The sound of my amp got thinner and brighter after I put the chasis into the combo!!!

[Alan0354]

This is strange. I really like the sound of my amp testing on the bench and hooked up to the combo cabinet using a 7' 16 gauge wires. I had my amp buttoned up and put it in the combo. The sound became brighter and thinner!!! Brittle might be the word.

1) I tested by adding the 7' cable between the amp and the speaker, it seems to be ever slightly less bright.

2) I removed the amp and use thick text books to block the opening to simulate the chassis is in to block the opening, it definitely got a little fuller sounding, not quite like before.

3) I even verify by having the amp driving another speaker inside the Pro Reverb, it was nice and full. So the amp behaved like before. The only difference is when the chassis is inside the combo cabinet. I have the WGS G12C/S which is a very full sounding speaker. I have the Celestion G12T-75 in the Pro Reverb. Before I put the chassis into the combo cabinet, it definitely sounded fuller and fatter with the combo cabinet than the Celestion in the Pro Reverb. But now, The Celestion sounds fuller!!!

everything are identical, only difference is the chassis is inside or outside.

Well, I built the amp into the chassis that fit the cabinet, I am pretty much stuck with this setup. So far I attempt to remedy by adding a resonance circuit to the power amp. It did get the sound thicker. I have to put it back in and test it loud tomorrow. I am hopping it will work.

Any other opinions or suggestions? I sure don't know acoustics and cabinet design. But I never thought it will make such a difference. I had the chassis of my Pro Reverb in and out all the time and it sounds the same. Never experience the amp sounds different when the chassis is in or out.

[frus]

is your listening position identical in both cases?

[Alan0354]

is your listening position identical in both cases?

Yes. sitting in the same chair and cabinet about the same position.

[Justin Thomas]

Any chance of a loose connection getting knocked about in between all the moving/reconfiguring?

Do you have any metal shielding in the top of the cabinet(s)? Matbe something is popping up & touching that?

Just for clarity, you are using a Pro Reverb as test speakers, and your build is supposed to be a 1x12" correct?

Justin

[g1]

Put your scope across the speaker to make sure it is an acoustics issue and not some electronic problem like Justin mentioned.

[Alan0354]

Any chance of a loose connection getting knocked about in between all the moving/reconfiguring?

Do you have any metal shielding in the top of the cabinet(s)? Matbe something is popping up & touching that?

Just for clarity, you are using a Pro Reverb as test speakers, and your build is supposed to be a 1x12" correct?

Justin

I don't believe I knock anything loose as I pull on all the wires, jiggle a lot of solder joints, tie the wires in bundles in the final checking. Scrapping the inside of the chassis then turn it up side down to shake out all the scraps, then use force air to blow out the rest. After that, I installed into the cabinet.

Yes, I have metal shield, but I have nothing conductive that come even close to the top.

The chassis and cabinet I use is an old KMD 1X12 combo. When I build and design the amp, I always had the chassis on the table, I use the cabinet of the KMD combo as speaker so I thought what I heard will be how it should sound when I put the chassis in. I always use the WGS G12C/S in the KMD from day one. I use a 7' long 16 gauge wires to connect from the chassis on the table to the KMD cabinet for testing all along.

I have a Celestion G12T-75 in the Pro Reverb. I did connect the chassis to the Celestion occasionally to test the sound. But it is for testing only. I notice from experience if the amp is not designed correctly, it might sound good with one speaker/cabinet, but it sound awful with another. If the amp is designed right, it should sound reasonably good with all different speaker/setting. Yes, it should sound best with one setup, but a correctly designed amp should sound reasonably good with others, just not as good. It should never sounds awful. So I always test the amp with a Celestion G12T-75 in the Pro Reverb, a Emence Legend in the other side of the Pro Reverb, a Celestion G12H-100 in the Marshall JCM900 Dual Reverb combo. Sometimes I even test in a Fender Rock Pro 1X12 closed cabinet.

WGS G12C/S is a very American sounding, fat and very big sounding speaker. When the chassis is out, the KMD always sounds fuller and bigger than everyone else. But when the chassis is in the KMD, I connected to the Celestion in the Pro Reverb, the Celestion sounded fuller than the KMD/WGS. That's another reason I don't think I knock something loose or touching the shield.

I am going to crank it up later today to see whether I can get back the fullness by adding the resonance circuit in the power amp. But because of adding the resonance circuit, I have some strange scratching sound when I turn the resonance pot almost to 0. I happened on both resonance pot for the clean and OD channel. That tells me it's not the defective pot as one is brand new. I need to take the scope out as the resonance circuit monkey with the NFB loop that can cause instability. It is not oscillating or doing any thing funny, it just has a "whoop" sound when I turn towards the end. The sound does not stay, as soon as I stop turning even at the sore spot, the sound goes away and the amp works normal.

This amp is strange!!!

[Alan0354]

Put your scope across the speaker to make sure it is an acoustics issue and not some electronic problem like Justin mentioned.

I'll try that later. Today is lawn mowing day!!!

I am not saying the amp sounds wrong, it just sound more British. When the chassis was out, the clean channel sounded fat Fender sound as the circuit is very similar to 59 Bassman. But in the KMD, it actually sound brighter, less bass, more like the AC30 from my memory. I am after the fat Fender in this amp, it turns more into a British one!!!!

[loudthud]

This may seem way out in left field unless it works. Reverse the phase of the speaker. If there is more than one speaker, verify that they are both still working and in phase with each other.

[Alan0354]

This may seem way out in left field unless it works. Reverse the phase of the speaker. If there is more than one speaker, verify that they are both still working and in phase with each other.

I only have one speaker in all the test.

Ha ha, at this point, I am more questioning my ears!!!

[Alan0354]

Ha ha, I solved the whooping sound problem of the resonance pot. It's not instability, it's the DC offset problem. The NFB is going to the tail of the PI and there's a DC voltage at the summing junction. When I add a pot for resonance in the feedback path, I change the DC offset by adjusting the resonance pot. Just a 100uF coupling cap to DC decouple from the ground reference OT secondary solve the problem. At least that's easy.......Another over sight in my part......Again!!!!

I am going to put back into the cabinet and see whether the resonance circuit can compensate the difference. Big boss has to take a snooze, so testing has to be on hold for an hour!!!

[Alan0354]

I have everything back in the combo. I did change the shaping circuits a little in the OD channel to anticipate the increase of the brightness when in the cabinet, that actually help quite a bit. The resonance does not do a lot on getting back my original sound, BUT it does add a dimension as I can make the sound very thick. That might be useful for another cabinet that has thinner sound.

As for the clean channel, it just doesn't seem to be different from my original Fender type sound, but I won't say it's bad, it's just more of a British sound with big bottom like the American sound. I might just have to live with it. I don't have the knowledge in acoustics to do much about the cabinet. Maybe it's time for another speaker again!!!

All in all, I think it's a success, the OD channel is getting close to what I set out to do. The clean channel is a little different from what I want, but it could be a lot worst compare to quite a few combos I tested in GC.


This has been a very eye opening experience, this is the first time I build an amp from ground scratch and it is so different from modifying an existing. Even when I ripped out the Bassman 100 front end is a lot easier, because I can always count on the power amp and the power supply. It is a completely difference when I have to choose all the components and build one. All the parts problems and all.

I want to thank you guys for the help and support, I don't think I can do it smoothly without you guys.

Thanks

[Pedro Vecino]

With these symptoms (including the effect to put chassis in the cabinet) you can have positive feedback on the amplifier. Not always a amplifier screams to warn it. With the deep control to minimum (in bypass), try disconnecting the feedback line of the speaker jack. If the volume declines slightly means that the primary of the output transformer is connected backwards.

[Alan0354]

With these symptoms (including the effect to put chassis in the cabinet) you can have positive feedback on the amplifier. Not always a amplifier screams to warn it. With the deep control to minimum (in bypass), try disconnecting the feedback line of the speaker jack. If the volume declines slightly means that the primary of the output transformer is connected backwards.

I put in the resonance control that literally decrease the feedback signal back to the PI and low frequency, it sure gets louder. So I am confident that the polarity of the OT is correct.

I don't think the power amp is unstable, I worked a lot on feedback stability, it does not show any sign of instability. The amp sounds normal, just brighter when the chassis is inside. I strongly suspect it's the cabinet acoustics. I can use thick text books to block the opening after I remove the chassis and make it a little brighter.

[Silvertone Jockey]

What are the chances that the magnetic field of the power transformer (or even just the core iron) is interacting with the speaker's magnetics'? are they in close proximity? Just a thought

[Silvertone Jockey]

and or the output transformer's field causing phase cancellation if it's in close proximity