• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ampeg SVT 3 PRO tube upgrade?

#21
Goingdownslow said:
This would be better over on the relevant thread SVT3Pro Bias Thread as you will see examples of others findings and maybe get a better understanding.

But ok, to check the bias voltage set your multimeter to the milliamps range and take a reading across the legs of the eight ceramic resistors that you will see adjacent to the mosfets on the heatsink.
Click to expand...

Nope, voltage, not current.
 
#24
kgmitchell said:
Checking the resistors for DCV. 25mV being the optimal average voltage per side.
Click to expand...

I assume you've now read all of Jerrold Tiers' posts on this? He designed many of the Ampegs of that vintage, so I would definitely listen to him. FWIW:

Jerrold Tiers said:
The resistors in question are the large ceramic rectangular items located in the middle of the heatsink.....

For the record, I actually don't recommend people do this adjustment..... people who can will know they can, others should probably not........ if you don't watch out, you can do expensive damage.

Without being too picky, may I suggest that people who can go ahead and adjust the bias are folks who would instantly identify and locate the resistors.......... A word to the wise.......
Click to expand...

Best of luck, and welcome to Talkbass.
 
#27
kgmitchell said:
Just fried my ampeg guys, thanks for nothing...
Click to expand...

I'm curious, what did you do to fry your head? I'm serious, I would like to see a description of what you did to destroy your head. It would also be helpful for people having the same type of problem.

I would assume the mistake occurred by over adjustment of one of the pots. Those are VERY sensitive, and that was mentioned several times in this post as well as the other posts that addressed the issue.

I'm not trying to give you a rough time, but I need to say this:
I saw your post this morning. I didn't have time to post a reply, it's very busy in my world and most other people too. I was going to reply after I got home from work, since I have performed this adjustment a few times on the SVT III Pro. When I realized there was a problem with my SVT III head, I read every post on the issue, and I even messaged Jerrold with a question. I took several measurements before before making any adjustments, and when I did make the adjustment, I was very, very careful in not overdoing it. You have to have the meter across the resistor when making the adjustment and take it slow, these pots are very sensitive.

Electronics is like most things, to do the job right, you need to have patients. In the long run, being patient and taking your time will save you in the long run.

I hope your amp repair is not very expensive. I also hope that you will take my post for what it is, an encouragement to learn from this lesson.

Best of luck.
 
#31
kgmitchell said:
Just kidding, set the bias voltage, it's still a bit low but I'm happy with it. I dropped in a new set of tubes. Sounds pretty good... Thanks guys.. lol
Click to expand...

Hey Mitchell, I'm glad you got your amp working and everything came out fine, however, the little thing about patients, it still is valid....

Just something to think about.

Take care and best of luck!
 
#33
Checked the voltage across all eight resistors located along the heat sink. It was low, averaging around .8mV. Shutdown, unplug, adjust trimpot several times before settling on about 22mV average. Installed new tubes, checked again. Full volume a few weeks ago is quarter volume today. Sounds pretty good actually. I'll know for sure tomorrow when I get a chance to push it a little...
 
#38
kgmitchell said:
Checked the voltage across all eight resistors located along the heat sink. It was low, averaging around .8mV. Shutdown, unplug, adjust trimpot several times before settling on about 22mV average. Installed new tubes, checked again. Full volume a few weeks ago is quarter volume today. Sounds pretty good actually. I'll know for sure tomorrow when I get a chance to push it a little...
Click to expand...

Can you give us a walk-thru of the mosfet biasing step by step?

It would be very useful to a few of us who own this amp. If you do post the procedure, I am sure it can become a sticky on the amp forum and a nice contribution to TB.
 

Latest posts

Top Bottom
Back