Help Ampeg SVT-3 Pro - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th July 2011, 08:09 PM   #1
cowpoke is offline cowpoke  United States
diyAudio Member
 
Join Date: Jul 2011
Default Help Ampeg SVT-3 Pro

Gentlemen,

It seems as though I blew up an Ampeg SVT-3 Pro while adjusting the bias.

I shorted the gate of R34 to ground. It blew all of the Mosfets except 1, also blowing R25 and D4, D5. Consequently R32-R38 were gone too.

Now I will be replacing the IRFP FET's as well as other parts. I would appreciate any advice or comments as I usually do not work on SS amps. My plan is to replace all silicon parts in the power amp section, acually all of the parts in red boxes. See attahced PDF.

I will then use the light buld in series trick, with the bias bot all the way down. I will hopefully bring it back to life. Any tips or tricks would be very helpful.
Attached Files
File Type: zip 42601H15.zip (211.3 KB, 105 views)
  Reply With Quote
Old 25th July 2011, 09:04 PM   #2
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Easy done, I dropped a metal screwdriver while adjusting the bias, there were lots of sparks before i could turn it off.
So i bought a plastic tweaking tool.

I just wiped out the output MOSFETs and everything else was OK.
However sometimes the output MOSFETs going can take out the drivers and/or the VAS too.
So I guess what your doing is probably best, to replace all transistors etc.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 25th July 2011, 09:57 PM   #3
cowpoke is offline cowpoke  United States
diyAudio Member
 
Join Date: Jul 2011
Nigel, did you use matched mosfets? If so, how did you match them? I was talking to a local tech and he said that he never matches them, they just pull them from stock and have had good results. I am fixing this amp to keep for myself, so I want to do it correctly. Did I miss any parts that you think should be replaced? See the pdf with all items to be replaced in RED boxes.
  Reply With Quote
Old 25th July 2011, 10:06 PM   #4
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
I have this note from ampeg:

If the mosfets are not matched for their Vgs(on), within 0.3V, they will never share well, and may fail at high currents. That was the factory spec, and the mosfets were sourced pre-matched and color dotted.

Before that procedure was followed there was a too-high failure rate. Once we introduced matched mosfets, there were a lot less failures.


I match them using methods similar to what Nelson Pass and Rod Elliot have posted on their websites. The amp will work just using mosfets out of the tube but...it WILL eventually blow up again especially if you(your customer) plays at high volumes!

I also change the gate resistor from 1K to 47 ohms as suggested by an ampeg tech and they seem to live a lot longer as well. some models have 1K and others seem to have 150, 100, 47 whatever parts they seemed to have laying around that day i guess in them.

ALSO.... I have found on some Ampeg models...that if the BIAS pot is turned too far one way or another they will oscillate and draw power on start up, even blow up if you are not looking for it. be careful. if you see the amp break into oscillation before you get to full line voltage. turn the bias pot a bit and it should go away. you can then power up all the way and set bias.

from Ampeg:


Adjust the trimpot closest to the mosfets for 25mv across the 0.47 ohm resistors in the mosfet source circuit. Average, so check each bank to be sure they are matching pretty well. No signal no load, obviously.

Be sure to have the pot at the LOW end of the bias voltage range to start with, or it will immediately draw a lot of current...........



Best of luck


Zc
__________________
It's not about what you can BUY...It's about what you can BUILD!

Last edited by Zero Cool; 25th July 2011 at 10:09 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Any Ampeg SVT-II (non-pro) schematics??? spirus Tubes / Valves 15 30th May 2010 11:00 AM
ampeg svt 2 pro preamp problem unsolved Tubes / Valves 9 1st March 2010 07:55 PM
Ampeg SVT-VR or SVT-CL Schematics mroovka Tubes / Valves 12 18th March 2007 12:21 AM
ampeg svt-3 pro schematic steven344 Solid State 0 23rd October 2006 12:09 PM


New To Site? Need Help?

All times are GMT. The time now is 09:37 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2