Bright red burning tubes.

[crysky]

I have a Stiletto Trident II and when it is running in 150 watt mode the 3 left tubes glow burning red. The notes all sound like they have a tremelo on them. Plus the amp has a motor boat type of background noise.
This is on the 150 watt setting using the silicon diode rectifier.
Know this all goes away when I put the amp on the 50 watt setting. The tubes glow normally and the sound is normal.
All preamp tubes seem to be fine as in not microphonic and good looking.
Any ideas ???

 

[BobL]

Keep using the amp as it is and kiss it goodbye. What you have is one or more failed or mismatched power tubes. The reason you don't have this issue when running at 50 watts is because the mismatched or failed tubes aren't being used. Replace the tubes with a matched set and you'll be good to go unless you've toasted some power supply componets along the way.

 

[Charlie]

You've got a bad power tube... possibly more than one. If your tubes have seen a lot of use you might want to replace the whole set. If they are fairly new or haven't been used a lot you could probably just get away with replacing a pair.

I'd take all of the power tubes down to a tech and ask him to run them through a tester to find the bad one and replace it.

 

[BMarchant]

Actually guys,
Crysky and I changed all power tubes and even the V5 preamp tube. I'm thinking either a screen grid capasitor, or I question one of the transformers might be bad. The amp actually makes a motorboat thumping sound. Anybody else see this happen? If so, how hard is it to get fixed.

Bmarchant

 

[BobL]

Actually guys,
Crysky and I changed all power tubes and even the V5 preamp tube. I'm thinking either a screen grid capasitor, or I question one of the transformers might be bad. The amp actually makes a motorboat thumping sound. Anybody else see this happen? If so, how hard is it to get fixed.

Bmarchant

Well it would have been nice if you had stated you had changed the tubes earlier. The red glow is the plate in the tube indicating incorrect idle voltage (BIAS) and it could be a capacitor but more likely an open resistor in the B+ side of the circuit. How hard it is to fix depends on the technician working on the amp and how quickly he can find the faulty component. I'm sure by now you've called Mesa (not on Fridays) or intend to call them and ask for the name of a few good repair stations.

 

[Monsta-Tone]

Sounds like your Bias Supply is faulty. Should be an easy fix for a good tech.

 

[domct203]

If the amp is under warrenty then make sure you get it to an authorized Mesa Tech.

TUBE AMPS CONTAIN HIGH VOLTAGE ( LEATHAL )
That's it for the disclaimer. Don't go fry yourself.

It could be an issue somewhere in the bias supply, measure voltage at pin 5, I believe it should be around -50V in 6L6 mode but don't quote me on that. If anything all 6 sockets should give about the same negative voltage on pin 5.

A common result of power tubes failing is that they will take out a screen grid resistor with them (usually located across the inside of the tube socket, or near by). These take the heat when the tube shorts and can fall out of specification easily. This will cause improper function of any power tube you place in the faulty socket -- if the resistor is open, the tube may as well not be in the socket! In any case, the amp will not be reliable until the screen grid resistor(s) have been replaced. The screen grid resistors can be checked using an ohm-meter to measure their resistance. Google their color codes so you know the original value (you can also use measurements from the known good side of the poweramp and compare). The measurement should be within 10% of it's marked value.

Remember, if it's under warrenty you'll need to have a Mesa Tech do the repair.

Dom

 

[Monsta-Tone]

Remember, if it's under warrenty you'll need to have a Mesa Tech do the repair.

Most definitely.


On a side note about power resistors, just because the meter (mine's a Fluke 87 III, which is used heavily by Andy & Ed at THD amps) says that the resistor reads correctly, does not mean the resistor is good.

I just replaced one of the Bias Resistors in my LSS yesterday. It read good, but looked terrible. The amp would crackle at mid volumes when I hit a chord. Replaced the resistor, and the problem vanished.

 

[BMarchant]

Hi guys,
Actually, Crysky posted his information, and I had showed up where the amp was, after he posted. We then went through everything that could be troubleshooted on the spot, like pre-amp and power amp tubes. Put the amp on silicon diod. When that didnt work, I then posted our findings. Its the first time I have seen an amp motorboat like that.

Thanks,

Bmarchant