# Tech question on cathode resistors



## keeperofthegood (Apr 30, 2008)

Hey oh

OK it is no secret we are looking to build an amp here 

We have been using the Champ 5C1 as our basis of design, and so far it looks sweetly blindingly simple and really upgradable.

Only thing is, I do no have a 6V6 handy. I have a less powerful 6F6 or I have a more powerful EL34 or 6CA7

The plans are for a 5500 ohm primary to plate, and 500 ohm resistor in the cathode. I cannot change the primary of the OT, however, I can add resistance to the plate, or change the cathode resistance. What would I be looking to do to put the EL34 or 6F6 in place of the 6V6?


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Hey oh
> 
> OK it is no secret we are looking to build an amp here
> 
> ...


Keeps, do NOT put resistors in series with the OT! Things would get very, very ugly! Don't even imagine it!

The 6F6 datasheets tell us that the values are all close enough for rock and roll. The plate voltage rating is a bit lower but for testing purposes you should be ok. The cathode resistor determines the bias for the tube. If things start to redplate you can bump the cathode resistor up to closer to 600 ohms.

The 6Ca7 is a different story. First off, the filament current is 1.5 amps, or over 3 times that of a 6V6! Can your power transformer handle the extra current?

To set the bias to a safe point, you should change the cathode resistor to 1000 ohms.

Actually, you will find that you won't really notice more power with the 6CA7. Just because a tube CAN handle more power doesn't mean it will give you more! Power is drawn, not pushed! This means that the tube draws power as it is driven to supply output power. Drive it harder and it will give you more output power, while drawing more power from the power supply. You have the same amount of preamp gain in your Champ no matter what output tube you have. The 6Ca7 is a bit more sensitive but still, there isn't enough preamp drive to make the output tube deliver that much more.

For testing, I'd just go with the 6F6.

WB


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## keeperofthegood (Apr 30, 2008)

Thank you Wild Bill! 

That helps lots! I have some 200 ohm 5 watt resistors, and some 330 ohm 3 watt I think.. going to scrounge about in my bits boxes too. @[email protected] I should just bread down and buy a 10 lot of tubes LOL gahhh next month


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## greco (Jul 15, 2007)

Wild Bill said:


> Keeps, do NOT put resistors in series with the OT! Things would get very, very ugly! Don't even imagine it!
> 
> WB


Keeps...Please Be Careful !!...this prototype/"redesign"/modification (specifically) is not worth (you) taking any risks to your safety. 
Not that any build is, obviously.

Dave


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## keeperofthegood (Apr 30, 2008)

HAHAHA Thank you Dave 

You know what the stupidest thing I ever did was?

I was about 10 years old. I had a counting thingy with PIXIE tubes in it (these are different than NIXIE, they are like neon lights with a sheet of paper infront of the lights with the numbers cut from the paper). I went looking for where a copper trace was (yes, it was circuit board with removable cards) and I stuck my hand in and 10 or 15 feet farther way doing the full body vibrating funky chicken the only thoughts in my head were 1) OW! and 2) I dun wanna die! LOLOLOL I got out my big meter and started to look for what bit me. My big meter could measure around 2kv, with heavy test leads and long probes. I found what bit me. 1600V rail that went from the power supply end, across the pcb, to a side plug and no where else. As they say, I got lucky, I could have fried myself, the current had to have been very very limited, or I think I would be dead. 

It sure threw me, put a scare into me, and damn that hurt too!!  Not the ONLY crazy stupid thing I have done in life (why I always make a grab for falling soldering irons I do not know), but it is probably the tops for "OMG what was that!" I say probably, being as we are guys we do have that piece of DNA they call "I cannot believe you lived to be an adult". I have fallen off cliffs, I have played in puddles in lightning storms, I even once mountain climbed my bedroom dresser and had it fall over with me under it


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