New Build Bass Control Issue

[mikeboone]

OK Guys...

I just finished a new build that is an AB763-ish single channel amp with a TMB stack. I used a PT from a 59 Bassman RI and an OT from a 50 watt Prosonic.

The amp sounds really nice, but there is an issue with an interaction with the Bass control and the Volume.

When cranked, the amp goes into a very low frequency oscillation, almost a "woof-woof" sound. Not really a motorboating sound that comes from power supply problems. I had that issue on my first build, bad cap.

If I back the bass off it goes away. and sounds really nice. It would appear as I have too much bass range, but I am unsure.

I have reviewed the schematic and layout servaral times and everything appears to be in order

Any suggestions on what may cause this and how to correct it.

[PRNDL]

In my experience, the main causes are lead dress, grounding and layout, which are really hard to figure out.

The simplest way to find it is to move things around while the amp is doing it (aka the chopstick method).

I just finished a new build that is an AB763-ish single channel amp with a TMB stack.

My tube amp mentor stressed that even minor changes to Fender amps could have dramatic results (usually not good).

If you can, post a photo and you'll get lots of helpful advice.

[mikeboone]

Photo of build. I tried adding it as an edit to my original post. Not sure if it worked.

Again, this is a frankenstien of features that I like. Most mid-powered (40W+ )Fender Clones and Kits don't come without the option of term and reverb. I dont like or use either. I like my playing to sound like my amp makes it sound.

[PRNDL]

I agree -- unique designs are awesome!

This is an superb build!

The only minor issue I can see is the third PS cap, which you've moved beside the preamp board. It's possible there's an issue with the shield of the coax being tied to its ground, but that's a long shot.

Bias might also be suspect.

[mikeboone]

Thanks for the compliment on my build. I gutted a Fender Blues Deluxe chassis for this. I have been perusing these forums for the last few years collecting ideas and observiing techniques.

I am an electronics tech with 20 years of field time and I hate to troubleshoot something that is a mess. When its done neatly, its easier to fix.

I got the idea of moving that 3rd filter cap down close to the pre-amp so that they are grounded together. I see it in other other types of amps. I did leave myself enough room on my filter board to put it back if there was an issue.

I have some more coax that I can make another lead and divert it away from that filter cap. We'll see.

I was also thinking of somehow limiting the range of the bass control. Even at low volumes anything past 12 o'clock is too boomy. It seems like I read somewhere that there are some mods to these 60's era Fender tone stacks to fix that boominess.

Thanks,
Mike

[pdf64]

Maybe one of your power supply caps is a duffer and you're not getting good decoupling? Sub in known good ones. If ok, then-
Strategy 1 - increase the power supply decoupling by increasing the value of that last cap and / or dropper resistor which feeds it.
Strategy 2 - reduce the value of 1 or more coupling caps. Need more detail of schematic to suggest which one.
Strategy 3 - reduce value of 1 or more cathode bypass caps.

Or a bit of each.
Peter.

[PRNDL]

I did leave myself enough room on my filter board to put it back if there was an issue.

There may be a grounding issue where the power supply return is flowing from that cap across the chassis, through your preamp grounds.

This is somewhat similar to other amps with a long-tail-pair and one preamp gain stage. Pro Jr, Matchless Spitfire and Vox AC15 come to mind, although they are without NFB.
The 6g3 deluxe has 2 preamp gain stages and divides the gain.

The hard part with these issues is keeping your sanity.

[mikeboone]

Thanks guys for the input. I'll dig into it this weekend and post the results.

pdf64 - The circuit is the normal channel of a Super Reverb with two mods

1. I added a mid control.
2. I excluded the feedback loop. The first build I did squealed and no value of resistor or even reversing the blue/brown on the OT would stop it. It would just move it out to higher volumes. So I used an infinite resistor which seems to have stopped it

All other values were used as per the AB763 schematic.

[imaradiostar]

Thanks guys for the input. I'll dig into it this weekend and post the results.

pdf64 - The circuit is the normal channel of a Super Reverb with two mods

1. I added a mid control.
2. I excluded the feedback loop. The first build I did squealed and no value of resistor or even reversing the blue/brown on the OT would stop it. It would just move it out to higher volumes. So I used an infinite resistor which seems to have stopped it

All other values were used as per the AB763 schematic.

This makes me question something in the phase inverter or power section. I'm not an EE but as a general rule negative feedback (up to a point) should correct minor issues and distortions created in the output section. It sounds like the power tube grounds are somehow altering the signal at the phase inverter or preamp grounds and generating a low frequency oscillation.

I'm no expert...it just seems strange that the amp didn't like any kind of negative feedback.

jamie

[mikeboone]

Interesting Discovery

Hey guys...

The amp in question here is the 2nd one that I have built like this, just made a few minor changes. Different trannies and added a mid control.

I cranked the volume and then the bass control on my first build (pictured attached) and it does the same damn thing.

It starts that low freq pulsing - maybe 2 times per second that hits the speaker with a bassy thump.

NOTE: There is a bright blue flash in the power tubes when it pulses.

On the 1st build, I guess I never had the volume and bass dialed in to this combination, theres no reason to.

I am completely confused and intrigued. What I ever I did wrong, I did it twice. I attached the schematic that I drew up to show just the normal channel of a Super Reverb. This is what I worked from to develop my layout.

[Bruce / Mission Amps]

Hey guys...

The amp in question here is the 2nd one that I have built like this, just made a few minor changes. Different trannies and added a mid control.

I cranked the volume and then the bass control on my first build (pictured attached) and it does the same damn thing.

It starts that low freq pulsing - maybe 2 times per second that hits the speaker with a bassy thump.

NOTE: There is a bright blue flash in the power tubes when it pulses.

On the 1st build, I guess I never had the volume and bass dialed in to this combination, theres no reason to.

I am completely confused and intrigued. What I ever I did wrong, I did it twice. I attached the schematic that I drew up to show just the normal channel of a Super Reverb. This is what I worked from to develop my layout.

The schematic is drawn wrong with respect to the tone controls.
You do not have the typical 6k8 mid control resistor, which grounds the tone controls.
Does the new amp have a mid pot that is grounded correctly?
Low speed thumping is usually described as motor-boating.
Frequently a symptom of poor power supply decoupling.
I'd be checking the power supply caps and their ground points first.

[mikeboone]

Bruce...

Thanks for the reply. I added the mid control to the 2nd build and connected as per the Fender Super Reverb layout. I see the mistake in the schematic. There is indeed a 6.8K R to ground on the bass pot of the first build.

On both builds the filter caps are grounded together and directly to the chassis via a short wire soldered to a ring terminal and screwed to the chassis.

The resistance from these grounds to the V1 preamp cathode ground is .2 ohms.

Is this good practice?

I am lost friends!

[mikeboone]

UPDATE!!!

Guys...

When I orginally put this concept together, using only the Normal Channel of an AB763, I decided to use a common cathode bypass for V1a & b.

That was a decision made by doing a bit of reading and asking the opinions of some more experienced guys.

I put in a separate bypass for V1b and problem is solved.

I tried every suggestion you guys put forth and learn a lot about my amp and just how little I really know.

Thanks again for the support.

Mike