Ampeg SVT CL trouble on tour

[HappyTom]

Hi guys!
Long time no see =)

I'm currently on tour and have a amp that dont behave.
Two days ago the amp started to sound wierd with alot of hum and random crackle, bias controls was impossible to get right so we used the spare amp on that gig. Yesterday I bought a new set of power amp valves and when I've put em in Bias 1 I had to crank all the way to get a green light, Bias 2 is acting normal. I thought there would work so the first two bands used it but when the third band flicked the standby on we blown the fuse, they used the spare amp.
Today i had some time to tinker with the amp so I switched the three power tubes on the left with the three on the right to see if the problem was a power tube but no difference.
The 12ax7 and 12au7 is brand new aswell.

Anyone who has any thoughts about this?
We have a day off tomorrow so there might be some time to look at this.
We are going from Wales down to France tomorrow and then we will go to Bordeaux if anyone who has a shop can help out.


Cheers
HT

 

[okgb]

Sounds like you'll have to get into it , open it up , look for anything obvious like a loose / broken  / burnt parts
maybe a resister has gone out of spec drifted in value from overheating  that  you can't get it with in range ,
just guessing . I don't think it's any fun getting into those things , Good luck !

 

[hobiesound]

on all the svt classics i service the resistors on pin 4 of the 6550's go out of spec or open over time. this will screw up the bias on that particular tube. When i replace those resistors the bias control will work again. The resistors i'm talking about are in parallel with the 1n4007 diodes and are usually mounted hovering on the 6550 pcb.  Value may depend on the revision (22 - 47 ohm on most).

greetings,

Thomas

 

[schmidlin]

In my experience, you're going to need to pull the 6550 board.  It's hairy, just make good notes on the cable hook-up.

Most often I see a hair-line crack in the solder where the tube pins meets the board.  Inspect closely, or just reflow them all.  I saw one that evidently developed a hairline crack on pin 3, and the arcing melted ALL the solder off.  Yikes.

Or a tube took out a screen resistor.  Check your tube pin tension, too.

Best of luck and stay safe around those things!

 

[CJ]

if this is the old svt then r35 and r36,  they need to be exactly 1.00 ohms each,

if the amp is not balanced it will self destruct because of the power,

i have the calibration procedure if you need more help,

but this is for the old stuff,

 

[domi62]

Hi HT

In Bordeaux you can call Guitar Shop ( +33(0)557 966 464 ) and ask for the guy who repairs the electronic stuff they sell, I can't find his phone number. Sometimes ago I brought him an Acoustic 320 bass head which needed to be serviced. As I was looking for an SVT he told me to avoid chinese ones as he had a lot to repair. Maybe that man could help you.

Cheers
Domi

 

[HappyTom]

Hi guys!
The last days we had no internet connection so I could not work on it. Got my dimm back home which make this a little difficult as i cant measure the resistors. We are still using the spare amp.

Today i checked in with my phone which may be kinda expensive haha, I  tried to find bad solder joints but didn't find any obvious so I resoldered all the solders on the left side of the board. This gave nothing.

The amp is not old, 10 or 15 years maybe which rules out CJ's tip, thanks though!
The amp on this site have exactly the same pcb's. http://www.bustedgear.com/repair_Ampeg_svt_classic_shutoff_5.html#TOP

I borrowed a meter in Wales a couple of days ago and measured the big resistors and they seemed fine.

The resistors that are in parallel with the 1N400* diodes I haven't checked yet though, might try that tomorrow if i get some spare time! Must buy a dimm...

Theres a tour schedule here www.tormented.nu if any of you are on that rout and want to make some extra money and know something about these amps.

Thanks
HT