Transistor Replacement on Wurlitzer Amp.

Thread Starter

Control Dave

Joined Sep 2, 2023
34
Hi All,

I am trying to repair a Wurlitzer amplifier and the two output transistors ( T5 & T6) need to be replaced. They are Motorola SP1271 GE type and are no longer available. I have done a little research and selected MJ2955 silicon type to replace the originals. Could anybody please advise me if they will need to be other modifications made to make these ones work, or recommend another type that may be more suitable. I have attached the amp schematic and pic of the original transistor and data on the MJ2955. Hoping someone can help.

Many Thanks

Dave
 

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MrChips

Joined Oct 2, 2009
30,432
Why do you suspect that the transistors need to be replaced?
The original transistors are made from germanium while the MJ2955 are silicon. You will have to change the bias resistors. I don't know if that will work. This would require some simulation.
 

Pyrex

Joined Feb 16, 2022
236
It's not a good idea to replace germanium transistors to silicone. Amplifier's performance will change. First of all, try to find SP1271 datasheet
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
Hi Mr Chips,
Thanks for the reply. I think the transistors are faulty because when tested with a multimeter there is a dead short between the collector, emitter, and base in any orientation. Is the MJ2955 a bad choice?
Regards
Dave
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
It's not a good idea to replace germanium transistors to silicone. Amplifier's performance will change. First of all, try to find SP1271 datasheet
Hi Pyrex,
I have tried to find the data sheet but nothing found. Found an article that suggested the MJ2955 one as a replacement.
Regards
Dave
 

Pyrex

Joined Feb 16, 2022
236
If datasheet is not available, more Wurlitzer data needed . What is power output and resistance? Circuit diagram is hard to read. Voltage on the output stage is about +28V, -28V, as I can estimate , as transformer produces 22.5V
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
If datasheet is not available, more Wurlitzer data needed . What is power output and resistance? Circuit diagram is hard to read. Voltage on the output stage is about +28V, -28V, as I can estimate , as transformer produces 22.5V
Hi Pyrex,
The diagram is from the service manual and is all that's available. I'm new to this sort of repair, so very grateful for any advice or help. I could replace the board with an ali express kit, but would love to get the original operating.
Regards
Dave
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
If datasheet is not available, more Wurlitzer data needed . What is power output and resistance? Circuit diagram is hard to read. Voltage on the output stage is about +28V, -28V, as I can estimate , as transformer produces 22.5V
Hi Pyrex
When tested voltage is 31.6v and -31.6v
Dave
 

MrChips

Joined Oct 2, 2009
30,432
I will do MJ2955 simulation and get back to you.
I am a bit suspect about the values of R24 and R26, 2.2Ω seems rather low.

I tried MJ2955 with R24 and R26 replaced with 10Ω resistors.
You can try MJ2955 transistors. If R24 and R26 are too low the transistors will be cut off into Class-B with severe cross-over distortion. Increase the value of R24 and R26 from 2.2Ω moving towards 10Ω, moving towards Class-AB amplifier. The transistors will tend to get hotter as you move from Class-B to Class-AB to Class-A but with reduced cross-over distortion. Do a listening test with audio source.
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
I am a bit suspect about the values of R24 and R26, 2.2Ω seems rather low.

I tried MJ2955 with R24 and R26 replaced with 10Ω resistors.
You can try MJ2955 transistors. If R24 and R26 are too low the transistors will be cut off into Class-B with severe cross-over distortion. Increase the value of R24 and R26 from 2.2Ω moving towards 10Ω, moving towards Class-AB amplifier. The transistors will tend to get hotter as you move from Class-B to Class-AB to Class-A but with reduced cross-over distortion. Do a listening test with audio source.
Hi Mr Chips,
Wow thanks for doing that. I will try this as soon as they turn up and get back to you with the results.
Many thanks,
Dave
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
It's OK, at no load.
Any data concerning output power and resistance? Usually it's 4 Ohm or 8 Ohm. Check a label on the Wurlitzer back
Hi Pyrex
As the Amp is mono, there is two 8ohm speakers wired in series, but not sure if this is original.
Regards,
Dave
 

Pyrex

Joined Feb 16, 2022
236
It's hard to say, which types are suitable for replacement from the list, as not all the data is known.
Anyway, the replacement should conform : Uce> 70V, F>100kHz, Ic>3A, P>30W...
2N3730 is a suitable replacement, imho
 

Thread Starter

Control Dave

Joined Sep 2, 2023
34
It's hard to say, which types are suitable for replacement from the list, as not all the data is known.
Anyway, the replacement should conform : Uce> 70V, F>100kHz, Ic>3A, P>30W...
2N3730 is a suitable replacement, imho
Hi Pyrex
Thank you for your advice. It's very much appreciated. I will contact you with the results.

Regards
Dave
 

Alec_t

Joined Sep 17, 2013
14,192
I think R4 and R26 are meant to be 22Ω. That value gives a reasonable collector current in an LTS sim. I see no reason why silicon power transistors couldn't be used instead of germanium. Increasing R24 and R26 to 33Ω would keep the collector current at about the same value.

Edit:
Scrub that. Sim shows the transistors would be dissipating too much energy.
 
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