What would cause a grid resistor to fry

[pdf64]

Pdf64 If interpreted as you have, if nominal is ~81 (which is actually 70% if I am correct)...

The nominal Vg1 in that table is -80V?
Note that both the June 1961 and Dec 1972 GEC KT88 info show the same 'typical operation' for fixed bias UL 560V HT 4k5 p-p load 50mA p+g2 idle current as now having an approx Vg1 of -75V.
With the plate and g2 at 550V, and plate + g2 current at 50mA, the total idle dissipation looks to be 27.5W, which works out to 67% of the 'plate + g2' max rating. Unfortunately the actual plate current isn't separated out, and the ratio of 'g2 current to plate current' tends to increase as plate current increases but at these idle levels it's reasonable to assume the plate dissipation is also around 67%. Which is a long way of saying yes 

...You say adjustable between 52 and 87?...

Just to be clear, -70V +-25% works out to -87.5V at one extreme and -52.5V at the other.

...I’ve run the Major over 90 on old tubes...

Sorry, do you mean that your amp idled at 90% plate dissipation or at -90V grid bias?

...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...

Yes, thanks, assuming that was 90%, it lines up with JUne 1961 and  Dec 1972 MO/Genelex/GEC KT88 info (previously linked to), which shows 'typical operation' table for fixed bias non UL with 560V HT as idling at 60mA plate current; that works out to 33W = 94% of the 35W plate rating!

...Is it coincidence tho that the nominal figures stated by the manufacturers work out to ~70%?...

No, if the designer's goal is to get high AB1 power at reasonable distortion, using a typical tube HT supply with a bit of sag, then the idle operating point tends to result in an idle plate dissipation of around 70%.

[pdf64]

...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...

Overall, I think that the 70% thing has been beneficial, as it's enabled musicians / folks without much competence in electronics to get some sort of a handle on a key aspect of the maintenance and operation of their tube amps; that's lead to a secondary beneficial effect of then creating a marketing point for amp manufacturers including bias test points.

Downsides include a muddled / confused understanding of tube operation (eg thinking that tubes work differently as plate dissipation increases) and terminology ('bias current' being a bugbear of mine).

For amps which have been designed to get a lot of AB signal power output from their power tubes (eg the EL34 in JCM800 1987 and 1959), idling them at 70% does tend to result in an appropriate operating point. However, where the manufacturer has apparently elected to try and get a longer power tube life by not squeezing the most AB signal power output from them (eg 6550 in JCM800 1987, 6L6GC in most 'AB763' models especially Tremolux / Vibrolux), idling them at 70% tends to work against that manufacturer's goal.

There's an interesting note 'Life Performance' on p4 of this KT66 info http://frank.yueksel.org/sheets/086/k/KT66.pdf
ie operational life should increase 25% if (continuous) dissipation can be limited to 73% of design max rating (~60% of abs max).
Also it's the only tube 'golden age' technical reference to some % of plate dissipation (well, plate + g2 total dissipation) I've yet found.

[pbman1953]

I think my overheated tube has become a casualty. I've put everything back together, plus installed the 4 new 47 ohm / 5 watt resistors.

I'm using the Bias Rite only to measure cathode current because the meter reading off the BR is not as accurate.

So my outside pairs are super close , with in 1 watt. My inside pairs are off by 8 watts

The nice thing is the amp is super quiet.

I may have to get a new tube

[PRR]

> 'Life Performance' on p4 of this KT66

Interesting.

I'll note that 8,000-10,000 hours is a VERY long time for a guitarist. If you play 40 hours a week (factory full-time) it is 4 years. Surely you earn enough in 4 years to pay for another set of tubes. Many players can't find 4 hours a week, so 40 years, which for many of us is a lifetime. Today's teenagers can buy another set in mid-life. However Golden-Age GEC is NOT the slap-together crap on the market today.

[Ritchie200]

Downsides include a muddled / confused understanding of tube operation (eg thinking that tubes work differently as plate dissipation increases) and terminology ('bias current' being a bugbear of mine).

Ok now you are describing me!   Muddled and Confused, isn't that a Led Zep song? (Sluckey don't even think about it!!!!)


BTW -90V for good old real Gold Lions. As PRR said, they certainly don't make them like that any more.


I wonder at what volume they figured "Life Performance"? Most performing guitarists will abuse their amps pretty good. I remember when the Scholz Power Soak came out all these bedroom guitarists were complaining that it was defective as they had to suddenly start changing power tubes and it was dangerous to use!!!!      That is a lot more abuse than listening to the Boston Pops on your Dynaco in your den.  I went through lots of tubes when I played out but the amp design beat the crap out of the screens and I played almost wide open into some soaks.


Jojo, give me a break - that is the ONLY amp I know anything about!  How do you think I knew to reference a 60yr old data sheet?


pbman1953, If your red plate tube is one of your inside pairs, time to order a new one.


Hijack...
PDF64, In Hanoi did you get to the "Freedom" museum?  I've always wondered how I would feel walking through that or the Vietnamese Air Force Museum.  Lots of Made in USA stuff on display.


Jim

[pbman1953]

I'll hunt around for another pair.


Now I need an opinion.


The picture I sent shows the rear tube with a unblemished base. The foreground tube has blemishes (rough to the touch) at the top of the base next to the glass.


Three tubes have this issue and one doesn't. The tube that has no blemish is not the over heated tube.


Please let me know if this is a concern


Thanks

[pdf64]

...I went through lots of tubes when I played out but the amp design beat the crap out of the screens and I played almost wide open into some soaks...

Yes, the greater the degree of overdrive, the greater the screen grid dissipation rockets up. More sag in the HT supply to them / higher value, beefier current limiting resistors to them are a big help in keeping the tube alive.

Hijack...
PDF64, In Hanoi did you get to the "Freedom" museum?  I've always wondered how I would feel walking through that or the Vietnamese Air Force Museum.  Lots of Made in USA stuff on display...

Would that be the Military History Museum? We had a good trek around that, my wife's tolerance for museums runs out fairly quickly but she did well and we saw most of it. The outside exhibits made the biggest impression, especially the aircraft wreckage https://en.wikipedia.org/wiki/Vietnam_Military_History_Museum#/media/File:Aircraft_wreckage.jpg
As a westerner, I felt very conflicted, torn between the awful experience and death those guys suffered, but also the horror they were delivering to others. Really that war unleashes hell onto the world (most especially the ordinary folks that get caught up / drawn in to it), can't be predicted or controlled, and that everything feasible must be done to avoid it.

[Ritchie200]

pbman1953 - that almost looks like a crappy plating job more than anything.  It may also have something to do with the process when they bond the base to the envelope?

Jim

[pbman1953]

Here's a response from a reseller-




"That coating on the metal base of the tube does not affect tube performance. As long as the vacuum is in tact (you would know if it wasn't because the tube would not function) and the internal parts are secure the tube should be fine."

[jojokeo]

What could make that resistor blow?

I came in late to bust Ritchie200’s balls but do not see anything regarding checking for DC leaking coupling cap(s)?!


Very common on older amps. Once DC gets through and turns the grid positive the bias gets thrown off leading to high currents, resistors heating/blowing, tubes red plating, and even taking out the OT with them.

[pdf64]

If a coupling cap was leaking dc, the issue would be likely to affect both tubes fed from that cap.

[jojokeo]

Sure as long as each set of tubes is fed by a single cap? In my book, never assume pdf - especially from correspondence on this site. Most things are never as simple as one might expect?

[sluckey]

I bet he has looked at the schematic.

[shooter]

Most things are never as simple as one might expect?

yup, I was doing fine for 30yrs, wound up here to find electronics in whole new analog light 

[jojokeo]

I was simply pointing out the obvious in a way that others might learn with a similar issue and don’t believe I saw anyone mention prior?

[sluckey]

Sure as long as each set of tubes is fed by a single cap? In my book, never assume pdf

That's not an assumption. That's a fact.

[pbman1953]

My new pair came in yesterday. After a warm up I got these figures


Voltage avg of all 4 was at 485 volts




Cathode average was 81


I didn't get a chance to really play through it yet. I'd think it should be good.

[Ritchie200]

pbman1953 - It sounds like you got this one! 

What could make that resistor blow?

I came in late to bust Ritchie200’s balls but do not see anything regarding checking for DC leaking coupling cap(s)?!


Very common on older amps. Once DC gets through and turns the grid positive the bias gets thrown off leading to high currents, resistors heating/blowing, tubes red plating, and even taking out the OT with them.

Hey!! Like I always say, I've never done anything here to deserve any retaliatory action!   If you have a problem with: my tele aversion, me making fun of SG's Flock of Seagulls hair, Cubs suck, and the Beatles were the most overrated band in the history of man, oh and the ST. LOUIS Rams were the greatest of all time - then that's your problem.

But as much as I hate to admit it....  If someone has this problem and does a search "What would cause a grid resistor to fry", this thread will probably pop up first.  The solution for this amp was only one possibility. Jojo's comment is good info for everyone. 

However, anything else he has to say, especially about my soundclips, you can ignore.

Jim

[jojokeo]

Sure as long as each set of tubes is fed by a single cap? In my book, never assume pdf

That's not an assumption. That's a fact.

I bet he has looked at the schematic.

Geeze, for some reason you feel the need to stick up for pdf with your snarky comments or did you just wake up on the wrong side of the bed yesterday?

[jojokeo]

pbman1953 - It sounds like you got this one! 

What could make that resistor blow?

I came in late to bust Ritchie200’s balls but do not see anything regarding checking for DC leaking coupling cap(s)?!


Very common on older amps. Once DC gets through and turns the grid positive the bias gets thrown off leading to high currents, resistors heating/blowing, tubes red plating, and even taking out the OT with them.

Hey!! Like I always say, I've never done anything here to deserve any retaliatory action!   If you have a problem with: my tele aversion, me making fun of SG's Flock of Seagulls hair, Cubs suck, and the Beatles were the most overrated band in the history of man, oh and the ST. LOUIS Rams were the greatest of all time - then that's your problem.

But as much as I hate to admit it....  If someone has this problem and does a search "What would cause a grid resistor to fry", this thread will probably pop up first.  The solution for this amp was only one possibility. Jojo's comment is good info for everyone. 

However, anything else he has to say, especially about my soundclips, you can ignore.

Jim

Ha! Innocent little angels by your name indeed!  Rather than having one on each, the little devils I think are always on BOTH of your shoulders?! >:) I think SG, Brad, and even Georgia herself (if alive, thank god she isn't) would all agree?!

[sluckey]

Geeze, for some reason you feel the need to stick up for pdf with your snarky comments or did you just wake up on the wrong side of the bed yesterday?

A little of both. Been like that a lot this year. You took a jab at pdf even though his analysis of your statement was right on. Your statement was true but in this case was not well thought out.