Hello all,
So I have a home brew 5F1 type circuit with a few switches and a master volume and with the preamp volume turned most of the way up to all the way up the amp will oscillate either like a very buzzy and thin raspy sound to just cutting out but with a 12AT7 instead of an AX7 it doesn’t do this. Also the the only 12AX7 I can use without any oscillation at any volume setting is a JJ ECC83.
So I’m wondering what are sources and/or causes of oscillation in a simple low-gain circuit such as this so I can know where to start diagnosing.
Thanks.
Causes of oscillation?
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Stevem
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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Causes of oscillation?
There are many possible ways that such a issue can be made!
In your case if the amp is layed out like a original then my bet since you don't have the issue when running a lower gain tube is that the source is the output of that first gain stage( pin 1) getting back into that gain stages grid , pin 2.
If your build had the two 68K resistors from the input jacks on the tag board like the original layed, then I would move them onto the input jacks themselves like most other amps have .
Next I use shielded cable to make the run from the Y point of those two resistors to pin 2 of that tube socket.
Note that on that cable you should only have the sheilding grounded at the input jack end, on the tube end just tape, or better yet shrink rap off the sheilding out of the way.
All grid wires unless they are less then 2 inches long should be sheilded.
If your amp is not layed out with those resistors on the tag board then please post up a few pictures of the whole build.
PS.
Sorry I for got that you have made some mods to this amp.
In that case we need pictures and just in general good build practice means all grid wires should be as short as you can make then .
For example when thes signal leaves the plate of a preamp tube once it gets to the down stream decoupling cap, then once again it becomes a grid wire.
Following good build practice these caps should be placed as close to the down stream tubes grid as you can get it!
Poor solder connections will make for a very high resistance point which can then act like a antenna and make for issues.
In your case if the amp is layed out like a original then my bet since you don't have the issue when running a lower gain tube is that the source is the output of that first gain stage( pin 1) getting back into that gain stages grid , pin 2.
If your build had the two 68K resistors from the input jacks on the tag board like the original layed, then I would move them onto the input jacks themselves like most other amps have .
Next I use shielded cable to make the run from the Y point of those two resistors to pin 2 of that tube socket.
Note that on that cable you should only have the sheilding grounded at the input jack end, on the tube end just tape, or better yet shrink rap off the sheilding out of the way.
All grid wires unless they are less then 2 inches long should be sheilded.
If your amp is not layed out with those resistors on the tag board then please post up a few pictures of the whole build.
PS.
Sorry I for got that you have made some mods to this amp.
In that case we need pictures and just in general good build practice means all grid wires should be as short as you can make then .
For example when thes signal leaves the plate of a preamp tube once it gets to the down stream decoupling cap, then once again it becomes a grid wire.
Following good build practice these caps should be placed as close to the down stream tubes grid as you can get it!
Poor solder connections will make for a very high resistance point which can then act like a antenna and make for issues.
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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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Causes of oscillation?
I found a picture of your build!
Try this .
Pivot around 180 degrees the coupling cap off of pin 1, and that wire that is running very close and parallel next to it, move it over to the left side of the chassis closer to the power supply filters.
Try this .
Pivot around 180 degrees the coupling cap off of pin 1, and that wire that is running very close and parallel next to it, move it over to the left side of the chassis closer to the power supply filters.
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!
Re: Causes of oscillation?
I need to post some updated pics of it, I’ve made a couple small changes since the last pics I’ve uploaded to the net. It most certainly is not laid out like the classic 5F1!
I appreciate the help.
I appreciate the help.
Re: Causes of oscillation?
Does it quit oscillating if you reverse the OT primary leads?
Re: Causes of oscillation?
Solder the 2 68k resistors to the grid, pin 7 of socket V1.
Last edited by TUBEDUDE on Tue Dec 07, 2021 6:02 pm, edited 1 time in total.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Causes of oscillation?
I am not 100% certain, but if your ot primary’s where hooked up the wrong way to play nice with the feedback circuit I think that it would still oscillate with a 12AT7 in there!
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!
Re: Causes of oscillation?
Positive feedback can exist without the conditions for oscillation being met. Consider that a weak valve in a trem oscillator circuit may result in the trem effect failing.
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Re: Causes of oscillation?
Thank you for the suggestions when I open her up I’ll try a couple things as well as post pics of the current circuit (I changed what one of the switches does and changed the tone knob to a master volume). You guys rock!
Re: Causes of oscillation?
Here are some gut shots of the amp as it currently sits, nothing changed yet per this thread.
The layout is pretty unorthodox as I’m using a turret board made for the Epi Valve Junior made by some guy or company I forgot the name of that apparently burned many people by taking their money and not delivering. Anyways the Valve Junior tone sucks compared to a Fender so I used up this blank turret board for a 5F1 build with only 1 input jack.
It looks like a spaghetti bowl as it’s my very first amp build but it’s quiet and the tone is AMAZING using a 6V6 and a ClassicTone 40-18031 OT. I ran low on parts building it hence paralleled 68k input resistors (one is a 47k on closer inspection, doh!) I have some 33k on hand now to replace it and move it directly onto the input jack as recommended.
Included in the build is a 3-way NFB switch (10k, 22k, and NFB OFF), a boost switch consisting of a 1uF bypass cap on the V1B cathode, and a 500pF bright cap on a switch. Also features a master volume that works great.
I’ve got a couple amp builds under my belt with satisfied customers and I want to revisit this first amp build of mine (which I’m keeping for myself) to iron out the kinks, namely the oscillation issue at high volume knob settings as well as possibly replacing some of the ugly wiring job, possibly with cloth wire as I’ve really taken a liking to such wire.
The layout is pretty unorthodox as I’m using a turret board made for the Epi Valve Junior made by some guy or company I forgot the name of that apparently burned many people by taking their money and not delivering. Anyways the Valve Junior tone sucks compared to a Fender so I used up this blank turret board for a 5F1 build with only 1 input jack.
It looks like a spaghetti bowl as it’s my very first amp build but it’s quiet and the tone is AMAZING using a 6V6 and a ClassicTone 40-18031 OT. I ran low on parts building it hence paralleled 68k input resistors (one is a 47k on closer inspection, doh!) I have some 33k on hand now to replace it and move it directly onto the input jack as recommended.
Included in the build is a 3-way NFB switch (10k, 22k, and NFB OFF), a boost switch consisting of a 1uF bypass cap on the V1B cathode, and a 500pF bright cap on a switch. Also features a master volume that works great.
I’ve got a couple amp builds under my belt with satisfied customers and I want to revisit this first amp build of mine (which I’m keeping for myself) to iron out the kinks, namely the oscillation issue at high volume knob settings as well as possibly replacing some of the ugly wiring job, possibly with cloth wire as I’ve really taken a liking to such wire.