M-125 question/suggestion

M-125 is described as having the possibility to use either 2 or 4 powertubes. Some
suggestions is to swap 2 tubes out when not needed, and swap them back when
the need arises.

Well, swapping tubes in and out is never beneficial. How to arrange a switchable
tube reduction that does not need to switch several 500VDC circuits without
creating dangerous POP's and flashes ?

Surprisingly easy. Disconnect the filaments on 2 tubes ! If disconnected when 4 tubes
are used, the 2 will slowly fade out. If connected while 2 tubes are active they
will slowly fade in.

The catch is that bias adjustment might change ( needs investigation).

Bob, is this a good idea ?

Peterh,
Don't over-think this.
Pull the back pair of output tubes -- power off of course -- then re-bias the front tubes. You'll drop to about 65 WPC but that's plenty for most speaks, anyways. No soldering nor un-soldering required. The filament end of the power supply will not be looking at a short if you do this and is not a worry.
Trust me, you won't break anything. But dial down the front bias accordingly.

But wait! I think it's not about horsepower so much as torque! Whilst 4 tubes gives you more top end power (akin to horse power, if you will), Isn't there a benefit down low (akin to torque?) in dynamics?

Dave, have you any experience going from 2 to 4 tubes per amp?

Tube Nube wrote:But wait! I think it's not about horsepower so much as torque! Whilst 4 tubes gives you more top end power (akin to horse power, if you will), Isn't there a benefit down low (akin to torque?) in dynamics?

Dave, have you any experience going from 2 to 4 tubes per amp?

Yes, I have. In our week or two of summer here in the Bitterroots I will pull the back tubes out, just to make everybody a bit cooler. At the SPL levels I listen at, I can hear no difference. Just, when you do a tube swap like that, the plate volts go up, so you must, must, must re-check the bias when you do it. Likewise, when you re-install the back tubes. Otherwise, it's a no-brainer

The goal with switching off filament for 2 tubes is to save tubes and sockets from wear. It
would also make the amp a "switchable 60 or 125w amp"
The need for re-bias is there whatever you do unless extra complexity is added that
connects / disconnects a pot that adjusts the bias when switching filament power. And the fact that
current filament wiring prevents break of filament to two tubes is a weak argument
that is cured by a tag strip.

This is only a suggestion, i have no M-125 myself, just wanted to contribute something that might
be of value.

Peterh,
If you're only doing this once or twice a year due to external temperatures or to save on your electricity bill nothing will be unduly stressed. Any time you pull or replace an output tube, it's a good time to re-tension the sockets anyway, then give them a short squirt of de-oxit before re-installing. This is especially important if you're changing out different tube types. Not all output tube pins are created equal in terms of diameter or circumference. The Russian pins are slightly fatter than the US-made Tung-Sols; I have measured them. So if you've been using KT-120s, for example, then replace them with British KT-88s or US-made TS 6550s, re-tensioning the socket holes is vital unless you enjoy fireworks as a couple of tubes red-plate and sink your bank account.
I am also told by a retired Fluke engineer and have mentioned elsewhere on this forum, gold pins and zinc sockets or vice-versa do not make for a happy conductive circuit. Zinc to zinc or gold to gold, but never the twain should meet.
I should add that all the filament voltage coming from the transformer will see in an empty socket is an open circuit. No need for a switch. It already knows it has nowhere to go, so it just sits in the transformer.