I built a single-ended amp with a preamp based on a Fender tweed 5E5-A. Normal & Bright Volume control and the standard Treble & Bass control (500k pots each) cathode follower tone stack. I'm having an issue with a pronounced squeal on my Treble pot when rotating it to 2/3 and past til full rotation.
Any suggestions or insights would be greatly appreciated.
Treble pot squeal
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funkgang49
- Posts: 130
- Joined: Sun Mar 25, 2012 4:36 pm
- Location: Northern New Jersey
Re: Treble pot squeal
It may be positive feedback resulting, given sufficient gain, in free running oscillation, due to excessive coupling between earlier and later stages.
With such matters, as well as the actual intended design schematic (?), the unintended but inevitable overlay of parasitic couplings resulting from any real world build constraints of layout and lead dress, will be significant.
So it may help to provide the actual schematic used (consider sketching it out and taking a photo) and build pics.
But outside of that, grid stoppers, ie mounted at the tube socket terminals, are very helpful in calming things down, I think they were basic good practice in the golden age, for one offs / prototypes. Once a design was proved good without them, they could be omitted.
With such matters, as well as the actual intended design schematic (?), the unintended but inevitable overlay of parasitic couplings resulting from any real world build constraints of layout and lead dress, will be significant.
So it may help to provide the actual schematic used (consider sketching it out and taking a photo) and build pics.
But outside of that, grid stoppers, ie mounted at the tube socket terminals, are very helpful in calming things down, I think they were basic good practice in the golden age, for one offs / prototypes. Once a design was proved good without them, they could be omitted.
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funkgang49
- Posts: 130
- Joined: Sun Mar 25, 2012 4:36 pm
- Location: Northern New Jersey
Re: Treble pot squeal
Thanks for the quick reply!
In the meantime, I adjusted my mixing resistor values to address a discrepancy in Volume output differences between Normal Volume and Bright Volume - went from 270K each as in 5E5-A schemo to 220K Normal & 330K Bright which evened the levels out a bit but still Bright channel is a tad louder. V1 (parallel) & V2 Cathode follower are 6SL7's. Cathode resistors/bypass caps - 1.5K/25uf@50v Normal, 1K/25uf@50v Bright. Coupling caps - Normal .022uf@500v, Bright .01uf@500v, no 100pf cap on Bright pot.
10K grid stoppers installed at grid pins in all appropriate locations.
Just played with Volume levels and noticed the the squeal minimizes and mostly disappears with amp Volumes at 12 o'clock or higher - ?
In the meantime, I adjusted my mixing resistor values to address a discrepancy in Volume output differences between Normal Volume and Bright Volume - went from 270K each as in 5E5-A schemo to 220K Normal & 330K Bright which evened the levels out a bit but still Bright channel is a tad louder. V1 (parallel) & V2 Cathode follower are 6SL7's. Cathode resistors/bypass caps - 1.5K/25uf@50v Normal, 1K/25uf@50v Bright. Coupling caps - Normal .022uf@500v, Bright .01uf@500v, no 100pf cap on Bright pot.
10K grid stoppers installed at grid pins in all appropriate locations.
Just played with Volume levels and noticed the the squeal minimizes and mostly disappears with amp Volumes at 12 o'clock or higher - ?
Re: Treble pot squeal
Regarding the treble pot squeal - you could have positive feedback from the 'NFB' circuit, which induces a squeal when the treble pot is dimed - try switching the OT primary winding ends around.
Regarding the variable vol pot squeal response - this could also be FB-loop-related, but triggered by something like bad grounding or a non-functioning filter cap decoupling node* to the pre-amp gain stage(s), making the vol pot act like a FB-phase-cancelling mechanism at different pot rotations, which is interacting with the 'NFB loop phase' being the wrong way around. - Check OT primary winding phase (as above) and ohm-out the filter cap -ve lead-to-chassis ground connections, and the +ve lead to HT supply rail, to see if any of them are open.
* insufficient power rail decoupling provides a signal-phase-cancelling feedback pathway between gain stages (through the power supply rail).
JM2CW of hunches based on the problem description given. YMMV
Regarding the variable vol pot squeal response - this could also be FB-loop-related, but triggered by something like bad grounding or a non-functioning filter cap decoupling node* to the pre-amp gain stage(s), making the vol pot act like a FB-phase-cancelling mechanism at different pot rotations, which is interacting with the 'NFB loop phase' being the wrong way around. - Check OT primary winding phase (as above) and ohm-out the filter cap -ve lead-to-chassis ground connections, and the +ve lead to HT supply rail, to see if any of them are open.
* insufficient power rail decoupling provides a signal-phase-cancelling feedback pathway between gain stages (through the power supply rail).
JM2CW of hunches based on the problem description given. YMMV
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funkgang49
- Posts: 130
- Joined: Sun Mar 25, 2012 4:36 pm
- Location: Northern New Jersey
Re: Treble pot squeal
Ahh, let me clarify:
The Normal/Bright Volume pots turned up diminishes the squeal of the Treble pot at the 2/3 rotation and beyond. There is no squeal at all in the Volume adjustments with the Treble pot at the 12 noon setting.
Also, this is not a new build. It has been running trouble free for a year, however it is a prototype that I have made minor tweaks and adjustments to often over that period. With that in mind, Tubeswell, I believe you may be onto something. I may have inadvertently damaged a connection in my recent tinkering. I'm going to dive-in a little deeper and check all my connections and report back.
Thanks for your insights!
The Normal/Bright Volume pots turned up diminishes the squeal of the Treble pot at the 2/3 rotation and beyond. There is no squeal at all in the Volume adjustments with the Treble pot at the 12 noon setting.
Also, this is not a new build. It has been running trouble free for a year, however it is a prototype that I have made minor tweaks and adjustments to often over that period. With that in mind, Tubeswell, I believe you may be onto something. I may have inadvertently damaged a connection in my recent tinkering. I'm going to dive-in a little deeper and check all my connections and report back.
Thanks for your insights!
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Stevem
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Re: Treble pot squeal
Rule of thumb number 1 , when working on something especially if something is working well, only make one change at a time, not a fist full so that if a issue arises you know where it came from!
Can you post a shot of the build?
If you disconnect one preamp from its mixer resistor does the amp still respond the same?
Sometimes a coupling cap orented a certain way near a input jack or a pot can make for oscillation.
Are you using a 6SL7 for V1 also because if so you have a good amount more gain to control then the stock amp would with 12AY7!
Can you post a shot of the build?
If you disconnect one preamp from its mixer resistor does the amp still respond the same?
Sometimes a coupling cap orented a certain way near a input jack or a pot can make for oscillation.
Are you using a 6SL7 for V1 also because if so you have a good amount more gain to control then the stock amp would with 12AY7!
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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!