I read that these tubes are bad performers and tend to cause shorts,was on the antique radio forum and the chap there is working on the same model Zenith as I am, fortunately for him his transformer is still good,LOL!!!Just wondering if there is any truth to that and if so what would be a good sub if any.

Hi Dale,
As I understand it, this is the real deal.
The problem with the 6X5 is a revision 1.0 problem.
When the 6X5G was introduced it had a maximum voltage spec on the heater to cathode of 90 volts (H-K max voltage). Some manufacturers, like Zenith, missed this and put it in circuits with H-K voltages that were higher. An example of this is the Zenith 5S527.
This over voltage would eventually cause arcing in the tube and an H-K short that would over load the transformer and cause it to fail.
When manufacturers re-designed the tube to be the 6X5GT, they increased the H-K voltage spec to 400 volts. As a result 6X5GT are OK to use.

Best Regards,

Bill Grimm

A max heater to cathode voltage of only 90 volts would have made the tube virtually useless for almost any application. Hardly seems likely the manufacturer would make it this way. The lowest published spec shows 400 volts. Where does this kind of stuff come from?

The tubes blow for the reason Norm stated.

:Hi Dale,
: As I understand it, this is the real deal.
: The problem with the 6X5 is a revision 1.0 problem.
:When the 6X5G was introduced it had a maximum voltage spec on the heater to cathode of 90 volts (H-K max voltage). Some manufacturers, like Zenith, missed this and put it in circuits with H-K voltages that were higher. An example of this is the Zenith 5S527.
: This over voltage would eventually cause arcing in the tube and an H-K short that would over load the transformer and cause it to fail.
: When manufacturers re-designed the tube to be the 6X5GT, they increased the H-K voltage spec to 400 volts. As a result 6X5GT are OK to use.
:
:Best Regards,
:
:Bill Grimm
:

Hi Dale

Agree with Bill. Some 6X5's are a problem especially in Zenith radios using two rectifier tubes. Believe part of the problem being first section of a 6X5 heating has to carry current for the entire radio.

Not all 6X5GT are equal. Early types can still short out. Preferred type have plates in a "X" shape.

Problem doesn't seem to be with 84 designed before 6X5G, similar tube as 6X5G with 5 pin base.

Norm

:I read that these tubes are bad performers and tend to cause shorts,was on the antique radio forum and the chap there is working on the same model Zenith as I am, fortunately for him his transformer is still good,LOL!!!Just wondering if there is any truth to that and if so what would be a good sub if any.
:

:Hi Dale
:
:Agree with Bill. Some 6X5's are a problem especially in Zenith radios using two rectifier tubes. Believe part of the problem being first section of a 6X5 heating has to carry current for the entire radio.
:
:Not all 6X5GT are equal. Early types can still short out. Preferred type have plates in a "X" shape.
:
:Problem doesn't seem to be with 84 designed before 6X5G, similar tube as 6X5G with 5 pin base.
:
:Norm
:
::I read that these tubes are bad performers and tend to cause shorts,was on the antique radio forum and the chap there is working on the same model Zenith as I am, fortunately for him his transformer is still good,LOL!!!Just wondering if there is any truth to that and if so what would be a good sub if any.
::
:
:
Thanx for the information, very interesting, I will assume at this point that bad filter caps destroyed the transformer in that 5S320 im working on, so when I do find a transformer for it I will be extra carefull and keep the rectifier tube in mind, so far it looks ok no shorts found with a meter.

I didn't know about this. I just replaced the transformer in a Hickok 288X Signal Generator which also uses the 6X5 tube. Coincidence? Should I replace it with the 6X5G version? I definitely don't want to have to replace that transformer again! ;-)

::Hi Dale
::
::Agree with Bill. Some 6X5's are a problem especially in Zenith radios using two rectifier tubes. Believe part of the problem being first section of a 6X5 heating has to carry current for the entire radio.
::
::Not all 6X5GT are equal. Early types can still short out. Preferred type have plates in a "X" shape.
::
::Problem doesn't seem to be with 84 designed before 6X5G, similar tube as 6X5G with 5 pin base.
::
::Norm
::
:::I read that these tubes are bad performers and tend to cause shorts,was on the antique radio forum and the chap there is working on the same model Zenith as I am, fortunately for him his transformer is still good,LOL!!!Just wondering if there is any truth to that and if so what would be a good sub if any.
:::
::
::
:Thanx for the information, very interesting, I will assume at this point that bad filter caps destroyed the transformer in that 5S320 im working on, so when I do find a transformer for it I will be extra carefull and keep the rectifier tube in mind, so far it looks ok no shorts found with a meter.
:

There is a lot of almost paranoia about 6X5's due to the excessive "negative publicity" on various forums. Zenith used them in a way that overstressed them. This often resulted in failures that took out power transformers. Other manufacturers of radios and test equipment used them in many applications without difficulty. Like any tube they will occasionally fail.

The GT version was improved over the G and has proven to be more reliable, but there is no need to panic just because a set (other than the Zeniths that use two) uses either version. The factory recommended fix was to use pilot lamps in both HV legs, either 0.15 amps or 0.25, depending on the set.

:I didn't know about this. I just replaced the transformer in a Hickok 288X Signal Generator which also uses the 6X5 tube. Coincidence? Should I replace it with the 6X5G version? I definitely don't want to have to replace that transformer again! ;-)

:There is a lot of almost paranoia about 6X5's due to the excessive "negative publicity" on various forums. Zenith used them in a way that overstressed them. This often resulted in failures that took out power transformers. Other manufacturers of radios and test equipment used them in many applications without difficulty. Like any tube they will occasionally fail.
:
:The GT version was improved over the G and has proven to be more reliable, but there is no need to panic just because a set (other than the Zeniths that use two) uses either version. The factory recommended fix was to use pilot lamps in both HV legs, either 0.15 amps or 0.25, depending on the set.
:
:
::I didn't know about this. I just replaced the transformer in a Hickok 288X Signal Generator which also uses the 6X5 tube. Coincidence? Should I replace it with the 6X5G version? I definitely don't want to have to replace that transformer again! ;-)
:
:
Well I think using a fuse in this case will also go a long way, will measure currunt when I finnish repairing and will get as close as possible so that in the future only fuse will blow.

The problem with using fuses is that the transformer will likely protect the fuse. That's why Zenith recommended the use of lamps instead. This was of course a klooge, but it worked.

Keep in mind that the main problem with 6X5's was the way Zenith used them. They had one tube in each leg of the HV. Unless both tubes warmed up at exactly the same rate one could be overloaded and blow. Goodbye power transformer.

Other manufacturers used them in the usual way, with one tube connected as a full wave rectifier. There were few problems with this setup although the G version still had some relaiablity issues. The GT has a different plate configuration and is more reliable.

An easy way to get away from 6X5's is to make an adapter and use a 6X4, or go solid state.

A solid state replacement can be made using an old tube base, two 1N4007s and a resistor. The resistor is needed to bring the B+ down due to the much lower drop of the diodes. Trial and error is requird here. Start with something like 200 ohms, 10W and go from there.

::
:Well I think using a fuse in this case will also go a long way, will measure currunt when I finnish repairing and will get as close as possible so that in the future only fuse will blow.
: