Switchable OT primary worth it?

[beasleybodyshop]

So im ordering up some more iron for a build, and the transformer company informed me that they can do multiple primaries for the OT.

I reasoned that it would be cool to make this switchable, so If i wanted to run 6L6s and say, swap over to some KT66/6550s or something with a drastically different impedance rating I could. my question is does the impedance mismatch make a huge difference in tone? Ive also read it's not good to push a lower impedance tube into a higher impedance primary. Are we talking serious tube life loss if that is the case?

[pops]

I have used 6v6 tubes in a 4x 6l6 amp and it sounded great. I don't think the impedance difference is that bad.

[beasleybodyshop]

I guess to provide some context here, its going to be a 100 watter with a quad of 6L6s in its original configuration. I was wondering what other power tube type would be a great contrast to the 6L6. Being that this amp is pure clean tone like a twin, I was thinking 6550s or KT88s if possible. My goal is to build something that would be good for both super warm clean guitar tone, but also making something that with little effort could provide the headroom for good pedal steel tones as well.

Also if anyone could point me to some info on what impedance output tubes require, it would be greatly appreciated.

Another question: in a 4 tube push pull amplifier, you are running two pairs of tubes in parallel right? So you would take the sum of each tube's output impedances and cut it in half to determine what impedance you needed right? Hope I worded all this nonsense properly lol.

[pdf64]

There will be voltage on the plates rather higher than any normal switch is rated for.
That would definitely preclude the operation of the switch whilst the B+ was present, even under static conditions.
Investigation would need to be made regarding the flashover voltage of the switch contacts; the 2 switch poles will be VB+ x 2 apart at max clean output, then flyback emfs will need to be considered when the amp gets overdriven.

in a 4 tube push pull amplifier, you are running two pairs of tubes in parallel right?

Yes.

So you would take the sum of each tube's output impedances and cut it in half to determine what impedance you needed right?

No, the output impedance (ie plate impedance) of a pentode / beam tetrode is very high; suitable load impedances are rather lower.

See http://www.hammondmfg.com/1608A.htm and http://www.hammondmfg.com/guitarLineOT.htm for normal tube type and load impedance (ie OT primary) combinations.
Pete

[beasleybodyshop]

Thanks Pete. The assumption would be that the amp has to be off before switching primary impedance settings LOL.

I will do some more research. I had thought about possibly running a 6K impedance so i could run 6V6s properly, but i think my plate voltages are going to be too high to run a modern set.

[pdf64]

I think that OTs are flash tested at 2kV, so the switch would need to cope with that or thereabouts.

I think that JJ6V6 plates are 500V rated http://www.jj-electronic.com/pdf/6V6.pdf

Pete

[Stevem]

I do not think its worth it as the thing we are after, that being tone gets put in the back the back seat somewhat as the added primary winding effects the primary to Secondary coupling to a degree.

[Firestorm]

Why switch the primaries? Secondaries are much easier. Even things like triode/pentode switches can arc between the switch connections at full power. That said, as long as you're running 6L6 appropriate voltages, I'd be inclined to run KT88s and 6L6s into the same load anyway. Unless you're willing to significantly decrease your grid load resistors, I'd stay away from 6550s: very unforgiving in that regard.

[beasleybodyshop]

Thanks for the insight everyone. I think I will just go with a 2.2K primary and adjust for taste if I want to swap in some other tube type.