Output transformer advice? Schematic attached.

[JW]

Hey folks,

I built one of these preamps because I had an Akai M7 mono block around, and this guy Jesse made the schematic available. He was pretty cool and encouraging about it, so I built it, and it sounds really good. It has two bottles, and EF86 and a 12au7. I'm wondering if the 4:1 output transformer is the best choice (?), or if a different ratio might be better? Right now, the non transformer  output sounds a lot better than the 10K:600ohm Carnhill that I have in there. But I like having a 600 ohm out for feeding my diy Pultec etc.

???

 

[Deepdark]

maybe a lower ratio, like 2K4:600 or something like that

 

[ruffrecords]

maybe a lower ratio, like 2K4:600 or something like that

I agree. If you want to drive something approaching a 600 ohm load you should adjust the 12AU7 bias to about 10mA of standing current to give it enough drive capability.

Cheers

Ian

 

[CJ]

that is a steel core transformer,

if using that 1 uf output cap, there is no DC on the core so you can use a nickel core transformer,

look at the LA2 compressor , the Jensen Ni core 4:1 sounds awesome in that circuit,

 

[JW]

Thanks guys.

So, the 220 ohm bias resistor should be changed to 23K? I'm pretty sure I have around the full 230V B+ at the 12au7 there.
So, for 10mA it would be

R=230V/.01A=23K?

The Carnhill transformer (VTB2291) can fabulously conveniently be wired 2K4:600ohm.

Thanks for the recommendation CJ. If I can solve the issue with the transformer I have though, I might just stick with it.

 

[ruffrecords]

Thanks guys.

So, the 220 ohm bias resistor should be changed to 23K? I'm pretty sure I have around the full 230V B+ at the 12au7 there.
So, for 10mA it would be

R=230V/.01A=23K?

The bias resistors are the two 680 ohms ones. Each triode has half the supply voltage across it. Looking at the 12AU7 datasheet I think the present bais point probably about right so it does not need changing.


Cheers

Ian