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PostPosted: 24 Mar 2017, 14:34 
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Vastly lower voltages and currents. IOW, more forgiving.

But, note that the 7199 is no longer being made - too complicated. There is neither the demand nor the skills to do properly. Back in the day, there were at least four major makers that listed that tube as available. The point being that when there are 500 workers on the tube line of a moderately sized factory, even short-run tubes (several hundred at a time) were no problem as it was only a small matter of pattern changes. These off-mainstream tubes were very often used as fill-in when orders were slow for main-sequence tubes.

The story of the 1L6 vs. 1R5 is illustrative. Sylvania, the only manufacturer of the 1L6, commissioned by Zenith as a miniature replacement for the 1LA6 Locktal (pretty nearly everyone made the 1R5) would set up the line for the 1L6, and often just keep it running for the 1R5 rather than retool as it was simply cheaper that way. So, today I scan any Sylvania 1R5 to see if it might be a disguised 1L6.

By the way - the 1L6 (close equivalent of your DK92) is a shortwave oscillator tube used in many portable shortwave radios from Zenith, Crosley, RCA, Hallicrafters, Silvertone and a few others. I keep a number of these radios and the tube lore around them is interesting.


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PostPosted: 24 Mar 2017, 18:34 
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Quote:
Have also tried a known set of KT88s with the same result:

This pretty much rules out the KT120's as the problem doesn't it.
Quote:
With the 1k screen resistors now installed it's 7.1watts screen dissipation which makes me think that it would be above 9 watts without the screen resistor

Too high. You're running ultralinear, so the dissipation is going to be even higher at idle without the 1k resistors . This is probably causing some kind of thermal runaway (for lack of a better term). 8 watts is the max screen dissipation on a KT120.
This leads me back to the output transformers.

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PostPosted: 24 Mar 2017, 20:30 
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blackdog wrote:
Quote:
Have also tried a known set of KT88s with the same result:

This pretty much rules out the KT120's as the problem doesn't it.

Unless they are from the same compromised source.


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PostPosted: 25 Mar 2017, 05:41 
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blackdog wrote:
Quote:
Have also tried a known set of KT88s with the same result:

This pretty much rules out the KT120's as the problem doesn't it.
Quote:
With the 1k screen resistors now installed it's 7.1watts screen dissipation which makes me think that it would be above 9 watts without the screen resistor

Too high. You're running ultralinear, so the dissipation is going to be even higher at idle without the 1k resistors . This is probably causing some kind of thermal runaway (for lack of a better term). 8 watts is the max screen dissipation on a KT120.
This leads me back to the output transformers.


Th 7.1watts is with KT150s. Max screen dissipation 9 watts. But yes, valves have been ruled out I think with the test with known KT88s. And if the transformers have the correct DC resistance-implying the 40% screen tap is spot on-what else could be wrong to give excess screen dissipation?


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PostPosted: 25 Mar 2017, 10:05 
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>> Th 7.1watts is with KT150s. Max screen dissipation 9 watts. But yes, valves have been ruled out I think with the test with known KT88s. And if the transformers have the correct DC resistance-implying the 40% screen tap is spot on-what else could be wrong to give excess screen dissipation? <<

How many taps on the OPT secondary?
Resistance is not impedance.

I will make a difficult "Ask":

Do you have a low-voltage AC source - say about 15-20VAC at 50 hz?

You will have to isolate the output transformer and short the AC source into each primary and secondary winding *section* in turn, then the entire secondary winding. What you are doing by this process is measuring what is 'induced' on either side, while not risking damage to the transformer. There should be a clear relationship at each step that should make sense in the aggregate. Both transformers should be identical to each other. Make a table:

X Voltage on primary = a voltage on common-to-4 ohms; b voltage on common to 8 ohms; c voltage on common-to-16 ohms; d voltage on 4-to-8 ohms; e voltage on 8-to-16 ohms; f voltage on 16-to-4 ohms.

Y voltage on secondary common-to-4 ohms = ...... You get the picture. All permutations and combinations.

Then, a low-voltage DC source - say... a 9V transistor battery. Go through the same process. You should get zero/nil/no induction at all. If you do, the OPT is toast. Exception: A spike each time voltage is applied and released. But no more.

These beasts are somewhere over 3,000 miles away, so I am looking for tests that do not need sophisticated instrumentation, do not require much specific understanding other than "what the meter says", and might discover new data.


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PostPosted: 25 Mar 2017, 13:00 
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Peter W. wrote:
>> Th 7.1watts is with KT150s. Max screen dissipation 9 watts. But yes, valves have been ruled out I think with the test with known KT88s. And if the transformers have the correct DC resistance-implying the 40% screen tap is spot on-what else could be wrong to give excess screen dissipation? <<

How many taps on the OPT secondary?

3
Resistance is not impedance.

It isn't, but it's a good indication of whether or not the transformer is basically sound.

I will make a difficult "Ask":

Do you have a low-voltage AC source - say about 15-20VAC at 50 hz?

You will have to isolate the output transformer and short the AC source into each primary and secondary winding *section* in turn, then the entire secondary winding. What you are doing by this process is measuring what is 'induced' on either side, while not risking damage to the transformer. There should be a clear relationship at each step that should make sense in the aggregate. Both transformers should be identical to each other. Make a table:

X Voltage on primary = a voltage on common-to-4 ohms; b voltage on common to 8 ohms; c voltage on common-to-16 ohms; d voltage on 4-to-8 ohms; e voltage on 8-to-16 ohms; f voltage on 16-to-4 ohms.

Y voltage on secondary common-to-4 ohms = ...... You get the picture. All permutations and combinations.



The amplifiers measure very well with low distortion, and sound very good indeed. So surely the only measurement worth doing is an AC test to test the primary impedance as that's the only parameter likely to cause the redplating?


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PostPosted: 25 Mar 2017, 13:40 
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Unless you have a "leak". Which is unlikely. .... But.


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PostPosted: 06 Apr 2017, 04:52 
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Geek wrote:
Hrm, perhaps....

I had a customer who kept blowing tubes. The amps worked fine, other than at his house! His house had built-in audio distribution from manufacture. He replaced the speaker cable as a last ditch effort and voila!

Turned out there was a painting hung in the wrong spot - nail through the in-wall cable.

A second customer had a similar problem, but the cable showed OK. Turned out to be a HUGE roll of leftover jammed in the drywall creating enough inductance to put otherwise perfect amps into oscillation.

Just a thought.


Shielded or non-shieled interconnects or very long interconnects or bad earth on cables?

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PostPosted: 06 Apr 2017, 07:08 
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mwhouston wrote:
Geek wrote:
Hrm, perhaps....

I had a customer who kept blowing tubes. The amps worked fine, other than at his house! His house had built-in audio distribution from manufacture. He replaced the speaker cable as a last ditch effort and voila!

Turned out there was a painting hung in the wrong spot - nail through the in-wall cable.

A second customer had a similar problem, but the cable showed OK. Turned out to be a HUGE roll of leftover jammed in the drywall creating enough inductance to put otherwise perfect amps into oscillation.

Just a thought.


Shielded or non-shieled interconnects or very long interconnects or bad earth on cables?


Interconnects were fine (we tried a few pairs). I'm talking about the speaker cables.

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PostPosted: 06 Apr 2017, 07:43 
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Geek wrote:
mwhouston wrote:
Geek wrote:
Hrm, perhaps....

I had a customer who kept blowing tubes. The amps worked fine, other than at his house! His house had built-in audio distribution from manufacture. He replaced the speaker cable as a last ditch effort and voila!

Turned out there was a painting hung in the wrong spot - nail through the in-wall cable.

A second customer had a similar problem, but the cable showed OK. Turned out to be a HUGE roll of leftover jammed in the drywall creating enough inductance to put otherwise perfect amps into oscillation.

Just a thought.


Shielded or non-shieled interconnects or very long interconnects or bad earth on cables?


Interconnects were fine (we tried a few pairs). I'm talking about the speaker cables.


Over capacitive?

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Projects: "Grace" - Psvane 6SN7-SE Globe preamp | | "VoXUno" - Single driver MarkAudio 12P speaker | | "Lagoon Take 2" - Single stage tube preamp | Website: retro-thermionic


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