Rebuilding a classic Ampeg SVT bass amp

[Ohioan]

In 1984, at the age of 17, I bought an Ampeg SVT from someone. I used it up until around 1991, and then set it aside. Over the last ten years it has been sitting in a barn, unfortunately. Today I retrieved it and removed the chassis. Here is a link to an album of photos I took today:

https://www.flickr.com/photos/194366...77720323179456

As can be seen, the chassis is a little rough, and there is remnants of a mouse nest.

The 6550 tubes were purchased in 1991, and have zero hours on them. Meaning... after I installed them, I never powered the amp.

My goal is to get this thing back to as close to 100% as practical. I am not in a hurry to do it, and I don't want to cut corners. And while I have next to no experience with guitar amp diagnostics & repair, I am well acquainted with electronics and testing. (I'm an EE and have been doing electronics-related stuff, both professionally and as a hobby, for 45 years.)

I have an ESR meter and decent DMM, and plan to measure the parameters of each component. For leakage current on some of the caps, I can borrow a high voltage power supply from work.

Am looking for advice, suggestions, etc.


Thanks,

Michael
Saint Paris, Ohio

[The Dude]

Too bad a mouse decided to make it home. The first thing I'd do is give it a good cleaning. Mouse pee is corrosive. The quicker you clean it up, the better!

[glebert]

I personally never bother measuring component values until I know if something isn't working right. For most amps (after cleaning) I will power them up slowly on a variac and light bulb limiter. The longer the amp has been sitting the slower I power it up. I will use small light bulbs at first, when I know it can get to 100% wall voltage with one size bulb I go up to the next size and slowly power it up again. For big amps I pull the power tubes at first. If I can get to 100% voltage with a big bulb (shouldn't glow much with no power tubes), I will repeat with power tubes, usually adding one pair at a time, but that may not work if your SVT has the tube protection circuitry.

If when powering up you see the light bulb flickering, hear popping sounds, etc, then back down a bit on voltage and see if it stabilizes, maybe even go back to a smaller wattage bulb.The goal with bringing it up slowly is to not slam the electrolytic capacitors with a huge voltage and current spike, and there is a notion that if the capacitors have dried out and gotten weak that limiting the current through them (that is the variac and light bulbs job) the capacitor can "reform" and perhaps be usable instead of blowing out completely. I recently did a Hiwatt DR103 that had been sitting for 25 years with the above procedure and it powered up just fine (though no mice on that one).

[J M Fahey]

I have an ESR meter and decent DMM, and plan to measure the parameters of each component. For leakage current on some of the caps, I can borrow a high voltage power supply from work.

Don´t.

You will cause way more damage or create new one by pulling and replacing parts for no good reason.
And roughly 40% of problems are NOT caused by "bad parts" anyway.

Ï personally never bother measuring component values until I know if something isn't working right.

THAT.

Instead, try to (carefully) turn amp ON, as described above, plug and instrument and play.

IF there is some kind of problem, post it here.

Just as an example: would you *fully* disassemble a car and measure ALL parts against Factory specs to make it roadworthy again?
How much of an error/tolerance would you accept?

What if all parts are fine, only there is a timing misadjustment or something?

Wouldn´t it be better just to oil it, charge battery, fill the tank and just try to start it?
And THEN decide on what to check, according to symptoms?

Same thing.

[drewl]

Looks like most of the Ampeg's I get in.

Probably works fine but may need filter caps.
Good luck!

[Tom Phillips]

Michael,

I have serviced several classic SVTs and I agree with the advice already posted. Start with careful cleaning including all tube socket pins, switches, pots and mechanical ground connections. Initial power up is best done slowly (over a few hours) with a variable line transformer ("Variac") without the tubes installed while monitoring the line current and the B+. Monitor the B+ voltage to insure that you don't over voltage the filter caps since the voltage could rise too high without the load of the tubes.

Before you start powering and testing the unit, obtain and study the proper schematics until you have a good understanding of the circuit operation. There are also adjustment notes on the SVT schematics. Many members here at MEF will also be able to answer any questions that come up. A search should find many past discussions containing valuable information. Use Google advanced search with "https://music-electronics-forum.com" entered in the "Site or domain" field.

Enjoy the experience and protect your back.

Cheers,
Tom

[g1]

Use Google advanced search with "https://music-electronics-forum.com" entered in the "Site or domain" field.

Off on a bit of a tangent here, but the 'search' box at the top of the page is vastly improved from what it used to be. I urge everyone to try it out, I don't bother with the google advanced search anymore if I want to search just on this forum.
(and there is a second 'search' box a little further down the page that will only search within the thread, don't use that one if you want to search the whole forum)

[Tom Phillips]

... the 'search' box at the top of the page is vastly improved from what it used to be...

I just tried it and Wow, "vastly improved" is an accurate description!

[g1]

I just tried it and Wow, "vastly improved" is an accurate description!

Plus, if you click on the arrow next to the box, you can click on 'advanced' and specify particular members, date range, etc.
I find it especially useful if I can't recall exactly what I posted about something previously.