Cold tubes in warm amp = smoke?

[20to20]

Originally Posted by bbogus

Thanks all for your insight!
It still seems strange that the EL34's worked fine before this and also that the manufacturer recommends then as a replacement?
I don't know if I'm allowed to on this site but I will attach a link to the amps website so that you all can see what I'm talking about.
If I'm not allowed to I trust that the moderator will catch it.

Link to amp site MP-301 MK2 Mini Tube Amplifier with Headphone Output

It sure looks like a plug and play, anything goes amp from the description.

[tubelab.com]

The manufacturer states:

"One amplifier supports 6L6 (6P3P), KT66, 5881 and EL34."

I know that it is possible to do this, since my SSE supports all of these tubes and the KT88 and 6550. So if the amplifier was properly designed there should be no issue plugging in EL34's.

As for the cold tubes in the hot amp.....The amp has no rectifier tube, so what is different between a cold amp and a hot amp?

You saw smoke...so something fried. That something is still waiting to finish frying. Look carefully for evidence of burnt parts. Does the amp still have a burnt smell inside, or does one of the EL34's smell burnt around the plastic base? A tube could have temporarilly arcced inside partially burning a resistor inside the amp. It will eventually fail. It is remotely possible that some crud inside the plastic base of the tube burnt. This usually happens with tubes that have been exposed to dirt or moisture.

[bbogus]

tubelab.com All valid points. I just finished talking to Gary at paradise and here's a quote from his response:" I can conclude one of the EL34 tubes you are using is defective causing excessive current draw to the amp. The big cathode resistor is heated up and generated some smoke. Since the cathode resistor is very high quality military grade Dale, it still survives. So, please stop using those defective EL34 tubes".
I guess that about says it all. I'm glad it isn't toast and that cold tubes in a warm amp weren't the problem.
Thank you all for your insite!

[tomchr]

Originally Posted by bbogus

" I can conclude one of the EL34 tubes you are using is defective causing excessive current draw to the amp. The big cathode resistor is heated up and generated some smoke. Since the cathode resistor is very high quality military grade Dale, it still survives. So, please stop using those defective EL34 tubes".

I guess that about says it all.

I don't think so. Military grade or not, if the part gets hot enough to emit smoke, it's probably drifted quite a bit out of spec by now. I would measure the cathode resistors (desolder one end and measure the resistance) before I plugged anything into that amp.

I'm a bit surprised the manufacturer actually "debugged" via email. If I was him, I'd have the customer ship the amp in for service before I made any statements about the condition of the components.

~Tom

[bbogus]

Interesting point tomchr. Although I have already used it with the old tubes in it and it seems O.K. do you think I should discontinue and have it looked at?

[Cassiel]

It happened to me with a 2W 100 ohm resistor, there was a short for a short time and the resistor started to smoke. I know you're supposed to change it but I forgot about it. No problems. One day I was doing a mod and spotted a resistor with a black wide stripe in the belly so I remembered the smoke scene. Still, measured OK and I did nothing. Now, I don't know if it drifts its value when it gets warm but I can't hear any difference. Having said this, I would replace your resistor because it's at the cathode of a power tube. You don't want to risk a failure there.

[20to20]

Originally Posted by bbogus

So , long story short, I decided to try the amp with the 6L6's then put in the EL34's while the amp was still warm(but off of course).

I did let the EL34's warm up for a couple minutes but as soon as I turned on the amp, ...

After warm up I turned the volume up a bit and smoke started coming out of the amp!

How did you let the EL34's warm up with the amp, "off?"

[tubelab.com]

Military grade or not, if the part gets hot enough to emit smoke, it's probably drifted quite a bit out of spec by now.

If the cathode resistor was wirewound it is probably OK. I don't use Dale....I use cheap Chinese Xicon. Why? These guys can smoke until the white ceramic turns brown and they don't die. They don't even change value more than 1 to 2% until you abuse them so much that they just go open. I have blown a few, but it usually involves more than 10 watts through a 5 watt resistor.

My previous comments about a smoking resistor was due to my experiences with making my SSE amps compatible with all the tubes listed above. I succesfully smoked the screen grid stopper several times with the EL34. The EL34 is a true pentode and some of the lesser quality versions do not have properly aligned grids which will cause the screen grid to draw excessive current when the amp is overdriven.

I test all my amps with a guitar preamp plugged in and dialed up to 11. if there is a weak link....this will find it. I had to change to 2 watt screen resistors in the SSE because of the EL34.

The cheap Xicon carbon film resistors can be smoked, but still live.....for a while. Then it will start to drift upward in value until it fries to an open. The distortion will increase slowly over time until the resistor runs away in a final puff of smoke and no sound from that channel. No other damage will result, and replacing the resistor will restore life.

[tomchr]

Originally Posted by bbogus

Interesting point tomchr. Although I have already used it with the old tubes in it and it seems O.K. do you think I should discontinue and have it looked at?

Personally, I'd measure the cathode resistors and verify that they're within spec. I would also measure the idle current of the output tubes after they've warmed up for a bit. Those things are easy to do if you have a multimeter.

I've cooked "military grade" Dale resistors. I think I dissipated 2x the rated power in one and it started melting itself onto one of my test leads (bastard!). It did reach the point where a little smoke was emitted and was about 20 % out of spec by then. I wouldn't use that resistor for a cathode resistor...

But I have no idea how out of spec the parts in your amp are - if they are at all. I'd have to have the amp here and measure it. But then, I have the gear, the skills, and am pretty anal about that sort of stuff...

Then again, a multimeter can be had for less than $10 these days. A good one for less than $100 (especially if you can find a good used one). I recommend the Fluke 70-series. It's a handy tool to have.

~Tom

[bbogus]

20to20. I phrased that wrong. I fired up the amp to let the tube warm up and as soon as I did it started to hum.
tomchar. I have a multi meter and know a bit about how to use it(a bit!) . I gather I'm looking for resistance? What should it read and how do I go about doing it? I appreciate any and all help.