@Control Dave , can you look at the actual R24 and R26 resistors and give us the colour codes on the resistor?
You can also tell us the measured resistance while still in circuit.

 

@Control Dave , can you look at the actual R24 and R26 resistors and give us the colour codes on the resistor?
You can also tell us the measured resistance while still in circuit.

Hi Mr Chips
The resistors do not have colour codes. They are black with 2R2 markings. See attached picture.

 

Ok, thanks. 2R2 is 2.2Ω.

 

Ok, thanks. 2R2 is 2.2Ω.

Hi Mr Chips
What is the effect of increasing the value of these to 100 ohm?

 

T

Hi Mr Chips
What is the effect of increasing the value of these to 100 ohm?

Doing so will increase the current flowing in both transistors. 100Ω is much too high. I expect that you don’t need to go higher than 10Ω.

 

Ok, thanks. 2R2 is 2.2Ω.

Hi Mr Chips
What is the effect of increasing the value of these to 100 ohm

T

Doing so will increase the current flowing in both transistors. 100Ω is much too high. I expect that you don’t need to go higher than 10Ω.

Would there bring an advantage of fitting variable resistors in place?

 

Hi Mr Chips
What is the effect of increasing the value of these to 100 ohm

Would there bring an advantage of fitting variable resistors in place?

Not really. I would buy a quantity of 1Ω resistors and experiment until you determine the value that works best. Then you can install a single resistor if you so desire.

 

Not really. I would buy a quantity of 1Ω resistors and experiment until you determine the value that works best. Then you can install a single resistor if you so desire.

Hi Mr Chips
Good idea. Some one has carried out other mods on the board for some reason. So once I got it back to the schematic and tested ill get back to you with the results.
Dave

 

The nice thing about the transformer coupled final stage is that the DC bias of the output power transistors is totally determined by just two resistors and not by a complex feedback loop which you find in modern solid state power amplifiers.

 

I've corrected my sim (I'd misread the schematic previously). 2.2Ω is definitely the right value if germanium transistors are used, but would need to be increased if silicon transistors were used, otherwise crossover distortion would increase significantly.

 

The nice thing about the transformer coupled final stage is that the DC bias of the output power transistors is totally determined by just two resistors and not by a complex feedback loop which you find in modern solid state power amplifiers.

Amplifier circuits are new to me, I do Pinball repairs for a hobby, and its all relay logic that I can understand, so really want to learn about these analogue circuits.

 

I've corrected my sim (I'd misread the schematic previously). 2.2Ω is definitely the right value if germanium transistors are used, but would need to be increased if silicon transistors were used, otherwise crossover distortion would increase significantly.

Hi Alec,
Thanks for the info. Slowly learning about these type of circuits.
Dave

 

I am a little late to this party,but...

I have 3 NOS packages of ECG121MP (Matched Pairs) if you are interested in them for somewhat exact replacements for the SP1271 transistors.

Dan

 

I am a little late to this party,but...

I have 3 NOS packages of ECG121MP (Matched Pairs) if you are interested in them for somewhat exact replacements for the SP1271 transistors.

Dan

Hi Dan,
Thanks for the response, I'm in two minds to replace the transistors with something easily available, or replacements. How much would a matched pair cost?

Dave

 

Best I have seen online for these is around $60 plus shipping. I have 3 sets. Contact me directly for details, but $50 per pair seems fair shipped.

Dan

 

..... or there's these.

 

If this is just an audio amp, you can replace the entire board with a modern class D amp for less than the price of a replacement transistor.

Edited to add: And get better sound out.

 

I think it is fair game at this point to try the MJ2955 substitution.
Hold on and I will give you a quick introduction to transistors.

 

I will begin with how the base-emitter junction needs to be biased for the transistor to start conducting across the collector-emitter. I will use an NPN 2N3055 high power transistor with a +30V power supply for this simulation because this fits your current situation.

For an NPN transistor, the base-emitter junction needs a positive voltage bias. The point of conduction is very critical and conduction reaches a tipping point of around 0.55V.



As you can see in the simulation, Q1 is only just starting to conduct, VCE = 22.4V, Ic = 7.6mA, when VBE = 0.544V
On the right transistor, Q2, VCE = 15V, Ic = 15mA when VBE = 0.564V, an increase of just 20mV or about 4%.

Hence the DC bias point is very sensitive to base-emitter voltage.
We will reduce this sensitivity by using a voltage divider at the base.


Now we have added a very "stiff" voltage divider to control the base voltage.
Rising the pull-down resistor slightly from 8Ω to 9Ω cause the transistor to go from cut-off to conduction.
But don't let the 1Ω increase fool you. This represents a 13% increase.

For a PNP transistor such as MJ2955, the circuit schematic and analysis is the same except that the supply voltage is changed. from +30V to -30V.

 

I will begin with how the base-emitter junction needs to be biased for the transistor to start conducting across the collector-emitter. I will use an NPN 2N3055 high power transistor with a +30V power supply for this simulation because this fits your current situation.

For an NPN transistor, the base-emitter junction needs a positive voltage bias. The point of conduction is very critical and conduction reaches a tipping point of around 0.55V.

View attachment 305104

As you can see in the simulation, Q1 is only just starting to conduct, VCE = 22.4V, Ic = 7.6mA, when VBE = 0.544V
On the right transistor, Q2, VCE = 15V, Ic = 15mA when VBE = 0.564V, an increase of just 20mV or about 4%.

Hence the DC bias point is very sensitive to base-emitter voltage.
We will reduce this sensitivity by using a voltage divider at the base.

View attachment 305108
Now we have added a very "stiff" voltage divider to control the base voltage.
Rising the pull-down resistor slightly from 8Ω to 9Ω cause the transistor to go from cut-off to conduction.
But don't let the 1Ω increase fool you. This represents a 13% increase.

For a PNP transistor such as MJ2955, the circuit schematic and analysis is the same except that the supply voltage is changed. from +30V to -30V.

Hi Mr Chips. Many thanks for taking the time to do that. Starting to make a bit more sense.
Regards
Dave