1/2 Power Switch Question...

[v00d00blues79]

Anyone have a schematic on how to wire up the 1/2 power switch? i'm looking at the available schematics and only see 50w versions.

Thanks,

Andy

[Bob-I]

Anyone have a schematic on how to wire up the 1/2 power switch? i'm looking at the available schematics and only see 50w versions.

Thanks,

Andy

Basically all you need to do is lift the cathode ground on 2 of the power tubes.

[dobbhill]

How does that affect plate load impedance?
D

[LooseChange]

It does not make a difference. Everybody asks that question. Try it, listen and tell us what you think.

[llemtt]

Disabling two out of four tubes doubles the output impedance of the amp while it also lowers the "open-loop gain" of the amp.

You can mismatch the speakers (8 ohm speaker on 4 ohm output) to compensate the impedance but as already said it's usually not needed because you can hardly hear a difference!

The "open-loop gain" change makes the presence control just a little less effective, another thing you can't notice unless you are running presence at 8-10.

Teo

[v00d00blues79]

also, what is the LED for on the back of the amp under the 1/2 power switch? is it to visually indicate the amp running in 1/2 power mode?

Thanks,

Andy

[Funkalicousgroove]

Yes, the LED indicates 50W mode, and YES YOU DO need to change the impedance, and you WILL hear a difference.

[LooseChange]

Funk knows better than me.. I concede.

[Funkalicousgroove]

Just trying to save later posts on blown OT's and arced tube sockets When you disconnect two tubes and leave the speaker load the same you are still running at the same reflected primary impedance.

EX: Twin OT, 2K primary with 4 tubes @ 4 ohm load, when you lift the 2 cathodes you still have a 2K primary but you have tubes that are wanting to see about 4K, so they end up dissipating more heat, they'll still operate, but they'll operate hotter and be biased differently. You only really lose 1/5 or so of the output power when the Load is a perfect 4 ohms, but most 8 ohm speakers measure about 6.2 ohms, so 2 in series actually end up being a 3.1 ohm load and throw the amp further out of whack. By simply doubling the load the tubes see the 4K they're looking for and everything is happy happy

[Robert]

Attached is the schematic for Mesa Mark IV power section showing their, presumably patented "simul-class/class A" switch, lifting the cathode from ground. I do trust your answer Funk, does Mesa just take their chances not recommending changing the ohmage and running the tubes biased to hot. I included this switch on my 100 watt build, but have never used it because it raised the bias of the two tubes with the lifted cathode ground to what I thought was an unexceptably high level.

[LooseChange]

I think if you've got an OT like a Hammond... You don't really have to worry much about mismatch.. Right Funk?

It's true that the B+ and Bias will rise if you diconnect one tube. I find with smaller tubes like 6V6's it's VERY minimal.

[Funkalicousgroove]

As far as the Boogie goes I'd say it depends on the OT, If you have a trannie with a 3k3 Primary it wouldn't matter much, because a pair of 6L6's will run just fine at that- but the closer it gets to dead short (Less than 1K) the greater the chances will be of overheating and eventually burning up your OT.

IF you are only mismatching 1/2 or 2/1 either way it doesn't hurt much anything, you just overheat your tubes and wear them out quicker, and your amp is less efficient. Where you really have Issues is a 1/4 or 4/1 mismatch like running the 16 ohm output into a 4 ohm load you're begging to burn up your OT, running the 4 ohm output into a 16 ohm load you're begging to arc your tube sockets.