I need help troubleshooting a EL84 push pull amp. - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 24th June 2005, 08:18 AM   #1
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
Question I need help troubleshooting a EL84 push pull amp.

I've just bought a second hand power stage. The circuit is a push pull with EL84, ECC83 phase inverters and ECC82 pre-driver. There are 2 problems:

A - The left channel is working, but the volume is very low.
B - The right channel sounds normal, but it makes white noise (even with the volume turned down).

I have tried the following:

1 - Check all connections.
2 - Change all valves for new ones: no difference here.
3 - Check all the voltages: they are very simmilar in both channels.
4 - Adjust the bias according to the manufacturer specifications (the instructions are labeled inside the amp).
5 - Remove the ECC83 valves: the noise is still there!. So problem B is not coming from the driver stage; it is some component of the output stage that is noisy (and it's not the valve) any clue here?


Thank you very much.
  Reply With Quote
Old 24th June 2005, 09:56 AM   #2
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
I've made a new test:

I've swapped the right/left output transformers: no difference here, so the two problems are not in the output transformers.

A curious thing: one of the many times that I've switched on the amp, the noise was not there! and it was still working (with the left channel very low, anyway) I've already "touched" all the parts while the amp is working, re-solder all the points to make sure that there are no weak connections, but I can't reproduce that situation again; the noise is back and constant
  Reply With Quote
Old 24th June 2005, 10:02 AM   #3
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
Replace the coupling capacitors on the control grids of the output tubes. It might be a good idea to replace all coupling capacitors in the amplifier.
__________________
Frank
  Reply With Quote
Old 24th June 2005, 10:35 AM   #4
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
Thank's Frank. I'll start with it.

Cheers,
Manuel
  Reply With Quote
Old 24th June 2005, 11:51 AM   #5
Banned
 
Join Date: Jan 2005
It could be a good idea to change all electrolytic caps too.
  Reply With Quote
Old 24th June 2005, 12:16 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
FWIW, the output transformers are the least likely culprits. But I guess you know that now.

After replacing the coupling and electrolytic caps, if things aren't working, it's time to do some basic voltage checks. Maybe even before starting to replace caps... Check the anode voltages on the EL84s. They should be in the 275-330V range. If there's a cathode resistor/electrolytic cap parallel combo in the EL84 cathode circuit, check the voltage across it- it should be something like 10V. Check voltage drops across the plate resistors of the 12AX7s- the voltage drop in volts should be something like 50-100% of the plate resistor's value in Kohm.

A scope will, of course, be a very powerful tool, especially for tracing the oscillation in the right channel.

If you've got an analog voltmeter and know EXACTLY what you're doing, you can load the amp's output with an 8 ohm power resistor as a dummy load, stick the voltmeter across it, then use your finger as a signal source causing the meter to swing when signal gets through. It's easy then to find where the signal stops. If you don't know EXACTLY what you're doing, this test method could cause you great harm, maybe death, but it's an old repairman's trick.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 24th June 2005, 02:40 PM   #7
diyAudio Member
 
pingfloid's Avatar
 
Join Date: Jan 2004
Location: Bangkok
Quote:
use your finger as a signal source


I prefer to use a sine wave from the PC soundcard

I've found the solution to problem A (low volume at left channel):
After long time checking the path of the signal with the scope, it was finally a short circuit at the speaker connector!

Now I'll concentrate fighting problem B. The noise is there about 3 of every 5 times that I switch on the amp (after cooling down)

Thanks for your help
  Reply With Quote
Old 24th June 2005, 05:28 PM   #8
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Its a solder joint, or bad tube connection.

Try wiggling the small signal tubes around with the amp on... Also try tapping them with a pencil, could be a noist tube.

If that doesnt work you could try, VERY CARFULLY, running the amp upside down. CAREFULLY, with a plastic pen or the plastic side of a screwdriver, push on various components until you find the source of the noise.
  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
Troubleshooting Push-Pull 2A3 (please help) tennisets Tubes / Valves 4 26th August 2009 02:48 AM
EL84 push pull slideman82 Tubes / Valves 4 19th June 2009 03:40 PM
modify parallel push-pull EL84 to single push-pull chungtat Tubes / Valves 12 3rd November 2005 11:25 PM
Troubleshooting EL34 push pull amp psysjal Tubes / Valves 12 15th October 2004 04:21 AM
EL84 Push Pull amp. hacknet Tubes / Valves 8 23rd September 2004 05:53 AM


New To Site? Need Help?

All times are GMT. The time now is 01:41 PM.


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