Tube Amps / Music Electronics
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|4/23/2005 10:48 AM|
|Joe||Cathode bypass switch for 5F1|
Does anybody have a good design for installing a switch (off/on) for the cathode bypass cap in the preamp of a 5F1 Tweed Champ?
|4/23/2005 11:18 AM|
You mean to ADD a preamp cathode bypass cap??? The 5F1 doesn't have one to begin with.
I've never seen a special drawing or schem for that. Seems simple enough though. If it's an actual Champ I wouldn't do anything permanent to it. You could set up a new input jack system. One standard with the cursory ground default, and the other with a switching arrangement to lift that ground while the second jack is in use that also switches in the bypass cap. You'll want to bridge a resistor (maybe 47k) across the switch for the cap to avoid a POP when you plug into it. I'll do a quick drawing to see if it can be done and post back.
|4/23/2005 1:54 PM|
Altough not mentioned in the schematic most 5F1s DID have a cathodebypasscap on the 1st cathode....
Hope this helps,
Love, peace & loudness
|4/23/2005 6:38 PM|
Thanks Chris. I was referencing the schematic. I've never even seen the inside of am early Champ.
|4/25/2005 3:45 PM|
Here are notes on the Kendrick Ruffneck mod for the 5F1:
If your interested, the Roughneck is a Fender 5F1 champ (Geralds favourite Champ, mentioned in his first book) with these mods:
1.The cathode bias cap is increased to 100uF@100v
2.The feedback resistor is increased in value to 56K, instead of 22K.
3.The first 12AX7 stage has the cathode resistor bypassed by a 25uF@25v cap.
4.Gerald doesn't use a 12AX7, he uses a 5751 tube instead.
5.The input stage doesn't consist of a 1Meg resistor and two 68K resistors going to the grid of the first tube.
The 1Meg resistor is removed, the 68K resistors are replaced with 470K resistors, one of these 470K resistors has a 250pF silver mica cap across it.
The shorting terminal on the input jacks,are connected to earth so that when one user is using the amp the first stage always sees a 470K grid leak resistor.
From the guitar output you get a 470K resistor in series followed by a 470K resistor to earth (grid leak). So the 250pF cap provides trble boost or shunt.
6. Gerald also recommends a silver mica cap across the volume control (100pf to 250pF) for those who require more brightness.
Regarding construction, the amp uses all 1/4 watt carbon comp resistors, the filter caps are Sprague Atoms, the two decoupling caps are Orange drops (forgot to check which series).
Here are relevant voltages;
Amp using stock 6L6 and GZ34 tube.
mains voltage at the time of testing; 245Vac.
rail to O/P txer; 404Vdc
rail to O/P valve screen grid; 361Vdc
rail to preamp; 306Vdc.
AC voltage across mains TX secondary; 667Vac.
voltage on preamp plates; 187Vdc.
voltage on preamp cathodes; 1.82Vdc.
Voltages using 5Y3 rect and 6L6 tubes.
rail to O/P tx; 368Vdc.
rail to screen grid; 325Vdc.
rail to preamp tubes; 279Vdc.
Voltages using 5Y3 rect and 6V6 tubes.
rail to O/P tx; 382Vdc.
rail to screen grid; 332Vdc.
rail to preamp tubes; 283Vdc
|4/25/2005 6:05 PM|
O.K. I drew it up on paper and it works. It's easy. You can switch the cathode bypass cap in and out on the input jacks with NO modification to the chassis. There is however a very minor cosmetic difference because the mod uses those black plastic Marshall type jacks. So you would have black sleeve rings instead of metal. If you can live with that, heres how it would work.
One input is the standard high gain input with the first stage cathode bypass cap in place.
The other is the same except with the first stage cathode bypass cap out of the circuit.
With both jacks in use they are both high gain with the first stage bypass cap out of the circuit.
You will no longer have a low gain input (but who uses it anyway?)
The mod still offers the ground default protection when nothing is plugged in so theres no run away tubes. If you are interested I can make a drawing on MSPaint and Email it to you.
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