Any Theories on 6U8A Hum?

Ok, I'm back and I have assembled the parts for the modification.
I just got done replacing a half dozen or so resistors that were way out of spec in the original circuit. (the unit had been re-capped when I purchased it several years ago.)
Tested the amp and I am now back to ground zero and ready to do the circuit modification.
Replacing these resisistors forced me to study the schematic in depth and now I have a better understanding what the circuit mod is going to accomplish.
I ordered enough components to make a voltage divider for each channel.
Should I proceed in that direction or just make one voltage divider and run both hum pots off of that?
I have no problem going in either direction, I just want to do it properly.
Thanks,
Jim
 
If you're unhooking the hum pots from the cathodes, you can tie both of them to a single voltage divider.
 
Success!
Gordon's calculation on values for the voltage divider are perfect.
I now have 37 volts of positive DC bias on the heaters. (The CL-90 drops the line voltage to 117 VAC which is what is spec'd as input voltage on the schematic)
I put in 2 fresh 6U8A's and am now ready to re-install the amp back into my component rack.
Have a 2nd ASR-433 which is going to get the same treatment.
Thank you to all for your responses and explanations. It really helps to understand the "why" for the fix.
Jim
 
Not suprisingly, the old tube still hummed (damaged, I believe)
Put in 2 fresh 6U8A's, the hum is gone!
Looking forward to many hours of hum-free performance.
 
Yes, as I've written once the alumina insulation becomes damaged the leakage will be unstoppable, and ever increasing, and performance is irrecoverably reduced and rapidly degrades. But tube amplifiers have many sources of hum, so it was important to identify non-elevated heaters as the cause. For the want of two resistors, far more expensive tubes were lost.

Elevated heaters are an important design factor which is honored more in the breach.
 
Yes, I've gone through at least 3 tubes, if memory serves me correctly, with a very limited number of operating hours.
A couple of resistors, a cap and a little labor is cheap in comparison. And that doesn't even count the frustration level...)
Great to put this problem to rest.
 
If I ever managed to get mine back together, a voltage divider is on the list of things to do for it. I really have no good excuse for it being in pieces as long as it has been, other than simple laziness on my part.
 
Nobody has a good excuse for omitting the elevated heaters, since it's no longer 1960, and you're not saving $0.25 per unit ($5 in today's money) times fifty thousand units.
 
Thought that I would add a side-note:
I used NOS Sylvania JAN 6U8A tubes for my initial testing.
Both hummed, the hum worsened if I had my hand near them and were very microphonic when tapped.
Replaced with NOS RCA's, problem solved
 
After searching AK on this subject, I have learned that a fair amount of people have experienced this problem
and some brands seem to hum, others don't.

My personal experience on the voltage amp/phase splitter 6u8 tubes in my ASR-433 has been that Westinghouse and RCA's were ok for a while then developed a hum. I tried a set NOS GE's that hummed right out of the box. When a hum in a tube develops, the hum follows that tube to the other channel when swapping L & R channel tubes.

Apparently, some people have tried 6GH8A, 6BL8, and 6EA8's as substitutes with some success.

My question is, does anyone have a theory as to what makes the 6U8 susceptible to hum or develop a hum?(not in my amp necessarily, just in general) Bad design of the tube?, the tube not being used in circuits for the purpose it was designed for? or?

Would running DC on the heater make any difference?

Thanks in advance for any discussion on this topic.
It's not the tubes themselves, it the circuit design. The great design engineers of yesteryear didn't know everything. Plus, this is a problem that usually creeps up slowly, insidiously. This is one reason I don't build Split Load Phase Inverters into my own designs anymore, I now use Differential Amps (long-tailed pairs).
The lower half cathode of a split load always has to be elevated because it's driven directly by the previous plate. You should float up a voltage on a voltage divider from the B+ that gives you about 75 Vdc. Place a 0.1uF film cap across it and connect that voltage to one side of the heater on the 6U8 and the problem should be alleviated. Note: The current 6U8 may be damaged now so try a new one.
 
The dicey part about elevating the heater too much is that all the other tubes are on the heater circuit too. The output tube cathodes are about +12v, the inverter some +75v, the rest are pretty nearly zero. It ends up being a balancing act to minimize things as much as possible. Probably about half the cathode voltage is as good as you're likely to get. That should get everything within 40v.
 
Good thread for my ASR-433: I've bought several JAN Philips 6U8's a few years ago and it didn't take a while before half of them began to hum... The two remaining good ones now resides in my 222D.
 
I just completed the heater circuit modification on my 2nd ASR-433. (and replaced out of spec resistors, re-cap was done around 6 months ago)
Another success, they are great amps!
 
Modifying the circuit was the solution.
I have complete confidence that this is the long-term solution but given that these amps get light usage my report on the effect of the circuit change will take a while.
However, I will report back at that time.
 
No, the Sylvania JAN's were microphonic, I replaced them with RCA's and all is good now.
 
I think that we are mixing apples and oranges here or are at least attempting 2 different discussions. Please allow me to clarify:
The 6U8's (didn't seem to matter what brand) developed a hum over time. As posted earlier in the thread, several knowledgeable members suggested that the dc voltage on the heaters needed to be raised to prevent the 6U8A deterioration over time, which I did.
The fact that the NOS JAN's hummed immediately and were microphonic was strictly due to the amp not liking them. (some time ago, I had some GE's do the same thing) I believe that some time ago when I searched AK for 6U8's that people liked in this amp, JAN's were not recommended, RCA's got a few kudos, so I have stuck with them and like them.
I ordered the JAN's before I started this thread to see if they would last longer but I found out as mentioned earlier that it was recommended that the heater circuit be modified, regardless of the brand of tube used.
Basically, I threw the JAN's in there as a test in case something went wrong after I modified the circuit (and used a dim bulb tester, of course!)
As I said, when I installed the NOS RCA's the amp performs wonderfully, I am confident that the hum that used to appear over time will not re-appear.
Hope that I have answered your question.
 
Yes, I see your point and am actually doing that. The tubes that I liked the sound of and which developed a hum over time were RCA's.
I have put in new RCA's after the circuit change and in that way will be able to judge the new circuit's effectiveness on tube longevity.
 
The 6U8's (didn't seem to matter what brand) developed a hum over time. As posted earlier in the thread, several knowledgeable members suggested that the dc voltage on the heaters needed to be raised to prevent the 6U8A deterioration over time, which I did.

The cumulative damage is the key point. Voltage stress gradually degrades the insulation causing leakage which increases until the tube is damaged or destroyed. Elevated heaters is the only solution and it completely solves the problem.
 
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