Used Ampeg SVT-CL, ~$700- Buy?

I assume that's a 1x15 (not 1x16) cab? Is it 4 ohms? SVTs are designed to drive a 4 or 2 ohm load. 8 is "okay" (according to the designer) if you're not pushing it to high volume. They will put 300W into that load played clean and up to 500-600W cranked into heavy distortion, can the cab take it? Is this the cab you intend to use with it?

For warning signs: get the owner's manual (available from the Ampeg web site) and familiarize yourself with the procedure for checking the bias. If the bias isn't isn't correct according to the lights on the back (after the prescribed warmup) you need to ask the seller some pointed questions. It could need tubes - or it could need more costly repairs.

Start with the gain knob all the way down, the tone controls all flat, and the master up full. Bring the gain knob up gradually as you play. With a reasonably efficient cab it should get "stupid loud" before it starts to distort significantly (this may be limited by your cab, though).

Try adjusting the tone controls, do they work as expected?

Look in the back of the amp while you're trying it out. An orange glow deep inside the output tubes is normal; if the large black structure of the tubes (the plate) starts to glow red, there's something wrong. Note that while you're playing the bias lights will flicker, but when you stop they should indicate correct bias.

Unplug your bass from the amp (nothing plugged into the amp at all). With the master still up all the way, there should be very little hum or other noise from the speaker. There may be slight hum or hiss if the gain knob is turned up. While everything is turned up, rap your knuckles sharply on the case, you may hear this a bit through the speaker, but if you hear it a lot there's probably a preamp tube or two that should be replaced. If a tube is extremely sensitive to vibration (microphonic), it may actually feed back, very unpleasant!

A well-running SVT is a thing of joy for a bass player. I love my SVT-CL, but generally don't get to use it (think wine bars and small brew pubs). I have a 4x10 4 ohm cab for it, it's definitely an outdoor toy.

 

^ @rickdog pretty much covered it. If you do buy the thing make sure you NEVER turn it on until you have the speakers hooked up. Oh yeah , $700 for a decent working Classic head sounds like a pretty darn good deal. I wouldn't sell mine for any less than a grand if I was even thinking about selling it.

 

You fry the power section in pretty short order. You might get a minute or two grace period if you're lucky, but you will trash the tubes and output transformer and pretty much everything else in the power amp by doing it.

 

Tube amps need a load, i.e. speaker, or you overload the circuits quickly.

 

I like to test the amp by letting someone else play while I concentrate on adjusting the settings and listening. Try all tone settings, make sure they all work. Wiggle the knobs while turning. Listen for any out of place noises.

-Ask how old the amp is. Is the seller the original owner.
-Ask if the amp has ever been serviced. If yes, what was done.
-Ask if any of the tubes have ever been changed.
-Perform a sniff test. Strong cigarette smoke is a deal breaker, it is very difficult to wash out.
-See if you can lift the amp.

For your interest, note where the amp was made (in order): USA, Vietnam, Korea, or China.

FYI, a complete retube is quite expensive. Factor the cost in, it may be necessary. It is rare that all the tubes would need to be changed.

www.thetubestore.com - Ampeg SVT-CL Amp

 

This is a very important point. I have a fairly large collection of amps (28 total). About 10 of them have solid state output sections and the rest are all-tube designs. I would estimate that about 60% of my tube amps had problems with output tubes either when they arrived, or within a month of arrival.

Only a few of my amps have had faulty small signal tubes.

SVTs often blow a screen resistor when the tubes fail. If this occurs, a technician will need to go in and remove/replace the faulty part before the amp will bias properly.

At the very least I suggest budgeting for a bench fee and the price of a new set of matched output tubes. Say $240 for the tubes and at least $100 for the bench fee should be in the ball park.

 

When was the last time you checked the battery in your active bass?

If by "break up" you mean power amp distortion, it'll be stupid loud by the time you get there. Or there may be a sudden silence as the voice coil of your single 18 goes flying by....

I like turning up the gain to add some warmth and thickness in the preamp, and use the master to control overall level. This runs the power amp pretty clean, and gives a pleasing, consistent, solid sound at any volume level.