Ampeg SVT Classic, new tubes and screen resistors, perfect green bias, no sound

Who installed the tubes and resistors?

I suggest a new tech take a look at it.

 

If i took it to someone and it came back dead,
I believe I'd go back and ask something
like 'Whiskey Tango Foxtrot'.

 

No offense, but the amp should have been tested before it left the shop

 

reputable or not, if it's not making noise after he fixed it, and everything is hooked up, he made a mistake and needs to fix it.

absolutely! and that's true of anything you take to any shop.

 

Why did he change the screen resistors?

Usually, if it's not bad you are better leaving well enough alone...

 

Since the power tube board was worked on, the connecting cable to the output of that board may have not been reconnected. This could be why the amp bias can be set, yet it is dead quiet on the speaker out.

I’m troubled by your comment that the screen resistors were “upgraded”. These resistors are intended to act like fuses and blow open when there is a problem. Putting the wrong resistor in there could result in a costly repair down the road. The spec is 220 ohm, 1/2W, 5%, flameproof.


To debug:

Reset the impedance selector switch, back and forth to where it should be set.

Plug you bass directly into the power amp input jack. You should hear something if the power amp is working.

Connect an instrument cable from the pre out to the power amp in. Test the amp and see what happens.

Connect an instrument cable from the preamp out jack to the power amp in jack of another amp. If you don’t have a power amp in, connect the preamp out into the instrument input of another amp, just for a quick test. Watch the volume levels. This will tell you whether or not the preamp is working.

 

I’ll go along with post #6. If the amp is dead when it should be working, take it back to the tech and demand that he fix it.

You did take the amp out of Standby yes?

 

I'm in the "why even touch it if it supposedly came back from the shop fixed"?
Once you are inside, the tech may claim no warranty.
And by the way; the tech is not good if a non-working, untested amp leaves the shop as a repaired amp.

 

preamp unplugged or something left unplugged. its kinda a pain in the ### to take that chassis apart

 

This is trivial, but sometimes you just forget to do the simplest things, is the cab plugged in, and did you flip the standby switch?

 

A while back (1995 or so) I was convinced my B15S needed new caps, probably some tubes and other work. It had developed a nasty hum and slight crackle. After a day or two of despondency i was going to try one more time and think the issue out so i could describe it to the shop. No audible problems, of course I was using a different instrument cable. Please not that I am not suggesting the issue at hand is that simple, just thinking it could be something relatively simple.

There is a repair shop in the area that does not test their repairs through speakers, they "scope them out". That testing procedure caused me to return an amp for repair and not utilize their services after that repair was complete.

 

Reputable, to me, means he does good work, and cares about his reputation. Part of keeping a reputation is admitting when you missed something, and apologizing for your error. Give him a chance to fix it. If he is reputable, and knows what he's doing, he'll fix it, and apologize for missing whatever it is he missed.

If he won't, then I completely diagree with your assessment of his reputation and skills.

 

Did it work at all when it left the shop?

 

Your tests confirmed that the issue is in the power amp.

The faint signal could mean that a driver tube is not working. On the bottom chassis there are three small signal tubes, a 12AX7 and two 12AU7's. Reseat these to ensure that they are properly inserted into the sockets. If that does't help, if you have spares, try swapping them in. If a driver tube is not working, you might hear a faint signal as you described.

If that doesn't help, the next step is to open the amp and check the cables on the power board and their destinations.

 

If you get signal form the pre and only weak signal from the power in jack try a new 12AX7 in the PI position. The jacks are just ahead of this tube. If the bias lights are functioning properly the issue probably isn't with the 12AU7s or 6550s as an issue with those tubes would cause problems with the bias LEDs.