Amp thinks it's a receiver ?? - 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 June 2009, 02:02 AM   #1
diyAudio Member
 
Join Date: Jan 2009
Question Amp thinks it's a receiver ??

Ya know...something tells me I know the answer to this, but it's buried back in the deep recesses of my mind...or what's left of it anyway !
One of my guitar amps...a Gregory MK X if that means anything to anyone...has this annoying habit. It thinks it's a receiver...TV or radio I'm not sure. I don't watch TV anymore...and it definitely ain't the Yankee game...but I digress. Anyway, it picks up stray signals from somewhere. None of my other amps does this...not even in the same spot. The building I live in is old with masonry walls at least a foot thick ..hard to believe TV would penetrate. Any ideas? When I shut it down it seems like the signal gets stronger ???
  Reply With Quote
Old 3rd June 2009, 02:26 AM   #2
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
It's common to use a high value stopper resistor (22k or more) on the input grid of a guitar amp to prevent this sort of thing. The guitar can act as a radio antenna.
  Reply With Quote
Old 3rd June 2009, 02:36 AM   #3
diyAudio Member
 
Join Date: Jan 2009
A very common value is 68K to the grid, and 1M from the input jack to ground. Of course a common way to boost gain is to cut these resistors' value...I've done it a couple of times. Can't remember if I did with this guy...I'll have to open it up and look. So will a larger value help correct this, Ray?
  Reply With Quote
Old 3rd June 2009, 04:07 AM   #4
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
I don't see how reducing the value of a grid stopper is going to increase the gain except, maybe, at high frequencies because of Miller.

I would expect 68k to be easily enough to stop any radio signals picked up by the guitar.

Other sources are possible, such as the speaker leads (if there's global NFB), or the PS (unlikely).
  Reply With Quote
Old 3rd June 2009, 04:53 AM   #5
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
It seems to me, that 68K is across the jack's switch, in a voltage divider, so one jack works for high-Z pickups, another one for low-Z pickups.

I may be wrong, since I did not touch guitar amps last 25 years.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 3rd June 2009, 05:31 AM   #6
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
There may possibly be a voltage divider using a 68k resistor, but this is different. It is connected exactly as a stopper should be, i.e. between the upper end of the grid leak resistor and the grid pin. It does not attenuate at low to middle frequencies, but it may well do so at higher frequencies because of the tube's input capacitance (Cgk + Cga x gain (=Miller)).

It serves the dual purposes of preventing parasitic oscillations and acting, together with the tube's input capacitance, as a high-frequency filter to block RF, such is bereanbill has been describing.
  Reply With Quote
Old 3rd June 2009, 06:13 AM   #7
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
In the two-jack input configuration, the typical old Fender input circuit used the jack cutout contacts to control those resistors. There was a 68k from each jack to the input grid. In the high gain jack, the two wound up in parallel for a 34k resistance as a stopper. If you plugged into the #2 jack, then the two resistors became a divider, cutting signal level in half.

This is an extremely common input jack configuration in guitar amps.

Another reason guitar amps turn into radio receivers is when the ground at the input jack has no integrity. SOmetimes backing off the mounting hardware on the input jack then retightening allows the toothed washer to find a fresh contact surface.
  Reply With Quote
Old 3rd June 2009, 05:48 PM   #8
pointy is offline pointy  United Kingdom
diyAudio Member
 
pointy's Avatar
 
Join Date: Mar 2009
maybe put a mu ring choke on the earth (ground) if it uses one and the inputs.
all you need is a 1" mu triode ring and then pass the wire through it 4 or 5 times. it can help to block RF
  Reply With Quote
Old 4th June 2009, 01:50 AM   #9
diyAudio Member
 
Join Date: Jan 2009
Enzo was on the money. My Gregory has an aluminum chassis, and I guess it got oxidized in it's 44 years on this planet. I took it out, cleaned it and the mounting surface, and voila ! No more radio. Thanks Enzo !
  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
Add pre-out to a receiver? Empty Solid State 3 1st October 2006 11:12 PM
uh oh me thinks me killed my cdp spock Digital Source 14 20th February 2006 02:04 PM
Speaker ignoramous thinks about ovals jmikes Multi-Way 1 9th November 2004 10:01 PM
Hey i need help with receiver........EXPERTS HELP(two channel receiver) Andre Baptiste Parts 1 1st May 2004 11:20 AM
diy receiver Randy Knutson Digital Source 0 15th January 2004 07:20 PM


New To Site? Need Help?

All times are GMT. The time now is 02:49 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