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Is 6X5GT prone to failure?
Is 6X5GT prone to failure?
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Old 9th November 2017, 03:38 PM   #1
sutantoroy is offline sutantoroy  Indonesia
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Default Is 6X5GT prone to failure?

Hi,

Regarding to the issue about 6X5 heater cathode shorts melting the transformer:
1. What causing it? I heard: that its related to the physical structure of the rectifier itself (heater cathode separated to close), not properly designed power supply especially regarding the impedance (or resistance??) of the secondaries power transformer to plate which also related to the value of 1st C input filter (maximum 40uf, typically 4uf), too high DC voltage handled by the rectifier causing leakage heater cathode, as indirect heated rectifier it cant tolerate high reverse voltage, some even say it caused by rectifier plate to plate or plate to cathode shorts.
2. What is the solution if still want to use them? (honestly i recently got the 6X5GT failure experience)

Thank you.

Regards,
Roy
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Old 9th November 2017, 03:49 PM   #2
kevinkr is offline kevinkr  United States
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Is 6X5GT prone to failure?
Use a dedicated filament winding and tie the cathode of the 6X5 directly to it, no potential difference, no cathode insulation break down issue. (Make sure the filament winding you use is rated for more than the highest voltage possible at the cathode of the 6X5 under high line, and before the other tubes start to draw appreciable load current)

You can also add a UF4007 or similar high PIV diode in series with each plate as is commonly done with the 5AR4.
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Old 9th November 2017, 04:15 PM   #3
leadbelly is offline leadbelly  Canada
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Where did you get the 40uF maximum from? The only datasheet I found that listed it said 32uF.

It's an 80mA rectifier. If you need to go bigger and have heater current capacity, the 6AX5 goes to 125 mA.
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Old 9th November 2017, 07:54 PM   #4
Eli Duttman is offline Eli Duttman  United States
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Consider either a Loctal socket/7Y4 or "hole shrinker"/9 pin mini socket/6X4. Current delivery ability is = and reliability problem reports absent.
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Old 9th November 2017, 08:04 PM   #5
stocktrader200 is offline stocktrader200  Canada
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fuse the heater winding
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Old 9th November 2017, 08:07 PM   #6
anatech is offline anatech  Canada
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Is 6X5GT prone to failure?
I have pieces of test equipment that use the 6X5 and have had zero failures. However, they designed the supply with recommended capacitance and resistance in line. That is the other spec that wasn't mentioned, minimum plate resistance. The power transformer makes some of this up.

If you drew enough current to wipe out the transformer, why didn't the line fuse blow?

-Chris
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Old 9th November 2017, 09:15 PM   #7
stocktrader200 is offline stocktrader200  Canada
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if it is like the SX 780 transformer, most of the power is not delivered on a single winding.
therefore if that winding shorts, it may melt and short with itself or adjacent windings before the line fuse blows
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Old 10th November 2017, 01:35 AM   #8
anatech is offline anatech  Canada
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Is 6X5GT prone to failure?
Well, over heating and melting is not an instant thing unless very high energy is involved (lightning for example). Under normal circumstances, the HT winding doesn't draw a lot of current, but in fault mode you might be surprised.

-Chris
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Old 10th November 2017, 05:25 AM   #9
sutantoroy is offline sutantoroy  Indonesia
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Quote:
Originally Posted by kevinkr View Post
Use a dedicated filament winding and tie the cathode of the 6X5 directly to it, no potential difference, no cathode insulation break down issue. (Make sure the filament winding you use is rated for more than the highest voltage possible at the cathode of the 6X5 under high line, and before the other tubes start to draw appreciable load current)

You can also add a UF4007 or similar high PIV diode in series with each plate as is commonly done with the 5AR4.
Is my draw wiring correct? I use separate transformer for heater, it has dual secondaries, one for the 6SN7GTA preamp, else the rectifier. Each secondaries winding rated for 2 amperes, its large enough right? ( i will use them to supply single stage 6SN7GTA with 300VDC HV). What is the function of UF4007 in series with each plate of 6X5GT?

Quote:
Originally Posted by leadbelly View Post
Where did you get the 40uF maximum from? The only datasheet I found that listed it said 32uF.

It's an 80mA rectifier. If you need to go bigger and have heater current capacity, the 6AX5 goes to 125 mA.
Quote:
Originally Posted by Eli Duttman View Post
Consider either a Loctal socket/7Y4 or "hole shrinker"/9 pin mini socket/6X4. Current delivery ability is = and reliability problem reports absent.
Quote:
Originally Posted by stocktrader200 View Post
fuse the heater winding
I do have many 6X5GT now, if its ok to use them, i want to still use them, the current for the HV is enough for single stage preamp of 6SN7GTA right? The 40uf is noted in Tung Sol datasheet, i do find each manufacturers have a little difference in mentioning C input notes, but basicly it have the same information to take precaution of the current surge from the transformer secondaries.

Quote:
Originally Posted by anatech View Post
I have pieces of test equipment that use the 6X5 and have had zero failures. However, they designed the supply with recommended capacitance and resistance in line. That is the other spec that wasn't mentioned, minimum plate resistance. The power transformer makes some of this up.

If you drew enough current to wipe out the transformer, why didn't the line fuse blow?

-Chris
Quote:
Originally Posted by stocktrader200 View Post
if it is like the SX 780 transformer, most of the power is not delivered on a single winding.
therefore if that winding shorts, it may melt and short with itself or adjacent windings before the line fuse blows
Quote:
Originally Posted by anatech View Post
Well, over heating and melting is not an instant thing unless very high energy is involved (lightning for example). Under normal circumstances, the HT winding doesn't draw a lot of current, but in fault mode you might be surprised.

-Chris
Just like what happened before on my preamp, i do test my new RCA 6X5GT, for about 8 seconds from i turn it on, i saw a some archs in the filament of the rectifier, so i turn it off. check the pin of the rectifier everthing is still ok, but physically notice the rectifier filament have some shiny parts as like it was scratched. Then i decided to use my daily used rectifier, there is humm coming out, try to replugging the rectifier and 6SN7GTA, there is no change. Then i smell burn out thing, turn off the amplifier and preamp, unplug the main cable, i touch the wood case of the transformer its warmer than usually. Decided to open the preamp, and find out the high voltage secondaries being short, 0 resistance in the winding, and find the out layer of the transformer melted. The main fuse is still ok. But i do find the C input of my preamp is to much bigger than the maximum, 180uf...
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Old 10th November 2017, 09:42 AM   #10
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by kevinkr View Post
Use a dedicated filament winding and tie the cathode of the 6X5 directly to it, no potential difference, no cathode insulation break down issue. (Make sure the filament winding you use is rated for more than the highest voltage possible at the cathode of the 6X5 under high line, and before the other tubes start to draw appreciable load current)

Strange. As the maximum heater to cathode voltage rating is 450 Vdc, there shouldn't be expected any issue, unless this and/or the maximum filter capacitor value ratings are violated. The 6X5 clearly appears to be designed to be operated from the same heater supply as the other tubes in the device. Maybe the specific tube itself was faulty? Tying the cathode to the heater eradicates the possibility of an issue here, of course.

Quote:
You can also add a UF4007 or similar high PIV diode in series with each plate as is commonly done with the 5AR4.

Really, and why? Maybe because a recent make's 5AR4 manufacturing quality hasn't too much in common with that of an original Philips/Mullard tube?


Best regards!
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