Charlie Wilson wrote: ↑Tue Jun 25, 2019 7:19 am
Whoa, not trying to start any trouble here. Just to clarify, I have never seen a Low Plate Classic in person. My only knowledge of them is from photos mostly on this forum. So Hoyt Axton, Joe Bonnamassa, Carlos Rios, and Borderland are the only ones I have seen. My hope was that maybe there is some more variations out there that people know about and they would chime in. Like Max did. Max do you recall if the .68 was a tantalum? I find, to my ears, the shift in frequency the 3k3/270 gives the presence cap a step in the right direction to where I want my amp to sound. I just need to find a presence cap value I really like. I said it before but I find it interesting that these early transitional accent switch amps were intended to be played with the presence out of the circuit unless you want the extra high end bump. Tony, check out the GNFB/accent switch on the crazy looking high plate amp you posted a photo of.
CW
Charlie, for me, my #40 second gen amp sounds best to me with the accent off (so the presence out of circuit) in OD mode, I tend to run it permanently like this and use the gtr volume to clean up.
regarding the presence control I agree it is a very subtle thing, I tend to just leave it at 12 O clock and never mess with it that much.
Tony, check out the GNFB/accent switch on the crazy looking high plate amp you posted a photo of.
CW
Looks to be a 2.8K w/280 ohm.(looks grey to me) Presence cap ? Judging from the size 1uF (but that's a guess) So this one would have a bit more NFB .090 or 9% w/switch.(Your right the amp is a HP "Classic" so add that one to the list) So having tried several different types of Presence caps. The one I like best is a Wima (FKC-2 Polycarbonate Film/Foil). Sometimes different cap types of the same value can sound different.You have a good set of ears and might notice this! So I would be interested to hear your opinion if you go there.
A long time ago we set up a relay to switch between 2 different NFB resistors for a player. So when you go to OD engaged the relay for a smaller NFB resistor. So the amp had "less" NFB on the clean to open it up and a bit more for OD to take the edge off and shave off some highs smooth it out. Not that you want to do this but it actually worked ot pretty well.
BTW. My post was not to flame you and am glad you pointed out the "classics" you have seen that have that FBC. am keeping count for you
Check your inbox
Tony
" The psychics on my bench is the same as Dumble'"
Charlie Wilson wrote: ↑Tue Jun 25, 2019 1:25 am
Martin, I am curious, how large would a presence pot need to be to completely defeat the effect of the presence cap? Hearing the Low Plate Classic without the presence cap is a bit of a revelation for me as the amp was intended by Dumble's design at the time to be heard flat and if you wanted some high end kick you flip the switch. The same with the 2nd and 3rd Generation amps.
If you make the presence pot 5x the tail resistance then its pretty well out, but not completely. That's what you see in the 2k pot with 390Ω tail, and modern Fender circuits where the tail is 5k and the pot is 25k. The trade-off is the bigger you make the pot, the smaller the useful part of the rotation is. The switch allows turning it off completely so a pot small enough to be effective over a large range of rotation can be used, a finer adjustment really.
talbany wrote: ↑Tue Jun 25, 2019 6:31 am
Why did you run that circuit?
Here are the GNFB circuits Dumble used and I thought we were discussing
3k3/390/4K7 1uF with a 2K pot This would be the "Skyliner" FBC
2k7/270/4K7 1uf 2k pot ..This would be (according to CW) the "Classic" FBC
3k3/270/4k7? .1 (104) w 500-ohm pot. This is called the "accent" control
There is another one that just has the " switch" (no Pot) I am so confused now I don't know what that one has maybe C.W can tell you.
There are four components involved: Rfb, Rtail, Rpot, and Cpres. You are listing an extra resistance... does that mean you want to compare the change due to e.g. 3k3 vs.4k7 Rfb in the Skyline case?
My intent was cover the circuit in general rather than model any specific case. The values used to make the plot are like the Skyliner amps (#124 is 4k7/390/2k/1u), except I dropped the pot to 500Ω to exaggerate the residual presence effect when the pot is at max resistance, and added the switch to the circuit to show how the cap can be removed as in the "Accent" switch. I included the equations so you are free to calculate the key parameters for any component values you like ;^)
AFAIR I remember there seem to exist some other configurations, too. Example: a low plate classic ODS 100W with this 3k3/270 configuration, but with a 1,5k presence pot and a 0.68 uF presence cap.
Max
Do you have an opinion on this circuit?
AFAIR I liked this presence pot configuration.
Charlie Wilson wrote: ↑Tue Jun 25, 2019 7:19 am ... Max do you recall if the .68 was a tantalum? ...
Very interesting read. I have been studing the GNFB, tail and presence cap at the Borderline Special and found the placement of the accent cap somewhat peculiar and different than #102 #124, etc.
On the layour of #102 #124 the tail resistor and presence cap meet at the ground and then you have the other side of the presence cap to the presence pot. In this case it looks like the presence cap is wired between the meeting point of GNFB, 24K and 390R and the presence pot
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Just to clarify, I have never seen a Low Plate Classic in person. My only knowledge of them is from photos mostly on this forum. So Hoyt Axton, Joe Bonnamassa, Carlos Rios, and Borderland are the only ones I have seen. My hope was that maybe there is some more variations out there that people know about and they would chime in. Like Max did. Max do you recall if the .68 was a tantalum? I find, to my ears, the shift in frequency the 3k3/270 gives the presence cap a step in the right direction to where I want my amp to sound. I just need to find a presence cap value I really like. I said it before but I find it interesting that these early transitional accent switch amps were intended to be played with the presence out of the circuit unless you want the extra high end bump. Tony, check out the GNFB/accent switch on the crazy looking high plate amp you posted a photo of.
