Had a weird thing happen today that I've never had happen before. I'm working on a Princeton style reverb circuit and I've got a super bad oscillation in the reverb circuit when I boost the preamp. As I turned up the volume higher and higher the squeal got worst of course and then the amp fell completely silent. Like dead quiet silent. I poked around thinking I destroyed a part like a resistor and when I measured the grid of one of the tubes with a DC multimeter the sound faded back in.
I did measure around 0.022V of DC on this grid. I checked the coupling cap feeding this grid and sure enough, it was not passing DC. I added a grid leak resistor to ground on the grid of this tube and I've not had the issue since.
Can someone tell me what happened here and is this the reason we install grid leak resistors in our amps?
Thanks so much!
DC on grid of preamp tube
Moderators: pompeiisneaks, Colossal
Re: DC on grid of preamp tube
The Reverb return in all Fender style amps has the return connected right to the grid of a 12ax7. If the grid is not grounded through the return circuit itself, the tube will run away. The 6G15 lacked this, but all Blackface amps have a 220K wired here. Tubes go crazy if there isn't a ground reference on the grid.
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Stevem
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Hhh
That circuit should have that resistor so why was it left off of your build?
The reason that the amp did not go silent, what happened is that the osscilation got up to such a high frequency and to such a strong level that (1) you could not hear it any more because the speaker could not reproduce it and (2) that signal level thru the whole amp was over riding your guitars signal by more than a 100 to 1 factor !
The reason that the amp did not go silent, what happened is that the osscilation got up to such a high frequency and to such a strong level that (1) you could not hear it any more because the speaker could not reproduce it and (2) that signal level thru the whole amp was over riding your guitars signal by more than a 100 to 1 factor !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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MarshallPlexi
- Posts: 69
- Joined: Tue Jul 26, 2005 9:55 am
Re: Hhh
Ah! that makes total sense! For what it's worth the grid leak resistors did help and I DID in fact have a HUGE parasitic oscillation that took me a while to find.Stevem wrote:That circuit should have that resistor so why was it left off of your build?
The reason that the amp did not go silent, what happened is that the osscilation got up to such a high frequency and to such a strong level that (1) you could not hear it any more because the speaker could not reproduce it and (2) that signal level thru the whole amp was over riding your guitars signal by more than a 100 to 1 factor !
Re: DC on grid of preamp tube
DC stabilization on a grid is not optional. Either DC coupling or a grid leak of some sort to ground or bias. I accidentally left one off once and I heard bzzzzzzzzzzzzt thru the speaker. Added it in an there was no damage.
If it says "Vintage" on it, -it isn't.