Low Ghost Notes in a Tweed Gibson - Intermodulation Distortion?

[rozrosner]

Please help me understand what causes very bad low ghost notes (intermodulation distortion?) when playing certain intervals (double stops) through my old Gibson amp and how I might solve this problem.

Someone posted about this exact problem in the vintage amps forum (with regard to his Ampeg Reverbrocket) and got almost no replies and none that seemed to address the problem with any precision. His description of the problem made it seem like the same phenomenon anyway.

My 1960 Gibson GA-18T gets very prominent low frequency ghost notes (notes not being played) when playing intervals high up on the neck, especially when playing major thirds on the B and G strings above the 8th fret. They are hideously bad but they are almost non-existent lower on the neck. If you pluck hard with a country-claw, pick-and-fingers approach, it makes the problem worse.

I am currently using a Weber made speaker that is a P10Q replica (Tungsten Amps). The problem is MUCH less prominent when I use a very efficient speaker in this amp. I was using an Eminence Ramrod speaker and the annoying low subharmonic problem almost disappeared, but the amp was way too loud and I prefer the vintage Jensen type sound in this amp.

Is this intermodulation distortion?

I have had the power supply filtering beefed up and I've experimented with different coupling cap values. Neither solved the problem.

This amp has a tiny output transformer. It's about the size of a Superbowl ring, maybe smaller. Could that be a factor? This is a very peculiar problem.

I think all amps have this problem to some degree, but it's really bad in this amp and I'd love to know why. I love the amp but this problem almost ruins it for me.

[Chuck H]

When you say the filtering has been "beefed up", what does that mean? Any old filters in the amp should be replaced. And it's possible that increasing filter value will help too.

Grounding can have an effect too. If your amp uses a "can" type filter with a common ground you may need to retrofit individual caps so that their grounds can be seperated.

It's also possible that when the filters were "beefed up" that some shinnanigans omitted a ground or filter for the power supply. The amp should brobably be returned to stock working order with known good parts before any further assesment.

Chuck

[Merlinb]

Grounding can have an effect too.

+1

If the PSU caps are all ok then bad grounding is the usual source of ghosting.

[rozrosner]

Not Sure Ghost Noting is the Corrrect Term

Thanks, guys. I think that intermodulation distortion is perhaps a more precise term for this phenomenon. Sum and difference frequencies, perhaps?

Here's a link to a GA-18T modification project that looks very interesting and may provide a good framework for a discussion.

Gibson GA-18

One interesting thing is that the pre-mod innards photo on the linked site is a photo of my amp, not the amp this guy bought on Ebay and modded. Years ago, I sent my amp to a guy in Texas (who was really into working on Gibson amps) and that guy posted some pics of it. I have no idea why the guy at the link below used a pic of my amp instead of a pic of the amp he modded.

I wonder if the power supply changes this guy did to his GA-18 would improve the intermodulation distortion in my amp if I made the same mods?? I wonder if they changed the tone of his amp to a significant degree?

I'll try to figure out what was done to the power supply filtering in my amp. I have a feeling it was brought "back to stock." As you can see (in the link) there was a non-original can cap in my amp before the power supply was worked on.

Also, Enzo who is, or used to be, a regular on this forum has worked on the trem circuit in this amp (which sounds great, by the way). Those two lovely light blue caps in the photo were installed by Enzo. I have great respect for Enzo but didn't want to bug him by asking him about this stuff directly (but I always appreciate his wisdom when he is gracious enough to offer it which is every time I have asked).

Also, a friend of mine has a Victoria "tweed deluxe" so I'm going to compare my GA-18 and his Victoria through the same speaker to see (hear, actually) how they compare when playing the intervals that cause my Gibson to spit out the ugly low sum/difference notes (harmonics?). Apparently, the GA-18 circuit is very close to the 5E3 Deluxe circuit and the Victoria is a close copy of a 5E3 (or so I've heard). I can't verify that the Gibson is very close to a 5E3, but you guys know if that is true.

I've been reading that some very popular and highly regarded amps tend to have this characteristic (like the Dr. Z Route 66) produce alot of these ghost notes. Some guys might actually like this characteristic. Some might think its a very subtle thing and would wonder why this bugs me. I once had a Dr. Z amp called a Maz 18 Junior and it did have this problem. I called and talked to Dr. Z (Mike Zaite?) and mentioned this to him. He said he never noticed it before and said he would look into it. I talked to him again, or emailed with him (I can't recall which) and he said he definitely noticed it and it was not subtle. I thought that was interesting because I noticed it right away when I got the amp. I'm not saying it was a flaw, but it bugged me.

Finally, I do acknowledge the possibility that the very aspects of the design of the Gibson that cause me to like it so much may also be responsible for the extra notes that bug me. Like so many things in life, it may just be a trade off situation. Fix this and deal with that. "Improve" one thing and make another aspect worse.
I've heard guys say that the minimal level of filtering in old Vox amps is a big part of why they sound like they do and why so many guys love them.

What I'm trying to find out is what the trade offs are.

[rozrosner]

Power Supply Filtering Info - GA-18T

There is a bundle of three big, light blue caps which I assume are the power supply filter caps. One is 33 uf and the other two are 22 uf each. The two orange drop coupling caps (I think they are the coupling caps) are 716P .047. I installed the coupling caps and kept trying smaller and smaller values in an attempt to mitigate the "ghost note" problem.

Thanks for helping to educate me on this stuff!

[rozrosner]

Oops! Coupling Cap Values

The coupling cap values are .0047, not .047. I just verified that.