Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on May 13, 2014, 09:28:01 pm
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Been a while all. Sort of just doing life lately. Question:
I built this 5E3 clone using a Bogen CHB-35 chassis and a 1963 CordoVox cabinet
It used to be in a head cab and i thought it sounded pretty darn good back then
Now the chassis is mounted to the bottom of the CordoVox cab as a combo amp and the EQ wires are running up to the top of the EQ panel using an octal socket for the preamp and 4 pin socket for the power switch
It sounds very different and is lacking that sparkle and shimmer. I used minature sheilded coax for the volume and tone knobs
Could the coax be changing the impedance of the preamp? The EQ coax is at least 22" long. Could there be serious insertion loss going on here? Pic's Below
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... the chassis is mounted to the bottom of the ... cab ... and the EQ wires are running up to the top of the EQ panel ...
It sounds very different and is lacking that sparkle and shimmer. I used minature sheilded coax for the volume and tone knobs
Could the coax be changing the impedance of the preamp? The EQ coax is at least 22" long. ...
What is the definition of a capacitor? It is 2 conductors separated by an insulator. What is a coax cable? It is 2 conductors (mounted "co-axially" or on the same axis, so one is around the other) separated by an insulator.
In other words there is capacitance from the signal-conductor to the ground conductor; the capacitance varies with different types of cables. There is a set amount of capacitance per unit length depending on the construction of the cable and the material used as an insulator between the conductors. The longer the cable gets, the more capacitance (pF/foot * more feet).
If you changed nothing else (no tube changes, no speaker change), maybe the extra cable capacitance and where it is placed in the circuit is rolling off your highs. The cable capacitance will very likely look like capacitance to ground.
You might try a different type of cable; George L's exists because it had less capacitance per foot than typical guitar cables, and seemed to restor highs. The absolute lowest-capacitance cable I've seen is from Gotham cable (but I think they have a single wholesale U.S. distributor, maybe only a single retail distributor).
If you used multiple coax cables for the preamp signal, you might be getting a double-whammy of cable capacitance over what you might have seen if you used a single multi-conductor shielded cable. Then again, I'd have to see exactly where in your circuit the different wires are, to know if it's feasible to have all conductors in a single cable.
Anyway, I wouldn't call it "changed impedance" except that now you have the equivalent of a 100-200pF cap to ground hanging on the circuit each cable is connecting.
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I had the same issue years ago with an old Shaftesbury Dallas amp that i tried to mod. Finally i get the solution doing like the original amp was.
First gain stage in the upper chassis close to tone and volume controls and .... P.I. and power tubes in the "floor" chassis.
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Capacitance* Ok i understand. That is a better explanation than impedance change HotBluePlates
I think i am going to put it back as a head config. Yes chocopower like some of those old Sano amps with the power supply and PI in the bottom of the cab and the EQ up top all on the same panel.