Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mr.Death on November 05, 2022, 12:58:57 pm
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Hello, I'm having motorboating issues that go away when I turn down the bass pot. Also I noticed when i turn down the bass pot the gain and volume lowers significantly. Bad bass pot or capacitor perhaps? Thanks.
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IMO it is not the pot
Wich amp ?
Schematic please.
Any mod on the amp or it is a home build
Post picture
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Amp is Traynor YBA 3 Custom Special. The amp has lots of mods that are on the schematic circled in red. I attached the original schematic as well as the one with the mods. The build is based off a hot rodded JCM800.
Also in my original post I said the motorboating goes away when the bass pot is turned down, I meant the issue goes away when turned UP. My mistake. I can also get the issue to go away when I choose the 10K V1 cathode resistor mounted to the board instead of the ones mounted to the switch. I did check the Cathode switch that I added and the grounds and connections are good etc. Will grounding that switch in a different place ( I grounded it to the chassis under the switch) compared to the 10k cathode resistor cause issues? Source of the motor boating? Thanks.
Un modded schematic below followed by mods.
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Mods
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Layout
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Switch, top of pic
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With many mods ! :BangHead:
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I can also get the issue to go away when I choose the 10K V1 cathode resistor mounted to the board instead of the ones mounted to the switch.
The way this is drawn, the 10K cannot be selected by itself unless there is a switch or switch contact missing (3-way).
The way it is drawn, the 10K would be in parallel with either the 3.3K or the 4.7K. Am I missing something?
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I can also get the issue to go away when I choose the 10K V1 cathode resistor mounted to the board instead of the ones mounted to the switch.
The way this is drawn, the 10K cannot be selected by itself unless there is a switch or switch contact missing (3-way).
The way it is drawn, the 10K would be in parallel with either the 3.3K or the 4.7K. Am I missing something?
My pic in the mod schematic was missing the middle lug that goes to ground. Apologies. The switch is 3 way. I tested it and when in the middle position I get 10 k, when up I get 3.3k and down I get 4.7.
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With many mods ! :BangHead:
Why so angry???
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Revised, sorry for my lack of schematic building skills. :dontknow:
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With many mods ! :BangHead:
Why so angry???
Mods are often the issue.
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My pic in the mod schematic was missing the middle lug that goes to ground. Apologies. The switch is 3 way. I tested it and when in the middle position I get 10 k, when up I get 3.3k and down I get 4.7.
[/quote]
No apologies needed :icon_biggrin: If the center position is truly grounded, then the 10K is in parallel with 0 ohms in that postion?
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Show picture of the pots wiring.
The layout of the wires is very important. From what I can see, there is room for improvement.
This is one of the reasons why I asked if there were any modifications because I had a Traynor amplifier with a similar problem recently.
A bad les dress act like antenna and may produce many kind of strange "noise" .
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My pic in the mod schematic was missing the middle lug that goes to ground. Apologies. The switch is 3 way. I tested it and when in the middle position I get 10 k, when up I get 3.3k and down I get 4.7.
No apologies needed :icon_biggrin: If the center position is truly grounded, then the 10K is in parallel with 0 ohms in that postion?
[/quote]
This is what the switch looks like to the best of my abilities. I rechecked my values and came close to what I got when using the resistor parallel calculator. Hopefully I didn't eff this one up. New to this stuff so bear with me. lol
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With many mods ! :BangHead:
Why so angry???
Mods are often the issue.
Mods made this tame bass amp sound like a high gain guitar amp!! Beastly! I'll post pics of the pots soon...
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With many mods ! :BangHead:
Why so angry???
Mods are often the issue.
Mods made this tame bass amp sound like a high gain guitar amp!! Beastly! I'll post pics of the pots soon...
By making certain appropriate modifications there may be "damage" that would have been caused.
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For a highish gain preamp, HT decoupling caps of 10uF seem kinda low, I suggest C32 & 34 be increased to 30 to 50uF.
As system gain is raised, the old school method of just using random connections between the circuit and the chassis, and using the chassis to conduct 0V common current, can result in excessive hum and possibly even oscillation.
Hence you may wish to consider the principles detailed in Merlin’s article http://www.valvewizard.co.uk/Grounding.pdf
Here’s a layout which implements that in a highish gain valve amp https://download855.mediafire.com/vvv53ryej8yg/arlsw02ata1j12m/Modulus+MR34+Layout.pdf
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Pics of pots as promised.
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pots
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I fixed the issue by changing the load resistor (R10) from 22K to 100K. Imagine that.