Help needed: hum noise in Eimac 250TL driven by 75TL Single End Amplifier - 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 3rd May 2012, 09:41 PM   #1
diyAudio Member
 
Join Date: Jun 2010
Default Help needed: hum noise in Eimac 250TL driven by 75TL Single End Amplifier

.

Last edited by HP8903B; 11th January 2014 at 01:01 AM.
  Reply With Quote
Old 4th May 2012, 05:26 PM   #2
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
that must be a seriuos amp. maybe filaments (AC or DC)? or bad ground wiring? power transformers polluting a too close OPT or other signal transformers?
  Reply With Quote
Old 4th May 2012, 08:56 PM   #3
diyAudio Member
 
Join Date: Sep 2004
Location: Oregon
Does the amp hum when the input is grounded? If no then try another source since your generator may be causing the problem. If yes, then things get more interesting. Go through the amp and re-solder the all ground connections, input, output, especially if the output transformer output winding ground is bad you get all sorts of weirdness going on and all of the connection on all four tubes. Those tubes run very hot and cook solder joints until they get bad. As Gluca said check all wiring to make sure you don't have an A/C cable bundled in with signal wires. Remember to try other tubes.
Since this is a stereo amp I am guessing it has a common supply for both channels. So if the power supply has a problem it will effect both channels. Pull the power tubes and tape up any loose wires. Safety first! Both of those tubes can have very large voltages which will kill you if you are careless. Turn on the amp and scope the output of the power supply to see what the voltage is and at the plate of the driver. If you see A/C trash on the driver plate and at the power supply then pull the driver tube and check again. If the trash is still there then the problem is in the power supply. Work your way back through the power supply cap string by disconnecting each section until you get clean DC. If you see trash on the B+ of the driver only when the tube is in place try another tube. If you still have the hum then you may have to trace out a schematic to see how the tubes are set up. This is just best guess with no schematic.
  Reply With Quote
Old 4th May 2012, 10:59 PM   #4
diyAudio Member
 
Join Date: Jun 2010
Quote:
that must be a seriuos amp. maybe filaments (AC or DC)? or bad ground wiring? power transformers polluting a too close OPT or other signal transformers?
Yeah, I think this amp has some potential.

The filament of both tubes is DC.
Perhaps.
I am certain that the hum is generating from the 75TL (driver).

I replace the loading resistor for the 75TL from 20k to 40k.
The plate voltage decrease and also current draw. Under this condition, there is 370mV hum (down from 600mV) at the grid of the 250TL and also at the plate of 75TL.
Therefore, there is only 100mV hum at the OPT secondary.

Last edited by HP8903B; 4th May 2012 at 11:03 PM.
  Reply With Quote
Old 4th May 2012, 11:10 PM   #5
diyAudio Member
 
Join Date: Jun 2010
Andy,

There is hum with the input grounded.

I will look into what you mentioned.

Both 250TL and 75TL use the same power supply. There are resistors, 40k total, in series with 75TL plate.

250TL plate is 1300V
75TL plate is 700V

There is 370mV AC at the 75TL plate.

Last edited by HP8903B; 4th May 2012 at 11:16 PM.
  Reply With Quote
Old 4th May 2012, 11:26 PM   #6
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The voltages here are positively lethal, and we have no idea how much experience you have with tube gear (possibly not a lot) particularly with these voltages.

Please affirm that you understand how to work safely at these voltages. (If not please read the high voltage thread here)

How exactly have you determined this is 60Hz - did you use a scope with a proper HV probe? Are you sure this is not 120Hz ripple from a bad power supply filter capacitor?

Are you certain that the DC filament supply is free of ripple?

Changing the 20K to 40K is not the right thing to do long term, but seems to point at the plate supply to the 75TL as a possible culprit..

How about a few pictures and a schematic. Who built the amp?
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 4th May 2012, 11:35 PM   #7
diyAudio Member
 
Join Date: Jun 2010
Kevin,

Thank you for you concern.

I have a Fluke 87. It can measure the frequency of AC voltage.
Notice I have not measure the AC voltage of the 250TL plate.

A friend of mine built this amp.
  Reply With Quote
Old 4th May 2012, 11:39 PM   #8
diyAudio Member
 
Join Date: Jun 2010
I make sure there is no voltage before touching the amp.

After I turned off and disconnected the amp, there is about 100V.
I drain the caps by connecting two Fluke 87 meters to the B+.

Also, I have a Fluke high voltage probe.
  Reply With Quote
Old 4th May 2012, 11:41 PM   #9
diyAudio Member
 
Join Date: Jun 2010
I am not certain that the DC filament supply is free of ripple.
  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
2A3 Single End Class A amplifier Musik007 Twin Audio 39 14th December 2013 08:26 AM
HELP: Eimac 75TL amp high frequency roll off HP8903B Tubes / Valves 71 30th April 2012 03:53 AM
6L6 class A single end amplifier Musik007 Twin Audio 33 28th April 2010 05:18 PM
Pure Class A Single End Amplifier Idea! Dxvideo Solid State 91 1st September 2008 10:54 AM


New To Site? Need Help?

All times are GMT. The time now is 01:30 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