JCM800 50W power tube melt and fuse blow

Neptune

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Hi guys

Dad offered help (much needed) and we spent the entire morning on the 4104: first of all we repeated the test with the tubes in place, the "darkened" JJ was obviously put in the "affected socket", and we saw that after a minute or so the insides of that tube start to glow very bright, which as you all know is not a good thing: in fact, with the multimeter on, we witnessed the failure of the new fuse that I had put in (not a big problem, I had bought a pack of 10). So, here's what we did after:

  • we checked all the pins of both power tube sockets without the tubes in place and measurements were all fine. Then we put the tubes back (and a new fuse, obviously) and tested them all again, flipping the bypass switch on and off to avoid tube melting and fuse blowing while we were measuring all values in both sockets and we found something strange: with the tubes in place, pin 5 (control grid) of the unaffected socket tested fine (minus 40something) while pin 5 of the affected socket tested zero (!)
  • we freed the pcb from the chassis and checked all the capacitors in the power section (first from the soldered pins below the pcb, then we desoldered them one by one and checked them again, then resoldered them): as strange as it may seem, particularly for some of them, even if all are stock from 1986 they were all in perfect working conditions;
  • we checked the output transformer and the total output was fine, and equal on both sides going to the sockets;
  • we found a resistor that seemed a bit ruined on the D.I. input/output jack (that I never used) but it tested fine;
  • we checked the resistors of the power section and they were all good;
After all the testing, desoldering and resoldering we wanted to understand why the plate current probe that I bought (see pics for details) tested zero, so we put the "good JJ" in the affected socket (worst case scenario: new tube burned, but only 20 bucks lost) and a NOS Philips Miniwatt in the unaffected socket: the plate current probe was in series with the JJ, connected to the affected socket. Switched it on and...voilà, it read a plate current. Excessive plate current (60ish). So we rolled back to minimum the bias trim pot and the value dropped to 50ish. After a minute or so, it spontaneously dropped to 30something. The online bias calculator (I put in the type of tube and wattage and the plate voltage measured with the multimeter) said that I should set the bias between 34.88 (60%) and 40.70 (70%), so I set it to 36 and let the amp switched on for like 15 minutes...uneventfully.
No bright glow inside the JJ connected to the affected socket, no fuse blowing, no burning smell---no nothing. And pin 5 measured fine on both sockets.

So, what is/was it, then?

I mean, what is the probability of having a vintage tube (with 35 years of honorable service on its shoulders) fail dramatically, buying a matched pair of new tubes and having the new one connected to the same socket shorted out of the box???

It's beyond absurd, but since everything tested fine inside the amp and now, with different tubes, it seems to work properly I can't think of anything else. Any other ideas?


P.S.: now the plan is to order another matched pair of tubes, putting them in and rebiasing; then, plug a guitar in and see what happens as far as electronics and sound are concerned.
 

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Gunner64

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It Could be 2 shitty tubes, but I still would be suspect of the socket as either being dirty, or perhaps a bad solder connection at pin 5. Myself I would clean the sockets, and reflow the solder connections just for peace of mind.

Good job!
 

Neptune

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Hi guys, I just want to thank each and every one of you for your support, knowledge and participation to my trubles with the 4104...thank you so much! You really gave me plenty of interesting food for thought: I will study more, and hope to be as helpful to other forum users as you've been to me.

P.S.: The new tubes are on their way from Germany to my home: in the end I ordered two matched pairs, one pair of TAD JJ EL34s and one pair of TAD E34L-CZs just to have a spare matched set in case something goes terribly wrong (the plan is to use the JJs to make all the electrical and biasing tests and then put the CZs in, verify the bias, leave them in as the "official power tubes" and keep the JJs as a backup pair).
 

Spanngitter

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E34L-CZs just to have a spare matched set in case something goes terribly wrong (the plan is to use the JJs to make all the electrical and biasing tests and then put the CZs

I do not want to cause some dissappoinment but -CZ denotes these are also JJs. The difference between these is that the JJ EL34 is a normal EL34 whilst the E34L-CZ is a more robust/rugged Version of the EL34. Later one is something I'll usually recommend for modern Amps as it is a tad more stiff in reaction and sounds a bit harsh compared to the EL34, however it whips out a nice amount of power, nearly 20% more then the EL34, and also has a really fat bottom end. They also tend to bias a bit different so there is chance that you might need to modify the bias circuitry to get them into range.
 

Neptune

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Hi guys, tomorrow I should receive the new tubes and I will try to test everything, reassemble the 4104 and I will tell you how it goes.

To Chris: I will follow your advice and tonight I will study lightbulb limiters to protect the tubes (now it is mandatory...see below for details on this)
To Spanngitter: I know the CZ are the "armored" version of the EL34 and that they are stiffer, but I was curious to test them against the JJs (modern EL34s) and the Philips Miniwatt (vintage EL34s) and see/hear how they compare "in the real world". They will need adequate break-in before the comparison and this is why I planned to keep the CZ in the amp.

Anyways, the real reason behind this update in the thread is to tell everyone of you out there reading this to try and buy the tubes you need as soon as possible: with export from Russia blocked (and the chinese factory that burned down last year still closed) it is now very hard to find tubes. I ordered them last week through Thomann (which is a very big retailer) but this week every single brand they usually sell is out of stock, that is to say all "made in Russia" tubes (Electro Harmonix, Tung Sol, Mullard etc.), but also "made in Czech Republic" tubes like JJ and TAD. Hope the info will help!
 

Chris-in-LA

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Hi guys, tomorrow I should receive the new tubes and I will try to test everything, reassemble the 4104 and I will tell you how it goes.

To Chris: I will follow your advice and tonight I will study lightbulb limiters to protect the tubes (now it is mandatory...see below for details on this)
To Spanngitter: I know the CZ are the "armored" version of the EL34 and that they are stiffer, but I was curious to test them against the JJs (modern EL34s) and the Philips Miniwatt (vintage EL34s) and see/hear how they compare "in the real world". They will need adequate break-in before the comparison and this is why I planned to keep the CZ in the amp.

Anyways, the real reason behind this update in the thread is to tell everyone of you out there reading this to try and buy the tubes you need as soon as possible: with export from Russia blocked (and the chinese factory that burned down last year still closed) it is now very hard to find tubes. I ordered them last week through Thomann (which is a very big retailer) but this week every single brand they usually sell is out of stock, that is to say all "made in Russia" tubes (Electro Harmonix, Tung Sol, Mullard etc.), but also "made in Czech Republic" tubes like JJ and TAD. Hope the info will help!
Any luck?
 

Neptune

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Hi Chris! :)

I finally rechecked the amp with the two different tubes and everything was fine; so I switched the tubes and put in the new JJ matched pair and made all the measurements again: everything was right, including the screen grid current in both sockets (pin 5). I set the bias to a medium value (65%), the amp runs fine without any trouble and the sound is really good. In the end, it seems that we should conclude that, as unlikely as it may seem, the problem was probably a faulty tube in the previous matched pair: it is absurd that the problem arose in the same way as the older tube and in the same socket, but I cannot think of any other reason.

As far as tubes are concerned, I will keep the JJs on for a while; I will store the CZs and use them in the future or as a backup. In the meanwhile, due to the shortages in tube supply, I will buy another used-but-new matched pair of tubes for the 4104 (and a matched quartet for the Laney Supergroup) from a guy here in Italy, just in case ;)

Thanks very much for your help and suggestions! Let's stay strong!
 


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