Dual bias help

[Xander8280]

Looking to build a standard marshall 100watter. Import 6550 specs, but I'm stuck between 6550 and KT88s. Is it possible to run a pair of each?

4 feed resistors and two bias pots?

If so how would I go about applying separate negative voltages to the different tube grids?

Thanks!!!

From what I've read the 6550 and KT88 yield similar responses but I don't want a middle ground of the two. Hope it's possible

[Stevem]

Both the 6550 and the KT88 are beam type tubes and at the power level they will be outputting I do not think you will hear much of a difference between having all 4 of one type in the amp or the other, so I do not quite understand why you would go thru this work when most people that do it want a combo of the different tube sounds and will use 2 EL34s since it is not a beam type tube!
For a easy duel bias set up take a look for example at a Fender AB765 schematic for its bias section and after the diode on the dc side is where you would Y things off to two of those bias circuits, but the 6550 and the 88 should bias in pretty close to each other!

Note that I would up the bias caps voltage to 75 or 100 volts!
Whatever you do, don't install the output tubes and fire up the amp until you confirm that you have a -52 volts on all four pin 5s on each socket!

[Xander8280]

Both the 6550 and the KT88 are beam type tubes and at the power level they will be outputting I do not think you will hear much of a difference between having all 4 of one type in the amp or the other, so I do not quite understand why you would go thru this work when most people that do it want a combo of the different tube sounds and will use 2 EL34s since it is not a beam type tube!
For a easy duel bias set up take a look for example at a Fender AB765 schematic for its bias section and after the diode on the dc side is where you would Y things off to two of those bias circuits, but the 6550 and the 88 should bias in pretty close to each other!

Note that I would up the bias caps voltage to 75 or 100 volts!
Whatever you do, don't install the output tubes and fire up the amp until you confirm that you have a -52 volts on all four pin 5s on each socket!

EL34's and 6550's together would be awesome. Are there any schematics of the dual bias style stuff?

[danman]

If you check over at Steve Millers website (home.comcast.net/~mamp17/index.html) he gives a layout for a dual bias setup for Marshall amps that mirrors the original. Check under the "Misc Mods" section.

[Xander8280]

Thanks guys!

This forum never ceases to make my days!

[Xander8280]

Both the 6550 and the KT88 are beam type tubes and at the power level they will be outputting I do not think you will hear much of a difference between having all 4 of one type in the amp or the other, so I do not quite understand why you would go thru this work when most people that do it want a combo of the different tube sounds and will use 2 EL34s since it is not a beam type tube!
For a easy duel bias set up take a look for example at a Fender AB765 schematic for its bias section and after the diode on the dc side is where you would Y things off to two of those bias circuits, but the 6550 and the 88 should bias in pretty close to each other!

Note that I would up the bias caps voltage to 75 or 100 volts!
Whatever you do, don't install the output tubes and fire up the amp until you confirm that you have a -52 volts on all four pin 5s on each socket!

Thanks for fender schematic! Read too fast