Are 6x5 Tubes problematic?

[johnny_fever]

Hi Guys,
I recently bought a Sansui HF-L60 (6L6) Amp. While researching the tubes I need I found a lot of bad press on the 6X5. According to the vintage radio collectors & techs the 6X5 has a bad rep for shorting out. I did hear the 6X5WGT was a better design over the GT. I also heard about the copper job. So I am looking for opinions here. Thanks Rick

Ps. My 6X5gt tube was weak but displayed no shorts.

 

[Big Harry]

I've never had a problem with a 6X5 as long as it is operated well within its limits. In the 1940's, Zenith had a problem with the 6X5 because of an undersized power transformer and running the tubes at their limits or above. I've worked on auto radios that use a 6X5 as the rectifier and more times than not the original equipment tube is still good. Supposedly the 6X5WGT is a better tube to use, but I don't know that is necessarily correct.

 

[Mike Stehr]

Maybe try a Bendix 5852.

 

[johnny_fever]

Thanks for your input guys. I'm still going to read more on this problem. Rick

 

[trobbins]

Part of the concern would be the collatoral damage when 6X5 cathode-heater shorting occurred as the original equipment would typically not have any more protection than a mains fuse.

Nowadays, a good restoration would include a power transformer secondary side fuse in the CT, and add series ss diodes to the anodes to give a better chance of saving the power transformer, and possibly the other valves from heater-cathode stress if a humdinger open-circuited.

 

[johnny_fever]

I emailed my tech. He wants to bypass the tube altogether. I think it will still glow. Some of these Sansui Amps are insanely rare. Maybe not as costly as others but defiantly more scarce. I sometimes wait many years for a mate to one of my Amps. Even with global shopping its not easy finding Sansui Monoblock's. Rick

 

[primosounds]

I emailed my tech. He wants to bypass the tube altogether. I think it will still glow. Some of these Sansui Amps are insanely rare. Maybe not as costly as others but defiantly more scarce. I sometimes wait many years for a mate to one of my Amps. Even with global shopping its not easy finding Sansui Monoblock's. Rick

I don't think you should change the circuit at all. Why pay a tech to do something that could be solved by changing the rectifier from 6x5 to the wgt model or similar. Changing out to solid state will incur other issues such as increased B+ because of much lower voltage drop thru the SSR. Since you are obviously a patient person, waiting for the 6x5wgt to turn up is no big deal.

 

[johnny_fever]

Thanks for your response. I was talking to a friend today and his opinion is the same as yours. I'm still researching all my options. My tech only offered to bypass it because I was ranting/freaking about the bad press of the tube in the first place. I can also agree the WGT Jan tube must have addressed any flaws the older tubes had.

 

[Joe Roberts]

There is no problem. 6X5GT was on the mil preferred list. It is a rock solid tube. I have been using them everywhere for 30 years. I can't think of one failure.

I don't know what they do with 6X5 in that amp. It's only a 70mA tube, not enough for a PP 6L6 amp. Separate LV supply perhaps?

 

[johnny_fever]

I am new to this Amp so I never heard much about it before. Here is a pic from another AK'er. Still learning about the good/bad about it. Thanks Rick

 

[Joe Roberts]

Separate screen supply with the 6X5, or rather separate supply for the 6L6 plates. Very fancy!

"L" must mean "Luxury!"

I never saw one of these in my life! Anyway, I wouldn't worry about the 6X5 but make sure that not a single original coupling cap remains. Japanese makers used some very dubious caps in that era, especially the ones that say "oil capacitor". on gray or white plastic shrink over the metal tube.

 

[johnny_fever]

L for Luxury. I like that. Yes The small caps are getting replaced. Were oil filled caps considered high grade back in the day? One friend says I should look into Bumblebee caps. I hear so many diff opinions. Its fun learning about these mono amps

 

[dsndblm]

I have a simple 6SN7 preamp and a Darling SE amp that both use the 6X5GT. The little preamp has seen daily use for over 10 years, with the same 6X5GT. The Darling runs its 6X5GT at around 85% of tube data sheet recommended current. It.'s been trouble free for a decade also. If I remember correctly, max current through the 6X5GT is 70 ma. If you keep the current below the rated limit, you should be fine.

 

[trobbins]

If you keep the current below the rated limit, you should be fine.

Perhaps ... you 'could' be fine .... is a better generic statement for 6X5. The HF-L60 is a rare beast in that the 6X5 has its own heater winding, and is just used for screen and preamp supply, so if that supply fails then it wouldn't suffer collateral damage to the valves (the same can't be said for a failed B+ supply in that amp so at least worth giving it some extra protection).

The sad part about valve failures is that no one nowadays can usually tell if they are a 'weakness' or an ad hoc failure or related to operational conditions, as we are usually discussing single events, or have some bias (eg. if you are a service tech and only see faulty amps). Unfortunately a valve diode is prone to taking out a power transformer as collateral damage, and failing output stage valves are prone to taking out power and output transformers. As I can modify my amps to add better protection against those failings then that's how I manage an amp's service life.

 

[CZ4A]

I had the same nervousness about the 6X5GT in my Eico capacitor bridge, but so far I've had no problems with the tube. One thing to note when shopping for a 6X5 is that the "stacked" plate tubes were more prone to shorts than the later parallel plate or "X-plate" tubes.

 

[primosounds]

I am new to this Amp so I never heard much about it before. Here is a pic from another AK'er. Still learning about the good/bad about it. Thanks Rick

The 6x5 in this amp powers the screens. If the screen voltage fails the power tube will essentially go into shutoff. There is not really any problem using any 6x5 in this amp.

 

[Joe Roberts]

The small caps are getting replaced. Were oil filled caps considered high grade back in the day? One friend says I should look into Bumblebee caps.

Good oil caps were always high-grade but the Japanese consumer-grade oil caps were quite junky. Anybody who works on old Japanese gear knows these leaky devils. Maybe they worked well for a couple years but not 50 years.

Bumblebee caps, real Sprague bumblebee caps, were next in line for being almost universally leaky. In that case, I think they had unrealistic notions of how well molded plastic would seal the cap from moisture ingress.

As for the 6X5 scandal, are there any reports of this actually happening or is it all third-hand info passed around like gospel?

I suspect the latter. Along with the Cold War era Armed Forces, I would use a 6X5GT without hesitation.

 

[johnny_fever]

Thanks guys. I knew nothing about the 6X5 tube before buying this Amp. Researching the tube led me to its possible SHORTcomings. I asked about the tube here for opinions/experiance. I try to be fair when it comes to people complaining about something on the internet. For every complaining person you might find a million happy campers. They say Bad news travels around the world before good news has its pants on. Thanks for all your thoughts. Rick

 

[AdamAnt316]

Good oil caps were always high-grade but the Japanese consumer-grade oil caps were quite junky. Anybody who works on old Japanese gear knows these leaky devils. Maybe they worked well for a couple years but not 50 years.

Bumblebee caps, real Sprague bumblebee caps, were next in line for being almost universally leaky. In that case, I think they had unrealistic notions of how well molded plastic would seal the cap from moisture ingress.

As for the 6X5 scandal, are there any reports of this actually happening or is it all third-hand info passed around like gospel?

I suspect the latter. Along with the Cold War era Armed Forces, I would use a 6X5GT without hesitation.

The first place I heard about the 6X5 being problematic was this page. There has also been much discussion about them on the Antique Radio Forum. I believe much of it has boiled down to this:

1. The internal design of (some of the?) early 6X5G tubes was flawed, as was also the case for the 84/6Z4 tube which the 6X5 was apparently based off of
2. Zenith ran the tube(s) too hard, exacerbating the problem
3. Zenith also used marginal power transformers in radios of this era
4. Later 6X5 designs are less prone to failure
5. The 6AX5 tube can be used to replace the 6X5(s), if the power transformer can handle the added filament current

So a combination of factors which has maligned the reputation of the 6X5 tube series. I myself havent used them, but i haven't had any issues with the related 12X4 tube as used in my Dynaco PAS-2 and PAS-3X preamps, nor have I heard of them being problematic in other people's PAS pres. I think that, as long as you stay away from the early shoulder-shaped 6X5G version and don't run the tube(s) too hard, you should be OK.
-Adam

 

[johnny_fever]

Thanks Adam.