Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 12th September 2012, 04:54 PM   #1
diyAudio Member
 
Join Date: Dec 2011
Default Ampeg J-12T 60's amp issues? Help!

Restoring an ampeg. When playing there is a fizzing that intensifies if I play harder. It also has some slight treble sweeping in the background. I tested all the preamp tubes and they appear to be good. I replaced the 7591 power tubes . The rectifier 5y3 checks out good. I measured all voltages and they seem good, however the V1 preamp tube which handles the trem circuit has an obscene amount of voltage on pin 10 (plate). It says per schematic that it should be at around 130volts- it measures around the full B+ 340volt. The plate resistor gives appropriate value when measured and the coupling cap I think is rated far below the voltage it's seeing. Could that be the issue? It doesn't seem burnt or physically damaged. Any pointers here???
I am wondering why the voltage is 340v before the plate resistor and about the same after it??? Could the value measure correctly and still not perform??
Thanks for the help.
  Reply With Quote
Old 12th September 2012, 05:02 PM   #2
diyAudio Member
 
firechief's Avatar
 
Join Date: Sep 2008
Location: Near Seattle Wa
Try locating the schematic and posting it here. You will get more responses. Also try replacing that subject tube.
__________________
"You can keep your insurance baby nothing is guaranteed" -Tom Petty
  Reply With Quote
Old 12th September 2012, 05:10 PM   #3
diyAudio Member
 
Join Date: Dec 2011
Click the image to open in full size.

Above is the variation on the schematic. Part values are Identical. Voltages are within 10%
  Reply With Quote
Old 12th September 2012, 05:11 PM   #4
diyAudio Member
 
Join Date: Dec 2011
Fire Chief I am led to believe it is in the parts rather than tubes. I swapped V1 and V2 around and got the same voltage issue with the tubes swapped.
  Reply With Quote
Old 12th September 2012, 05:25 PM   #5
diyAudio Member
 
firechief's Avatar
 
Join Date: Sep 2008
Location: Near Seattle Wa
OK Could it be that the tremulo switch is not really grounding the bottom of the 270K resistor in the cathode? Bad switch/connection?
__________________
"You can keep your insurance baby nothing is guaranteed" -Tom Petty
  Reply With Quote
Old 12th September 2012, 09:20 PM   #6
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Blog Entries: 8
Quote:
Originally Posted by firechief View Post
OK Could it be that the tremulo switch is not really grounding the bottom of the 270K resistor in the cathode? Bad switch/connection?
That'd be favourite IMHO

A resistor with no voltage drop says to me the other end ain't grounded.

Chris
  Reply With Quote
Old 12th September 2012, 10:43 PM   #7
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
FUnny voltage on the oscillator tube plate won;t likely make fizzies on your sound. Have you connected the amp to a different speaker? A rubbing speaker voice coil can be described as making a fizzy sound.

However, just to be sure, ground pins 3 and 5 - should already be wired together - of V1 to eliminate any influence from the trem circuit.

I just hate those Joe Piazza drawn AMpegs schematics. I see no voltage references. Are you using a different schematic?

With the trem switch OFF, the tube can;t conduct, so no voltage drop across its plate resistor. Any 130v plate reading would have to occur when the trem was running. With switch off, full B+ voltage would be normal there.

The trem switch shows a 270k cathode resistor. Really? I'd believe a 2.7k. Is yours actually 270k?
  Reply With Quote
Old 13th September 2012, 04:51 PM   #8
diyAudio Member
 
Join Date: Dec 2011
Enzo: I tried a different speaker with the same problem. So I am pretty sure it isn't the speaker. I do wonder if the OT could be responsible.
I do have a schematic on the inside of the amp but it is pretty tore up so I have been piecing it together from different sources. The Schemo internally has most of the voltages posted so I am pretty sure Im close in all the B+ voltages. Should I just start swapping parts? It's a real phantom problem, with my experience being a newb.
I kind of think it might be related to the cathode resistors. It's that weird under/over biased preamp tube sound (fizz). However, I am not sure what that whirling sound in the background is?
The Resistor you speak of is I believe a grid resistor (270k), the cathode in my amp has a 3.3K resistor. And funny you mention that resistor because it is the only non stock part in the amp. So it for sure has been serviced at that point. Should I try to swap it?
Thanks for all the insight everyone.
  Reply With Quote
Old 13th September 2012, 05:06 PM   #9
diyAudio Member
 
firechief's Avatar
 
Join Date: Sep 2008
Location: Near Seattle Wa
re read post #5 and #6. Dont go chasing ghosts.
__________________
"You can keep your insurance baby nothing is guaranteed" -Tom Petty
  Reply With Quote
Old 13th September 2012, 05:27 PM   #10
diyAudio Member
 
Join Date: Dec 2011
Hahaha Firechief you are correct!!! I just thought since she's stock and old, and Im in there, Maybe I should recap it.
Also, the cathode of the suspect tube is showing about 7volts instead of the low voltage that should be present.
  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
60's era op-amp modules? theAnonymous1 Everything Else 6 21st May 2010 04:39 AM
60's Peerless 12" Drivers P Zolt Multi-Way 9 3rd February 2008 12:31 PM
AMPEG amp convert satelitis Instruments and Amps 6 30th March 2006 04:56 PM
Need help for ampeg tube amp transformers ... Ahmad_tbp Tubes / Valves 3 13th May 2005 06:18 AM
Ampeg Amp Value audiopro Instruments and Amps 0 2nd August 2004 03:09 PM


New To Site? Need Help?

All times are GMT. The time now is 07:28 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