Mark III biasing

[AtomicMassUnit]

Just a quick question. I am trying to get some good sounds out of this amp. I used the shunt method to check bias and I'm just wondering if the readings I'm getting are for pairs of tubes like a Twin, or if it's single tubes, because when I measure the outer EL34 I get 22ma, and the 6L6 next to it is getting 19ma, same for the other two tubes. B+ is 485. So, that's cold if it's just the reading for one tube, but crazy cold if it's the pair. Also, would the simul-class/class a switch change the bias?

[Amp Kat]

So, that's cold if it's just the reading for one tube, but crazy cold if it's the pair. Also, would the simul-class/class a switch change the bias?

Yes, flipping the switch changes the bias. If you would put your meter on pin 5 or at that voltage divider resistor on the terminal board you'll see it changing. In class A mode it will only use two tubes and I think it's the EL-34's. It lifts the Cathodes of the 6L6's and changes the bias. When in Simulclass you should then divide by two but as you probably notice theres not a big difference in loudness unless you really crank it. This is also the case in a 4 tube amp when you pull two you only get a -3dB reduction.

[AtomicMassUnit]

Thanks for the reply. By the way, I'm out in Lafayette, haven't seen your shop yet, have to drop by when I'm in Baton Rouge. So far the "class A" setting sounds better. So I guess what I should do is set it to the class A mode and set the bias to around 35ma per tube, then check it in the simulclass setting and make sure it's ok. I'm hoping that would smooth out the dirty sound a little bit.

[Amp Kat]

Hey man what a small world ! It would be great to see you come in and check out the shop. I love Lafayette and it's just a cool place. The other Co. I work for has an office on Eraste Landry Rd. and I go there some times and do some tech work. Give me a shout sometime and I'll show you around the place.