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 Post subject: Replace Horizontal output damper with HV diode?
PostPosted: Apr Fri 16, 2021 9:41 pm 
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Hi guys,

Got a Setchell Carlson C101 TV. Seems to be all intact except for missing a damper tube I believe. (one right below the horizontal output). SAMS says its a 6AX4GT. Is it possible to stick a standard silicon HV diode in there or just go find a replacement 6AX4GT? Is it even safe to power the set without it installed?

Thanks

-tp4by3


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Fri 16, 2021 10:26 pm 
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The power for the horizontal output flows through the damper. So without the damper that circuit will essentially be disconnected. The lack of a damper will not hurt anything, but other parts of the set might not like being powered up without having some parts replaced first. As a minimum you should use a dim bulb tester. Google that to see what I mean.
You would need a very high voltage diode to replace the damper. Better to just get a tube.

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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Fri 16, 2021 10:39 pm 
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There is a significant pulse whizzing around in that circuit. And it's buzzing around at approx 16000 times a second. Solid state devices easily fry with such activity. 6AX4 couldn't care less. Craig


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Fri 16, 2021 10:54 pm 
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Tom Schulz wrote:
The power for the horizontal output flows through the damper. So without the damper that circuit will essentially be disconnected. The lack of a damper will not hurt anything, but other parts of the set might not like being powered up without having some parts replaced first. As a minimum you should use a dim bulb tester. Google that to see what I mean.
You would need a very high voltage diode to replace the damper. Better to just get a tube.


I have a variac to start it up with. And okay, ill just order a tube. Does the exact one matter like brand wise? Also 6AX4GT vs like a 6AX4GTA??


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sat 17, 2021 12:20 am 
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The GTA and the GTB will have higher voltage ratings and current ratings. Perhaps important for a color TV. The brand should not matter.

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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sat 17, 2021 7:58 am 
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analog.tv wrote:
There is a significant pulse whizzing around in that circuit. And it's buzzing around at approx 16000 times a second. Solid state devices easily fry with such activity. 6AX4 couldn't care less. Craig


The first thing to understand, to be able to make a remark or comment on a suggested modification to a circuit, is to understand the principles of the energy recovery rectifier.

Then it might be more obvious if a solid state device (like a diode) can be inserted in place of a vacuum tube diode in the specific application.

It might be in the particular circuit configuration that it could be beneficial, when replacing a vacuum tube diode, with a semiconductor one, that some series resistance is added, to better mimic the behavior of the vacuum rectifier.

So I would suggest, if using the silicon rectifier, initially at least start with a minimum of a 10 Ohm 2W resistor in series with it, and adjust that as required. It may not require the resistor, but it would be a matter of experiment.

It is also important that the PIV rating of the rectifier is very high, two type BY228 or BY448's in series would suffice.

The topic of the energy recovery rectifier (efficiency rectifier in TV H scan output stages) was not well covered in many textbooks, for this reason I wrote an article on them and you can read about it here:

http://worldphaco.com/uploads/TELEVISIO ... ODE_OR.pdf

The original idea for it was patented in the UK by Blumlien, but developed in RCA laboratories by Otto Schade and culminated in the design of the RCA 621TS TV set.


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sat 17, 2021 4:01 pm 
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Considering how cheap 6AX4GT tubes are, why bother? It should be technically possible though. A few tube sets did have a solid state damper (like the RCA CTC22).


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sat 17, 2021 6:57 pm 
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ACORNVALVE wrote:
The first thing to understand, to be able to make a remark or comment on a suggested modification to a circuit, is to understand the principles of the energy recovery rectifier.

Then it might be more obvious if a solid state device (like a diode) can be inserted in place of a vacuum tube diode in the specific application.

I think it is reasonable to start out by pointing out that the normal rectifier diodes that people have laying around will not work and give an idea of why they will not work.
Perhaps we should have continued to mention that there are diodes made that can be used, but that you would have to understand the circuit and select a high voltage and fast recovery diode to do that.
It is reasonable to point out that it would be easier to just use the tube that the manufacturer used.

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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sat 17, 2021 11:39 pm 
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Many years ago I replaced a temporarily unavailable damper in a British TV with a string of 800piv diodes and it worked.

For your 6AX4GTx, I would use the later and more readily available replacement, the 6AU4GT.

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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sun 18, 2021 10:39 am 
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Tom Schulz wrote:
ACORNVALVE wrote:
The first thing to understand, to be able to make a remark or comment on a suggested modification to a circuit, is to understand the principles of the energy recovery rectifier.

Then it might be more obvious if a solid state device (like a diode) can be inserted in place of a vacuum tube diode in the specific application.

I think it is reasonable to start out by pointing out that the normal rectifier diodes that people have laying around will not work and give an idea of why they will not work.
Perhaps we should have continued to mention that there are diodes made that can be used, but that you would have to understand the circuit and select a high voltage and fast recovery diode to do that.
It is reasonable to point out that it would be easier to just use the tube that the manufacturer used.


Yes I agree, but you only partially quoted my post, in which I made reference to the requirement of a high PIV rating and suggested some type numbers that have the required properties, including recovery time, and are easy to get, but unlikely sitting in somebody's junk box.


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sun 18, 2021 7:26 pm 
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Location: Colorado Springs, CO 80917
I have a stash of NOS 6AX4GTB. PM if interested.


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sun 18, 2021 8:29 pm 
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You'd have to use a high voltage Schottky diode with series resistors


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sun 18, 2021 9:35 pm 
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Location: Great Bend KS
I remember the RCA CTC22 damper diode was a high failure item. Later on the replacement from RCA was two in parallel!


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 Post subject: Re: Replace Horizontal output damper with HV diode?
PostPosted: Apr Sun 18, 2021 10:23 pm 
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Posts: 1561
6E5 wrote:
You'd have to use a high voltage Schottky diode with series resistors


I have never seen a Schottky rectifier with suitable properties to be used as an energy recovery rectifier in TV's H output stage, generally they lack the very high required PIV ratings and the lower fwd drop is of no help. They are better suited to much lower voltage transistor or mosfet power supplies.

The safest method would be to replace the 6AX4 tube.


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