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Is 6X5GT prone to failure?
Is 6X5GT prone to failure?
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Old 10th November 2017, 01:47 PM   #11
anatech is online now anatech  Canada
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Is 6X5GT prone to failure?
Hi sutantoroy,
What you are seeing is a classic failure due to excessive capacitance. Use a little lower capacitance than the maximum for starters. Then, pay attention to the minimum plate resistance specification as well. If you have to add resistors, that's fine. Manufacturers have done that in their designs for ages.

The lower capacitance will allow a higher amount of ripple on the DC line, but that beats the higher frequency pulses that the high capacitance will generate. The lower frequency ripple is a lot easier to get rid of with a regulator than the high frequency hash created when capacitance is too high.

-Chris
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Old 11th November 2017, 06:54 AM   #12
sutantoroy is offline sutantoroy  Indonesia
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Hi sutantoroy,
What you are seeing is a classic failure due to excessive capacitance. Use a little lower capacitance than the maximum for starters. Then, pay attention to the minimum plate resistance specification as well. If you have to add resistors, that's fine. Manufacturers have done that in their designs for ages.

The lower capacitance will allow a higher amount of ripple on the DC line, but that beats the higher frequency pulses that the high capacitance will generate. The lower frequency ripple is a lot easier to get rid of with a regulator than the high frequency hash created when capacitance is too high.

-Chris
Thank you Chris. So, what do you think about separate secondaries winding for rectifier heater and tying the heater pin to cathode, is it necessary doing that? What is the side effect by doing that (tying the heater pin with the cahode)

Regards,
Roy
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Old 11th November 2017, 06:28 PM   #13
anatech is online now anatech  Canada
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Is 6X5GT prone to failure?
Hi Roy,
That all depends on the rectifier you are using. Normal heater-cathode breakdown is on the order of 100 ~ 200 VDC (either polarity, peak). Some rectifier tubes were designed with very large heater to cathode voltage tolerance, so they could be run off the same heater line as all the other tubes (in this case, 6X5 has a breakdown of 450 VDC). That means that you can run this tube from a ground referenced heater line with all the other tubes as long as that potential difference remains below 450 V peak.

Tubes that use a filament like a 5U4 must run a separate filament winding that is at the same potential as your B+ supply. This one needs 5 V at 3 amperes. That is why you will see tube power transformers that commonly have a 5 VAC winding good for 2~3 amperes.

So, your power supply does not require you to run your rectifier tube off a separate winding. You can even float your entire heater supply up to 30 ~ 50 VDC to reduce hum if you want with zero ill effects for the 6X5.

-Chris
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Old 13th November 2017, 03:16 PM   #14
sutantoroy is offline sutantoroy  Indonesia
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Hi Roy,
That all depends on the rectifier you are using. Normal heater-cathode breakdown is on the order of 100 ~ 200 VDC (either polarity, peak). Some rectifier tubes were designed with very large heater to cathode voltage tolerance, so they could be run off the same heater line as all the other tubes (in this case, 6X5 has a breakdown of 450 VDC). That means that you can run this tube from a ground referenced heater line with all the other tubes as long as that potential difference remains below 450 V peak.

Tubes that use a filament like a 5U4 must run a separate filament winding that is at the same potential as your B+ supply. This one needs 5 V at 3 amperes. That is why you will see tube power transformers that commonly have a 5 VAC winding good for 2~3 amperes.

So, your power supply does not reuqire you to run your rectifier tube off a separate winding. You can even float your entire heater supply up to 30 ~ 50 VDC to reduce hum if you want with zero ill effects for the 6X5.

-Chris
Thank you again Chris. To make sure that i do get know what you are telling, so i can connect the heater pin to ground reference or just leaving it not connected to the ground reference (leaving the transformer secondaries heater winding just connected to the tube heater pin, no conection to the ground reference). Both is ok, is it right?

For the kind like 5U4 (is it called direct current rectifier?), the reason it needs separate winding is it because the current requirement for the rectifier heater is relatively very high to other tubes or other electrodes current capacity, so it can be a very high inrush current damaging other electrodes. By doing separate qualified winding will likely as current manufacturer which is supplying the rectifier heater high current demand, is it right?

Regards,
Roy
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Old 13th November 2017, 04:53 PM   #15
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Is 6X5GT prone to failure?
Hi Roy,
You can ground the centre tap of the heater winding. Here it is a question of background hum, so try floating it, grounding it and floating it to a specific voltage below say - 50 VDC. Make sure you have at least a 10 uF capacitor to ground if you tie the heaters to some voltage above ground.

A filament tube like the 5U4 has it's filament at full raw B+ potential. This is by design and you can't escape that connection. Have a look at a schematic to see what I am saying. Since this must be so, the filament does require its own dedicated winding. Just remember that in these tubes, the "heater" is combined with the cathode to make it a filament.

-Chris
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Old 13th November 2017, 08:17 PM   #16
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Originally Posted by sutantoroy View Post
so i can connect the heater pin to ground reference or just leaving it not connected to the ground reference (leaving the transformer secondaries heater winding just connected to the tube heater pin, no conection to the ground reference).
Roy, the heater for a rectifier valve needs to be tied to some potential, not left floating.

For directly heated cathode rectifiers (such as 5U4), where the cathode is the heater, then one end of the cathode/heater is connected to (typically) the first filter cap positive terminal (typically B+), which has a defined voltage relative to 0V ground.

For indirectly heated cathode rectifiers (such as 6X5), the heater has to still be connected to a defined reference potential - which could be 0V, or an elevated DC voltage (as when trying to minimise hum, or due to the circuit having valves with cathodes that are at a high DC voltage), or to the rectifier's cathode (such as when a separate heater winding is just being used for that rectifier valve's heater supply).

For indirectly heated cathodes, the datasheet shows the voltage difference that allowed to be applied between heater and cathode.

Last edited by trobbins; 13th November 2017 at 08:20 PM.
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Old 13th November 2017, 09:00 PM   #17
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Is 6X5GT prone to failure?
Hi trobbins,
The 6X5 with it's 450 Vpk H-K rating is designed to share the heater power with the rest of the tubes in the device. I've never seen signal tubes left floating, but I can't imagine anything untoward would happen to a 6X5 that had an unreferenced heater source. We don't like the idea, but I doubt there would be any repercussions if you let it float completely. Leakage current would do that job for you anyway (reference the heater to some voltage, probably B+).

-Chris
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Old 13th November 2017, 10:18 PM   #18
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Hi Chris, yes I agree with your view on what would likely happen with an unconnected heater. For forum chit-chat I'd prefer to present a more clear-cut path for diyers as the concept of a floating circuit would likely be new to many, along with reconciling what could happen and what some docs (such as safety standards) dictate as to how to interpret what insulation etc to apply for such a floating circuit.

From a practical perspective, the configuration that minimises heater-cathode voltage difference is likely to reduce quirky processes that are not too definable, such as how heater-cathode resistance changes over time.

Ciao, Tim
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Old 14th November 2017, 01:23 AM   #19
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Is 6X5GT prone to failure?
Hi Tim,
You have that right! I've studied various forms of heater-cathode insulation failure. The topic was well written up way back in time. Only zero potential difference may avoid that fate, or an early open heater.

Ever see an arc between the heater and cathode? I have, and not in a rectifier tube either. It's very dramatic and results in a dead tube very quickly.

-Chris
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Old 14th November 2017, 02:55 AM   #20
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I found it interesting reading the 1962 RCA digest of papers, and the onerous process of identifying impurities and processing/purity requirements of heater coating, and the investigations of metal migration, and the physical jostling that goes on during turn-on/off between moveable heater and fixed cathode tube. No wonder there is such a wide distribution of heater-cathode resistance found in vintage signal tubes.

I recall a failed PT from what appeared to be 6X5 failure - I'll have to look up my notes on that one.

Was the arc in an output tube from pin 2-3 connection?
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