I changed my RM100 to JJE34L tubes on sunday. I have biased the 6L6's several times since I got the amp and swapped out preamp tubes and know exactly how to bias the RM100. Well, 10 seconds after the 34's are in and the bias is all the way down a fuse blows. I think maybe I have a fuse that was wacky so I replaced it. I started over and no problems after that. Got them set at 38 and played Sunday for about an hour then monday rechecked them and all was good and played for another hour. Then get home tonight(tuesday) turn on the amp, play for 10 seconds, hear a pop, then no sound and another fuse blows. Well now the bias section has no volts and when I try and read it, and the amp has no sound. The amp has full power but no volts running through the bias area and all the tubes are glowing and getting juice. I am fixing to go down the Sam Ash and start yelling about a bad quad of a SUPPOSED "matched' set of tubes just wrecked my amp. Any ideas? Am I missing something?
I think my amp may be dead!! HELP!!
- Thread starter schlagdog
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tehuk
Well-known member
Maybe your output tranny is fried.
Julia
Well-known member
You may have blown a fuse in one of the less accessible places in the amp. You may have fried a screen grid resistor. You'll probably have to see an amp tech.
* I will never put E34Ls in my amp. They came with them. The tubes lasted 3 months. I had a quad in a Marshall last 3 weeks. E34Ls are 25W tubes. The manual has them biased too hot IMO. 33 mV tops. See the JJ web site if you don't believe me: EL34, E34L, KT77 = 25 W max plate dissipation. I've run the calculations. 33 mV is the 70% figure that you really never want to exceed with these tubes in this amp. Yes some people can get lucky and run them hot. To me the difference in the tone isn't worth it. It is very subtle.
* I've seen brand new quads arc on power up. They blew a fuse. They're gone. I will not try them again. They go back to the store as a bad quad.
I will not give a quad of tubes a second chance. If they fail, the bad tubes are marked and if they're in the warranty period they go back. If out of warranty, the bad ones get tossed and the good ones kept for backups and test purposes.
There is a reason a fuse blows. They are protecting other circuitry in the amp. They tell you that you have a bad tube if they're those on the back of the amp. It isn't worth messing with this problem. If I see a flash of light from the tube it's gone. If I hear a fuse pop the offending tubes are gone.
* I will never put E34Ls in my amp. They came with them. The tubes lasted 3 months. I had a quad in a Marshall last 3 weeks. E34Ls are 25W tubes. The manual has them biased too hot IMO. 33 mV tops. See the JJ web site if you don't believe me: EL34, E34L, KT77 = 25 W max plate dissipation. I've run the calculations. 33 mV is the 70% figure that you really never want to exceed with these tubes in this amp. Yes some people can get lucky and run them hot. To me the difference in the tone isn't worth it. It is very subtle.
* I've seen brand new quads arc on power up. They blew a fuse. They're gone. I will not try them again. They go back to the store as a bad quad.
I will not give a quad of tubes a second chance. If they fail, the bad tubes are marked and if they're in the warranty period they go back. If out of warranty, the bad ones get tossed and the good ones kept for backups and test purposes.
There is a reason a fuse blows. They are protecting other circuitry in the amp. They tell you that you have a bad tube if they're those on the back of the amp. It isn't worth messing with this problem. If I see a flash of light from the tube it's gone. If I hear a fuse pop the offending tubes are gone.
Julia knows the deal. I'm running E34Ls pretty hot, but luckily no problem. And I have done a lot of gigs.
I definitely think you got a fuse problem where it's not obvious. I had the same problem and had to get a tech to fix it.
I definitely think you got a fuse problem where it's not obvious. I had the same problem and had to get a tech to fix it.
guitarcomet
Well-known member
Julia is right!
I'll also add that when you go messin' with something that is already working just fine, you're asking for trouble. But guys do it every day.
I'm the type that takes it to an amp tech for anything major that I don't feel comfortable doing myself. (fuses & tubes I can handle)
I've also heard horror stories of 6L6/EL34 switches that ended up "not so good".
Ever since I turned on a defective 2000w Crown P.A. amp and saw/heard it smoke. crackle & pop....I've had a healthy respect (read: Fear) for high powered electronic equipment.
Now, I cringe every time I have to turn a high-powered amp on.
I'll also add that when you go messin' with something that is already working just fine, you're asking for trouble. But guys do it every day.
I'm the type that takes it to an amp tech for anything major that I don't feel comfortable doing myself. (fuses & tubes I can handle)
I've also heard horror stories of 6L6/EL34 switches that ended up "not so good".
Ever since I turned on a defective 2000w Crown P.A. amp and saw/heard it smoke. crackle & pop....I've had a healthy respect (read: Fear) for high powered electronic equipment.
Now, I cringe every time I have to turn a high-powered amp on.
Anyone know if there is an easy way to get to the internal fuses? I don't want to tinker with it but if it's accessible I'd look at the fuses. The amp runs like normal but no volts running through the tubes. They power up but no sound and no volt on the meter. Sam Ash said they would return the tubes but now I'm annoyed I will more then likely have to pay the tech there to change a $1 fuse. For the hour I got to play the amp I think I liked the EL34's better. They were way crisper and tighter then the 6L6's but did
lack a ton of low end.
lack a ton of low end.
Julia
Well-known member
Now this is a two person job. Take the amp out of the chassis and take a look around on the PCB that holds the tube sockets. If you see any scorch marks you've got an amp tech issue.
You'll have to do that to get at the internal fuses anyway. If you've never done it before or don't have anyone available to help you see a tech.
You'll have to do that to get at the internal fuses anyway. If you've never done it before or don't have anyone available to help you see a tech.
I actually took the amp apart an hour ago and found the 2 fuses I blew right behind the MIDI section and fixed it all up. I must have got lucky. because the whole job took me about 20 mins to take the chassis out and replace the fuses and put it back together. I impressed myself at how easy it was. I'm thinking it was beginners luck. My friend told me it unscrewed from the bottom and I unscrewed and got it all set up. After being very annoyed I guess I was very lucky that tube didn't fry anything and just blew 2 internal fuses. Thanks for all the info and ideas on what was wrong everyone. It's good to go so now I just need to return my tubes and I am all set.
Julia
Well-known member
I called Randall at one point to get info on the transformer for calculating bias ranges. They told me the transformer supplies approx 530V. So I got some formulas from the Aiken Amps site and a few others and checked the tube manufacturers' sites.
The bottom end is 50% and the top end is 70% max plate dissipation. One is not better than another, although the hotter the bias the shorter the tube life. I stick with around 60% just because it allows for some wiggle room since bias fluctuates a bit during tube burn in.
EL34/E34L/KT77 = 25W and bias in the range of 24 to 33 mV
5881 should bias in the 25W range
6L6GC and JJ 6L6 = 30W and bias 28 mV to 38 mV
6550 = 35W and bias 33 mV to 46 mV
(I just noticed something interesting -- JJ/Tesla tubes are listed for sale on the New Sensor web site and thus may indicate that JJs are now a division of New Sensor/Sovtek)
If you can get Ruby Tubes they have a 6 month warranty. If you're going EL34 I'd stick with the Ruby winged C from SED in St. Petersburg, available from www.dougstubes.com, or go with their 6550C.
The bottom end is 50% and the top end is 70% max plate dissipation. One is not better than another, although the hotter the bias the shorter the tube life. I stick with around 60% just because it allows for some wiggle room since bias fluctuates a bit during tube burn in.
EL34/E34L/KT77 = 25W and bias in the range of 24 to 33 mV
5881 should bias in the 25W range
6L6GC and JJ 6L6 = 30W and bias 28 mV to 38 mV
6550 = 35W and bias 33 mV to 46 mV
(I just noticed something interesting -- JJ/Tesla tubes are listed for sale on the New Sensor web site and thus may indicate that JJs are now a division of New Sensor/Sovtek)
If you can get Ruby Tubes they have a 6 month warranty. If you're going EL34 I'd stick with the Ruby winged C from SED in St. Petersburg, available from www.dougstubes.com, or go with their 6550C.
