'99 Tremoverb Head Sounds Killer....for about 15 minutes.

[Motocooney]

Messing with a Tremoverb head. Works perfectly and sounds fantastic for about 15 minutes or until it reaches full temp.

At this point, it starts cutting out slightly. Almost pulsating, hissing and thumping. Almost like a bad connection or cable. Harder it's played the worse it gets.
I've checked and swapped all tubes with good ones. Bypassing preamp makes no difference.

Checked for cold solders and bad connections. Reflowed any spot that looked questionable.

When I got it, it had a non-functioning screen grid resistor but was still working.
I replaced all 4 with 1k 5w resistors.


When it starts cutting out and making hiss and thumping sounds, the bias reading from the power tubes jumps up and down +/- 15ma.
The plate voltage jumps abruptly also +/- 50vdc.
When the amp is first turned on and warm the bias is about 32ma and the plate voltage is 470vdc.

Makes no difference what channel or setting the amp is on.

I'm currently running 6CA7 power tubes and Mesa branded 12ax7
I've ordered all new filter caps and mains caps.

An additional issue is that the tremolo doesn't work on either channel. I swapped in a new IC chip today. This brought the tremelo back for a few seconds before it fizzled out and cut all signal.
Switching the tremolo off brings the signal back to normal.

The amp really sings and feels great when it's functioning normally.

Any thoughts???

 

[Motocooney]

Anyone?

 

[thediavlo]

Anyone?

I had a similar problem with my mark V at one point. took it to an amp tech and it was a 5 dollar fix. Can't remember exactly what was wrong with it but I guarantee your local tech can fix it.

 

[Motocooney]

Kinda wanting to fix it myself.
I'll figure it out eventually on my own. But I was just hoping maybe someone had a similar issue with their dual rec or Tremoverb.

 

[Motocooney]

Went through lastnight with the schematic and checked every test point.
Everything checked out. No weird readings. Hmmm.

 

[Shemham]

Have you tried wiggling the power tubes in their sockets when the problem sets in to see if it causes any changes? If it does, the pins could be simply loose and then it would be just a matter of tightening them.

Have you checked the power tube sockets themselves to see if there's any carbon residue between the pins or on nearby traces? That might cause arcing between pins. It'd seem possible with the amp previously having a bad (burnt?) screen resistor.

If you wanted more help, you could take decent photos of the circuit and share them here.