Two separate bias voltages?

[Lectroid]

I recently viewed a Marshall DSL40 (4xEL34) schematic.  It has a thing I've never seen before: a bias with two adjustment pots.  It looks like they can each supply an independent voltage. Snippet attached.

Why do this? Are these used to bias the power tubes more closely?  Is it like buying only matched sets of power tubes? It seems like it might be a noticeable improvement for maybe an hour's work.   If it is, why isn't it done more often?

Thanks!

[kagliostro]

Yes, it is a way to achieve a better match on PP amps so to be able to use not so well matched tubes

BTW in the far past times there was near no attention on matching tubes (also a litte mismatch sometime is considered to give a "pleasant" component to the tone on guitar amps

Franco

[Lectroid]

Yes, it is a way to achieve a better match on PP amps so to ve able to use not so well matched tubes

Franco, thanks for confirming that.

BTW in the far past times there was near no attention on matching tubes (also a litte mismatch sometime is considered to give a "pleasant" component to the tone on guitar amps.

That is certainly true. But this seems like a neat trick and I wonder why it isn't used more often?  Is it just not that useful after all?   

[Merlin]

That is certainly true. But this seems like a neat trick and I wonder why it isn't used more often?  Is it just not that useful after all?   

Well worth it IMO, especially with quartets (and more) of tubes, since matched sets are expensive. Bias balance pots are quite common in hi-fi because it maximises output stage performance. But it's hard enought to get guitarists to understand biasing at all, without giving them two pots to worry about

[Lectroid]

[But it's hard enought to get guitarists to understand biasing at all, without giving them two pots to worry about

I’m living proof of that. 

So This is probably just a case of another great idea that couldn’t outrun the bean counters.

[d95err]

The benefits of two bias pots would be lost on 99% of guitarists. It would however add components and conplexity, i.e. higher costs.

For a DIY or high-end boutique anp, it makes sense. For a mass-market amp, it’s just not worth it.