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Old 7th January 2017, 10:32 AM   #1
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Default Question for Papa about Cascoding Devices

I noticed in your sony 40 year anniversary vfet amp you used TO220 bipolar devices as cascodes.
Is this because they are generally superior to the TO92 bipolars or is there a dumber reason (eg you already have a truck load of them that are begging to be used in something)

The reason I ask is today I came across a box containing a stack of 2sc5171/2sa1930 and 2sc4793/2sa1837, they are not going to be used in anything else so if these are great devices for cascodes (ie better than TO92), then I can finally put them to use.

Feel free to say anything else on this as I'm sure to learn something new.
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Last edited by 2 picoDumbs; 7th January 2017 at 11:25 AM.
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Old 7th January 2017, 11:22 AM   #2
AndrewT is offline AndrewT  Scotland
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To94 is a stud mounting device.
Is that what you have?
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Old 7th January 2017, 11:27 AM   #3
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Quote:
Originally Posted by AndrewT View Post
To94 is a stud mounting device.
Is that what you have?
One of those brain fart moments. Hahaha.
I corrected it to say TO92
Thanks
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Old 7th January 2017, 03:04 PM   #4
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Quote:
Originally Posted by 2 picoDumbs View Post
I noticed in your sony 40 year anniversary vfet amp you used TO220 bipolar devices as cascodes.
Is this because they are generally superior to the TO92 bipolars or is there a dumber reason (eg you already have a truck load of them that are begging to be used in something)

The reason I ask is today I came across a box containing a stack of 2sc5171/2sa1930 and 2sc4793/2sa1837, they are not going to be used in anything else so if these are great devices for cascodes (ie better than TO92), then I can finally put them to use.

Feel free to say anything else on this as I'm sure to learn something new.
I believe the 2sc4793/2sa1837 were Papa's choice for cascodes in the F5 turbo article. I wil be using them in a cascode for the BA3 preamp I have to increase the voltage to 36V. (straighforward copy of the f5turbo cascode section)

They have higher dissipation values than the to92 this might be a factor in some designs such as the F5turbo depending on rail voltages. I don't remember the rail voltage in the sony amplifier but if these must drop lots of voltage the To92 parts might be too small.


And he probably has a truckload of those...
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Old 7th January 2017, 03:18 PM   #5
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Originally Posted by MASantos View Post
I believe the 2sc4793/2sa1837 were Papa's choice for cascodes in the F5 turbo article. I wil be using them in a cascode for the BA3 preamp I have to increase the voltage to 36V. (straighforward copy of the f5turbo cascode section)

They have higher dissipation values than the to92 this might be a factor in some designs such as the F5turbo depending on rail voltages. I don't remember the rail voltage in the sony amplifier but if these must drop lots of voltage the To92 parts might be too small.


And he probably has a truckload of those...
Question: Can you just lift the cascade circuit off one amp and install it on another? Excuse my lack of knowledge here, but I thought resistors and so forth had to be calculated for each application. I will be happy to be wrong here!

Russellc
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Old 7th January 2017, 03:51 PM   #6
6L6 is offline 6L6  United States
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Pico - I bet Papa's answer will be his normal, "they are of proper dissipation and I had 10,000 of them"

Russell - Basically yes. In theses amps the cascode usually sits around 1/2 the rail voltage, to protect the Jfet from seeing big rails in a higher-voltage amp. Look at the F5Tv3 for a good place to start.
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Old 7th January 2017, 04:51 PM   #7
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well , decent Hfe and not too big capacitances , are two things worth caring

for EU boyz , if obtaining decent BD139/140 is possible , they're good and cheap solution
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Old 7th January 2017, 04:51 PM   #8
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Quote:
Originally Posted by Russellc View Post
Question: Can you just lift the cascade circuit off one amp and install it on another? Excuse my lack of knowledge here, but I thought resistors and so forth had to be calculated for each application. I will be happy to be wrong here!

Russellc
I got the idea from a member who built an awesome BA3 power amp using 36V rails.

He used the F5 turbo cascode circuit with good results. The input stage of the BA3 and F5 turbo is very similar and in this case the voltages are similar so i can get away with the same resistor values. If you change the voltages it is just a matter of adjusting the resistors to make sure you have the right voltage going into the jfets.

BTW I have been running my BA3 input jfets at 30V for the last 4 months with no problems, they are at 40-44 C.
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Old 7th January 2017, 07:38 PM   #9
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Originally Posted by 6L6 View Post
Pico - I bet Papa's answer will be his normal, "they are of proper dissipation and I had 10,000 of them"
.
Yes I suspect this is the correct answer also.
Just don't want to miss the opportunity of possibly learning something new.
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Old 7th January 2017, 07:45 PM   #10
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My understanding is, hearing this from the partners at Pass Labs, his normal conversation goes something like, "So how many of them do I need to buy to get the price I want?"

And this is why I saw eye-watering amounts of long out of production amazing transistors on the shelves in their production storeroom.

Very cool.
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