Fender 112SE BIAS setting

[Jazz P Bass]

With +1.11 Vdc & -1.16 Vdc on the TIP bases, I would tend to believe that they are barely On.

The question is: is this happening at a very low output amplitude or across the full power range?

Can it be heard?

[Dave H]

With +1.11 Vdc & -1.16 Vdc on the TIP bases, I would tend to believe that they are barely On.

I think you are right. The Stage 100-160 schematic below has the same output transistors and the base voltages are marked as +1.08V and -1.08V dc.

[Enzo]

it was asked before: do you HEAR a problem?

When you said crossover distortion, did you see it on a SCOPE, or do you just hear some distortion and call it crossover? A rubbing voice coil sounds a lot like crossover distortion, for example, but won't look like it on a scope.

[Feufeu]

it was asked before: do you HEAR a problem?

When you said crossover distortion, did you see it on a SCOPE, or do you just hear some distortion and call it crossover? A rubbing voice coil sounds a lot like crossover distortion, for example, but won't look like it on a scope.

Ho Yes; we hear especially this distortion when play chords. I can see this crossover distortion on my scope only when amp is plug on 8R resistive load.
With no load and only scope the output signal is good...

For me It's not normally amp working with crossover distortion on output signal ... the sound is very dirty

-> the original transistor is Motorola TIP 142 / TIP 147 and Were replaced by ST brand :

http://www.st.com/content/ccc/resour...CD00000914.pdf

TIP142 Fiche technique pdf datenblatt - Motorola, Inc - 10 AMPERE DARLINGTON COMPLEMENTARY SILICON POWER TRANSISTORS ::: ALLDATASHEET :::

I think just maybe need recale BIAS setting for news transistors ? Or try to found same reference :Motorola ... ?

it's the first time have this problem on transistor amp ...

Thanks

[Dave H]

I think just maybe need recale BIAS setting for news transistors ? Or try to found same reference :Motorola ... ?

The transistors look to be equivalent. They both have the same low value base-emitter resistor on the output transistor. As you can hear the crossover distortion I'd try adjusting the bias current by adding a small resistor to the bias diode chain as I suggested in post #13.

[Enzo]

A TIP142 is a TIP142, the brand doesn't matter.

[Dave H]

A TIP142 is a TIP142, the brand doesn't matter.

I thought that until I checked a few data sheets. For example the ON data sheet has the internal base emitter resistor as 40R but ST has it as 150R.

[Feufeu]

I thought that until I checked a few data sheets. For example the ON data sheet has the internal base emitter resistor as 40R but ST has it as 150R.

Yes ! you have speak on this internal resistor upper in the thread

Can you explain how to adjust the bias with the resistances of 2R2?
where and how to install this resistor in the circuit, I'll try this solution ...

Otherwise I have other messages in forum for exactly same problem, when replacing the TIP transistors crossover distortion Appeared ... and this problem are never resolved ...

thank you very much

[Dave H]

Can you explain how to adjust the bias with the resistances of 2R2?
where and how to install this resistor in the circuit

Just add a 2.2ohm resistor in series with one of the bias diodes (CR13-CR16). The current through the diodes is about 5.5mA (set by R92, R93) so 2.2ohms will increase the bias voltage by 5.5 x 2.2 = 12mV. Assuming the output transistors are just cracked on an extra 12mV across the 2 x 0.47ohm series emitter resistors will increase the bias current by 12/(2 x 0.47) = 12.75mA for each pair of transistors.

[Enzo]

There may be subtle internal differences, but I have never found it to matter at all in guitar amps.

[Dave H]

There may be subtle internal differences, but I have never found it to matter at all in guitar amps.

Yes, I think it's a red herring in this case as the original Motorola transistors have a lower value Rbe than the replacements.

[Enzo]

What matters is current through the junctions, the value of a resistor is minor.

[Dave H]

What matters is current through the junctions, the value of a resistor is minor.

In post #10 the OP says "I have around 4.3mV on R106 to R111..." That implies a bias current of 4.3/0.47 = 9mA for each pair of output transistors but how much of that is current through the junction? (i.e. real bias current). To just start to turn on an output transistor it needs a Vbe of 0.5V (say) but that 0.5V is also across the internal 40R base-emitter resistor (Rbe) so the Rbe current is 0.5/40 = 12.5mA which is more than the 9mA total current (real bias current + Rbe current) through the 0.47R emitter resistor so the real bias current is zero for an internal Rbe of 40R. If the Internal Rbe is 150R there will only be 0.5/150 = 3.3mA Rbe current and as the total current is 9mA the real bias current is 9 - 3.3 = 5.7mA.

[Feufeu]

Just add a 2.2ohm resistor in series with one of the bias diodes (CR13-CR16). The current through the diodes is about 5.5mA (set by R92, R93) so 2.2ohms will increase the bias voltage by 5.5 x 2.2 = 12mV. Assuming the output transistors are just cracked on an extra 12mV across the 2 x 0.47ohm series emitter resistors will increase the bias current by 12/(2 x 0.47) = 12.75mA for each pair of transistors.

OK, i have put 2R résistors ( 1R+1R série because haven't got 2R2 in stock) . put between CR13 & CR14 .

- the problem stay the same, always this crossover distortion, but through the 0.47R now i have 4,65mV .

- with amp on Bulb limiter i have put my decade box between CR13 & CR14 et up slowly résistors values, with 34R i have à good signal without distortion ...

-> Throught .047R now i have 6.6mV and have 1.21 & 1.19 on TIP base.

do you thinks is good setting for this transistors ? or is dangerous setting ?

thanks

[Dave H]

So it's gone from 4.3 to 4.65mV? That's not much is it. Perhaps I didn't do the calculations correctly or the output transistors really were running without bias current. At 4.65mV they are probably still off. Try increasing the value of the 2ohm resistor until you measure 10mV across the 0.47R emitter resistors. That will be 21mA bias which should be enough to power the 40ohm Rbe and just turn on the outputs.