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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 30th Aug 2022, 8:32 pm   #1
60 oldjohn
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Default Roberts RT1 Driver transistor?

The OC71 tests as being very noisy. I intend swapping it out. I have a NOS OC71 with an HFE of 51 (Min for OC71 is 50) I also have an OC75 HFE 75. Which transistor would you use? I could experiment but the copper traces are so delicate on these 1950s sets I don't intend trying them.

John.
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Old 30th Aug 2022, 10:06 pm   #2
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Default Re: Roberts RT1 Driver transistor?

If both work either will do in that set John.

The HFE won't matter.

Greg
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Old 30th Aug 2022, 10:36 pm   #3
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Default Re: Roberts RT1 Driver transistor?

Thanks Greg, I thought the higher HFE would make the set louder. If it does not make any difference I will use the OC71 and keep the set original.

John.
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Old 31st Aug 2022, 10:00 am   #4
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Default Re: Roberts RT1 Driver transistor?

There won't be any audible difference.
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Old 31st Aug 2022, 10:09 am   #5
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Default Re: Roberts RT1 Driver transistor?

Thanks Paul for the confirmation.

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Old 31st Aug 2022, 11:09 am   #6
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Default Re: Roberts RT1 Driver transistor?

Which position would choosing the highest gain transistor out of a batch make the largest difference to the performance of a Transistor set? Talking about a basic MW / LW set from around 1960.

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Old 31st Aug 2022, 11:19 am   #7
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Default Re: Roberts RT1 Driver transistor?

Again, it's not going to make a significant difference. Early Ge transistors had a wide hfe spread, and circuits were designed to work accordingly. You'll only notice differences with transistors that are completely out of spec - low hfe and they just won't work, high hfe and they'll become unstable.
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Old 31st Aug 2022, 11:53 am   #8
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Default Re: Roberts RT1 Driver transistor?

Thanks Paul, Not what I would have thought, but hey, you live and learn.

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Old 31st Aug 2022, 12:08 pm   #9
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Default Re: Roberts RT1 Driver transistor?

The reason it doesn't change the gain of the amplifier is that it's in a feedback loop: the feedback ratio defines the overall gain, independant of the amplifier gain without feedback.

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Old 10th Mar 2023, 11:12 am   #10
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Default Re: Roberts RT1 Driver transistor?

Thread re-opened at OP's request
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Old 10th Mar 2023, 11:38 am   #11
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Default Re: Roberts RT1 Driver transistor?

In response to another wanted thread about a Roberts RTI, the Driver transistor on this set was noisy. But on another RT1 it could be almost any component. I would identify the noisy component by using Freezer spray, a quick spray on a suspect component and after a few seconds the noise should disappear. I would try the transistors in the output stage first, if no joy try the resistors in the output stage. If you don't have freezer spray you could try an ice cube in a plastic bag "a real faff"

Best to get the right component first time, these 60+ year old first generation PCB don't take well to reworking.

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Old 10th Mar 2023, 12:40 pm   #12
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Default Re: Roberts RT1 Driver transistor?

Noisy AF transistors are a very common source of background hiss in radios of this era though.
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Old 10th Mar 2023, 3:09 pm   #13
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Default Re: Roberts RT1 Driver transistor?

Quote:
Originally Posted by 60 oldjohn View Post
The OC71 tests as being very noisy. I intend swapping it out. I have a NOS OC71 with an HFE of 51 (Min for OC71 is 50) I also have an OC75 HFE 75. Which transistor would you use? I could experiment but the copper traces are so delicate on these 1950s sets I don't intend trying them.

John.
As far as I remember, the OC75 was a lower noise version of the OC71 (in other words, the same transistor, off the same production line but specially selected when tested and renumbered). In short, either will work and probably won't make any difference to the sound.
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