Sound City 120 Bias Doubler

[g1]

Here is link
http://www.rig-talk.com/forum/downlo...4282&mode=view

Your link is not working, so I don't think anyone can see what the circuit looks like.

[vmazz]

Ok Try this one. Look at the completed mod pic of able to. Rig-Talk ? View topic - Lickliter "Rage mod" Sound City 50+ re-work!

[g1]

Well there's no schematic so not much to go on unless someone wants to draw it out.
I only see 2 trimpots total, so I would guess one is bias and the other is balance.

[Dave H]

Well there's no schematic so not much to go on unless someone wants to draw it out.

It looks like it's the (Two controls) DR103 Doubler circuit in post #9 minus the 220k resistors but with 10u caps added from pot wipers to ground.

[vmazz]

That is what I was seeing. I was just wondering if anyone would know of the advantage of doing it this way as opposed to the exact dr103 doubler circuit. It looks like the doubler attaches to a stock Marshall bias circuit of some sort. Of course with dual bias pots.

[vmazz]

Mhuss quick question. Is there an error in the dr103 two Control schematic? There is a 25kwritten after the diode junction but no resistor symbol. Is that omitted?

[Dave H]

Is there an error in the dr103 two Control schematic? There is a 25kwritten after the diode junction but no resistor symbol. Is that omitted?

It looks OK to me. I'd delete the 25k. I don't think anything is missing.

[g1]

I don't think there is any 25K missing there either. Only pots are 25K there and none are missing. I think it was just a copy & paste error.

I was just wondering if anyone would know of the advantage of doing it this way as opposed to the exact dr103 doubler circuit.

To me, the circuit without the 220K 'safety' resistors is inferior. Without the resistors, any poor connection between the wiper and the carbon track of the pot will result in loss of bias and tube meltdown.