Is it ok to remove the slave pot from a boogie?

[texwest]

I have a Boogie mark iic+ and I want to remove the slave pot and replace it with a 25k bias pot. It seems like it shouldn't be a problem. Just remove the pot and clip the wire going to the presence pot and speaker. I just want to clear it with some experts before I do it. Are there any unforeseen circumstances of doing this? Thanks ahead of time!

[cap47]

Originally Posted by texwest

I have a Boogie mark iic+ and I want to remove the slave pot and replace it with a 25k bias pot. It seems like it shouldn't be a problem. Just remove the pot and clip the wire going to the presence pot and speaker. I just want to clear it with some experts before I do it. Are there any unforeseen circumstances of doing this? Thanks ahead of time!

Do you have the schematic?

[AR-305]

Yes! Simple as that. I often replace the slave out pot with the R2 master volume on Mark III's. I recommend you use one of the fender type screw driver adjustable bias pots instead of a standard shaft pot to avoid accidents. Also note that many have tried unsucessfully to put adjustable bias pots on the simulclass amps. It is very difficult to get the Class A tubes and the AB tubes to each bias optimally at the same time with a global bias voltage adjust. The IIC+ amps have a very high plate voltage from the "105" power transformer and so a cooler bias is best anyway to get any life from your tubes. Proceed with care the IIC+'s are getting harder to come by.

[texwest]

Thanks AR, that's what I thought. And yes I'm thinking about a fender style pot. Mine has the lower voltage transformer with about 450 on the plates. There are extra 220k resistors that go to the "class A" power tubes to change their bias compared to the AB tubes. A guy on the boogie board changed these to 150k to even out the current flow between all the tubes. What do you think of this?

[AR-305]

I'm not sure I would want to "balance the current flow" remember the whole idea behind the simulclass output is "mixing" the class A pair with the Class AB pair via the ultra linear transformer. In other words, if you look at the simul schematic the primary of the output tranny has one center tap to the B+ then four seperate taps for the plates. These two pairs having different impedance. You would be better off the just swap to a standard 100watt output tranny with a single adjustable bias voltage. I've never seen the simul class set up respond well to a single global bias adjustment. I've even done a complete rewire for a seperate bias on both seperate pairs but still because they share an ultralinear transformer changing the current draw on one pair will affect the other. Do you see where this is going.

[texwest]

Thanks for your info. I have two of of these amps: one simul; the other not. I definitely want the bias pot on the non simul, but the simul one is a bit of a mystery. The "class A" tubes are el34's and they are running at 31 and 28 ma, so they are not running in class A at all. The 6l6 AB tubes are running at about 26 ma. All this seems a little cold to me. I jumpered in some resistors in the bias circuit and got it running a little warmer. If I get the 6l6's tubes up to 34ma, the el34's tubes go to 45 and 40. It's interesting how the current of the el34's doubles in comparison to the 6l6's. They are sylvania 6ca7's so I think they could handle this. It doesn't really seem like class A at all because they are all running at the same plate voltage ~450. What do you think of all this?