Differing MC275 Output Tube Temps! Normal?

[atkinsonrr]

I just retubed the output tubes on my MC275 Mk VI (see my post on Psvane tubes).

I just checked KT 88 tube temps with an infrared thermometer and temps are all over the place. From left to right, V8 is 130F; V9 is 230F; V10 is 290F; V11 is 321F.

Is this normal? Or do I have a bias problem? Or is this a tube problem?

The vendor tested these tubes before sending out and they are within 5% for Ip.
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[joeinid]

Could the spot at which the tubes are measured vary the output temperature value? I only ask because even on a heat sink for my SS amps, temperature values can vary greatly depending on where I point my infrared thermometer. I assume within the tube itself, hot and cooler spots would be common/inevitable.

 

[mhardy6647]

I don't know, but I suspect that you may be using a tool to ask the wrong question of the tubes.
I'm not sure I'd worry about it as long as the voltages and currents to which all four tubes are subjected (i.e., their operating points) are all correct and the tubes themselves are good.

Were it me, I'd hold my hand over each of the tubes and sense the temperature. If one of them is cold (i.e., 130 degrees vs. 300-ish degrees, 1) it will be obvious. Very obvious. If so -- the cold tube is dissipating no power (that 130 degrees would be from the filament, "warm" not "hot"); it is not working.

In the absence of tube socket 'extenders' to permit easy measurement of voltages, as long as none of the tubes is redplating and both channels sound the same when fed identical signal (e.g., monaural audio) -- all is probably well.

Conversely, a redplating tube would be sign of severe trouble with either that tube or the amplifier and must be attended to immediately. A redplating tube is dissipating far more power than it is rated for; the excess heat causing the plate to glow red hot. Very bad.

Here's a pretty good photo of redplating EL34s (particularly the one on the left, although both appear to be redplating). The tubes are upside-down 'cause this photo is from a guitar amplifier

In fairness, it is surprising how good a push - pull amplifier can sound with one half of the push-pull pair not operating at all. BTDT. But, generally, it still will sound different in a head to head comparison with an amp (or channel) that's working properly.

 

[atkinsonrr]

Thanks for the replies and reassurance.

I did verify that the one cool tube is bad. I cant see a glow and when measured after the amp was on only 10 minutes it measured 96 degrees! So will be returning that one. And it really IS amazing how good the amp was sounding with one tube out! Including the channels appearing completely balanced.

As for the other 3, I was able to measure the tubes in roughly the same spots on the tubes (for sure Joe there is big difference depending where on the tube you measure) and in various spots. After doing it over and over, I'm pretty sure there is a real and not insignificant diff in their temps. I put the stock tubes back in and their temps are very consistent, all right in the 250 range.

And mhardy, I too am doubting that the info is telling me anything meaningful. And I'll take your advice and wont fret about it. But I guess I'm left wondering why tubes drawing the essentially the same current (according to their tested Ip) would vary so much in temperature. Logically it seems there should be a direct relationship between temp and power.

 

[joeinid]

Sorry for your problem but ecstatic that we own wonderful amps that don’t self destruct with a bad tube and sound awesome as well.

Good luck!

 

[mhardy6647]

Thanks for the replies and reassurance.

I did verify that the one cool tube is bad. I cant see a glow and when measured after the amp was on only 10 minutes it measured 96 degrees! So will be returning that one. And it really IS amazing how good the amp was sounding with one tube out! Including the channels appearing completely balanced.

As for the other 3, I was able to measure the tubes in roughly the same spots on the tubes (for sure Joe there is big difference depending where on the tube you measure) and in various spots. After doing it over and over, I'm pretty sure there is a real and not insignificant diff in their temps. I put the stock tubes back in and their temps are very consistent, all right in the 250 range.

And mhardy, I too am doubting that the info is telling me anything meaningful. And I'll take your advice and wont fret about it. But I guess I'm left wondering why tubes drawing the essentially the same current (according to their tested Ip) would vary so much in temperature. Logically it seems there should be a direct relationship between temp and power.

If one tube was dead, the other tube in the push-pull pair is probably under some sort of stress -- to Mac's credit, they have historically run their tubes conservatively, which may be while you'll be OK.

Were I you, I'd be extremely troubled by receiving a brand new and tested (matched) tube from a reputable dealer that was DOA. That doesn't augur well for those tubes (the brand in general or that particular production lot), I'd suspect.

 

[atkinsonrr]

Executive Summary: The Psvanes are now running 9 degrees cooler than the Factory tubes and the variance in their temperatures is the same as the Factory tubes. I think I'm gonna call it all normal, exchange the one bad Psvane and be happy!

If anyone's interested in how I got here, OR if you have a keen interest in tube temps in a MC275 VI OR you're obsessive like me, here's what happened: I decided to check and reset the bias and see the effect. While I had the amp lying on its side and out of the shelf I found I could get a clear shot at the center top of each tube with the infrared thermometer. And this gave more consistent results.

I had it running with 3 Psvanves and 1 Factory tube (in place of the dead Psvane) and took temps before and after resetting the grid bias from where I found it at -54VDC up to -61VDC where it should be (Mac specs -60 to -62VDC). Interesting, the 3 Psvane tubes dropped in temp an average of 18+ degrees after the bias adjust! The Factory tube stayed the same temp.

After the bias adjustment I took the temps of each Psvane a few times 5-10 mins apart, averaged across the temps I got for each tube, then averaged across the three Psvanes. Average temp was 141 degrees F. The spread between the coolest and hottest Psvane was 8+ degrees with the coolest tube running at 94% the temp of the hottest tube.

I then put all 4 factory tubes in place and did the same with them. Average temp across the 4 Factory tubes was now 150 degrees. And, with the more reliable way of taking temps, the Factory tubes no longer looked perfect. The spread between the coolest tube and hottest was 8+ degrees with the coolest tube running at 94% the temp of the hottest tube. There you have it. Based on a set of 2, I think I'm gonna go out on a limb and say some variance in output tube temps is normal and OK.

Oh almost forgot. Someone mentioned operating points as a variable to consider. So I checked voltages at the output tube sockets. Have no way to check current. Control grid voltage was slightly different than the grid bias as measured at the bias checkpoint. It varied from -61.2 to -63. Plate voltage varied closely around 500 volts, from 499.3 to 500.9. The screen grid voltage was only a volt or so less, and spot-on consistent across all four tubes at 498.5. I tried to correlate the small differences in bias voltage and plate voltage with the temp differences but saw no coorelation.
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[sKiZo]

Be interesting to see your setup ... having the tubes all get hotter going in one direction usually indicates some sort of ventilation problem. Lots of obstacles closest to the hottest tube - that sort of thing.

That said, you're talking some pretty radical temp differences ...

 

[atkinsonrr]

Hi Skizo- I think you're on to something. Once I got the amp out of its resident shelf the tube temps measured more consistently. Its an open front but in-wall shelf unit with two fans built in the wall pulling air from the open front and across the amp and preamp (each on their own shelf with their own fan). I thought about that having an effect but the tube closest to the wall with the fans was not always the coolest one, I couldnt see a pattern. But then, air can move in strange and mysterious ways, I guess.

 

[atkinsonrr]

If one tube was dead, the other tube in the push-pull pair is probably under some sort of stress -- to Mac's credit, they have historically run their tubes conservatively, which may be while you'll be OK.

Were I you, I'd be extremely troubled by receiving a brand new and tested (matched) tube from a reputable dealer that was DOA. That doesn't augur well for those tubes (the brand in general or that particular production lot), I'd suspect.

Yep, I hear ya. I am not too pleased about it either. If they were truly tested, (and they looked like they were, with stickers for Ip and Gm on each box) how did I get a bum one? And they were packed well, so why would a tube that's supposed to be a 'premium' tube self-destruct in shipping?

....but DAMN! they do sound good!

 

[mhardy6647]

well, maybe from getting bumped and bruised in transit. Still, I'd expect indirectly heated cathode tubes (like the one you're talking about) to be pretty darned rugged. Heck, they used vacuum tubes in guided missiles in the good old days (no kidding)!

In fairness the guided missile vacuum tubes were generally a bit ruggedized.
Smaller, sometimes, too... there are modern hifi components using such "subminiature" tubes, though

http://www.nj7p.org/Manuals/PDFs/Tubes/Sylvania-Guided-Missile-Tubes.pdf

 

[twiiii]

I could almost always touch the envelope on the KT-88 by Gold Lion on my 275's back in the late 60's and early 70. The 240's with the 6L6GC were the tubes that ran hot, particularly the two tubes near the power transformer.. MC 60's ran pretty cool, too. But you must remember your amps are running at close to 500 volts where mine were around 440 to 450.

 

[atkinsonrr]

Yep, about 500 volts exactly. I'm comfortable with the way the temps are running now (or will be running -- still waiting for the replacement Psvanes to come in). But still can figure out how/why the Psvane tubes came down in temp almost 20 degrees by changing the bias voltage by 7VDC. Meanwhile the stock tube stayed the same.

 

[damacman]

Yep, about 500 volts exactly. I'm comfortable with the way the temps are running now (or will be running -- still waiting for the replacement Psvanes to come in). But still can figure out how/why the Psvane tubes came down in temp almost 20 degrees by changing the bias voltage by 7VDC. Meanwhile the stock tube stayed the same.

Thou shalt not mess with the bias settings without a firm understanding of the outcome ... both short and long term.

You’ve taken a pefectly functioning amplifier, swapped good tubes for defective ones, and have now adjusted the amplifier to work with them and quite likely altered reliability in the process.

This I do not understand.

 

[4-2-7]

I did verify that the one cool tube is bad. I cant see a glow and when measured after the amp was on only 10 minutes it measured 96 degrees! So will be returning that one. And it really IS amazing how good the amp was sounding with one tube out! Including the channels appearing completely balanced.

The MK VI will shut down and not work with a bad power tube, that's what the Sentry monitor does. The small signal tubes light up red in front of a bad power tube and shuts the power off. So how can the amp sound good if the tube is bad and not working?

I decided to check and reset the bias and see the effect

After the bias adjustment

There is no user bias adjustments on this amp, what are you doing?

Lots of assuming I think, You test tubes with a tube tester, not a thermometer. You power tube was fine if the amp was staying on and not tripping the safety shutdown.

 

[4-2-7]

Thou shalt not mess with the bias settings without a firm understanding of the outcome ... both short and long term.

You’ve taken a pefectly functioning amplifier, swapped good tubes for defective ones, and have now adjusted the amplifier to work with them and quite likely altered reliability in the process.

This I do not understand.

Yeah, this is a strange situation that never needed to be in the first place, tube swap or not.

 

[atkinsonrr]

Sorry damacman and 4-2-7 if you've lost the plot. But I obviously did have one bad tube (regardless of the amp not shutting down). A cold unlit tube is non functioning dontcha think? And the Psvanes are not 'defective' tubes, its not all that unusual to get a bad tube now and then. There is a bias adjustment -- sorry if you dont know about it. And I didn't mess with it without knowing what I was doing. The service manual calls for the grid bias to be set at negative 60-62 VDC and that is exactly what I set it at. And that bias setting remains the same with the factory tubes or the new ones, so I dont think your point about setting the bias to work with defective tubes makes much sense. And if you think adjusting to the factory-spec'd bias setting, resulting in a cooler running amp results in reliability issues I'd suggest Thou Shalt better not mess with your bias settings until you have a firm understanding of the outcome...both short term and long term. All in all, seems its not me who's doing "a lot of assuming I think".

 

[4-2-7]

Sorry damacman and 4-2-7 if you've lost the plot. But I obviously did have one bad tube (regardless of the amp not shutting down). A cold unlit tube is non functioning dontcha think?

Either the tube was fine or the amp is not functioning correctly in Sentry Monitor circuit, it's as simple as that.

Show me anything credible stating to know a tube is bad is the temperature it puts out, please...

 

[Pio1980]

Either the tube was fine or the amp is not functioning correctly in Sentry Monitor circuit, it's as simple as that.

Show me anything credible stating to know a tube is bad is the temperature it puts out, please...

Early grid emission bias problems will show up as increased conduction and heat. The only fix for this is a replacement tube.
If you have an IR temp pistol, it wouldn't be a bad idea to occasionally check transformer and tube temps.

 

[damacman]

It would be a shame for a typical McIntosh tube amplifier customer to stumble upon this thread.