New Output Transistors for a QUAD 405 (2)

[mihaiil]

Hi guys!
I have Quad 405-2 (12565-6) with one pair of transistors RCA17556 (original) and another one 2SC1116 (not original). The amp works fine, but I want to change the transitors to MJ15003G. Can I do it safely without measurments, or it is not recommended. I have only multimeter.

 

[Mooly]

I would say not recommended. There are threads on here where this kind of substitution was done and there were big problems. The output stage of the Quad is uniquely susceptible to the transistor types used and the original slower devices are a part of the overall design for stability.

 

[p_haslam]

Interesting thread guys

Out of interest, I have one of these on the bench and ocne of the 17556 has shorted and blown the fuses. Gues it would be best to replace both 17556's on that channel - was going to go for MJ15003 - but would it be wise to change the other channel as well so taht they are matched?

cheers

 

[Mooly]

I can only speak for myself but I would always do the same to the other channel in anything I am working on if it involved fitting a different part to the original.

 

[p_haslam]

thanks man, sounds wise

 

[Nisbeth]

Agree, but it makes sense to replace the dead channel first and confirm that it is working properly so you can check against the original (voltages/currents) if something seems odd following the replacement. If you change both channels at the same time you risk been left without a proper reference point

 

[ljm_ljm]

Almost all amplifiers. The output transistor will not have any impact.
Also includes QUAD.
We may be concerned about the output transistor because it is relatively large. Easy to detect with the eyes.

 

[huggygood]

Almost all amplifiers. The output transistor will not have any impact.
Also includes QUAD.
We may be concerned about the output transistor because it is relatively large. Easy to detect with the eyes.

 

[p_haslam]

Thanks team. I agree change the dodgy side and compare initially then I’ll take it from there.

 

[p_haslam]

Update for interest, bought 4 and replaced the broken side first. Compared the output, IMD and THD+N with the working R channel with original transistors in and both were the same. Opted in the end to save the guy £19 and left it at that. Will see what it's like testing with his set up
P

 

[Mooly]

Sounds like a good result

I'm not surprised there is no difference tbh but there is just the satisfaction (and more so if its your own of course) of having things matched.

If you had a preamp that used NE5532's in one channel and say LM4562's in the other would you be able to tell a difference? and would you do it

 

[p_haslam]

ha, yes. if it was mine.....!

 

[ejp]

All other things being equal and assuming stability, a transistor with higher gain will lower the THD. I have measured this many times, but you don't have to measure to know this: it follows from first principles ..