6GH8 substitutes

[Aperna1985]

Hi Guys, I had to take a break from my Zenith 16H23 because I'm getting the house ready for our first baby. Before I put it away I was having Horizontal sync problems. The set will be rock solid and look good, then the picture starts to bend at the top and bottom, then the horizontal goes nuts, I readjust it and it holds for a while then it starts again. It uses a 6GH8 for the horizontal oscillator. I noticed the output of pin 1 was low. I put in a 6EA8 and got the proper 130v coming out of the tube on pin 1 but the lock was even worse. I'm wondering if it's not a good substitute for this application or if there is another problem in the set. Is 6GH8 as bad of a tube as I've heard? Is there a good strong substitute?

[Chris K]

6U8A possibly?

[Yamamaya42]

The 6GH8A is not a bad tube in the general sense, it's a very common workhorse that was used in the 60s, there is nothing really wrong with it, where it gets a bad reputation is in longevity, they tended not to last long, but if you have known good NOS ones, the problem is likely something else.

https://en.wikipedia.org/wiki/6GH8

[Aperna1985]

I don't have any new ones. However I was going to order some off eBay and give it a shot. I did test the tube in my tube tester and it comes up as decent emissions and no grid leakage. But I don't know if this is one of the tubes that will test good and actually be bad. The set is high hours, and I've had to replace quite a few week tubes in it. The 6gh8 is actually a Zenith tube possibly original.

[kvflyer]

6U8 as Chris has said. Also, 6BL8.

[Zenith26kc20]

I would look for a bad horizontal AFC diode. The selenium ones go leaky. Replace the two halves with 1N4148 diodes.
Also, check plate resistors on the 6GH8 for going up in value.

[old_coot88]

If any set has weak horz lock and has an AFC dual diode, replace it out of hand.

[vortalexfan]

Originally Posted by Aperna1985

The 6gh8 is actually a Zenith tube possibly original.

Zenith didn't make their own tubes they usually sourced them from Sylvania and then had their name slapped onto the tube.

But I would guess that it probably is an original if not the second to original tube to the set (if the original owner of the set used Zenith Branded tubes as was suggested by Zenith back then on their TVs and radios.)

Was this the Zenith "Bugeye" TV? If so I had worked on one of these once and that was a pain in the butt TV to work on but when I got it going it worked really well.

With Horizontal or Vertical circuits the oscillator tubes need to be pretty much brand new (NOS) or else you're going to have oscillator stability issues in the vertical and horizontal sweep circuits because they are critical circuits.

I would definitely try getting some NOS tubes for the TV and try that out and see what that does with your TV.

[kvflyer]

Side note, RCA manufactured the 6GH8 and used many in their color sets. They worked them hard and found that they failed often. In later production, they sourced the 6GH8 from Sylvania and of course, they were labeled as "RCA". You can tell which ones are manufactured by Sylvania easily. The tube number is obviously the ink and font used by Sylvania as opposed to the octagonal number surround.

I have tubes manufactured by both companies.

[DavGoodlin]

Some later designs used a 6KD8, similar but not as common.

[Aperna1985]

Originally Posted by Zenith26kc20

I would look for a bad horizontal AFC diode. The selenium ones go leaky. Replace the two halves with 1N4148 diodes.
Also, check plate resistors on the 6GH8 for going up in value.

Originally Posted by old_coot88

If any set has weak horz lock and has an AFC dual diode, replace it out of hand.

I actually ordered one from Talon Electronics. It looks similar to the integrators you'd find in the vertical section on Zeniths except the schematic doesn't show a resistor.

Originally Posted by vortalexfan

Zenith didn't make their own tubes they usually sourced them from Sylvania and then had their name slapped onto the tube.

But I would guess that it probably is an original if not the second to original tube to the set (if the original owner of the set used Zenith Branded tubes as was suggested by Zenith back then on their TVs and radios.)

Was this the Zenith "Bugeye" TV? If so I had worked on one of these once and that was a pain in the butt TV to work on but when I got it going it worked really well.

With Horizontal or Vertical circuits the oscillator tubes need to be pretty much brand new (NOS) or else you're going to have oscillator stability issues in the vertical and horizontal sweep circuits because they are critical circuits.

I would definitely try getting some NOS tubes for the TV and try that out and see what that does with your TV.

It's actually a 1962 console TV but my next guess is that I need atleast a NOS tube in the horizontal. Funny part is the TV had a great picture when I started. It just had slight vertical collapse, after replacing the cap in the vertical the picture looked great for 10mins then the horizontal problem started.