A tube fried my amp. . .??? !!!

pjchappy · « **on:** 7 Feb 2003, 02:04 am »

I have a Foreplay Preamp. . .well, today I switched tubes to a tube I have used before (Tesla ECC 82) just to try something different. Well, when I flipped on my amps after my Foreplay had been on for a minute or so, a horrible noise came out of one of my speakers. . .and then, my amp smoked and then I saw a flash. Stupid me (very, very stupid me--no lectures please), trying to figure out what caused this, switched tubes to the other channel and flipped it on. . .same noise. Luckily, it was only on for about 1/5 of a second. SO, my other monoblock is fine. I took apart my amp and a cap is blown and the metal attachment to its heatsink is MELTED!

The upper right cap (yellow one) is blown/black (it is the output IC). The barely visible little silver metal prongs in the middle that attach to the heatsink (8 of them) are damaged. One is completely melted.
Well, as I said, I switched tubes and the noise/problem followed the tube. Also, the preamp is working fine w/ different tubes into a HT receiver.

I am just wondering why a tube would do this (a 12au7). The tube was purchased new, only has a few hours on it, and looks perfectly fine after a long visual inspection. WHAT THE HELL HAPPENED??!!!!
oh, my speakers seem to work fine. . .luckily. . .

thanks!

p

DTH · « **Reply #1 on:** 7 Feb 2003, 05:11 am »

Sorry I don't have an answer for you. There are some folks here ho have a better idea than me, but you might also want to try the Bottlehead forum on Audio Asylum, there is a whole lot of Foreplay gospel that happens there.
http://www.audioasylum.com/forums/bottlehead/bbs.html

I am very curious to know what happened as I am going to start my Foreplay project soon.

What amps are/were you using?

JohnR · « **Reply #2 on:** 7 Feb 2003, 06:13 am »

Ouch

Don't know what would have caused that... somehow the tube made the preamp unstable - ?

Curt · « **Reply #3 on:** 7 Feb 2003, 07:10 pm »

Is there a DC blocking cap in the preamps output stage?

If not the tube may be putting out a high DC offset which could have been the reason the output IC blew-up.

Also, there maybe an oscillation problem with that tube (the noise you heard) which can destory DMOS transistors, they don't like high freq oscillations.

Xi-Trum · « **Reply #4 on:** 7 Feb 2003, 07:16 pm »

Hey, that looks like an unmodded LeAmp!

pjchappy · « **Reply #5 on:** 7 Feb 2003, 07:45 pm »

What would cause a tube to do that? It is almost brand new and worked fine before. . .

As I said, the preamp works fine. . .I just put in new tubes and am using my HT receiver as an amp until I get my amp fixed. . .

p

tkp · « **Reply #6 on:** 7 Feb 2003, 07:59 pm »

Since the problem followed the tube, the only possibility I can think of is high frequency oscillation. Re-Design said specifically on their website that some tube preamp put out high frequency oscillation that could destroy amplifier.

http://www.redesignsaudio.com/warrexpl.html

DiNo · « **Reply #7 on:** 7 Feb 2003, 08:06 pm »

Quote from: Xi-Trum

Hey, that looks like an unmodded LeAmp!

Indeed, it's the LeAmp. Take a look at the properties of the image

pjchappy · « **Reply #8 on:** 7 Feb 2003, 08:11 pm »

I am just not clear on this issue. . . why would a tube cause this? What could have went wrong with it?

Now, I am afraid of this happening again. . .

p

Xi-Trum · « **Reply #9 on:** 7 Feb 2003, 08:31 pm »

This is the warranty statement from R.E. Designs:

"If the LNPA 150 Amplifiers are connected to tube equipment other than Audio Research, Conrad Johnson, McIntosh, or Audible Illusions the above warranty is null and void."

Wow!!!

I would take a very serious look at the Foreplay preamp.

Is there a way to measure the high-frequency oscillation to determine for sure that that's what is happening in the Foreplay?

MaxCast · « **Reply #10 on:** 8 Feb 2003, 03:25 am »

I'm using a Foreplay with a pair of LeAmps. I'm serarching for new tubes....now I'm worried. You say they worked before hopefully it was just a rogue tube.

pjchappy · « **Reply #11 on:** 8 Feb 2003, 03:42 am »

I hope it was just a rogue tube, too. . .

What sucks is this happened the day after I got my Mensa DI/O from Wayne. . .

If it happens again, I start breaking things and throwing stuff out of windows!!

p

Hantra · « **Reply #12 on:** 8 Feb 2003, 03:50 am »

PJ:

Were they new Teslas? I just bought some NOS Tesla ECC082S from The Tube Store. I like them a lot!

Where did you get the tube? Was it tested, and burned-in at the store?

Sorry about the amp. .

B

pjchappy · « **Reply #13 on:** 8 Feb 2003, 04:00 am »

They were new ones that I got from parts express. . .and as I said before, I had used them for several days before. . .I just wanted to try them out w/ my new Mensa DI/O. ..

Oh, at least the amp will be cheap to fix. . . shipping will probably be more expensive. . .and, I hate waiting!

p

Curt · « **Reply #14 on:** 8 Feb 2003, 04:30 am »

Quote from: Xi-Trum

Hey, that looks like an unmodded LeAmp!

Right on Xi-Trum, good eyes. I didn't even look

This removes the DC issue and points more towards the HF oscillation. If the tube is not a good match for the circuit it can easily become unstable and oscillate. Transistor output stages can do the same thing if the circuit is not balanced correctly.

This problem happens more with tubes because people like to "tube roll" because it's easy, they can not do that with transistors.

Some of the designs out there do not have enough stability designed in to swap major parts, phase margin etc...

This is why many manufacturers don't like customers tweaking everything after they finally get the designs the way they want them.

Electronics desgns are much more complicated than they look, and many times are dependent on the sum of all the components to operate correctly.

Anyway, don't worry we will fix it as good as new. Look at the good side, now you can order the DC coupled Mod while your getting the repair. That is if you send in both channels. Just a thought...