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 Post subject: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 8:58 pm 
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I hope this is the best place to ask this. I am working on a Heathkit stereo that looks to have a bad transistor in the power supply section. It uses a germanium transistors housed in a metal can package, and I am measuring about 15 ohms between the base and the housing. The base connects to a -27V node on a voltage divider, and the housing is mounted in a holder tied to ground – and this is shorting that -27V to ground.

I am thinking that a more modern transistor would be easier to find, and might even perform a little better. The part in question is X14 on the schematic below. Unfortunately my experience with PNP transistors is very limited, so I am hoping someone here can give me some advice. The circuit looks to be simply providing a low impedance -27V source to the transistor above it (X15). Would a 2N3906 or something similar work just as well? Looking at the specs, it looks to handle the voltage and current with no problem, but the Hfe is a little higher. Is there anything I am missing that might pose a problem if I were to give this a try?


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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 9:13 pm 
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That’s mainly an active low-pass filter to provide DC voltage without AC hum. It isn’t a regulated power supply. It will be perfectly OK to replace the 2N408 with any modern small signal PNP silicon transistor with similar collector voltage and current ratings.

There’s nothing all that special about germanium transistors. The only application where they cannot be replaced by silicon transistors is in circuits that operate on extremely low voltages, such as a single 1.5V battery.

The voltage drop from base to emitter is very low for germanium transistors (150 to 300mV). For a silicon transistor it is 600mV. In some cases changing from germanium to silicon will require changing the base bias resistor. But for this Heathkit power supply no modification is needed to change from germanium to silicon.

-EB

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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 9:15 pm 
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A 2N3906 should be fine. X13 and X14 are in Darlington configuration. They are used as a capacitance multiplier. The 100uF capacitor at the base of X14 (can't read its reference number) is multiplied by by the composite current gain. Let's say that if the gain is 10,000, it's like having a 1F capacitor at the emitter of X13.

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Last edited by bb.odin on Jan Mon 13, 2020 9:21 pm, edited 1 time in total.

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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 9:19 pm 
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If I remember correctly the metal can of the 2N408 might be directly connected to the collector lead. So in this case the mounting clip would need to be insulated from chassis ground.

This transistor shouldn’t run hot enough to require a metal clip as a heatsink. So perhaps you could just pull it out of the clip and that might correct the short circuit.

-EB

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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 9:35 pm 
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Thanks for the quick replies. I tried to look around for documentation that stated if the metal can was tied to anything, but wasn't able to find much. This part, as well as 2 other 2N408s in the power supply section, are installed on special terminal strips that include a wire coil that holds the transistor in place, so I am sure it was built as Heath specified. I know the power supply of this amp worked when I got it (had a dead channel but did have power), but it's been waiting for me to work on it for a *long* time (like 20+ years) so whatever is causing this current problem is due to age.

I'll see if I can find a 2N3906 in my parts stash, and will leave the 2N408s out there for someone else who needs them. Thanks!


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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Mon 13, 2020 11:05 pm 
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Quote:
That’s mainly an active low-pass filter to provide DC voltage without AC hum.


In the old days, these were sometimes called "filter cap multipliers." The pass transistor acts as a filter without the need for a big filter cap ahead of it.

Zenith used this circuit in audio amp power supplies.

Rich


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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Tue 14, 2020 2:44 am 
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Rich, W3HWJ wrote:
Quote:
That’s mainly an active low-pass filter to provide DC voltage without AC hum.


In the old days, these were sometimes called "filter cap multipliers." The pass transistor acts as a filter without the need for a big filter cap ahead of it.

Zenith used this circuit in audio amp power supplies.

Rich
They still are :wink: Rather, just "cap multiplier."

They are particularly useful in tube amplifiers because the AC line voltage can vary and while that might seem to be a reason to use a regulated B+ supply that would mean all the other voltages, such as bias, would need to be regulated as well (or else bias shifts all over the place as B+ remains constant but the bias voltage changes). Then, a regulated supply has to have headroom (to cover ripple) but that headroom must be at the lowest expected AC, meaning at 'nominal' or the max V the headroom is huge leading to wasted power dissipation and more expensive parts.

The second issue is where they shine because a 'cap multiplier' naturally 'floats' to the average B+ so only the ripple headroom need be catered for.


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 Post subject: Re: PNP transistor for power supply
PostPosted: Jan Tue 14, 2020 5:56 pm 
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Joined: Nov Wed 27, 2013 5:59 am
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Location: Metzger Oregon
Thanks everyone for explanation of how this circuit works, I'll have to remember that in case it comes in handy in the future.

As an update, I tried a 2N3906 in the amp last night, that and a couple new caps fixed the power supply issue, thanks everyone! I got one channel half-working, now I need to track down the issues with the amplifier circuits...


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