Weird EH12AY7 Problem - 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 25th February 2009, 06:44 PM   #1
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Default Weird EH12AY7 Problem

I use a pretty good number of 12AY7's in guitar amps, mostly Fender Stuff.

Just recently I have been getting an unusual hum issue with EH12AY7's

The hum increases with the volume control, and also if you put your hand anywhere near the glass envelope. The tube shield also increases the problem/noise.

It is almost like a ground is lifted, just not as loud.

If I replace the tube with other 12A_7's type tubes the problem stops.
So I guess it is safe to say they have a recent production issue?

I have 16 of these here out of the last 2 purchases. (10 one month, 10 more the next) 2 started out ok, but only for about 30 minutes.

Normally these are dead silent great sounding tubes which really had me thinking I was missing something.
They test fine in my testers, but clearly something is not right with the current batch from a Known Distributor.

Any thoughts?
  Reply With Quote
Old 25th February 2009, 07:10 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Looks like an RF oscillation. Grid stoppers?
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 25th February 2009, 07:19 PM   #3
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Quote:
Originally posted by Miles Prower
Looks like an RF oscillation. Grid stoppers?

This is in the V1 position of a tweed circuit, these have 68K resistors to grid.
Schematic

Thing is, any other brand tube, 12AX7's ect are not doing this. It is almost like they changed something or had a production mistake.

The ones I bought 6 months ago do not exhibit this problem at all.
  Reply With Quote
Old 25th February 2009, 07:52 PM   #4
diyAudio Member
 
Join Date: Feb 2004
Location: Hamilton, ON
Sorry, I don't have a solution. But I ran into a similar problem with the 12AU7EH. In one particular line of stereo amp, in the phase splitter circuit, after about a month of use the amps started to hum. Not too loudly but audible. Replacing the tube, or even just swapping out the phase splitter tube for the first stage tube in the same channel would eliminate the hum. This problem still exists and is one reason I no longer use EH tubes. No other brand of tube does this.
Other than tube replacement, I have yet to find a solution.
__________________
Dan Santoni
www.dtsaudioelectronics.com
  Reply With Quote
Old 25th February 2009, 10:52 PM   #5
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Quote:
Originally posted by audiopro
Sorry, I don't have a solution. But I ran into a similar problem with the 12AU7EH. In one particular line of stereo amp, in the phase splitter circuit, after about a month of use the amps started to hum. Not too loudly but audible. Replacing the tube, or even just swapping out the phase splitter tube for the first stage tube in the same channel would eliminate the hum. This problem still exists and is one reason I no longer use EH tubes. No other brand of tube does this.
Other than tube replacement, I have yet to find a solution.

Right now, EH is the only new production 12AY7 available.

Many of the Tweed era amps use these in the V1 position and other than hunting down older vintage tubes(not practical) there is really no solution I guess.

Using the 12AX7's in that position really limits the tonal quality.

Wonder what the heck happened, Those EH12AY7's were outstanding for seems like ages.
  Reply With Quote
Old 26th February 2009, 12:51 AM   #6
diyAudio Member
 
Join Date: Feb 2004
Location: Hamilton, ON
They must be having some quality issues. I used all kinds of EH tubes for years. Now I can't get quiet 12AU7's, reliable EL84's or good 12AX7's from them. I've since switched to JJ for my purposes.
12AY7's are not that popular. I would imagine NOS versions would not be that expensive.

Have you tried a 12AT7 in that position?
__________________
Dan Santoni
www.dtsaudioelectronics.com
  Reply With Quote
Old 26th February 2009, 12:52 AM   #7
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Does sound like those 12AY7A are oscillating, checked the circuit with a scope to see? Some minor lead dress and plate/grid stoppers may help. Does sound like there may have been a recent change or it may just be their transconductance is just a bit higher than usual. (I have had NOS 12AY7 oscillate under some circumstances as well.)
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 26th February 2009, 01:36 PM   #8
Trout is offline Trout  United States
diyAudio Member
 
Trout's Avatar
 
Join Date: Nov 2004
Location: Midwest Madman
Quote:
Originally posted by kevinkr
Does sound like those 12AY7A are oscillating, checked the circuit with a scope to see? Some minor lead dress and plate/grid stoppers may help. Does sound like there may have been a recent change or it may just be their transconductance is just a bit higher than usual. (I have had NOS 12AY7 oscillate under some circumstances as well.)

I put a few 6021's, RCA 12AY7's, and GE 12AY7's in the amp last night, no hum.
12AX7's work perfectly also, but they have more hair than needed/wanted.

I guess it must be a batch issue, I contacted the distributor these came from, but no response yet.
  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
weird problem sberube Solid State 15 22nd October 2008 06:27 PM
NAD 306 weird problem Juko Solid State 1 4th May 2007 08:21 AM
Weird Amp Problem Stewart8989 Car Audio 3 6th April 2007 06:54 AM
Weird PSU problem phn Power Supplies 8 6th February 2007 02:42 AM
Weird amp problem Etnoo Car Audio 5 22nd August 2006 06:26 AM


New To Site? Need Help?

All times are GMT. The time now is 05:59 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