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PostPosted: 08 May 2017, 12:03 
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Just a thought Bruce. Have you considered sending 2 known good tubes to the problem amp and have them send the problem tubes to you so both the tubes and the amp could be evaluated in one go? I know crazy pricing but may be worth the cost to find out.


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PostPosted: 08 May 2017, 21:11 
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Hi, Actually yes to both. The biggest concern right now with that is sending tubes half way around the world. You can not be sure that how they worked on departure is how they will behave on receipt. Our shippers can damage granite rocks. :blush:

Good listening
Bruce

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PostPosted: 16 Aug 2017, 15:42 
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Hi everyone, the saga continues. It seems there are at least two versions of the tube out there. Some with "U" shaped heat radiators in the top and some with square ones. The "U" shaped ones I have all work fine. I'm getting reports that the square ones do not. The North America Distributor (New Sensor) told me that they don't know of any issues and few tubes are returned. My email and forums tell me different, you decide. I do not have any of the square version and plan to order some next week to test them. If they fail then it really does raise questions on the viability of class A amps based on them. More to follow.......

Good listening
Bruce

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PostPosted: 17 Aug 2017, 00:59 
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I'll keep an eye out for them on the Rogue units we get in the store.

Thank you!

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PostPosted: 17 Aug 2017, 01:28 
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Great Work Bruce

I'm trying to get my head around the "U" and "Square" radiators

In this image of a KT150,
Is this what you mean by a "U" shaped radiator?


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PostPosted: 17 Aug 2017, 08:44 
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Hi, That one seems to have the "U" shaped heat radiator. Others have a large flat piece from what is reported to me. I have not personally run into any yet. IMO it would seem that either ought to work, but there may be other internal changes that mess them up that are not obvious. The square fins may just be a way to tell them apart. I wonder if the spacing of the internal supports and screen grids are different. Possibly thinner anode metal. These are after all beam tetrodes and the shape, location and target area for the beam matters. Change any of them and the performance is altered. If I cant find a way around the tubes...if indeed all future ones are sub par, I may start looking a other tube types to get the 40-50 watts wanted. I wold rather not as that is sure to increase costs.

Good listening
Bruce

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PostPosted: 23 Aug 2017, 20:54 
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what a disheartening trend of events, but don't worry, even this will pass.


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PostPosted: 23 Aug 2017, 21:28 
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Hi, I expect so. But in the meantime my company has several of the amps out there and if someone needs tubes they may be SOL. Worse we can only sell a few more before we run out of existing stock of tubes. True the amps will run with KT88s and similar tubes. But only ar 25-30 watts not 40-45. Rather dissapointing for the DIY community as well. I will be getting a new quad of tubes from New Sensor shortly to see exactly what they are selling. Cross your fingers they are OK.

Good listening
Bruce

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PostPosted: 29 Sep 2017, 18:05 
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Hi Everyone. This has been a long road and a not so great conclusion. To avoid an legal entanglements I must state that the following is my personal opinion and is not provided to cause any companies or individuals problems. The findings apply to amps of my design and may or may not apply to others. That said:

Here is the scoop on using Tungsol KT120s in class A U/L mode push-pull amplifiers. The company has clearly in my assement changed the design of the tubes. Maybe it will handle the specified dissipation in some modes, but U/L class A is not one of them. All of my earlier amps that have tubes made in the first few years work fine at a combined screen and anode dissipation of 55.5 watts. My personal set has been running at 60.1 watts for about 4 years as a test bed for longevity. They do not “red plate”. FWIW the new tubes in the Oddblocks get really red at the 60 watt level. The very best I can get from the newer tubes is 50.0 watts dissipation. Above that level they all red plate. I ran them for hours at that setting with no obvious signs of color. Ones with higher GM (like 8600 or so) tend to be a bit worse. The ones of recent manufacture that work best have GM in 7400 range (lower probably even better). This is odd because early tubes as high as 8600 were just fine. My sense is that the newer tubes have been optimized for push-pull class AB pentode operation. They would be fine IMO doing that. But any time you use U/L mode and especially in class A they will not work as well. It is likely due to the behavior of the screen. I suspect it has been modified to allow use at significantly lower voltage than the anode. Reducing the voltage on the screen in U/L mode is possible, but it carries a price. The effectiveness of the U/L mode is diminished. It is likely that similar problems will occur with triode modes. A possible work around for both U/L and triode modes would be to run the tubes at lower voltages and higher currents. Operation possibly at about 300 volts for B+ as opposed to the 450 range in my current amplifiers might work OK. A change in the output transformer impedance should also be considered.

A summary of some of the more relevant values I measured during the tests is as follows:

At B+ of 450-462 in the test amp (a stock high output Oddblock) with an eight ohm passive load and 1K HZ test frequency.

KT120s at 117ma each, GM of 7400 matched pairs, Dissipation 50 watts per tube (no red showing)
Max clean output (under 1-1.5%THD) 24 watts RMS, max output (at 4% THD) 40.5 watts RMS, max output at
Soft clipping 48 watts RMS. Please note that these amps soft clip as opposed to many that hard clip.
Frequency response at 1 watt RMS, 8 ohm was +/- 0.5db from 10HZ to 70KHZ, -1db at 75KHZ. The lower -1 db point was below my test set up.

KT120s running as KT88s (everything the same except idle current was 95ma and dissipation 44 watts)
Max clean output was 20 WRMS, max output at 4% THD was 28 WRMS, soft clipping at 37 WRMS
Response was unchanged.

JJ KT88s as KT88s (everything the same as the KT120s as KT88s)
Max clean output was 20 WRMS, Max output at 4% THD was 23 WRMS, Max output at soft clipping 36 WRMS

The discrepancy for the difference in THD for the JJ KT88s was due to the differences in drive requirements. They “bias” at a lower voltage for the same B+ and as a result the input signal will swing below the threshold for the CSS to operate effectively and the result is a small kink in the lower half of the output trace. This could be eliminated in several ways. The easiest is to apply about 4-5 volts of positive DC to each of the grids. This will shift the “bias” point upward and eliminate the problem. This is done in the later EL84 amps of the Oddwatt design. In the High Power Oddblocks since KT88 operation was secondary this was not done so as to not complicate the design. The level of distortion was only slightly higher above the kink, but 4% is the threshold I use for max power output regardless of source or type.

Clearly something has changed, but if you keep the dissipation at or below 50 watts (combined screen and anode) the tubes are OK in this application. Some may show a very little red, but I don’t believe it will be harmful to tube life.

Good listening
Bruce

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PostPosted: 30 Sep 2017, 02:17 
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Thanks for the update, Bruce!
The more I read, the more I love the EL34 ;)

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