What would cause a grid resistor to fry

[pbman1953]

Here's I have my Sunn 2000s which has been running great. I wanted to spot check the bias and one tube red semi red plated. I saw it start to " red up" so I turned it off. I moved tubes around to isolate if it was the tube of the behind the socket.


It's not the tube because the issue didn't move. I flipped the amp over I noticed that one of the 5w resistors was discolored. Then I looked at the other 3 , which were fin,to compare color and I could tel it was toast.


What could make that resistor blow?


Thanks

[pompeiisneaks]

Usually when you get redplating, it's one of two things:

1. The tube has gone bad
2. the bias has gone south. 

That could roast a lot on the power tubes components. 

I'd pull all power tubes, and check that you're getting the listed -55VDC at E between the two 100k resistors there.  If not, then you've got something that's failed in that pathway, and I'd check all resistors for being in spec, as well as make sure that both 50uF/150V caps seems to look okay.  Make sure the pot is able to sweep back and forth and get a decent range of negative voltage and set it for the requested -55VDC as a starting point.  Then if that all looks good, it is possible that one or more of the power tubes have failed. 

~Phil

[pbman1953]

I have to say that I have 2 bias pots , 1 per pair.


From the first measurement, one is at - 61.9 and the other is -63. Then i moved each one and they are now matched at -55. No power tubes installed


Let me remind that only one tube is plating. The pairs in this amp are inside pair and outside pair. The issue tube is part of the outside pair   I swapped with another tube from the inside pair and the problem didn't move. The tube that was glowing, after being moved, was fine. I have to report that  I saw a mistake I may of made. Pin 3 is supposed to get the 5 w /42 ohm resistors and pin 4 were supposed to get the 2w version. I had them reversed: 2w on 3 and 5w on 4.


 I'll buy new resistors today and reinstall in the proper locations.

[sluckey]

I would use 47Ω 5 watt for plate and screen. Total of 8 resistors.

[John]

What is the purpose of the plate resistors? Just curious.

[pbman1953]

I would use 47Ω 5 watt for plate and screen. Total of 8 resistors.

On the list, thanks.


From what you've read does anything stand out?  I picked this up Sat morning from a play space, a basement. When used there the amp was amazing. I wanted to use it at home and as soon as I turned it on there was some crackling noise. After checking that the tubes were well set, the 6Kn8 was noisy one. I pulled it in and out a couple times and it seemed to go away. Then I see that one tube lighting up. Then I did some musical chairs with the tubes to see if the problem moved, to se if it was that one tube. But the issue remained in the same spot. Then I decided to report it.

[pdf64]

Are the actual measured resistance values of all 8 47ohm resistors, also the 4 x 1k control grid stoppers, ok?
Their (and the 1k control grid resistor) function is to limit bandwidth and so suppress oscillation; additionally, the screen grid resistors limit current flow and mitigate for excessive screen grid dissipation.

Regarding the red-plating, I suggest to retension the female contacts of the affected tube socket.

That a plate resistor overheats is probably an effect of the red plating, not the cause, ie the resistor gets hot because the tube is drawing too much current.

What idle plate or cathode current was each tube drawing? 1 ohm cathode resistors are extremely beneficial for safely sensing the cathode current.

It would have been useful to see if the red plating stopped with the tube in V2 removed, as that would have indicated that the issue was oscillation.

If the tubes were otherwise running ok before, I really wouldn't operate them with a lower bias voltage without immediately verifying that the resulting idle plate / cathode current was appropriate. The risk being that they might all run too hot.

[pbman1953]

Before I left for work I checked all of the 47ohm - 2W and 5W. Plus I checked the ones on  pin 5, and all they were all good at 1k.


All of the 5W's were all on target at 47 ohm


Three of 2w's were 47-48 and the one under the plating one tube was 50 ohms. I could retention but these are newer , with in 1 year old, Belton sockets. The cathode current for most of them hovered around 59-61ma. I got that with the Bias Rite

[pdf64]

What about the cathode current of the tube in the red plating socket?

[ALBATROS1234]

Too much current passed through it. Lol

[pompeiisneaks]

I'd check the negative bias at pin 5 of all tubes, especially the one that was redplating.  It's possible if it's socket specific that either as was mentioned the socket needs re-tensioning, or the negative bias is good at the pots but somehow is lost by the time it gets to that one tube?   Either way redplating in one socket only is likely the socket or the path of the negative bias.

~Phil

[pbman1953]

I'd check the negative bias at pin 5 of all tubes, especially the one that was redplating.  It's possible if it's socket specific that either as was mentioned the socket needs re-tensioning, or the negative bias is good at the pots but somehow is lost by the time it gets to that one tube?   Either way redplating in one socket only is likely the socket or the path of the negative bias.

~Phil

Tubes out?

[pompeiisneaks]

yes tubes out.  you don't want to risk any more 6550's until you're sure it's getting solid grid bias

[pbman1953]

Just wanted to be sure.


Is there a certain figure you're looking for?

[pompeiisneaks]

The negative bias settings you had before are more negative than the schematic, which means they were biased a bit cooler than spec which is even more puzzling.  Basically make sure that on pin 5 of every tube you're seeing the -55VDC or thereabouts.  whatever you see on the output of the bias pots.  (well it does go through a few resistors so it will drop a bit getting there I'm sure, just don't want it WAY lower).  I am a bit confused that you say there are two pots.  The schematic doesn't show that, but maybe it was a mod?  In the schematic there is only one VR. the 10k above the 18 bias resistor.  At any rate, just make sure you're getting that on the pins.  Also make sure all of pin 5 pins are re-tensioned to ensure the pins are getting good contact to the pins as well. 

Basically if you've not got the right negative bias on pin 5 of those tubes, then the tubes will go into redplating very quickly. Since this only happened on one tube that stays with the socket, and didn't follow the tube, you've likely got a problem with that socket, that pin 5, or the negative grid bias isn't getting to pin 5 somehow

~Phil

[pbman1953]

At pin 5 :


Inside tubes were -49.7


Outside tubes were -50.1


As far as the pot , you're right. A few years back , from the help of thew forum, I was able to install a second pot to separate the pairs.


Compared to the -55VDC , you're right, but I'm using KT120 tubes which is a much stronger tube and say it draws more than a 6550. I'll take a look at the individual socket and try to tighten.


Thanks!

[pompeiisneaks]

Cool so it looks like you've got the right bias coming to the tubes.  I'd ensure the tube pins on that socket are good, and if it's still happening, you may also want to clean the socket out a bit.  some deoxit and a dental scrub pick may clean it out.  Either that or deoxit in the socket and push in and remove a tube a few times to clean it out. 

[tubeswell]

That a plate resistor overheats is probably an effect of the red plating, not the cause, ie the resistor gets hot because the tube is drawing too much current...

Agree. It's a bias issue on that socket (especially as the OP swapped the tubes around and the issue remained on the socket). Check socket pin clamp tension.

[pbman1953]

Welllll,


Since I made the tests and they seem to come back ok, I tried to clean the sockets and attempt to retention. The Beldon sockets I have are the fork type, not sure if I explained that properly. They are not circular sleeves.


I installed the tubes last last night and there were no issues. I'm guessing some friction , cleaning & tensioning that I had better connections. I run a dehumidifier in my basement and I'm sure if the other place did. Maybe that had an effect.




So now my voltages are around 505 and cathode is reading 77. The highest tubes is reading 77ma. Sorry I forgot to get a negative bias reading. I can come back with that.


Do any of you use this site for biasing?


https://robrobinette.com/Tube_Bias_Calculator.htm




Thanks for the help, now onto installing the new resistors.

[pdf64]

Adjusting bias to achieve some specific plate dissipation seems wrong headed to me, and nonsensical if, as in your case, much beefier tubes have been used than the amp was designed for.
Plate dissipation spec is just a limit, not a characteristic.
Aiken suggests the 'idle up to 70%' figure as an upper limit, not a target, in fixed bias AB to avoid breaching the limit at higher signal levels http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent
People seem to have taken that and read way more into it than was said.

[sluckey]

In conclusion, Aiken has this to say, "If the plate load impedance is not known, setting the bias to 70% of the maximum allowable dissipation is usually a safe spot for a class AB amplifier, as shown by the graphs.   This "rule-of-thumb" will work in most cases for commercial guitar amplifiers."

I think that is the 'take home' info. If you only remember one thing from today's lesson, let that be it. 

[pompeiisneaks]

Adjusting bias to achieve some specific plate dissipation seems wrong headed to me, and nonsensical if, as in your case, much beefier tubes have been used than the amp was designed for.
Plate dissipation spec is just a limit, not a characteristic.
Aiken suggests the 'idle up to 70%' figure as an upper limit, not a target, in fixed bias AB to avoid breaching the limit at higher signal levels http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent
People seem to have taken that and read way more into it than was said.

Although what you're saying is 100% true, also the overall tube life gets shortened the higher the bias, so basically, a ton of touring musicians that love to bias hot just bring along a lot of extra tubes knowing they'll eat them like candy.  I can't recall who I'm thinking of but I read or watched a video about a player that had his amp tech buy surplus power tubes because he loved the tone of his amps at about 80-85% diss and they just swapped tubes every week or so because they had a few blow somwhere near that timeframe so they're beating them to the kill point.  So yes, an exact bias isn't a huge deal, but knowing what it is, and understanding the consequences is a good idea :) 

Also older tubes could take a lot more abuse than current new ones can. 

~Phil

[Ritchie200]

In conclusion, Aiken has this to say, "If the plate load impedance is not known, setting the bias to 70% of the maximum allowable dissipation is usually a safe spot for a class AB amplifier, as shown by the graphs.   This "rule-of-thumb" will work in most cases for commercial guitar amplifiers."

I think that is the 'take home' info. If you only remember one thing from today's lesson, let that be it. 

I don't claim to know a minutia of Mr. Aiken or anyone else around here for that matter.  However, the tube manufacturers also lay out their operating limits to produce the best possible water pump based on the time tested 70% rule and have done so since the beginning of time. Now it's suddenly irrelevant? I would also question Aiken's program parameters.  What wave frequency is he basing his graphs and conclusions on?  Reproducing a pure tone is quite different than the complex waveforms of amplitude and frequency of a guitar or "Hi-Fi" or even vocals.  So unless you are playing Gypsy's, Dead and Gone (listen to that song on utube if you don't get my drift) every night, 70% or more will probably be fine depending on what country you bought your tubes from.  As Slukey suggested, after all that gyration and warnings of the article, Aiken finally admits that at 70% you may be popping above the limit on occasion but should be ok....  Ok.....?  Seems like a lot of mud in what was a pretty clear well defined stream.  As Pompeiisneaks said, many artists (with money) run their tubes "over spec".  Heck Ritchie Blackmore ran his Majors over 30% higher than spec.  They actually did just fine.  I understand the education, just not sure about the content on that one.  Just MVHO.

Jim

[sluckey]

Yep. I know Richie likes it hot. I heard that he was actually involved with the fire at Montreux!   

[pdf64]

Crikey, that 70% kool aid sure is a winning formula!
I couldn't have asked for better examples of the wrongheaded thinking I previously referred to.
Please note that the OP has put KT120 in an amp designed for 6550; does idling them at 70% really seem like a good idea?
Best keep the fire extinguisher handy!

[sluckey]

In conclusion, Aiken has this to say, "If the plate load impedance is not known, setting the bias to 70% of the maximum allowable dissipation is usually a safe spot for a class AB amplifier, as shown by the graphs.   This "rule-of-thumb" will work in most cases for commercial guitar amplifiers."

I think that is the 'take home' info. If you only remember one thing from today's lesson, let that be it. 

[Ritchie200]

pdf64, I'm not sure what you are inferring? You are calling everyone out for a "nonsensical" practice and then post a link that supports that same "wrongheaded" practice - however convoluted that journey was. What % kool aid would you ascribe to? I'm just having a hard time understanding what is so horribly wrong with this apparent mythical starting point (and a historically generally accepted one at that - by designers and tube manufacturers) for getting the maximum safe performance out of a tube over average operations?  You state that plate dissipation is just a limit, not a characteristic.  Well it most certainly IS a characteristic of performance.  Government Motors would have a hard time selling Corvettes with a 20mph top speed. Or MO Valve with a KT88 that puts out a whopping 2 watts but costs as much as a 6550.

As far as the OP, he has been running these tubes for over a year.  Many other 2000s owners have been doing the same. This grid resistor issue is not the norm, but the exception.  As we have seen, it was isolated to a possible component issue.

Sluckey, I have never seen or heard of any fires associated with Ritchie Blackmore.  I have no idea what you are talking about.  Please get your facts straight.  You are turning into SG.

Jim

[shooter]

"nonsensical" practice

IIRC there was a track & driver write-up on some new beemer, a note to buyer says something like; "6 figures and NO cupholder"!  guessing that's where it comes from 
oh, don't worry, SG can't be emulated 

[sluckey]

Everybody knows how handy he was with a flare gun.

[pdf64]

...What % kool aid would you ascribe to?...

The 70% thing seems to have become a mantra to such a point that it may work to prevent critical thinking; folks just seem to focus on the tube's capabilities and ignore that it's a component in a system.

...I'm just having a hard time understanding what is so horribly wrong with this apparent mythical starting point ...

The idle current resulting from 70% of a 60W tube will be a lot higher than 70% of a 35W tube; my concern is regarding that increased loading on the amp's power supply.

...a historically generally accepted one at that - by designers and tube manufacturers) for getting the maximum safe performance out of a tube over average operations?...

I've not seen any references to the 70% thing predating the 90s, or from a tube manufacturer; I'd be grateful if you could point me at them.

...the OP...has been running these tubes for over a year.  Many other 2000s owners have been doing the same...

OK, thanks, I wasn't aware of that.

[shooter]

Pdf, what's in the background of your avatar, I keep getting flashbacks of the monument to discoveries in Lisbon.

[sluckey]

There was no mention of 70% biasing back in the '60s. Back then only techs worked on this stuff. And a bias check simply meant set the bias pot for the voltage shown on the schematic, or follow the manufacturer's bias instructions, if any. The 70% deal is a more recent development. It gives a way to establish a baseline that has been universally accepted. I like it.

You also have to consider that today for every good technician out there, there are 1000 boneheads that don't know which end of the iron is hot but they want to bias their own amp. And roll tubes through it. And buy fancy bias probes to use with their $10 harbor freight DMM. So 70% bias point has opened up a new market so that anyone can buy some bias probes and use an online bias calculator or phone app and tweak their amp for that perfect tone, which BTW, lives on the bleeding edge of tube performance, not back down the hill in safe, sensible tube operation.

So let's all get on board the 70% train and go roast some mushrooms down at Montreux. Richie will be there with his flare gun.

[shooter]

there are 1000 boneheads that don't know which end of the iron is hot

I use the lawyers by the minute pricing for them 

[pbman1953]

My ears were ringing when I read Sluckey's last one. I'm guilty and I'm a weekend warrior repair dude. But I love it so it didn't hurt. I understand his point and appreciate all the help the forum gives out. I really do  It's a great community of some talented technical people.

[sluckey]

I like to think that you have risen above bonehead. 

[pdf64]

Pdf, what's in the background of your avatar, I keep getting flashbacks of the monument to discoveries in Lisbon.

We took a big vacation to the far east a couple of years ago, for my wife's 50th. I found that statue / monument just outside the centre of Hanoi's old town area. It's in the soviet iconic style, celebrating the proletariat taking up arms and freeing themselves of the colonising oppressors 

[shooter]

HA.
the last time I seen this place was 40yrs ago, but every time I see your background I "see" the guy on the bow 
the brain, a great thing to F$%#K with 

[pbman1953]

I like to think that you have risen above bonehead. 

I've graduated! 


I do have to say, and make a small plug, that we do have a very cool small electronics company here outside of Boston in Needham.. You Do It Electronics has been at the same location for almost 40 years. A full row of small parts from resistors   caps and tools and beyond . Solder stations and meters and test equipment  A fun place

[sluckey]

That does sound like a fun place. That reminded me of a mechanic shop in Savannah called shade tree. 6 bays with lifts, all the tools you would likely need. And a well stocked parts store in front with specialized tool rental. You provide the labor. Beats doing a brake job in the driveway.

[pompeiisneaks]

That does sound like a fun place. That reminded me of a mechanic shop in Savannah called shade tree. 6 bays with lifts, all the tools you would likely need. And a well stocked parts store in front with specialized tool rental. You provide the labor. Beats doing a brake job in the driveway.

Savannah GA?  I was stationed near there in the Army.  In Ft Stewart GA.  I loved Savannah, such a cool city.

sorry major thread derail lol.

~Phil

[PRR]

> I've not seen any references to the 70% thing predating the 90s

+1.

> it may work to prevent critical thinking

So are we talking "well designed" amplifiers?

I've tried to debunk "70%" and found that, for the general class of amplifiers we use for stage-amps, competently designed, there's no good reason but it is almost always a fine starting point.

An "objection" is that OVER-volted amplifiers will have crossover distortion at 70% or even 100%. I have had an amp cranked-up to 140% before the crossover really got small. And as it was a hi-fi amp, I could hear the sound improve. Started to smell. Didn't leave it that way.

OTOH we have "throw together" amps. Some experimenter puts KT-190s or HK257 in a 6V6 DeLuxe. Dialing-up to 70% of 90W will sag and *smoke* the power supply. Power output actually less due to PS sag. (However crossover will be nil....)

Saw another, real-world, "experiment". Someone found a tube intercom. It ran one of the usual 110V 9W radio tubes, capable of 2W, at low voltage and high impedance to get a half-Watt. Working near 30% Pdiss. He thought there was more potential on tap if the idle heat was higher.

[Ritchie200]

Page 2. Circa 1959


https://frank.pocnet.net/sheets/084/k/KT88.pdf


There is a hot end of a soldering iron?
Jim

[Ritchie200]

To Slucky’s point I remember reading this years ago!  I was hoping it was still out there....


Jim


http://www.marshallforum.com/threads/biasing-70-vs-90-vs-plate-current-vs-cathode-current.10890/

[pdf64]

Page 2. Circa 1959
https://frank.pocnet.net/sheets/084/k/KT88.pdf...

If referring to the note '*A negative bias range of 70+-25% is recommended' I'm afraid you've misunderstood; from a modern perspective I understand how it might cause confusion, but it actually means that the bias voltage, nominally -80V, should be adjustable over the range -87.5 to -52.5V. It should be adjusted to achieve the stated 50mA plate+g2 idle current.
Consider, how could a negative % of plate dissipation be possible?
I suspect that the '70' in the note is probably a typo, and should actually be '80', as per the Vg1 (approx) figure in the table above.

The same note can be found in most all vintage manufacturer's KT88 info for 'typical operation' fixed bias operating conditions, but is usually worded better, eg see https://bms.isjtr.ro/sheets/086/k/KT88.pdf
'*It is essential to provide two separately adjustable bias sources, having a voltage adjustment range of +-25%'.

Also see the note in https://bms.isjtr.ro/sheets/084/k/KT88_GEC.pdf at p23 of the pdf (p11 of the Circuit Supplement) '*A bias voltage range of at least +-25% is recommended'.

[SILVERGUN]

There is a hot end of a soldering iron?
Jim

Yeah, if you used one in this century you might remember

[shooter]

wow, you woke up and joined the party, now I can go outside and play 

[Ritchie200]

Pdf64 If interpreted as you have, if nominal is ~81 (which is actually 70% if I am correct) and you have stated 70 as a misprint, yet the numbers work. You say adjustable between 52 and 87?  I’ve run the Major over 90 on old tubes and Ritchie even higher (or lower!) on an amp designed by the THIS tube manufacturer without modifications to the bias circuit.  The KT66 data sheets references +-25% of “this value” (-Vg1). I don’t know. I guess we could both make a convenient assumption based on our views.  Is it coincidence tho that the nominal figures stated by the manufacturers work out to ~70%?  Hey I’m here to learn. If my assumptions have been wrong all these years, I want to know why.


SG it sounds like you have a fever. And the only prescription is NOT a cowbell.... Please reference my Colaboration tune for some guidance to change your outlook on life.


Jim

[shooter]

I’ve run the Major over 90

you know this is an apples n oranges debate, engineer vs artesian.  The 2 cannot by all laws of physics and meta-physics EVER agree 

[jojokeo]

All this KT88 and Blackmore talk...you are insufferable. But what else is new...oh wait! You have a soldering iron???    Now that you mention it I overheard your wife the other night mentioning you only putting out 70%? 


Your pal keo 

[jojokeo]

Pdf, what's in the background of your avatar, I keep getting flashbacks of the monument to discoveries in Lisbon.

On my iPhone I thought they were statues of Dorothy 👠👠and the cowardly lion 🦁 ??