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6X5 bypass question - Printable Version

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6X5 bypass question - OldRestorer - 12-21-2016

Hey all,
 I just finished a Zenith 6-S-511 and even though it is all good now, I dont want to leave the 6X5 in there...
I had to bypass them in my Zenith 10-S-464 but adding 2 4007 diodes and 2 150 ohm 5W resistors...

  I was thinking that since this radio only has one 6X5 I could use a single diode and a 150ohm 5W resistor by connecting the anode from the wire that went to pins 3 and 5, connecting the cathode to the 5w resistor in series and ending at the wire that was connected to pin 8. this way the tube will still light up but will be safe...

 I was talking with someone and they had said that in this set they thought it would be better to run it the same as the Zenith 10-S-464. Meaning that I should double everything. I would then run one diode and resistor for each plate of the 6X5 joining the 2 anodes and then joining the ends of the 2 resistors. He also recommended using 10W resistors too...

  What do you guys think?
SCH:
http://www.nostalgiaair.org/pagesbymodel/230/M0025230.pdf

See diagram

RE: 6X5 bypass question - morzh - 12-21-2016

If you join two anodes (that is two xfmr wires that come there) your xfmr will blow up. Why would you join them anyways?
What's wrong with running the diodes from the corresponding xfmer pin (or pin 3 and 5) and then joining the outputs (with resistors or without)?

https://www.tedweber.com/wx5

RE: 6X5 bypass question - OldRestorer - 12-21-2016

Yes sorry... misread the way to do it...

One off each side of the transformer secondary

Ugh...

Me

RE: 6X5 bypass question - morzh - 12-21-2016

Better.

RE: 6X5 bypass question - TV MAN - 12-22-2016

I am curious, what is wrong with the 6X5? Bad design or something?

RE: 6X5 bypass question - morzh - 12-22-2016

Some say that 6X5GT is more reliable. Has Russian analog 6Ц5С (6C5S).

RE: 6X5 bypass question - OldRestorer - 12-22-2016

The 6X5 has a very high failure rate and when it fails... ByeBye transformer..
I know the GT is better but it is still not "safe"
I want it safe so I will bypass the 6X5G. It will still glow but it will be non functional as a rectifier...

Mike, is 150 ohm 5 watt good? someone said to use 200ohm 10watt resistors...

Me

RE: 6X5 bypass question - morzh - 12-22-2016

The Plate impedance of the tube is 150 Ohm. So to emulate it that's what has to be added per diode.
The max output current of the tube is 70mA, which is 35mA/resistor, which dissipates 0.184 Watts. This makes 3W rating for the resistor more than enough. 2W will do fine.

If however you want to go by dropping 22V as it is what the tube does, than the resistance has to be increased.

RE: 6X5 bypass question - Mondial - 12-22-2016

I think the failure rate of the 6X5G has more to do with its age and hours of service rather than an inherent defect. The basic design is identical to the 84/6Z4, 6X5GT and the miniature 6X4. 

A 6X5G is just an 84/6Z4 with an octal base.

Since the 6X5G has not been made since the early 40's, chances are any 6X5G you find in a radio has many hours of use and is therefore more prone to failure. Of course, running the tube the way Zenith does at or close to its maximum ratings doesn't help either.

Typically what happens is that the filament insulating coating breaks down from the repeated expansion and contraction of heating and cooling and eventually shorts to the cathode. This shorts the B+ line to ground and takes the power transformer with it.

A new unused 6X5G should be just as reliable as a new 84/6Z4, 6X5GT or 6X4.

RE: 6X5 bypass question - OldRestorer - 12-22-2016

Thanks guys,
I get some people who say just put a GT in there and then those who say I need a 20 watt resistor at 5000 ohms..
Here, I get facts and an explanation...

Me

RE: 6X5 bypass question - morzh - 12-22-2016

Mondial

if a spare winding could be added to a transformer to separately feed the filament of the 6X5, than that would not be a problem. You could even explicitly short them together.

RE: 6X5 bypass question - TV MAN - 12-22-2016

Would adding a fuse to the primary or even the secondary prevent this from destroying the p-trans? What if the replacement silicon diode shorts? This was a common problem when I was servicing TVs for a living.

RE: 6X5 bypass question - morzh - 12-22-2016

Fuse could potentially help, but in the secondary the voltage across it could exceed the rated fuse voltage causing arcing across it if opened. It is a complex process, the voltage could not be that high in the first place as the voltage after the fuse is that of a charged cap that would discharge slow plus the load is bnot purely inductive, but still it has to be analyzed.

In the primary fuse could help.
In any case one has to remember that fuse is a SAFETY device and not an equipment protection device, which even if it does, then only as a side effect. The main goal of the fuse is preventing fires. Even the fast action fuse might not be fast enough to prevent damage to the electronics if something goes wrong.

RE: 6X5 bypass question - Mondial - 12-22-2016

(12-22-2016, 01:24 PM)morzh Wrote:  Mondial

if a spare winding could be added to a transformer to separately feed the filament of the 6X5, than that would not be a problem. You could even explicitly short them together.

Yes, that would certainly work, but it would add to product cost, and reducing cost was why the 6X5 rectifier was used in the first place. 

In fact, the 84/6Z4 and 6X5 were originally designed as rectifiers for use in car radio vibrator supplies, not in AC powered sets.

Manufacturers like Philco and Zenith soon found it was cheaper to use these tubes instead of an 80 or 5Y3 which required a separate filament winding in inexpensive low power sets. Why they continued to use the 84 and 6X5 in larger consoles is another story. It could only be to save money.

Putting 400VDC between the grounded filament and the cathode of a rectifier tube is really not a great idea. The insulation will eventually break down and short. Its just a matter of time.

The 5V4G and 5Z4 also had heater-cathode construction for low forward voltage drop, but they both purposely tied the cathode to the filament internally so one had to use a separate isolated filament winding.

RE: 6X5 bypass question - morzh - 12-22-2016

(12-22-2016, 03:21 PM)Mondial Wrote:  Putting 400VDC between the grounded filament and the cathode of a rectifier tube is really not a great idea. The insulation will eventually break down and short. Its just a matter of time.

Yep first time I saw that, even before I knew of 6X5 problem, I thought that was not a good thinking behind that idea.
Direct heater rectifiers did not have that problem.
The only thing was, they were usually high filament current types. Like 80 with its 2A. Or 5U4 with its 3A.