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dropping voltage in Champ?

6/18/2000 10:13 PM
dropping voltage in Champ?
Anyone know the exact wattage rating (of a zener diode) I should use if I want to drop the B+ in a SF Champ by 50-75V? It's running about 396V on the plates of the 6V6 (NOS Mazda) which start to glow red after a while.  
Thanks for any info.
6/18/2000 11:25 PM
Peter S

Use a 50watt 50volt reverse biased Zener diode.  
6/19/2000 12:13 AM

What's the reason for the 50 watt? Why not a 10W or 25W?
6/21/2000 11:33 AM
Peter S

meant to say 5watt, although a 50watter would work just fine.  
6/19/2000 5:22 AM

Assuming you know what an overheated power tube looks like when it's "glowing red"..... :D  
It's probably something else other then just the plate voltage.  
Stomp box on the front end or bare guitar?  
Are you using a NOS RCA or GE 5Y3GT or a new SOVTEK 5Y3... IMHO (bad) for these amp.  
Is that 396 volts on the plate to ground or on the plate to the cathode Cole?  
Usually these amps have about +20v or so on the cathode.  
The actual plate voltage is the the difference of the plate to cathode.  
Yours would probabaly be about 375vdc.  
That means you should be idling the 6V6GT-6V6GTA  
tube at around 34ma to 38ma or so.  
Do you know what the idle current is yet?  
If the tube is glowing with those 33ma-37ma numbers, then I'd say check your screen voltage and make sure it is not too high.  
You could drop that a bit with a singular 1K 2W resistor from the board right to the tubes socket on pin 4, eliminating the solid wire there now.  
I've found, many times, that simple 1K 2W trick and replaceing the original cathode bias resistor with a 510 ohm 2 watt metal oxide one instead, will take some of the load off the tube.  
If you need a zener then pick a lower voltage then 50v to 75v!!  
That will suck the life right out of that amp.  
You should be able to use a 5w 20v zener here with no trouble.  
Slightly adjust the cathode bias reistor and the screen resistance value to get the tube where it should be... at idle current.  
The other main factor in killing these tubes is using a stomp box with HUGE square wave preamp signals going into the amp.  
That is the kiss of death on a SE class A amp that is near it's limit anyhow.  
6/19/2000 2:27 PM

Thanks for the info to all!  
The rectifier and preamp tubes are stock, and I've been playing around with the output tube. Here's weirdness if I've ever seen it: the amp burns up 6V6EHs but it runs fine for hours with a Marconi 6V6 (*metal* envelope). I've done the screen resistor mod (1k, 5W) which actually improved the tone, to my ears.  
I've been doing some reading and figure the tube is just running too hot, there's about 22.4V on the cathode, and with the drift of the cathode resistor (414 ohms) it's running at more than 50mA, not too good!
6/20/2000 2:30 PM

"Here's weirdness if I've ever seen it: the amp burns up 6V6EHs but it runs fine for hours with a Marconi 6V6..."
Your lack of experience betrays you! :)  
There is no newly manufactured 6V6, including the new SOVTEK 6V6EH, that will stand head to head with an old NOS 6V6.  
Change the cathode resistor to a 510 ohm one and see if that helps before you go any further.  
Your running that poor tube at close to 18 watts at idle!!  
For 12-14 watt tube, 18 watts at idle sure doesn't give it much of a chance to cool down after peak power bursts.  
What is the measured screen voltage?  
Have you replaced the cathode bypass cap too?  
Probably a good thing to do if not.  
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