VVR and SS inductors

[rzenc]

Hi,

I’m building a stand alone all tube guitar preamp for metal purposes... I would like to experiment with VVR and SS inductors.

SS inductor is as presented in Merlin second book, pg.83, fig.4.18. Is there benefits regarding lowering noise floor by employing multiple cascaded stages? would it be worthy the trouble?

VVR is duplicated from Tubenit’s 56T. It’s 3x12AX7 and 1x12AU7. I will scale the whole preamp B+.

Do you guys think it could work?

Thanks in advance!!!
Best Regards,

R.

[rzenc]

Howdy guys!

Mouser listed NTE2973 as not recommended for new designs... Is there a substitution part avaiable?

Or, could we use IRF820??

Thanks in advance

Best Regards

R.

[zendragon63]

Hey rzenc, I have used the STP2NK90Z instead of the NTE2973 though the specs as I understand them look like overkill for your application. I haved used it successfully in several VVR applications from 10W to 40W amps. Compared to the 2973, its pretty cheep cheep at $1.18 each at Mouser here in the US. It also has zener protection built in so you can dispense with the 10R 3W and the 12V zener. Still needs to be heat sinked.

 http://www.mouser.com/ProductDetail/STMicroelectronics/STP2NK90Z/?qs=sGAEpiMZZMvAvBNgSS9LqkIU81bJj8Iw

If you want to compare specs, there is a data sheet link in the above you can reference. I am no expert on power MOSFETS but I suspect that there are probably dozens that could be used to vary the higher voltage feeds.

Others may chime in as to what they are using. Regards

dennis

[jazbo8]

Hi,

I’m building a stand alone all tube guitar preamp for metal purposes... I would like to experiment with VVR and SS inductors.

SS inductor is as presented in Merlin second book, pg.83, fig.4.18. Is there benefits regarding lowering noise floor by employing multiple cascaded stages? would it be worthy the trouble?

VVR is duplicated from Tubenit’s 56T. It’s 3x12AX7 and 1x12AU7. I will scale the whole preamp B+.

Do you guys think it could work?

Thanks in advance!!!
Best Regards,

R.

I think they will work, but just curious, why scale the preamp and use a SS choke for low power app? Usually the VVR is used to scale the PA, sometimes the PI, and yes the whole amp sometimes, although that's not as common. Since the current requirement is low, a small choke should be pretty cheap, so you are not saving that much with a SS choke. Then again, there is nothing wrong with experimenting. 

Jaz

[FYL]

Most mosfets are designed for switching applications. They use a very small die with quite poor thermal coupling to the case/tab/... and behave quite badly with steady state B+ in the 200 mA+ range, even if they are rated at 2 A or more.

[rzenc]

Thanks for the answers guys!!

Since it's a preamp stand alone unit, I calc around 30mA at most...
SS electronics are new ground to me...I would like to experiment with these ideas...

I checked IRF820 specs and it suggests 50W of power handling...Current consumption will be greater with higher B+, which I plan to be around 400V. I would like to be able to drop it to 1/3B+ if possible, which is around 130V. I also believe it will be necessary to try and adjust it to taste or to feel it's effect.

I got this idea after studying some high gain preamps and noting how B+ changes from one design to another...

The idea of SS choke comes from building a lighter unit and also to try to understand how and why it works like it's supposed...theory madman!!

But I also think you guys are right, a 30-40mA choke would be cheap.

What called my attention is the fact that this SS choke Simulates a 47H choke..which would be expensive to get made...and also the possibility to cascade multiple stages and drop noise floor considerably... This preamp is going to a recording studio...

So I have multiple goals in mind:

a) be able to vary B+ and so headroom;
b) lowest noise floor I can manage to get;
c) light unit;
d) keep cost down.

I will keep posting results since I will start building it next week.

All thoughts are welcome and greatly appreciated!!

Thanks guys!!

R.

[jazbo8]

Since the current requirement is so low, I think a VVR for B+ will do, not sure you will even need a choke, electronic or otherwise - less parts, less cost, etc. Of course, you can also just change the Rk's to play with the headroom and the clipping level and leave the B+ alone...

Jaz

[rzenc]

After some more reading and brain-cooking, I decided to try on thing at a time. I will start by trying VVR.
Does this drawing looks alright to you?

Thanks in advance!
Best Regards,

R.

[rzenc]

Regarding the installation of STP2NK90Z or any other type of mosfet for VVR application, do I need additional protection like mica insulation to employ heat sink?

Thanks in advance

Best Regards

R.

[kagliostro]

Without the use of an insulator the heat transfer is better, however you will have voltage on the heat sink

I've seen montages without the heat sink, but only on low voltage projects

with voltages on the order of hundreds volts (like in guitar amp) I think you must use an insulator also if the heat sink in inside the chassis, an  unaware technician might suffer the consequences

K

[rzenc]

What you guys suggest?

[rzenc]

Found this:

http://www.keyelco.com/pdfs/M60/M60p109.pdf

[Willabe]

I like the thermal pads, not the mica. Mouser and Digikey both have them.

Mica can crack and short to the chassis.


              Brad