MK3 and gold lion KT88 reissue problems -bad luck or...

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MK3 and gold lion KT88 reissue problems -bad luck or...

Postby junkwaffel » Sun Jun 06, 2010 9:56 am

... something else going on here. One of the KT88s on each amp gone into fireworks mode, first one amp and then a 1/2 hour later the other amp. Line voltage at the time was 124 volts. This happened first thing this morning. The tubes were biased at .7mA each at a line voltage of 121 volts at that time.

Here is the history of these amps since Feb this year:

I installed Shannon Park's Poseidon boards on my MK3s. One of the features of this board is DC balance adjustments - I used 1/4 watt 10 ohm metal film resistors on each socket for individual bias monitoring. The MK3s were originally rebuilt by me back in 2000 including hexfreds to replace the selenium rectifier and new sockets (ceramic with silver plated sockets)as well as the SDS PS cap boards. After the Poseidon installation, I initially ran the amps with a quad of low milage US Tung Sol 6550s without incident. I purchased from Jim McShane 2 matched pairs of the Genelex Gold Lion reissue KT88s and installed them at the end of Feb. Since I am home all day these amps are played typlically 8 hours/day with line voltage between 118 volts and 124 volts (ususally 120-121 volts).

Around mid March, one of the KT88s started to glow bright orange with a nasty hum through the speaker. I turned the amp off and found that one of the bias resistors was open. Which brings me to a question regarding bias resistors acting like a fuse that I will ask later. I emailed Jim on the matter and sent him the pair. Jim replaced the one bad tube free of charge with another and in our email correspondance, suggested that I increase the bias resistor rating to a min of 1/2 watt since he felt that the 1/4 resistors could be stressed when music is played. So I replaced the bias resistors on the one amp to 1/2 watt metal glaze (IRC). I got the pair of KT88s back from Jim and they played flawlessly until now.

A month later, one of the tubes in the other amp glowed bright orange with the nasty hum. This tube had excessive grid leakage and Jim graciously replaced that tube with another free of charge. The bias resistor on the socket of the bad tube measured over 20 ohms so it was about to open. I replaced the bias resistors with 1/2 watt metal glaze IRCs. I got the new tube from Jim and they played flawlessly until now when one of them (the original, not the new tube just replaced) went into fireworks mode just like the other one. Like before, the one bias resistor on the socket of the failed tube measures 22 ohms.

So to recap, 2 months have passed with the amps playing on average 8 hours/day without any issues until today, when a tube on each of the amp went into fireworks mode, a half hour from each other. I will replace the bias resistors, and perhaps I should go with the 3watt metal glaze IRC resistors I have on hand. Of course, I will bench check all voltages but when I did check the voltages before, everything measured fine.

Am I cursed or could there be something else going on? It is quite a coincidence that tubes in both amps would fail the same way within a 1/2 hour of each other, even with a line voltage of 124 volts.

On the matter of the 1/4 watt bias resistors acting as a fuse, if a resistor opens, according to what I read, that tube should "shut down". What does that mean, as in each case when the resistor opened, the tube continued to act up, first with the orange glow and hum and then with the fireworks display. Doesn't seem to me that the whole fuse idea really works. After all, this noise is being played by the speaker attached to the amp and the last thing I need is for my speaker to be damaged. So, what is the expected behavior of the tube when the bias resistor opens?

Jerry
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Postby battradio » Mon Jun 07, 2010 2:37 am

Hi ,

The tube can heater cathode short and the tube can still conduct , that is one of the problems of the 10 ohm resistors in cathode ciruit . When the 10 ohm resistor opened the cathode goes to B+ potental , which is greater that the 200 volt peak heater cathode spec .
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Postby dcgillespie » Mon Jun 07, 2010 6:43 am

Hi Jerry --

A couple of things you might consider:

1. I have not tried the reissue tubes and cannot speak to their durability or reliability, but obviously this is one element to consider. When the MK III was in production, it was never known to have such issues due to it's design, and it's extremely doubtful that the new driver board is causing this concern, so the tubes themselves must be considered.

As for the cathode resistors themselves, 1/2 watt units should be sufficient, and will act as semi-fuses under the most adverse conditions. While it appears that your cathode resistors were certainly damaged, it also appears that they never completely opened either. You might try resistors of different composition to help with this cause. The flame proof metal film resistors will typically burn open every time, rather than merely increase in value as yours appear to have done.

Additionally, if the cathode resistor opens due to a loss of grid #1 bias, the cathode voltage will increase to near B+ levels and damage the heater-cathode insulation as Battradio indicated. On the other hand, if other gremlins are at play and the negative bias is still present when the damage occures, the cathode voltage (when ungrounded) will merely rise up to such level as would normally provide "cutoff" conditions for the tube -- in this case, no more than say +100 volts which the heater-cathode insulation can easily withstand.

2. One very important thing to do when increasing the power supply capacitance as you have, is to include screen stability resistors. These need be no more than about 100 ohms in series with each OPT lead, mounted close to the tube socket of each tube. The original power supply had enough impedance to it to prevent internal screen arcing without the resistors. But when the PS impedance is lowered by way of the increased PS capacitance, screen arcing is always a possibility unless the stability resistors are added to add a little impedance back into the screen circuit. This is especially important in fixed bias amplifiers. Make sure these are included in your amplifiers, as if they are not, that could very likely be the cause of your problems. Good luck!

Dave
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Postby Ty_Bower » Mon Jun 07, 2010 7:51 am

I don't have much to add, other than to say I've been running the New Sensor re-issues in my Mark III for quite a while now. I've got other amps which often come into rotation, so the hours on the Mark III are relatively low. I've not had any problems whatsoever with the re-issues.

I've recently starting running my vintage amps on a bucking autoformer to get the line voltage down closer to the original specifications. My voltage runs high (125~126VAC). I believe the Dynaco manual indicated a design voltage around 117VAC. Running on a high line voltage not only increases B+, it's also going to raise all the heater voltages as well.
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Postby junkwaffel » Mon Jun 07, 2010 2:06 pm

Thanks all of you for the replies. I have been in touch with Jim McShane (whom I bought the KT88s from) and he also suggested the extra screen stoppers on pin 4- he is sending me 470ohm 3watt resistors. This confirms what Dave is suggesting and since I am using the SDS PS board, this most likely what might be happening. Anyway, I ordered another quad of the GLs from Jim and will install the extra screen resistors and keep my fingers crossed.
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Postby Ty_Bower » Mon Jun 07, 2010 3:11 pm

junkwaffel wrote:I have been in touch with Jim McShane and he also suggested the extra screen stoppers on pin 4- he is sending me 470ohm 3watt resistors. This confirms what Dave is suggesting and since I am using the SDS PS board, this most likely what might be happening.


I used the SDS cap board in mine, and I did add resistors at the screens. I think I used a pretty small value (grey-red-black, is that 82 ohms?) only because that is what I had in the extra parts bin. It seemed like good insurance.

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Postby Jim McShane » Tue Jun 08, 2010 8:02 pm

dcgillespie wrote:Hi Jerry --

A couple of things you might consider:

1. I have not tried the reissue tubes and cannot speak to their durability or reliability, but obviously this is one element to consider. When the MK III was in production, it was never known to have such issues due to it's design, and it's extremely doubtful that the new driver board is causing this concern, so the tubes themselves must be considered.

As for the cathode resistors themselves, 1/2 watt units should be sufficient, and will act as semi-fuses under the most adverse conditions. While it appears that your cathode resistors were certainly damaged, it also appears that they never completely opened either. You might try resistors of different composition to help with this cause. The flame proof metal film resistors will typically burn open every time, rather than merely increase in value as yours appear to have done.

Additionally, if the cathode resistor opens due to a loss of grid #1 bias, the cathode voltage will increase to near B+ levels and damage the heater-cathode insulation as Battradio indicated. On the other hand, if other gremlins are at play and the negative bias is still present when the damage occures, the cathode voltage (when ungrounded) will merely rise up to such level as would normally provide "cutoff" conditions for the tube -- in this case, no more than say +100 volts which the heater-cathode insulation can easily withstand.

2. One very important thing to do when increasing the power supply capacitance as you have, is to include screen stability resistors. These need be no more than about 100 ohms in series with each OPT lead, mounted close to the tube socket of each tube. The original power supply had enough impedance to it to prevent internal screen arcing without the resistors. But when the PS impedance is lowered by way of the increased PS capacitance, screen arcing is always a possibility unless the stability resistors are added to add a little impedance back into the screen circuit. This is especially important in fixed bias amplifiers. Make sure these are included in your amplifiers, as if they are not, that could very likely be the cause of your problems. Good luck!

Dave


Hi Dave,

Thanks for helping Jerry out. I've sent him some resistors for screen stopper duty. I like a little beefier part than you do, but we agree on the importance of them being there.

It also made me go back and re-read your Audio Express article - VERY well done, thanks a lot for the effort you obviously put forth to write it.

The KT-88 Genalex is not an unreliable tube - but it is also not immune to the same tube killing forces that have been present since the beginning. The vast majority are providing excellent service and great music. But as you say, when you make changes to the circuitry, you MUST pay attention to the seemingly small details that make or break the results.

Thanks again for weighing in!
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Postby dcgillespie » Thu Jun 10, 2010 6:35 am

Thanks Jim --

The KT88 as originally produced was an absolutely wonderful tube. I have read glowing reports about the sound and build of the new reissue tubes, but know precious little about their ability to act, last, and actually deliver the same measured performance as the originals. Your comment about their reliability certainly helps. Can you shed any light on the other areas? I have some MK IIIs myself that currently still have original production KT88s installed, but nothing lasts forever, so your input would be greatly appreciated!

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Postby TerrySmith » Thu Jun 10, 2010 7:38 am

I can't speak for the Genelex KT88, I can say that the reissue Tung-Sol 6550 seems to be rock solid and half the price of the Genny's. They are biased at 55ma and I'm using 5R4WGB rectifiers to knock down the B+ close to original levels.

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