Bassman 5F6-A extreme oscillation - diyAudio
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Old 16th May 2011, 11:08 PM   #1
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Default Bassman 5F6-A extreme oscillation

Howdy!

I just built a clone of a Bassman inside an old solid state Marshall 2x12 combo. My only deviations from the original design are a SS rectifier and moving the filter cap board inside. It sounds killer... just not through the two speakers built in. When I connect it to a 16-ohm 4x12, located about 6 feet away, it sounds normal, but when I connect it to it 16-ohm internal load it makes a very high pitched squeal before switching to a deep WOOOOOOOOOOF! Terrifyingly loud, even with the volume all the way down. THe only thing that seems to affect it is fiddling with the presence knob. The negative feedback is connected to the 4-ohm jack, though I have tried connecting it to both the 8 and 16 to no avail.

I am utterly stumped and would appreciate any help.
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Old 16th May 2011, 11:11 PM   #2
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Could be a feedback circuit problem.

Alternatively could be poor decoupling between stages causing feedback through the power supply.
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Old 17th May 2011, 01:24 AM   #3
ChrisA is offline ChrisA  United States
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Originally Posted by OrganicBud View Post
Howdy!

I just built a clone of a Bassman inside an old solid state Marshall 2x12 combo. My only deviations from the original design are a SS rectifier and moving the filter cap board inside. It sounds killer... just not through the two speakers built in. When I connect it to a 16-ohm 4x12, located about 6 feet away, it sounds normal, but when I connect it to it 16-ohm internal load it makes a very high pitched squeal before switching to a deep WOOOOOOOOOOF! Terrifyingly loud, even with the volume all the way down. THe only thing that seems to affect it is fiddling with the presence knob. The negative feedback is connected to the 4-ohm jack, though I have tried connecting it to both the 8 and 16 to no avail.

I am utterly stumped and would appreciate any help.
First off it is a basic law of physics that oscillation requires positive feedback. The trick is to find the path

The simplest mistake is to connect the NFB to the wrong side of the secondary so it becomes a positive feedback. Let's hope it's that simple.

But before you do anything, What if you pull out all the preamp tubes (leave in the phase splitter, it's part of the power section.) If you still have the problem with the preamp tubes out then you've proved the problem is down stream of the master volume pot. Maybe a backwards NFB?

But if moving the speaker fixes this then I'm thinking it might be in the preamp and maybe one of the tubes is microphonic and the feedback path is acoustic. The speakers are simply shaking the bad tube. Microphonic tubes are not uncommon
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Old 17th May 2011, 01:29 AM   #4
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Where do you connect ground wire of an external speaker, and where do you connect ground wire of the internal one?
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Old 17th May 2011, 01:40 AM   #5
eyoung is offline eyoung  United States
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Hola, Howzabout a schematic of your clone....maybe we can figure out your problem athough it sound like a microphonic tube. tapping on the tubes will reveal the offending tube. what tubes are you using and what is your bias on the output tubes???

El
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Old 17th May 2011, 04:17 AM   #6
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Originally Posted by ChrisA View Post
First off it is a basic law of physics that oscillation requires positive feedback. The trick is to find the path

The simplest mistake is to connect the NFB to the wrong side of the secondary so it becomes a positive feedback. Let's hope it's that simple.

But before you do anything, What if you pull out all the preamp tubes (leave in the phase splitter, it's part of the power section.) If you still have the problem with the preamp tubes out then you've proved the problem is down stream of the master volume pot. Maybe a backwards NFB?

But if moving the speaker fixes this then I'm thinking it might be in the preamp and maybe one of the tubes is microphonic and the feedback path is acoustic. The speakers are simply shaking the bad tube. Microphonic tubes are not uncommon
I realized another change I made was putting a 12BH7 in as the PI. I always thought it sounded killer there in Fender amps.
The power tubes are EH 6L6s.
Forgot to mention, the OT is a Hammond 1650NA, which has their easy wire secondary, so I don't think thats hooked up wrong.

I've currently got the thing opened up so I can redo some grounds, so I'll have to wait till tomorrow to see if any of the tubes are microphonic... which would be sad. Brand spankin new EH 12AX7 Golds. I will readily admit that I'm new at most of this, and I'm not sure how I could have wired up the negative feedback backwards, as I thought it was just a wire going from the tip connector of the speaker jack to a 27k resistor on the board to the presence knob. I also realized that some of the wires going to the PI are about 3 inches long; could that be picking up anything?

Thanks for all your help guys!

Last edited by OrganicBud; 17th May 2011 at 04:25 AM.
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Old 17th May 2011, 04:29 AM   #7
6L6 is online now 6L6  United States
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Is this the schematic you are using?

Click the image to open in full size.
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Old 17th May 2011, 06:04 AM   #8
ChrisA is offline ChrisA  United States
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Originally Posted by OrganicBud View Post
... I'm not sure how I could have wired up the negative feedback backwards, as I thought it was just a wire going from the tip connector of the speaker jack to a 27k resistor ...
Yes from the tip connector but which wire on the transformer is "tip"? You have a 50/50 chance of getting it right. Actually I suspect it is right or you'd be having a much worse problem.
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Old 17th May 2011, 10:05 AM   #9
DF96 is offline DF96  England
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Have you used the correct values for coupling and decoupling caps around the driver and output stage? 'Improvements' here can easily cause LF stability problems, as the CR rolloffs and the OPT rolloff all have to cooperate together to keep the loop stable.
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Old 17th May 2011, 10:05 AM   #10
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The fact it works with an external speaker but not with an internal one leads me to suspect you have an acoustic feedback problem. With the external speaker connected try tapping each of the tubes in turn with a pencil to see if any is particularly microphonic.

Cheers

Ian
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