# Switching tubes question



## Imported_goods (Mar 4, 2006)

I've got a question concerning the output tubes on my amp, any help would be appreciated.

The amp is an all-tube cathode-biased amp, with 2 6L6's for the output (tube rectified and 2 12AX7's in the preamp). There are 2 output jacks, one for 4 Ohms and one for 8 Ohms.

The amp is pretty loud, and I was told that I could safely swap the 6L6's for 6V6's, since the amp is cathode-biased. I was also told that the impedance values for the outputs would double if I switched tubes, i.e. the 4 and 8 Ohm outputs would become 8 and 16 Ohm outputs respectively.

Is this true? And since the amp was designed around 6L6's, will the current or whatever be too high for 6V6's and shorten tube life or amp life?

:confused-smiley-010 Thanks!


----------



## Wild Bill (May 3, 2006)

*It's always something...*



Imported_goods said:


> I've got a question concerning the output tubes on my amp, any help would be appreciated.
> 
> The amp is an all-tube cathode-biased amp, with 2 6L6's for the output (tube rectified and 2 12AX7's in the preamp). There are 2 output jacks, one for 4 Ohms and one for 8 Ohms.
> 
> ...


Maybe you can and maybe you can't!  We need to know a few things first.

I don't see what cathode biasing does to guarantee you can switch those tubes. In many circuits 6L6's and 6V6's use the same or close value of cathode resistor but that's not the important part. We need to know the plate voltage on those tubes!

Most 6L6 amps run plate voltages on these tubes of 450 volts DC or more. This is much higher than the maximum spec for 6V6's. You can push this voltage rating somewhat (Leo Fender did with the Deluxe Reverb) but it could easily be asking too much. If this is the case, 6V6's will likely die early. You do pick up a little margin with cathode biasing in that the tube actually sees a bit less than the supplied plate voltage because the biasing method lifts the cathode up from ground by the amount of the bias voltage but not likely enough to make things safely work.

As for the impedance thing, again if we know nothing about your specific amp then we can only make a wild ass guess! 6L6's are most often designed in with a 6600 ohm plate load. The output transformer steps this down in a FIXED ratio to the 4,8 or whatever speaker taps.

Doing what you describe with higher value speaker loads will get stepped up back through the transformer to a load of over 13k ohms. This is much higher than 6V6's like to see, who usually run from 6k to 8k and would be perfectly happy with the speaker loads being run the same as a 6L6.

We need to know more about your amp. You're talking about the number of pistons and horsepower but telling us nothing about the compression ratio!

What kind of amp? Or at least, what is the plate voltage?

If you're not comfortable safely measuring the plate voltage, please get someone knowledgeable to do it for you. We'd like you alive to continue this thread!


----------



## Imported_goods (Mar 4, 2006)

Thanks very much for your response, Wild Bill. Your depth of knowledge in this field never ceases to impress me!

You're quite right, there IS quite a lot of information missing, and unfortunately it's because the amp is a hand-built affair by a really nice guy who happens to be on vacation right now, thus my inablility to go directly to the source for answers!

I had no idea that my question involved so many variables, which goes to show how ignorant I am about all things electric  

I'll bide my time and wait for the builder to get back from summer vacation, because I feel I definitely WILL injure myself if I start to try measuring plate voltages...


----------



## Wild Bill (May 3, 2006)

*We're all weary pilgrims...*

Glad to hear you've got local support available!

Don't ever feel embarassed about needing to ask basic questions. How else do we ever learn?

And beware the man who claims to know everything! He's only trying to scam your vote!


----------

