Ampeg SVT6 Pro trouble with V1....

[Tilman]

Hi,

the volume drops out suddenly and no sound comes out of the amp anymore. Changing V1 (12AX7) helped each time.
The same thing happened 3 or 4 times now with several weeks in between these drop outs.
The tubes taken out measure OK on my tube tester.....
I did exchange the V2 socket (this was my first guess).
The heater for both V1 and V2 seems to by DC with both tubes wired in a row.
The heater to cathode voltage is bellow 25V at each cathode.

The only unusual thing is a 2.5-3V DC voltage at the grid 7 of V1. The voltage between 7 and 8 is -1,2V though. I guess it is somehow biased like a follower with the grid tied to a split R from cathode to ground. The stage amplifies normal.
The amp did never drop out in my workshop. Even if I mistreat the thing and beat it up....it never drops out. The rehearsal room the amp is normally in is quite cold I was told. Maybe a temperature problem ? But what kind of ?

Any ideas ?
Maybe someone had a similar problem with that amp ?

Thank you!
Tilman

[g1]

Changing V1 (12AX7) helped each time...I did exchange the V2 socket (this was my first guess).

Did you resolder or change the V1 socket?

[Tilman]

I did now. In fact I did re solder the whole preamp board (V1+V2) just to make sure there is no cold solder joint.
During the re soldering I did not see any solder joint that looked suspicious...

Tilman

[Enzo]

Changing V1 helped each time. Did you ever try removing V1 and then simply reinstalling the same tube? When repeated tube changes seem to help for a short time, rather than suspect a string of bad tubes, it seems more likely that it was the act of changing the tube rather than the tube itself that helped. You have to take it apart, move things around, things get flexed, etc.


You have preamp out and powr amp in jacks? next time it happens, plug a cord from preamp out to power amp in. If this "external bypass cord" restores the sound, your power amp jack needs service.

FX loop jacks and power amp jacks are THE #1 cause of sound dropping out in amps.


Here is a test. without putting ANY presure on the contacts, measure the resistance betwen the tip contact and its cutout contact on the power amp jack. BY "no pressure" I mean put your probes on the solder pads instead of on the contact themselves. You should measure less than half an ohm - ideally zero ohms. If you measure 1 or 2 ohms ( or more, of course), yes the jack will work, and that small resistance won;t get in the way. But the simple fact it is 1 or 2 ohms instead of close to zero tells us the contacts are dirty or oxidized, and while they are 1 ohms at the moment, tomorrow they could measure a lot higher. Clean them.


Dropouts? DO not assume automatically it is the amp. In your shop are you using the same speaker cord and plugging into the same cab as at the gig? Same axe? At the gig are you using any floor toys or anything else between instrument and amp? ANy of that stuff can become intermittant.

[Tilman]

Hi Enzo,

thank you!
I just did the simple test and the meter shows 0,2 ohm. The amp is very clean inside. Unfortunately it never dropped out in my shop and I told the guy to do the power amp in test when it drops out before. He did not do the test but installed a new tube.......musicians ;-).
Of course I use my own cables, speakers and guitars here in my shop. This is a huge problem. I think I am searching for equipment faults on good equipment 10% of my time because of that.
Tilman

[Enzo]

I'd say that is a fair estimate. I usually estimate 10-15% of the stuff coming in has nothing wrong with it.

Whenever I hear intermittant sound or sound cutouts, I usually invite the customer to bring in all his cords for a quick check, and he is welcome to bring in the cab as well.