20.0
300mV	13.5mA	22.2
350mV	14.4mA	24.3

Obviously, your bulb will be different (especially if the 'cold' resistance is 46 ohms) but it illustrates the general idea of the equivalent resistance going up as the voltage increases.


It might be sacrilege, but a JFET would do basically the same job. Something like the J111 (https://www.onsemi.com/pdf/datasheet/j111-d.pdf) with gate & source tied together. At low voltages it behaves like a resistor, but the current is limited as the voltage increases. The resistance increases with temperature (datasheet fig 9) so it gives thermal stability.

The main aim here is to get the current through Q78 and Q79 the same on the repaired amp (when everything's warmed up) the same as on the working amp.

 

[jokeramik]

It´s the same configuration like the transistors 74 and 75.
I think it forms a constant current source.
Which transistors are used?
Best regards!

 

[Voyager10]

Q74 is 2N3703, Q75 is 2N3705. The part numbers are listed on pages 3 and 4 of the PDF.

 

[vintageampfixer]

The lamp's used as a current limiter - so that when the current in the output stage passes a threshold, the lamp's filament starts to heat up and would present a dramatically higher resistance, limiting the output stage current. IF you're lucky, you might see a glimmer from the lamp but in normal operation I would expect it to stay dark. A simple resistor or a diode cannot be used to sub here because they would behave differently. I agree these aren't readily available but at least here in the UK, Google finds a few suppliers although generally the lamps are in LES packages rather than wire-ended.

I'd try any bulb that's similar - a good auto store will have small lamps to backlight dashboards, etc. If you can find something similar - 1w to 2w for the test would probably be fine - put the new part in circuit and see what happens. For my money, you'll either see 'nothing much' and find the amp works ok-ish or you'll see the lamp illuminate strongly and possibly blow given the 24v rail. If the lamp is bright, that would indicate a fault, probably in the output stage or that something's upsetting the output stage's bias.

Beware of 'testing' with (say) a 10w lamp because that might be fine (for a while) if the amp does work - to JR's earlier comment, lamps do randomly fail - but it won't have the right shape knee in its resistance curve to give the protection the lamp is intended to bring to that circuit.

 

[Rob Flinn]

The lamp's used as a current limiter - so that when the current in the output stage passes a threshold, the lamp's filament starts to heat up and would present a dramatically higher resistance, limiting the output stage current. IF you're lucky, you might see a glimmer from the lamp but in normal operation I would expect it to stay dark. A simple resistor or a diode cannot be used to sub here because they would behave differently. I agree these aren't readily available but at least here in the UK, Google finds a few suppliers although generally the lamps are in LES packages rather than wire-ended.

I'd try any bulb that's similar - a good auto store will have small lamps to backlight dashboards, etc. If you can find something similar - 1w to 2w for the test would probably be fine - put the new part in circuit and see what happens. For my money, you'll either see 'nothing much' and find the amp works ok-ish or you'll see the lamp illuminate strongly and possibly blow given the 24v rail. If the lamp is bright, that would indicate a fault, probably in the output stage or that something's upsetting the output stage's bias.

Beware of 'testing' with (say) a 10w lamp because that might be fine (for a while) if the amp does work - to JR's earlier comment, lamps do randomly fail - but it won't have the right shape knee in its resistance curve to give the protection the lamp is intended to bring to that circuit.

yes, thanks. I have managed to get the same spec bulb (at great expense), so once it arrives hopefully that will be the end of it.