It is not necesary UL tap , just the internal winding end/start . Probably the primary is made from 4 interleaved windings , if is good for UL operation remains to be seen
330-470ohm resistors in series with each G2 could save the tube in this situation ... will limit the current and burn
330-470ohm resistors in series with each G2 could save the tube in this situation ... will limit the current and burn
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Naming the tap as a 'UL tap' is just for nominal identification - it has no bearing on the issue or the amp - it was used to indicate an intermediate wiring tag for the primary winding - there would be no intent to determine at what % turns location it was made.
The amp schematic shows a stiff regulated common screen supply with no individual screen stoppers. The original designer may well have shunned adding any additional individual stopper resistance - we don't know. We also don't know the regulation and overload capability of the screen supply regulator, which imho makes it difficult to easily assess what individual screen over-current protection could be provided (given that perhaps it shouldn't be a significant level of resistance), and what fault scenarios could be managed by that or other forms of over-current fault protection.
The amp schematic shows a stiff regulated common screen supply with no individual screen stoppers. The original designer may well have shunned adding any additional individual stopper resistance - we don't know. We also don't know the regulation and overload capability of the screen supply regulator, which imho makes it difficult to easily assess what individual screen over-current protection could be provided (given that perhaps it shouldn't be a significant level of resistance), and what fault scenarios could be managed by that or other forms of over-current fault protection.
In the case of UL it may be worth finding a sacrificial resistor between the UL tap and the screen. Needs to be low enough that it does not affect the sound or output but will die in the case where a plate connection is lost.
May need to be flame proof or placed in some way that it can happliy burn up without burning up other things.
May need to be flame proof or placed in some way that it can happliy burn up without burning up other things.
Every tube amplifier needs to be made bulletproof. Ha Ha.
At some point, if you do that, then there is more protection circuitry than for the original unprotected amplifier circuitry.
I use Both a Slow Blow fuse and a Fast Blow fuse in the power transformer's primary circuit.
Example, one of my mono-block amplifiers has a 0.6A (600mA) slow blow in case there is a constant overload current;
and that fuse is in series with a 1.25A fast blow that will blow if there is an unusually large inrush current at power up, or at a Hot Start condition.
(ask me if you do not know what a hot start is).
Other than the series fuses, using very conservatively rated parts (*), that is the only other protection I use.
(*) Examples:
I use 500V electrolytic caps (550V surge) on a B+ supply that will not go higher than 500V even when all the tubes are pulled out (or Dead Open).
I use resistors that are rated for at least 3 times of the power they will dissipate, and usually rated for 5 times the power they will dissipate (keep cool).
I use output tubes at quiescent conditions that are well below their voltage, current, and dissipation ratings (keep cool, keep from arcing, keep from thermal run-away).
I use choke input B+ filters whenever possible. Keep the power transformer running cool.
Consider as many failure modes as you and your friends can think of, when you look over your amplifier circuit.
Then revise the circuit as necessary.
You are building an amplifier for yourself, Right?
You are not setting up a commercial production line where you have to make money, Right?
I do not attempt to make my power amplifiers work like a Formula One Race Car.
Design and build your amplifier any way you want to.
Your Mileage May Vary.
Just my opinions.
At some point, if you do that, then there is more protection circuitry than for the original unprotected amplifier circuitry.
I use Both a Slow Blow fuse and a Fast Blow fuse in the power transformer's primary circuit.
Example, one of my mono-block amplifiers has a 0.6A (600mA) slow blow in case there is a constant overload current;
and that fuse is in series with a 1.25A fast blow that will blow if there is an unusually large inrush current at power up, or at a Hot Start condition.
(ask me if you do not know what a hot start is).
Other than the series fuses, using very conservatively rated parts (*), that is the only other protection I use.
(*) Examples:
I use 500V electrolytic caps (550V surge) on a B+ supply that will not go higher than 500V even when all the tubes are pulled out (or Dead Open).
I use resistors that are rated for at least 3 times of the power they will dissipate, and usually rated for 5 times the power they will dissipate (keep cool).
I use output tubes at quiescent conditions that are well below their voltage, current, and dissipation ratings (keep cool, keep from arcing, keep from thermal run-away).
I use choke input B+ filters whenever possible. Keep the power transformer running cool.
Consider as many failure modes as you and your friends can think of, when you look over your amplifier circuit.
Then revise the circuit as necessary.
You are building an amplifier for yourself, Right?
You are not setting up a commercial production line where you have to make money, Right?
I do not attempt to make my power amplifiers work like a Formula One Race Car.
Design and build your amplifier any way you want to.
Your Mileage May Vary.
Just my opinions.
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FYI: Never to Old
Thomas Mayer has a website http://vinylsavor.blogspot.com/ where for more than 10 years he has had a tube of the month article. Among other things in the article he breaks open and dissects tubes, and posts the pictures with an explanation.
Steve
Thomas Mayer has a website http://vinylsavor.blogspot.com/ where for more than 10 years he has had a tube of the month article. Among other things in the article he breaks open and dissects tubes, and posts the pictures with an explanation.
Steve
It's a good idea to put sacrificial resistors either in the screen and cathode leads of power tetrodes and pentodes, independingliy if it is an UL arrangement or not. These resistors need to be flame proof or being mounted in a way that they won't burn something else.
Best regards!