100W OT in 50W amp?

[Justin Thomas]

Hello,

I have a SF Showman/Twin Reverb OT that I'd like to use to make a small bass (single channel 1x12" ~50W) combo. As I understand, if I have an amp with four output tubes and I pull two, I also want to halve the speaker load to keep the matched ratio? Since this is basically what I am doing (using a 4x6L6 OT 4-ohm sec. w. 2 6L6s), I would want to run this at a 2ohm load, which would be rather impractical since 2ohm speakers are rare and don't want to use 2 speakers. Is that correct? I'd rather not blow up this transformer doing something silly, but if I can use it, it was free... Are there other output tubes that might provide a better match for this OT in a pair? I'm still learning the basic concepts in amp-math, impedance matching, what specs apply to what application. What spec would I check as I surf other output tubes? I'd like to do some research and learn some basic concepts. I'm comfortable with 6CA7, KT66, the usual suspects. 6AR6 might be attractive & cheap... others? Nothing TOO weird. :P

Thanks in advance,

Justin

[Diablo]

Nope, you need to double the 4 ohm speaker load if you run only 2 output tubes. Use an 8 ohm speaker and two - 6L6 tubes and you have a perfect impedance match.

[Justin Thomas]

thanks!

Hey Diablo,

Thanks for the correction. Upon further reading, halving the impedance has to do with the selector switch if your amp has one. In the case of Fenders, you'd disconnect a speaker, which is the same as doubling. Sorry for the confusion.

So say I have the specs for an OT. In the case of the one above, 2000 primary, 4 secondary. What tube spec would I use to find out whether a certain tube complement would work? I've seen both "Load Resistance" and "Plate-to-Plate Resistance" listed. I'm thinking plate-to-plate? I can figure out the math with further reading, just need to know which numbers to plug in!

Thanks again,

Justin

[Diablo]

If you look at the tube manufaturer's data sheet, pick out the load resistance for the type of amp you intend to build. Let's say I want to build a push pull class AB1 amp. Go to the 6V6 tube data section for push pull class AB1 amp for "characteristics and typical operation". Read out the plate voltage (285 volts in this case), and read out the load resistance (8,000 ohms) - that's for two tubes. A typical operating load for a pair of 6L6 tubes in push pull AB1 is 4,000 ohms.

[Justin Thomas]

Diablo,

Thanks, that's the spec I was looking for - load resistance! That's what I was leaning toward. I know some say it's not rocket science, but any-old-part-will-do is not my style. Now to read up on my math a bit, do some practice, and learn some more!

And glad to know I can put this piggy to good use.

Justin