Tube amp (Trace Elliot Hexa Valve) slowly going silent after 30 mins

D

DiMarco

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Feb 4, 2010
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#1
Hi, I bought a Trace Elliot Hexa Valve from 1991 through ebay. It sounds AWESOME!

But there is a problem... After some 30 minutes of running, the input level starts lowering over a period of 15-30 seconds before completely going silent.

The 12 band EQ can still be heard, you notice the hissing when maxxing out the top 3 frequencies so that part of the preamp, and the power section have no problem.

The input level indication leds also go dark so the signal is lowered somehow.
When I switch off the amp for about 30 minutes and then back on, all is working fine again.
Is this some kind of overheating protection kicking in for the two 12ax7 preamp tubes?

The amp sounds really really good and I want to start using it as my main amp, replacing the Mesa Bass Prodigy I also own (the Trace sounds a lot more hefty and less boomy).

Can anyone give me some advice what to do? Thanks!
 
#2
You could try putting in new preamp tubes, since that is not totally expensive. Might work. Otherwise, you will need to get it onto the bench of a qualified tech. Good news is that the circuit is not complicated, highly serviceable, and uses available components.

Feel free to PM if you head down that road and need the schematic. I believe I still have one buried in the files somewhere.
 
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#3
bucephylus said:
You could try putting in new preamp tubes, since that is not totally expensive. Might work. Otherwise, you will need to get it onto the bench of a qualified tech. Good news is that the circuit is not complicated, highly serviceable, and uses available components.

Feel free to PM if you head down that road and need the schematic. I believe I still have one buried in the files somewhere.
Click to expand...
I think, normally there not are pre-amp protection circuits like there are for output stages. But that amp might be different. If there were, I would expect it to cut out more abruptly rather than fade out. I agree about trying a new tube first. Relatively inexpensive and should be fairly easy to do. If it has the original tube, it's probably on it's last legs anyway.
 
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#4
There's two brand new Tung-Sols in there right now. The amp had blown its fuse 5 minutes after I got it. After replacing the fuse, the preamp signal was dead so I replaced the original TE branded 12ax7's with the Tung-Sols.

So with the two TE tubes it is permanently silent and with the two Tung-Sol tubes it goes silent after 30 minutes. I do not have a clue what is going on here but overheating is my gut feeling about this. I am no expert of course.

Really want this amp to work. It sounds phenomenal!
 
#6
DiMarco said:
There's two brand new Tung-Sols in there right now. The amp had blown its fuse 5 minutes after I got it. After replacing the fuse, the preamp signal was dead so I replaced the original TE branded 12ax7's with the Tung-Sols.

So with the two TE tubes it is permanently silent and with the two Tung-Sol tubes it goes silent after 30 minutes. I do not have a clue what is going on here but overheating is my gut feeling about this. I am no expert of course.

Really want this amp to work. It sounds phenomenal!
Click to expand...
Oops! Blowing fuses. Changing tubes doesn't fix. May be more than one problem here?
Does it have a tube rectifier or solid state? If tube (though likely it doesn't) you might want to try changing that too.
You might need a new set of ps filter caps. Something that old, they could be drying out.
If you take her to the shop, send along both sets of tubes so the tech can try with either since it does something different with each set.
 
#8
OK Tech time it is. Thanks for chiming in guys!
I'll let you know how things go.

I also have an RAH600SMX with powersection problems (blows up fuse when the crossover is engaged) and an AH300-12 with a cooling fan and a 50hz buzz problem. Guess the tech will be looking at a package deal.
 
#11
A qualified tech. Will look at the operating conditions by measuring voltages in key locations? Look at signals throughout the signal path before and after the loss of signal occurs. The differences between the two conditions will suggest where the fault(s) are located, and analysis will identify the likely component(s). This is a straight ahead procedure for a professional tech who understands the circuitry he is working on.
 
#12
A qualified tech. Will look at the operating conditions by measuring voltages in key locations? Look at signals throughout the signal path before and after the loss of signal occurs. The differences between the two conditions will suggest where the fault(s) are located, and analysis will identify the likely component(s). This is a straight ahead procedure for a professional tech who understands the circuitry he is working on.
 
#15
Sounds to me that it might be the power supply upstream from the capacitors are cutting out, and that 15 second to no volume is draining the caps.

It's not complex, but there will be some high voltage that presents danger if you don't follow safety procedures.

Looking at a gut shot of the amp, the problem is going to be the sheet metal housing that protects the circuitry for the vacuum tubes, both pre-amp and power amp sections. Plus it looks like the leads from the power transformer are contained in that.

It's going to be a hassle, and will present some safety challenges. But it will be the same for any tech. They will likely up-lift the charge to inspect and repair.

If the amp has external bias test points, I'd use that for the first test.

Don't just ask advice here. I've hung out at AX84 and Metroamp for a decade before going here. There is also plexiplace forums, and others you can ask knowledgeable DIYers and professional techs. Check for a Trace Elliot forum for tech.

The guitarist for Jefferson Starship went from knowing nothing to building one of the best plexi for EVH brown sound for at Metroamp.

Here you're going to get saturated with the following typical replies:

1- Don't work on vacuum tube amps because of the high voltage (repeatedly).
2- Don't work on any amp, but take to a repair shop (repeatedly, obviously).
3- Don't work on vacuum tube amps, but only solid state (probably the most dangerous advice because solid state amps can have dangerous high voltage).
 
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#16
74hc said:
Here you're going to get saturated with the following typical replies:

1- Don't work on vacuum tube amps because of the high voltage (repeatedly).
2- Don't work on any amp, but take to a repair shop (repeatedly, obviously).
3- Don't work on vacuum tube amps, but only solid state (probably the most dangerous advice because solid state amps can have dangerous high voltage).
Click to expand...
Not true. That kind of advice is only given to people who who demonstrate zero knowledge or aptitude for digging into their amps. Show a little aptitude and you will get some great answers by very knowledgeable people.
 
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#17
74hc said:
Sounds to me that it might be the power supply upstream from the capacitors are cutting out, and that 15 second to no volume is draining the caps.

The guitarist for Jefferson Starship went from knowing nothing to building one of the best plexi for EVH brown sound for at Metroamp.
Click to expand...

Classic Ghost Busters.
Filter caps are neither up-stream or down-stream from the power supply, they are 'cross-stream.

Too bad Jefferson didn't remain an Airship.
In space you can't hear them play. Which actually became a good thing.
 
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#19
74hc said:
Sounds to me that it might be the power supply upstream from the capacitors are cutting out, and that 15 second to no volume is draining the caps.

It's not complex, but there will be some high voltage that presents danger if you don't follow safety procedures.

Looking at a gut shot of the amp, the problem is going to be the sheet metal housing that protects the circuitry for the vacuum tubes, both pre-amp and power amp sections. Plus it looks like the leads from the power transformer are contained in that.

It's going to be a hassle, and will present some safety challenges. But it will be the same for any tech. They will likely up-lift the charge to inspect and repair.

If the amp has external bias test points, I'd use that for the first test.

Don't just ask advice here. I've hung out at AX84 and Metroamp for a decade before going here. There is also plexiplace forums, and others you can ask knowledgeable DIYers and professional techs. Check for a Trace Elliot forum for tech.

The guitarist for Jefferson Starship went from knowing nothing to building one of the best plexi for EVH brown sound for at Metroamp.

Here you're going to get saturated with the following typical replies:

1- Don't work on vacuum tube amps because of the high voltage (repeatedly).
2- Don't work on any amp, but take to a repair shop (repeatedly, obviously).
3- Don't work on vacuum tube amps, but only solid state (probably the most dangerous advice because solid state amps can have dangerous high voltage).
Click to expand...
I'm not sure what graphics you are looking at. The QuatraValve had a stainless steel case; but the HexaValve had a more standard wooden rack enclosure. Removing the chassis makes everything quite accessible. The circuit is very simple. I gigged two of them for several years and had to restore one of them. The Tech's comments were that he was inclined to start building his own with that layout, since it was so simple and straightforward. They are great amps.
 
#20
No need to be cynical towards or about other forum members guys.
We all have the same hobby but everyone has their own insights depending on their experience. Not every answer will be the same and the TS (me) can pick bits of information from the several useful answers.

My wage comes in later this week (spent too much on my new car last month) and I will contact a tech in Amsterdam to see if he can help me.

I will use the input from this forum and my experiences with the amp to inform him about the problem. It indeed seems to be some small problem inside the pre section, in between the instrument input jack and the 12 band EQ.
 
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