Zlofengir
Well-known member
I think I toasted a power tube while playing Ch 1 in Fat mode... The volume dropped, and there was a slight burnt smell; but the amp stayed on.
Thoughts?
Thoughts?
On 10w? Seriously?
- Thread starter Zlofengir
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Zlofengir said:
Were you running standard Mesa 6L6's or EL34's?
Not that it should make much difference, but I understand 34's are biased hotter. Hopefully there is no damage to the board and a simple replacement should fix it :?: :?:
Zlofengir
Well-known member
They were 6CA7's... Fortunately, I had a spare set of 6L6's handy; flipped the bias switch, and everything is groovy. Just had to re-tweak each channel...
Glad it worked out good for you.Zlofengir said:
Out of curiosity, what brand were the 6CA7's?
Dom
Zlofengir
Well-known member
They are Electro Harmonix Dom; great sounding tubes, definitely a different mojo than 6L6's.
Had similar issues with the JJ 6CA7, sounded great but they were toast in 2 weeks of use.
NOTE: You may want to check the screen resistors in the amp. If you know what burning phenolic smells like, and if it smells like that, it is the carbon composition screen resistors that use a phenolic resin as the insulator. What I have seen when the screen resistor blows out is a crack or open split along the body of the resistor. Even after the resistor kills itself, the amp will still work. However, the current flowing though the tube may increase. Tone will also change and red plating tubes will be common place until the resistor is replaced (if damaged). The following image shows the screen resistors after they failed while using EL34 type tubes. The tubes themselves are in great shape, but the screen resistors are not. I replaced them with metal oxide power resistors and so far all is well. Not sure how long the one resistor was compromised but when it blew, based on the smell I knew right away what it was. Amp kicked out for 2 seconds then back on.
Note: there are some tubes that will not survive the 10W mode. JJ EL34L for instance. The bias on the amp is much hotter than most Mesa amps. Fixed Bias for 6L6GC (-51V), Bias for EL34 (-39V). When Compared to the Mark IV the bias is (-57V) for the 6L6. Still have not gotten a bias probe yet to check out the currents. Considering how many tubes I have burned up in this amp (usually at 45W in variac power setting) and the overall tone, the bias is too hot for longevity. Only tubes I have been able to go long term are SED=C=6L6GC or the Gold Lion KT77. All the tubes I have purchased for the Mark V were requested as "for hot bias".
Sooner or later I will end up adding a bias mod to the Mark V. I already know what parts to change. I can either keep it fixed bias or adjustable. I may go the adjustable route. First I need the bias probe than I will proceed with bias fix. It is hard to remove the ice pick tones from the amp while the bias is running on the hot side.
NOTE: You may want to check the screen resistors in the amp. If you know what burning phenolic smells like, and if it smells like that, it is the carbon composition screen resistors that use a phenolic resin as the insulator. What I have seen when the screen resistor blows out is a crack or open split along the body of the resistor. Even after the resistor kills itself, the amp will still work. However, the current flowing though the tube may increase. Tone will also change and red plating tubes will be common place until the resistor is replaced (if damaged). The following image shows the screen resistors after they failed while using EL34 type tubes. The tubes themselves are in great shape, but the screen resistors are not. I replaced them with metal oxide power resistors and so far all is well. Not sure how long the one resistor was compromised but when it blew, based on the smell I knew right away what it was. Amp kicked out for 2 seconds then back on.
Note: there are some tubes that will not survive the 10W mode. JJ EL34L for instance. The bias on the amp is much hotter than most Mesa amps. Fixed Bias for 6L6GC (-51V), Bias for EL34 (-39V). When Compared to the Mark IV the bias is (-57V) for the 6L6. Still have not gotten a bias probe yet to check out the currents. Considering how many tubes I have burned up in this amp (usually at 45W in variac power setting) and the overall tone, the bias is too hot for longevity. Only tubes I have been able to go long term are SED=C=6L6GC or the Gold Lion KT77. All the tubes I have purchased for the Mark V were requested as "for hot bias".
Sooner or later I will end up adding a bias mod to the Mark V. I already know what parts to change. I can either keep it fixed bias or adjustable. I may go the adjustable route. First I need the bias probe than I will proceed with bias fix. It is hard to remove the ice pick tones from the amp while the bias is running on the hot side.
Zlofengir
Well-known member
Someone made me an offer that I couldn't refuse... The V has gone bye bye.
Enjoy your Two Fenders. I thing that was a good deal.
Zlofengir
Well-known member
bandit2013 said:
Trust me, I will... I am.
crane
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Great idea to make an adjustable bias for tha amp. Share some pics if you do it !
I had to order the coldest EH EL34 ever in order to run them under 80% of their max power in the middle slots... My beloved STR-416 where running at 110%....
I had to order the coldest EH EL34 ever in order to run them under 80% of their max power in the middle slots... My beloved STR-416 where running at 110%....
I am still in debate on this. I have not factored in all the variables yet. I believe the -39V for the EL34 is about right. Only way to find out is to get a bias kit with tube sockets. Without measuring current though the tubes, any mods are pointless. The bias mod itself seems easy enough to do but what the overall effect will be is unclear especially in 10W mode. I am only considering a range of -51V up to -57V (Mark IV), All depends on how hot the tubes are as is. I should at least measure the bias voltage to confirm it is where it should be. I do not mind having a fixed bias, but I would like to be able to at least use Mesa tubes without red plating. My RA100 and Roadster are perfect and perhaps a bit colder on the bias then say the Mark IV. The Mark V is a tube toaster. At the moment, I am stuck with using SED =C= or GL KT77. The TAD 6L6GC tube seem to fair well but have not used them long enough. Getting the current measurements would be a great starting point.
Understood, Not an easy amp to work on.
OldTelecasterMan
Well-known member
Hmmmm... I run in 10w a lot. My last 3 jobs actually so that is 12 hours and then practicing between. So far, no problems.
crane
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Got the bias/voltage probe.
I'll take some measures tonight and post the results here.
Probably with the stock Mesa 6L6 and the EH EL34, maybe the Mesa STR-416.
EDIT.
Here we are.
EH EL34, rated 28mA for the inner tubes, 36mA for the outer pair
Outer pair : Va-k = 434 V, Ik = 46mA so plate dissipation around 80%
Inner pair : Va-k = 434 V, Ik = 52mA so plate dissipation around 90%
G2 current can be quite important on a EL34. According the schematic (haven't measured it) it could be between 3-6mA on this amp, so it lower the plate dissipation to 80% and 75%, which is too much anyway.
Mesa 6L6GC AC10 "BLUE"
Outer pair : Va-k = 434* V, Ik = 43mA so plate dissipation around 62%
Inner pair : Va-k = 434* V, Ik = 48mA so plate dissipation around 69%
G2 current is very low in 6L6GC, so the plate dissipation may be 1-2% lower than calulated
STR-416 6CA7
Outer pair : Va-k = 434* V, Ik = 15/32mA so plate dissipation around 26%/55%
Inner pair : Va-k = 434* V, Ik = 64mA so plate dissipation around 111%
One of the outer tube may be bad, the current s very low. But the inner pair run very very hot and red plated in less than 1 minute.
So my conclusion is that the MkV is a EL34 killer with the stock bias. The EH EL34 I have are the lower grade I could find (28mA) and run too hot. The 36mA rated pair is a "Mesa bias selected pair", running at 100-105% in the iner sockets.
I think I'll do the bias mod, only one resistor need to be changed for the EL34 bias without altering the 6L6 position. And another one could be replaced with a trimpot for adjusting the bias to any matched quartet.
*tubes are too big to put the voltage reader, so I assume the plate voltage is around that value (same as the EL34 value)
I'll take some measures tonight and post the results here.
Probably with the stock Mesa 6L6 and the EH EL34, maybe the Mesa STR-416.
EDIT.
Here we are.
EH EL34, rated 28mA for the inner tubes, 36mA for the outer pair
Outer pair : Va-k = 434 V, Ik = 46mA so plate dissipation around 80%
Inner pair : Va-k = 434 V, Ik = 52mA so plate dissipation around 90%
G2 current can be quite important on a EL34. According the schematic (haven't measured it) it could be between 3-6mA on this amp, so it lower the plate dissipation to 80% and 75%, which is too much anyway.
Mesa 6L6GC AC10 "BLUE"
Outer pair : Va-k = 434* V, Ik = 43mA so plate dissipation around 62%
Inner pair : Va-k = 434* V, Ik = 48mA so plate dissipation around 69%
G2 current is very low in 6L6GC, so the plate dissipation may be 1-2% lower than calulated
STR-416 6CA7
Outer pair : Va-k = 434* V, Ik = 15/32mA so plate dissipation around 26%/55%
Inner pair : Va-k = 434* V, Ik = 64mA so plate dissipation around 111%
One of the outer tube may be bad, the current s very low. But the inner pair run very very hot and red plated in less than 1 minute.
So my conclusion is that the MkV is a EL34 killer with the stock bias. The EH EL34 I have are the lower grade I could find (28mA) and run too hot. The 36mA rated pair is a "Mesa bias selected pair", running at 100-105% in the iner sockets.
I think I'll do the bias mod, only one resistor need to be changed for the EL34 bias without altering the 6L6 position. And another one could be replaced with a trimpot for adjusting the bias to any matched quartet.
*tubes are too big to put the voltage reader, so I assume the plate voltage is around that value (same as the EL34 value)
crane said:
If you can try some Red or Yellow Mesa STR440 6L6s. That will get your plate dissipation down to something a little more reasonable. Blue is the second hottest of the Mesa colors, White (which I don't even think I've ever seen) would be the hottest.
