The great Sino-Soviet cook-off

I am sufficiently impressed with PSVanes and Black Treasures in the 6SN& holes, it's time for the next jump. MY bias circuit on one of the Holger M-125s cooked four of my favourite Genalex KT-88s to the tune of $400, so I've got four holes to fill. Black Treasures roll in at about the same price, so I'm going to give them a swing. Again, no flies on the Genalex KT-88s; they actually withstood 1.0 v each on the recommended bias setting of .55 or so until one of the bases shattered from the heat, so definitely a component failure upstream.
Since these guys are going on the bench to find the errant component, and not knowing how the other KT-88s weathered this storm it's time to re-tube the whole smash. So I've ordered eight of the Black Treasure KT-88s and provide a sitrep viz the the Russians as is warranted, assuming the M-125s ever get fixed.

Hi Dave,

I'll be interested to know what Holger has to say about the situation there! I hope it's not a Gold Lion issue, as there are a bunch of us who have embraced those tubes for their great sound, but I realize it's not really that long ago that, I think it was, Junky Jan who lead the charge! Jan? Amigo? Where are yu, and are you still using GL kt 88s?

These Black Treasures seem to be the latest tubes to make a big splash. I know that "our friend" Don, as the mafiosi say, is a fan. Not just for the 6SN7, but KT88s too.

Please keep us informed of your experiences. I love the GL kt88, but We're hardly committed in holy matrimony. I'll readily "swap" them for the special delights of Black Treasures like it was the 60's all over again, and I had no Fear of Flying!

I currently have the GL KT88 tubes in my ST-120. So far no problems. My first tube amp is a Yaqin MS30-L that came with blue Shuguang 6CA7 tubes. I decided to splurge and got a quad of Black Treasure 6CA7's for that amp and they are wonderful tubes; lots of bass authority. When the time comes, I will certainly take a long look at the Black Treasure KT88's. As an aside, I have also used some KT120's with my ST-120 and I prefer the KT88's.

There are no flies on the Genalex KT-88 Gold Lions. Best tube I've ever heard. Just tryin' to stretch for puuuurfection. If the Chinese can povide, so be it. Both PSVanes and the Black Treasures make the finest-sounding 6SN7 tubes I have ever heard. Time will be the test. Flash in the pan, or in for the long haul?

Tube Nube wrote:Hi Dave,

I'll be interested to know what Holger has to say about the situation there! I hope it's not a Gold Lion issue, as there are a bunch of us who have embraced those tubes for their great sound, but I realize it's not really that long ago that, I think it was, Junky Jan who lead the charge! Jan? Amigo? Where are yu, and are you still using GL kt 88s?

These Black Treasures seem to be the latest tubes to make a big splash. I know that "our friend" Don, as the mafiosi say, is a fan. Not just for the 6SN7, but KT88s too.

Please keep us informed of your experiences. I love the GL kt88, but We're hardly committed in holy matrimony. I'll readily "swap" them for the special delights of Black Treasures like it was the 60's all over again, and I had no Fear of Flying!

It is EXTREMELY unlikely that four Genalex KT-88s would simultaneously fail. It is also worth noting that I have VERY few that i send out fail as well. In fact it has been a couple months since I've had even one fail.

Jim McShane wrote:

Tube Nube wrote:Hi Dave,

I'll be interested to know what Holger has to say about the situation there! I hope it's not a Gold Lion issue, as there are a bunch of us who have embraced those tubes for their great sound, but I realize it's not really that long ago that, I think it was, Junky Jan who lead the charge! Jan? Amigo? Where are yu, and are you still using GL kt 88s?

These Black Treasures seem to be the latest tubes to make a big splash. I know that "our friend" Don, as the mafiosi say, is a fan. Not just for the 6SN7, but KT88s too.

Please keep us informed of your experiences. I love the GL kt88, but We're hardly committed in holy matrimony. I'll readily "swap" them for the special delights of Black Treasures like it was the 60's all over again, and I had no Fear of Flying!

It is EXTREMELY unlikely that four Genalex KT-88s would simultaneously fail. It is also worth noting that I have VERY few that i send out fail as well. In fact it has been a couple months since I've had even one fail.

Brother James, this has nothing to do with the Gold Lion KT-88s, which I love. They are superb tubes. The bias ran amok on all four tubes in one of the Holger M-125s, indicating either a power supply or bias circuit issue common to all four tubes. I'll tell you how extreme this is: all four bias measurements, set at .55, jumped to 1.10 at the same time. And bless their little hearts, they kept pumping out clean sound, but that's a bit rough even on them. So I'll order a re-supply, but want to check out the Sino tubes as well. Again, some resistor or cap or winding failed, it weren't the tubes!

deepee99 wrote:

Jim McShane wrote:

Tube Nube wrote:Hi Dave,

I'll be interested to know what Holger has to say about the situation there! I hope it's not a Gold Lion issue, as there are a bunch of us who have embraced those tubes for their great sound, but I realize it's not really that long ago that, I think it was, Junky Jan who lead the charge! Jan? Amigo? Where are yu, and are you still using GL kt 88s?

These Black Treasures seem to be the latest tubes to make a big splash. I know that "our friend" Don, as the mafiosi say, is a fan. Not just for the 6SN7, but KT88s too.

Please keep us informed of your experiences. I love the GL kt88, but We're hardly committed in holy matrimony. I'll readily "swap" them for the special delights of Black Treasures like it was the 60's all over again, and I had no Fear of Flying!

It is EXTREMELY unlikely that four Genalex KT-88s would simultaneously fail. It is also worth noting that I have VERY few that i send out fail as well. In fact it has been a couple months since I've had even one fail.

Brother James, this has nothing to do with the Gold Lion KT-88s, which I love. They are superb tubes. The bias ran amok on all four tubes in one of the Holger M-125s, indicating either a power supply or bias circuit issue common to all four tubes. I'll tell you how extreme this is: all four bias measurements, set at .55, jumped to 1.10 at the same time. And bless their little hearts, they kept pumping out clean sound, but that's a bit rough even on them. So I'll order a re-supply,  but want to check out the Sino tubes as well. Again, some resistor or cap or winding failed, it weren't the tubes!

no doubt it will probably end up being something fairly 'simple' which has caused the bias 'runaway'. Once I get my hands on that amp, all will be sorted and fixed, good for many years to come!

I'm in solidarity with you Deepee...  The ST-120 has Shuguang's BT types in all slots (CV-181, 12AU7, KT88-T).   And they take a lot time to run in is my only complaint.  Sound was very strange for the first 50 hours though.  Initially VERY bright sounding but have settled down in the past 2 months.

I will keep you posted; Holger likely will figure it out in short order.
Something was still holding the bias back to about ~1.0 VDC, and none of the four tubes red-plated, nor was there any magic smoke or odour. Had this not been a routine bias check I would have noted nothing amiss.
Two lessons thus far: DO regularly check bias. And the Gold Lion is one beefy tube -- if you really want to get some power out of 'em at 1.0 volt, be my guest. Not sure for how long, though.
I've found the best way to "break in" a Black Treasure is to put them in the back holes of the SP-14 and M-125s until they get the requisite hours, then put 'em in the front slots. The PSVanes (at least the 6SN7 iteration) for some reason sound broken in right out of the box, so I put them in the front holes. After ~100 hours or so, I can't hear a difference 'twixt 'em in the preamp.

Are you checking the bias while music is playing?  That would make make your bias jump.  I thought the best way to check is with inputs shorted.  Or with no signal present.

corndog71 wrote:Are you checking the bias while music is playing?  That would make make your bias jump.  I thought the best way to check is with inputs shorted.

Nope. If the preamp is on I have the selector set to a source that is off. Also check house voltage, too, as that can affect bias as well, though not by much, and check it after the amps have been running for at least an hour; checking them right after power-up will give you a lower reading than after they've warmed up.

I've had those little blue pots go bad on me.

[quote="corndog71"
I've had those little blue pots go bad on me.[/quote]

Four, all at the same time? Doubtful.
Nevertheless, that's one thing I'd adopt on the M-125s as standard; a beefier bias pot rather than the 50-cent Radium Shack variety.
Holger's BB/M-125 amps each have 4 bias pots (one for each tube), hard to believe all four went south at the same time and at the same rate. So I'm back to some common component or voltage issue.
Turns out (according to Holger) there's a 10-turn mil-spec pot that will fit the driver board solder holes, and give much greater bias precision, but the slotted wiper doesn't quite line up with the holes in the chassis as they are off-centre a bit. Worth it to me to drill some bigger holes in the top and use 'em.
This would not be a problem with those using stock VTA/T4HiFi with driver boards exposed. I will have this mod performed even though I don't think the pot(s) were the problem, unless one going haywire would screw the whole bias system up.

I'm curious to know if this was resolved? I'm getting prepared to start my M125 build and knowing any potential problems and their solutions is always a good thing...

Congrats on your purchase of those bad boys, the M-125 twins!

I look forward to your test drive report.

deepee99 wrote:[quote="corndog71"
I've had those little blue pots go bad on me.

Four, all at the same time? Doubtful.
Nevertheless, that's one thing I'd adopt on the M-125s as standard; a beefier bias pot rather than the 50-cent Radium Shack variety.
Holger's BB/M-125 amps each have 4 bias pots (one for each tube), hard to believe all four went south at the same time and at the same rate. So I'm back to some common component or voltage issue.
Turns out (according to Holger) there's a 10-turn mil-spec pot that will fit the driver board solder holes, and give much greater bias precision, but the slotted wiper doesn't quite line up with the holes in the chassis as they are off-centre a bit. Worth it to me to drill some bigger holes in the top and use 'em.
This would not be a problem with those using stock VTA/T4HiFi with driver boards exposed. I will have this mod performed even though I don't think the pot(s) were the problem, unless one going haywire would screw the whole bias system up.
[/quote]

Dave,

The stock VTA M-125's do not use the 50 cent Radio Shack bias pots. They use the more expensive Spectrol (tan colored) bias pots that are "Made in England". IMHO The Spectrol is the best bias pot out there right now. These have been 100% reliable for me. I will continue to use these as long as they continue to be available .. See photo below ...

Bob

Bob Latino wrote:

deepee99 wrote:[quote="corndog71"
I've had those little blue pots go bad on me.

Four, all at the same time? Doubtful.
[/b][/size]

To clarify, I had used a similar pot in a different amp and had it go bad. I've had no problems with the pots in my VTA 120. Didn't mean to disparage your part choices, Bob.

The Spectrol trimmers (50K ohm p/n M63P503KB40) are not stocked at Digikey.  Mouser has them and both places want about $4.50 each.   Allied Electronics has them in stock for $1.28 each.
Spectrol 50K ohm trimmers at Allied Elect

I recently installed some 4-turn trimmers that makes adjusting bias a breeze.  The only problem is that the adjustment is backward - CW colder, CCW hotter.  Almost cooked my KT-88s first go until I figured out I was going the wrong way.  I also had a bad trimmer after installation that would not adjust so be careful.  You can verify installation easily with no output tubes installed and measuring the voltage on the output socket(s) pin 5.  They each should adjust between 0 and -70V with no tubes installed.   And the more negative voltage the colder the bias for starting points.  

When looking for trimmers the pitch between three pins is on 0.1" increments.  Metric versions will be slightly off.

The tranny and chokes are OK, thank goodness. Oddly, no fuse failed, and Holger uses 3a slow-blows on the mains as opposed to the 5as that come stock.
And again, Holger's iteration of the M-125 (Big Boppers) use a bias pot for each tube rather than one for each pair, so you can catch a hinky tube quicker. The obvious (to me) problem is a bulgy cap in the pwr supply, but my local tech friend who has the hood open says that while the cap was at or near failure, that would not necessarily fudge the bias.
Here is my local guy's recent report:  "I see the plate currents drifting up and down between 50 and 150 mA, which translates into a lot of heat at 500V. There's also a 60 Hz "hickey" that I'd have to show you on the scope."
So I've fried four reasonably new tubes and neither he nor I have any clue yet as to why. We swapped out rectifier tubes with the same results, so it ain't them.
Bob, Roy, Holger, Skiz, any bright ideas? Many thanks, And is there preventative maintenance for the other amp should it want develop the same issues at some future point?
Dave

The bulging electrolytic cap is a sign the part is stressed.  Is this cap in the bias supply circuit?  Due to the unregulated nature of these power supplies it would not take much impedance change of the cap to throw voltages way out of whack.

I would remove the tubes/ rectifier and stick a voltmeter on the bias supply to see if its voltage remains constant as the cap warms up.  Freeze spray can be useful in isolating flaky parts but use with caution around hot parts if tubes are installed.

Same can be said of an electrolytic cap in the B+ supply.  Would not take a lot of impedance change to affect B+.  But in this case B+ would be less and more than likely less cathode current.  Really sounds like the bias supply is not stable.

likely either a bad capacitor or a bad solder connection in the bias circuit

Heat (& time) seem to be the constants here. Longer the amp runs the hotter the bias gets. And I don't mean the little bit that it creeps up over the course of a couple of hours, i.e. bias set at 0.5 will creep up to maybe 0.54/tube. But these guys went double and I could not get them down to 1.2/tube, which would be 2.0 or more on a stock VTA amp with one pot per PP line.
To his credit, Holger has offered to make good on his warranty and even replace the tubes, but postage or shipping is a killer. Plus, I'd really like to know what went south.
That fat cap, I think, was a consequence, not the cause, of the problem But ya never know.
Thanks for the input, guys.I'll let you know the outcome.