Cathode mod?

JohnnyCrash · June 25th, 2008, 04:46 PM

I gotta big mouth and a small brain, so...

I'm hoping one of you amp guys with a larger brain can double check my math here.

I am going to mod my 5E3 yet again. This time I want to be able to swap preamp cathode values while the amp is on. My current switch setup is as follows:

CURRENT CATHODE SETUP:
Both of V1's cathodes (pins 3 and 8) are tied together, then instead of going to resistor and bypass go to a DPDT switch. The switch then goes to either a 2.7k and 0.68uF cathode setup, or the stock 820 and 25uF setup - both of which are grounded.

DRAWBACK:
I cannot flip the switch without turning the amp off since momentarily the cathode will loose it's load.

WHAT I'D LIKE TO DO:
I'd like to always maintain a resistor on the cathode and just engage another one in parallel so that I can switch while the amp is on. I would also have to swap out bypass caps in the process.

MY QUESTION:
My thought was to wire up a 2.7k and a 1k in parallel. This should give me 830 ohm resistance, right?

I'm hoping I can wire one side of the DPDT switch to engage or disengage the ground for the 1k resistor and on the other side engage or disengage the ground for either cap.

Something like this (DPDT on my tone switch):

1 1
2 2
3 3

On the left side pin 1 goes to the 0.68uF cap, pin 2 (center pin) goes to ground, and pin 3 goes to the 5E3 stock 25uF cap.

On the right side pin 1 goes to nothing, pin 2 goes to ground, and pin 3 goes to the 1k resistor. The 2.7k resistor is always grounded.

When pushed in, the switch engages pins 2 and 3, grounding the 25uF on the left side and the 1k on the right side (with the 2.7k always grounded the 1k in parallel makes 830 ohms). When pulled out, the switch engages pins 1 and 2, grounding the 0.68uF cap in circuit and shorts the 1k from ground, leaving the 2.7k.

This way I always at least have a 2.7k resistor on that 12AY7's cathodes at all times. I should be able to switch with the amp on (without having to power down), without dropping the cathode's resistor at all.

Does this sound good or did I have too much coffee? Again, rememebr I am a complete moron :)

teleamp · June 25th, 2008, 05:04 PM

The "momentary" loose of the bypass cap will not hurt the tube. That is how some boost switches work.

JohnnyCrash · June 25th, 2008, 05:19 PM

Quote:

Originally Posted by teleamp

The "momentary" loose of the bypass cap will not hurt the tube. That is how some boost switches work.

Yes. This is how many of my "Champ"-like builds have a "boost" as well. With my 5E3 on the other hand, I currently have it wired to momentarily lose cathode resistors. What I'm worried about is my math...

I'm hoping my math is correct (and I have an extra 1k laying around), so I can get a 830 ohm when in parallel - an 830 ohm is close enough for normal 5E3's/12AY7's given 5-10% +/- resistor tolerances anyway.

SO...
Does my parallel Resistor math look correct? 2.7k parallel with 1k equals 830 ohms, right?

This way I can always keep the 2.7k in-circuit and simply drop the 1k in or out to give me stock 5E3 values and not lose any resistors during the momentary swap in/out. The caps I have no worries about swapping - it's the resistors.

Scott S · June 25th, 2008, 05:31 PM

I get 730 ohms, but close enough.

JohnnyCrash · June 25th, 2008, 05:38 PM

Quote:

Originally Posted by Scott S

I get 730 ohms, but close enough.

Damn.

I'd like to get closer to the stock 820 ohms if I can...

Oh well... time to do it the hard way: bust out the meter and start measuring other resistors in parallel with a 2700 HAHAHA!!

I'm also realizing I can simply always leave the 0.68uF cap in-circuit, and simply swap the 25uF one in/out. A 25.68uF bypass isn't off 5E3 spec at all (again, given tolerances +/-).

teleamp · June 25th, 2008, 05:49 PM

I don't know what I was thinking.

Scott S · June 25th, 2008, 05:52 PM

By my math, you'd need a 1178-ohm resistor, assuming the other one is perfect. They *do* have 1200-ohm resistors...

- Scott

teleamp · June 25th, 2008, 05:58 PM

Here ya go!

http://xpertvid.startlogic.com/573/615.html

JohnnyCrash · June 25th, 2008, 06:01 PM

I've got it.

A 1200 in parallel with a 2700 will have a resistance of 830 ohms.

I am wiring my switch now to always have a 2700 and 0.68uF in-circuit, then I will have the 1200 and 25uF added in or out to give me either 2.7k+0.68uF or 830+25.68uF.

That'll do it!! Good thing I drilled my own board... I added another eyelet when I was fooling with my cascade switch mods (since torn out).

I'll be back to post my results and measurements.

JohnnyCrash · June 25th, 2008, 06:03 PM

Quote:

Originally Posted by teleamp

Here ya go!

http://xpertvid.startlogic.com/573/615.html

Thank you!! Those are exactly what I'm looking for, except those turn off the switch when closed. At least I'm on the right track now. "Switch safety covers." Cool...

Thanks guys! You two are always helping everyone out!

JohnnyCrash · June 25th, 2008, 07:15 PM

Finished.

Works great. I can get fat, barking tweed Deluxe mids, or slightly thinner/brighter Marshall mids at the pull of a Tone knob now... without having to power it down.

Jumpered channels (essentially V1 in parallel) with Marshall cathode switched on is an in-between midrange thickness.

Thanks guys!! Measured 830 ohms in Fender mode and 2,733 ohms in Marshall mode.

Now with this and my rectifier switch I can go from saggy, syrupy thick Fender to tight bottomed SS rectified (still essentially Fender, just with less fart and more volume/push).

JohnnyCrash · June 25th, 2008, 07:18 PM

Oh, and I didn't even need to wire a DPDT, all I would have needed was a SPDT. I simply ground or un-ground the 1,200 ohm resistor and 25uF cap from one switch lug.

11 Gauge · June 25th, 2008, 07:32 PM

Quote:

Originally Posted by JohnnyCrash

I am wiring my switch now to always have a 2700 and 0.68uF in-circuit, then I will have the 1200 and 25uF added in or out to give me either 2.7k+0.68uF or 830+25.68uF.

Actually, I now pair all of my e-caps on the cathodes with a small poly - it allows me to resort to some of the cheaper brands when I'm in a pinch, and still have them sound great.

I was getting this goofy DC leakage noise from my Fischer-inspired build when I rolled down my guitar's volume and played an Ab. I bypassed the 22 uF caps with a pair of 1 uF polys, and the noise is gone, and the amp sounds even better! I LOVE that amp!

Scott S · June 25th, 2008, 09:23 PM

Quote:

Originally Posted by JohnnyCrash

Finished.

Works great. I can get fat, barking tweed Deluxe mids, or slightly thinner/brighter Marshall mids at the pull of a Tone knob now... without having to power it down.

Jumpered channels (essentially V1 in parallel) with Marshall cathode switched on is an in-between midrange thickness.

Thanks guys!! Measured 830 ohms in Fender mode and 2,733 ohms in Marshall mode.

Now with this and my rectifier switch I can go from saggy, syrupy thick Fender to tight bottomed SS rectified (still essentially Fender, just with less fart and more volume/push).

Sound clips or it didn't happen.

- Scott

JohnnyCrash · June 26th, 2008, 01:52 AM

Quote:

Originally Posted by Scott S

Sound clips or it didn't happen.

- Scott

I'm busy tomorrow (or is it "today" now?), so Friday for sure.

I'll do some clips with a Tele/Esquire, my Les Paul Junior (P-90 bridge, minihumbucker neck), and my '57 RI Les Paul... both in stock and in SS rect/Marshall cathode modes.

That Jensen C12N is a beast, so that'll hopefully give folks a taste of a 5E3 with that speaker as well.

June 25th, 2008, 04:46 PM	#1 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	Cathode mod? I gotta big mouth and a small brain, so... I'm hoping one of you amp guys with a larger brain can double check my math here. I am going to mod my 5E3 yet again. This time I want to be able to swap preamp cathode values while the amp is on. My current switch setup is as follows: CURRENT CATHODE SETUP: Both of V1's cathodes (pins 3 and 8) are tied together, then instead of going to resistor and bypass go to a DPDT switch. The switch then goes to either a 2.7k and 0.68uF cathode setup, or the stock 820 and 25uF setup - both of which are grounded. DRAWBACK: I cannot flip the switch without turning the amp off since momentarily the cathode will loose it's load. WHAT I'D LIKE TO DO: I'd like to always maintain a resistor on the cathode and just engage another one in parallel so that I can switch while the amp is on. I would also have to swap out bypass caps in the process. MY QUESTION: My thought was to wire up a 2.7k and a 1k in parallel. This should give me 830 ohm resistance, right? I'm hoping I can wire one side of the DPDT switch to engage or disengage the ground for the 1k resistor and on the other side engage or disengage the ground for either cap. Something like this (DPDT on my tone switch): 1 1 2 2 3 3 On the left side pin 1 goes to the 0.68uF cap, pin 2 (center pin) goes to ground, and pin 3 goes to the 5E3 stock 25uF cap. On the right side pin 1 goes to nothing, pin 2 goes to ground, and pin 3 goes to the 1k resistor. The 2.7k resistor is always grounded. When pushed in, the switch engages pins 2 and 3, grounding the 25uF on the left side and the 1k on the right side (with the 2.7k always grounded the 1k in parallel makes 830 ohms). When pulled out, the switch engages pins 1 and 2, grounding the 0.68uF cap in circuit and shorts the 1k from ground, leaving the 2.7k. This way I always at least have a 2.7k resistor on that 12AY7's cathodes at all times. I should be able to switch with the amp on (without having to power down), without dropping the cathode's resistor at all. Does this sound good or did I have too much coffee? Again, rememebr I am a complete moron :)

June 25th, 2008, 05:58 PM	#8 (permalink)
teleamp Tele-Holic Join Date: Oct 2006 Location: Central TX Age: 49 Posts: 592	Toggle switch safety cover. Here ya go! http://xpertvid.startlogic.com/573/615.html

June 25th, 2008, 05:04 PM	#2 (permalink)
teleamp Tele-Holic Join Date: Oct 2006 Location: Central TX Age: 49 Posts: 592	The "momentary" loose of the bypass cap will not hurt the tube. That is how some boost switches work.

June 25th, 2008, 05:31 PM	#4 (permalink)
Scott S Tele-Afflicted Join Date: Dec 2005 Location: SW CR IA US NA PE Age: 28 Posts: 1,576	I get 730 ohms, but close enough.

June 25th, 2008, 05:49 PM	#6 (permalink)
teleamp Tele-Holic Join Date: Oct 2006 Location: Central TX Age: 49 Posts: 592	I don't know what I was thinking.

June 25th, 2008, 05:52 PM	#7 (permalink)
Scott S Tele-Afflicted Join Date: Dec 2005 Location: SW CR IA US NA PE Age: 28 Posts: 1,576	By my math, you'd need a 1178-ohm resistor, assuming the other one is perfect. They do have 1200-ohm resistors... - Scott

June 25th, 2008, 06:01 PM	#9 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	I've got it. A 1200 in parallel with a 2700 will have a resistance of 830 ohms. I am wiring my switch now to always have a 2700 and 0.68uF in-circuit, then I will have the 1200 and 25uF added in or out to give me either 2.7k+0.68uF or 830+25.68uF. That'll do it!! Good thing I drilled my own board... I added another eyelet when I was fooling with my cascade switch mods (since torn out). I'll be back to post my results and measurements.

June 25th, 2008, 07:15 PM	#11 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	Finished. Works great. I can get fat, barking tweed Deluxe mids, or slightly thinner/brighter Marshall mids at the pull of a Tone knob now... without having to power it down. Jumpered channels (essentially V1 in parallel) with Marshall cathode switched on is an in-between midrange thickness. Thanks guys!! Measured 830 ohms in Fender mode and 2,733 ohms in Marshall mode. Now with this and my rectifier switch I can go from saggy, syrupy thick Fender to tight bottomed SS rectified (still essentially Fender, just with less fart and more volume/push).

June 25th, 2008, 07:18 PM	#12 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	Oh, and I didn't even need to wire a DPDT, all I would have needed was a SPDT. I simply ground or un-ground the 1,200 ohm resistor and 25uF cap from one switch lug.