[jameshuls]

As per my previous post, I am selling my hammond transformer to have one custom-wound by Jack Elliano at Electra-Print for my Cornet 2 build. He asked me a few questions that I am not sure of the answers to. If anyone can help me answer Jack that would be great. This is (part of) his response to my inquiry when I quoted him the specs for the currently specified Hammond 370BX:

550vct @ 60ma
50v bias tap
5vct @ 3a
6.3vct @ 2a

Do you need a bias tap??
Do you need a center tapped 5v??
Do you need a center tapped 6.3v??
If so this will cost more.
Let us know what is really needed.  Give us a list as shown above of the real need for this trans.

Cheers!

James

[GRD]

Do you need a bias tap??  No, it's not used in the C2.

Do you need a center tapped 6.3v??  No, the center tap is not used in the C2

Do you need a center tapped 5v??  Not really, but the CT is used in the C2.  You could tie the B+ to one leg of the 5v.  But a real purist might keep the center tap so I'll let others chime in on that.

[jameshuls]

Thanks GRD. I understand the hammond is a commercial transformer designed for flexibility under a range of scenarios, so it is inevitable that there will be design comprimises. I am aware of what the various taps are used for. However, my understanding of transformers at this stage in my learning curve is limited so my ability to differentiate what is necessary for the circuit from what is built into the hammond is lacking.

I greatly appreciate any help offered .

James

[poty]

I fully agree with GRD - he has clearly answered all the questions.
I should add only, that there is one more important parameter - the power of the transformer. I think you can add some extra power to your new one just to be shure it won't be the weakest point in your system. Say 1,5 times...
Maybe, if you have already entered this way, you can switch to LCLC power supply (like in Cornet3=Sakura). Then (to save the remaining schematic of Cornet 2) it is advisable to have more voltage on the secondary (hi-voltage one).

[Bernie]

Jack will build you a great transformer for the cornet two. He did one for me on the cornet one, but I have not used it yet. However, does anyone know exactly how much current the Cornet two draws from the power supply?

Bernie.

[jameshuls]

you can switch to LCLC power supply (like in Cornet3=Sakura). Then (to save the remaining schematic of Cornet 2) it is advisable to have more voltage on the secondary (hi-voltage one).

So, maybe 300-0-300 instead of 275-0-275?

BTW, would the LCLC be difficult to implement in the Cornet 2? Also, what sonic benfits are there to this vs the RC (I believe?) that is currently used in the Cornet?

[GRD]

LCLC really makes for a nice, clean B+  About the best you can get.

But you'll have to fit in the 2 inductors in the chassis.  And since the first L gives 90% of the transformer RMS voltage, you'll end up needing  around 375 volts to yield the final 330 volts needed.  And you'll have to figure the inductor and capacitor sizes unless it's already been done for the C3  (Duncan Amps has a good power supply design tool). 

So, it would be some work.  Would the result be materially better than the RC supply?  I have no first hand experience but I expect you'd be able to hear a difference.  But the RC circuit sounds pretty good to me already.

[GRD]

Bernie,

Current draw:

12AX7s - 4ma (about 1 ma per section)
12AU7 - 12ma (about 6 ma per section)
Bleeder resistor - 5ma

I come up with about 21ma.  So I would guess 20 - 25ma.

[jameshuls]

If I were to get the higher voltage transformer but did not implement the LCLC at first I assume I could drop the volatge with resistors until implementing it?

[GRD]

Caution.  With a higher B+ transformer (say 375v) you'll exceed the voltage ratings of the specified capacitors in the CRC circuit at turn-on.  The reason is that the tubes are cold and won't draw current until they warm up and start to conduct.  Only then can the current draw through the resistor drop any voltage.   Peak voltage is = 1.4xRMS.  And you need to account for wall voltage variations and transformer regulation (transformer voltage is a bit higher when no current is being drawn).

I think you can run into the same problems with a LCL circuit.



   

[poty]

Caution.  With a higher B+ transformer (say 375v) you'll exceed the voltage ratings of the specified capacitors in the CRC circuit at turn-on.  The reason is that the tubes are cold and won't draw current until they warm up and start to conduct.

There is heating time for 5Y3 too, delaying full secondary AC to the capacitor for 3-4 seconds. Then (for LCLC circuit) the first choke will have rather big resistance to AC part of rectified voltage (around 10kOhm for 15H in Sakura) - also delaying the first capacitor charge for another 1-2 seconds. There is R107-R108 (in Cornet2) or LED resistor (in Sakura), which constantly will draw 2-5 mA. There is current for charging C207 (Cornet2) through R221+R221 (which will be delayed from charging C108) - another 2-3 mA. Heating time for 12AX7 and 12AU7 is around 11 sec, the tubes started to conduct around half this time (say 6 sec), which is just in time for fully charged capacitors.
For CRC circuit you onle need to add resistance before the first capacitor with the same effect.

But I agree, the first capacitor I'd get with higher voltage, just in case (say 600V).

And you need to account for wall voltage variations and transformer regulation (transformer voltage is a bit higher when no current is being drawn).

There is RC directly connected to the secondary, which account about 2-3 mA.

[GRD]

Safe design would include the possibility of a failure in the H+.  So all components should be able to stand the possibility of the resulting B+.

So, pick the transformer you need based on the PS you want to build.  The lower voltage C3 is looking more attractive to me.  I would leave designing a LCLC to feed to C2 to the experts.   

[poty]

Safe design would include the possibility of a failure in the H+.  So all components should be able to stand the possibility of the resulting B+.

Excellent precautions. But if you are afraid of problems you should buy not DIY, but manufactured things. There are many ways to go: correcting parts values, sticking to Sakura changes, combining several changes...

I would leave designing a LCLC to feed to C2 to the experts.

In Russia there is a saying: the burning of pots is not gods' job. In English it should be something like: any man can do what another man has done.