Filtering overtones?

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azatplayer
Posts: 556
Joined: Thu Feb 26, 2009 7:59 pm
Location: Great Southland

Filtering overtones?

Post by azatplayer »

Hey. I just love my Rocket conversion on a twin build i did. It sounds so good, that my brothers amp i built him ( KT66 UL deal), well he decides he wants me to mod that to a rocket too.
The thing with this 66 amp, is it has always made these overtone/ ghost notes. It has pretty substantial filtering, but ive not been able to erradicate them. Now that ive modded it to rocket board, which sounds pretty amazing!, im remembering the ghost problem as its still alive and well.
Oddly, ive built 2 of these 66's, both do it, but at varying levels depending on location.
My house is bad, my mates guitar store is almost not present.
Im using a hammond 1650H UL OT.
Original build, first filter is a 50/50 can. OT fed from first 50/standby switch, then second 50 drops and feeds the PI direct then a 47uf drops and fed V1.

Ive changed the 47 to a 22 to feed V2 then dropping to another 22 for V1.

Im just thinking as im looking at it, the OT is only filtered by the single 50UF, which being a UL OT is the filtering for both Plate and Screen.
Im wondering if this is enough, and maybe the cause of my issue.

The ghosting is not a pleasant octave, its like a 7th below or so. Not hugely present but still pretty annoying.

Im thinking i may add 30uf to the first stage, making it 80uf, change the standby around to the rocket style, so the 80uf can get filled without a load on it, try and save the GZ34 getting that initial bump, like the rocket, and see what i get. Failing that, i will drop the UL and feed the screens the old fashioned way. Been on my mind to do for some time. And as the rocket has no neg f/back, like this amp originally, all is good.

I have to add, an octal rocket is a hell of an amp ;)
ampdoc1
Posts: 669
Joined: Sat Feb 26, 2005 12:42 am
Location: Tulsa, Oklahoma

Re: Filtering overtones?

Post by ampdoc1 »

I only throw this up, because it happened to me a few years ago! A guy brought in an amp for repair and complained of the same ghosting/harmonic problem. I went through it, fixed/changed everything that might cause such a problem. I tried it out as well as one of the of the guitar techs,..no ghost notes. The customer picked it up!

A couple days later he's back complaining of the same thing. Nobody in the shop could make the amp do it. So the customer back and starts playing, and, lo and behold, there are the ghost notes.

It turned out, his picking technique (perhaps his thumb brushing the string) was causing the "problem". He changed his pick attack and no more "ghost" notes.

ampdoc1
azatplayer
Posts: 556
Joined: Thu Feb 26, 2009 7:59 pm
Location: Great Southland

Re: Filtering overtones?

Post by azatplayer »

I know exactly what youre talking about, i do that myself on cramped necks.
I followed thru this morning and rearranged the filters. Added 16uf on to the first stage, no change. I decided to drop the UL taps off the screens, as its bothered me for sometime, i mean ive spent many hours rearranging various things.
I left the first 50uf/ plates, sent that to a new 47uf cap thru a 500R to the screens, then back to the filter line.
Well first thing i noticed was the amount of extra volume. I have read how UL attenuates your output to some extent, this is more than noticeable.. Has given it so much more output.
Oh, and the ghosting has gone. I may be wrong, and its only an assumption, but perhaps 50 uf filtering for both plate and screen is a little low on a 40 watt amp. It now has near 100uf fro both duties, may be too much, as its lost a bit of touch response. Im gonna try a JJ 40/20/20/20 cap in place of the 50/50 soon. Will prolly just grab a choke and throw it in too. I did all this mod with what i had in my parts drawers. So really to get to a rocket i need to totally overhaul the filtering.
May just try dropping both to 40uf and see how it gos. It is clean tho, and has some thump, with a zen in front its an amazing rock sound. Way cool.
My brother will try it tomorrow, he may like it the way it is.
Funny how you build these things and they kinda become your children, you try and bring them up a certain way but lo and behold the buggers are individuals with their own personality! Doesnt mean you like em any less!
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jelle
Posts: 2391
Joined: Sat Feb 04, 2006 7:55 pm
Location: New Jersey

Re: Filtering overtones?

Post by jelle »

With the current draw of KT66's you'll need more than 50uF on the center tab of the OT. Look at the Fender silverface series...eg the 2x6L6cg bandmaster reverb with an UL output stage and observe the filtering. That is twice what you used.

I know that this fender PS does not ghost.


Hope this helps.

jelle
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