New IIC, Need an Expert!

[Blueracer]

Blueracer,

I'm curious why a Simulclass amp would not get the same benefit of an adjustable bias? I can understand the difference in the way the inner and outer tubes are configured. Would it not still be beneficial to have the capability, knowing it would be to dial in the collective bias of both pairs?

I ask this because I've been considering whether my MKIVa would sound better configured properly with some NOS 6L6's.

The inner and outer pairs run different bias currents, so you can't get both pairs biased "right" using a single bias pot. You'd probably need to have bias pots for each pair of tubes.

For the Mark IVa, get a quad of STR 420s. I replaced =C=s with 420s and never looked back.

I agree with dodger916 on this issue and said as much in a previous post. A counter agrument to this is that you can at least optimize one or the other set of tubes with a single bias control. This is a legitimate point of view and in principle could be done. To me it isn't worth the effort hence the advice to glawk to revert to stock. This is a judgement call on my part. If someone wants to do this by all means give it a try and let us know how it turns out. I am all for experimentation. I will point out glawk's experience when he tried to adjust the then unknown control. He said "I tried moving it...doesn't seem to do anything". Well almost nothing. Anyway that is the rationale for what I suggested. I can go into more technical detail if you like that further supports my advice.

 

[Mongo1]

I definitely understand the desire not to give the opportunity for someone to jack with the bias setting if they are not monitoring the current draw and have an understanding of the dissipation the tubes are experiencing. No need to give someone the tools to blow up their amp...

If a Simul amp is running very cold on both sets of sockets (on both settings), having the ability to move it into a warmer range doesn't seem unreasonable. Granted, the outer sockets will determine the upper limit of the range. I'm not suggesting reconfiguring a Simul to have individual pair bias settings, or changing the circuit in any other way other than to make the existing fixed setting, variable.

I updated my Simul-Sat in this way and it made a marked improvement.

 

[Blueracer]

I definitely understand the desire not to give the opportunity for someone to jack with the bias setting if they are not monitoring the current draw and have an understanding of the dissipation the tubes are experiencing. No need to give someone the tools to blow up their amp...

If a Simul amp is running very cold on both sets of sockets (on both settings), having the ability to move it into a warmer range doesn't seem unreasonable. Granted, the outer sockets will determine the upper limit of the range. I'm not suggesting reconfiguring a Simul to have individual pair bias settings, or changing the circuit in any other way other than to make the existing fixed setting, variable.

I updated my Simul-Sat in this way and it made a marked improvement.

I think we are in total agreement (read above). Like I said my advise to glawk was more of a judgement call for a point of diminishing returns for putting in a bias control on a simulclass. However you do have my curiosity. What sort of improvement did you get and under what conditions. How did you set up the bias. What sort of tubes did you have. Any sort of before and after recordings. BTW I believe most Boogie power tubes run on the cold side if I remember some posts I read a while back (like several years ago). They do bias a bit more negative than say a blackface Fender Super Reverb or something similar which would constrict their bias current. Kind of makes sense for a company that wants to cram in their own power tubes into our amps and have a design that doesn't include an adjustable bias control. More tubes would pass Mesa's screening this way. Other's experience here would be interesting.

 

[dodger916]

Blueracer,

I'm curious why a Simulclass amp would not get the same benefit of an adjustable bias? I can understand the difference in the way the inner and outer tubes are configured. Would it not still be beneficial to have the capability, knowing it would be to dial in the collective bias of both pairs?

I ask this because I've been considering whether my MKIVa would sound better configured properly with some NOS 6L6's.

The inner and outer pairs run different bias currents, so you can't get both pairs biased "right" using a single bias pot. You'd probably need to have bias pots for each pair of tubes.

For the Mark IVa, get a quad of STR 420s. I replaced =C=s with 420s and never looked back.

Where did you get the 420's? Were they Mesa Labeled or just Chinese tubes?

I got them on eBay, and they are Mesa-labelled.

 

[mule#1]

For the Mark IVa, get a quad of STR 420s. I replaced =C=s with 420s and never looked back.[/quote]

Where did you get the 420's? Were they Mesa Labeled or just Chinese tubes?[/quote]I got them on eBay, and they are Mesa-labelled.[/quote]

Much Thanks! I'll scout out a pair of 415's and 420's!

 

[Mongo1]

I think we are in total agreement (read above). Like I said my advise to glawk was more of a judgement call for a point of diminishing returns for putting in a bias control on a simulclass. However you do have my curiosity. What sort of improvement did you get and under what conditions. How did you set up the bias. What sort of tubes did you have. Any sort of before and after recordings. BTW I believe most Boogie power tubes run on the cold side if I remember some posts I read a while back (like several years ago). They do bias a bit more negative than say a blackface Fender Super Reverb or something similar which would constrict their bias current. Kind of makes sense for a company that wants to cram in their own power tubes into our amps and have a design that doesn't include an adjustable bias control. More tubes would pass Mesa's screening this way. Other's experience here would be interesting.

The Simul-Sat was running very cold. I believe the idle current was running around 17ma. I haven't decided where to set it yet because I'm still in the process of putting a system together. The Simul-Sat is the center section of a wet/dry/wet rig. I'm running the outer channels from a fifty/fifty. The source for both sections is a Studio Preamp. I replaced the caps in all of them and needed to make multiple repairs on the Simul-Sat. It had cooked some screen resistors and some toasted wiring. I still have much to do on the rack before I get to a place of dialing in the bias. Initial tests with some of my existing STR-420's sounds promising.

 

[Blueracer]

I think we are in total agreement (read above). Like I said my advise to glawk was more of a judgement call for a point of diminishing returns for putting in a bias control on a simulclass. However you do have my curiosity. What sort of improvement did you get and under what conditions. How did you set up the bias. What sort of tubes did you have. Any sort of before and after recordings. BTW I believe most Boogie power tubes run on the cold side if I remember some posts I read a while back (like several years ago). They do bias a bit more negative than say a blackface Fender Super Reverb or something similar which would constrict their bias current. Kind of makes sense for a company that wants to cram in their own power tubes into our amps and have a design that doesn't include an adjustable bias control. More tubes would pass Mesa's screening this way. Other's experience here would be interesting.

The Simul-Sat was running very cold. I believe the idle current was running around 17ma. I haven't decided where to set it yet because I'm still in the process of putting a system together. The Simul-Sat is the center section of a wet/dry/wet rig. I'm running the outer channels from a fifty/fifty. The source for both sections is a Studio Preamp. I replaced the caps in all of them and needed to make multiple repairs on the Simul-Sat. It had cooked some screen resistors and some toasted wiring. I still have much to do on the rack before I get to a place of dialing in the bias. Initial tests with some of my existing STR-420's sounds promising.

Well your experience seems to support what I remember about Boogies being on the cold side. You should let the group know how your biasing experiments turn out and how you go about it. We are all interested in improving our tone and how to go about it.