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Old 1st March 2009, 03:49 AM   #1
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Default Cathode-heater leakage

I find triode-pentodes useful for many applications. Types ECF80/6BL8 and ECF82/6U8 are fairly obtainable here and not too expensive.

Getting some hum in a recent design, I hooked up a circuit to test for cathode-heater leakage for either triode or pentode, and got the shock that few tubes are 'kosher' in this department. Of about 10 tested, only 2 had negligible heater ac transfer to the signal output. (The test was done with the alternative of switching over to 6V clean dc on heaters. Heater potential was at some +70V wrt common.)

The 7199 may be suggested as a more reliable alternative, but the gm is a little low for my purpose and they come expensive here. Most of the triode-pentodes with higher gm were designed for rf use, where no particular care was taken to keep heater leads away from signal leads. I could also not notice significant differences in internal layout with a magnifier, so much of the undesired transfer must take place inside cathodes.

The alternative would be to use separates e.g. EF86 + triode(s) or a dc heater supply, but I am still puzzled at the inconsistancy of the test results.

Does that mean that one had better steer away from the typical tv application tubes? I do see them in audio circuits. I would appreciate anyone's experiences here.
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Old 1st March 2009, 08:25 AM   #2
Tweeker is offline Tweeker  United States
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Ive read pushing the HK rating greatly increases leakage, but if they have a common heater there might not be much you can do in your app.
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Old 1st March 2009, 03:55 PM   #3
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A trick is temporarily up the heater volts to 7V; clout the tube and see if noise drops.That would indicate some impurity between htr & cathode.
As you mention, the other solution left is to DC all heater volts and lift 50V+ off ground or supply heater from separate source.

Going to 7199's; I abandoned this move some yrs ago as these are getting rare and pricy. What RCA and GE stock I have remaining have similiar same htr & cathode leakage problems and far worse, screen voltage variations. I find Sovtek's 7199 vary sonically.

In price comparison I find ECL series, from Aero, Ei, and 7643 group quite consistent.

richy
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Old 1st March 2009, 04:17 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by richwalters
A trick is temporarily up the heater volts to 7V; clout the tube and see if noise drops.That would indicate some impurity between htr & cathode.
As you mention, the other solution left is to DC all heater volts and lift 50V+ off ground or supply heater from separate source.

Going to 7199's; I abandoned this move some yrs ago as these are getting rare and pricy. What RCA and GE stock I have remaining have similiar same htr & cathode leakage problems and far worse, screen voltage variations. I find Sovtek's 7199 vary sonically.

In price comparison I find ECL series, from Aero, Ei, and 7643 group quite consistent.

richy
Hi Rich,
What do you mean by "clout the tube," even as a native English speaker I am not sure I have a clue as to what you mean. I suspect non native speakers might be even more confused. Do you mean tap the tube or something else?

I also found that Sovtek 7199s hum uncontrollably in a lot of applications in addition to their generally very mediocre sound quality.
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Old 1st March 2009, 04:56 PM   #5
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I switched from 7199 to 6GH8A for my Dynaco amp due to availability and price. I had to use adapter plugs but it works well if you find the right 6GH8A. I had hum problem with supposedly reputable RCA (NOS) but since it was so cheap, it didn't bother me much to toss them. Then I went with Amperex brand and it works fine. Again, it was very cheap.
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Old 1st March 2009, 06:04 PM   #6
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Sorry Kevin;
"clout the tube" I really mean't = tap it fairly hard with a pencil. It seems we agree with the Sovet 7199.

It seems tubes destined for communications equipment front ends weren't designed for min hum. It wasn't an issue.

richy
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Old 1st March 2009, 08:34 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by richwalters
<snip>

It seems tubes destined for communications equipment front ends weren't designed for min hum. It wasn't an issue.

richy

My observation would have to be the same on that one. I don't toss them as otherwise they are fine, I just save them for an application where that is not an issue or heat them with dc.

The Sovtek 7199 are a major disappointment, I've had ones that had internal shorts in the connections that were visible to the naked eye. That is when I could still see with the "naked" eye.. Unfortunately no longer..
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Old 1st March 2009, 08:48 PM   #8
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Quote:
Originally posted by kevinkr

The Sovtek 7199 are a major disappointment,....
Hi Kevin ,

I agree with that !!

Johan , Kevin , Richy ,

Does anyone have tried to use the old and very reliable 6AN8 ???
I think that its performance is better than 6BL8 , 6U8 or even
the Sovtek 7199 .

Regards ,

Carlos
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Old 1st March 2009, 09:01 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by refference


Hi Kevin ,

I agree with that !!

Johan , Kevin , Richy ,

Does anyone have tried to use the old and very reliable 6AN8 ???
I think that its performance is better than 6BL8 , 6U8 or even
the Sovtek 7199 .

Regards ,

Carlos
Hi Carlos,
Yes I have had experience with the 6AN8, quite commonly used in various audio amplifiers made here from about the late 1950s onwards. It is an ok tube, most NOS types I have encountered don't have the hum issue, and it sounds ok.
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Old 1st March 2009, 10:59 PM   #10
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Thanks to all for input.

Darn it, @#%$&%! I will have to go to our local supplier and investigate every such topology (tube kind ... I must now and then use a fancy word to make others think I am Engresh) for internal construction to see if anything better exists. Not to many of them; I will start with 6AN8 - thanks Carlos.

Some further detail: I was going to use this in an RIAA tube pre with a hi-Gm triode at high current for low noise, as input. Testing as stated earlier, I found an up to 20x variation in hum induction via the triode cathode with 1K bias resistor to which RIAA feedback also went. The few good ones introduced nothing.

This is not a one-off; the circuit will get published, so tapping etc. no go. One might be forced to use a separate triode; hope ECC88 has no similar shenanigans.

Quote:
Originally posted by Tweeker
Ive read pushing the HK rating greatly increases leakage, but if they have a common heater there might not be much you can do in your app.
Well, this is opposite to what I was told! That is that hum can be the result of unwanted emission from heater to somewhat positive cathode. Making the heater positive with regard to all cathodes (say +40V) would obviate that. I have experienced that in the past. But by 'pushing the HK rating' you might literally mean running at maximum spec or higher - then agreed, but that is not what I meant (and which one should not do).

But then the Scottish blood in me says involving an extra tube will be more expensive than making a dc heater supply, with other advantages of the latter .... decisions, decisions...

(why don't I just go to SS) Oops, sorry!
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