Mosfet CF and mini-TOS

[tubenit]

Guys,

The other day I was using my TOS with the FX-Reverb unit that I built that goes with it, and it sounded great.

And then for the fun of it, I tried the TOS withOUT the FX-Reverb unit and discovered that the cathode follower behind the 5879 still gave it a really super nice transparent tone when the OD was engaged.  Without the OD engaged,  the clean channel does NOT sound as good (running the Boss digital delay) as when it's plugged into the FX-Reverb unit.

In other words, the passive effects loop with the delay seems to need a CF in front of it.

So, my thinking is why not install a CF on both the clean and the OD channels?  Geezer has been kind enough to interact with me around this idea in using a Mosfet on the clean channel to accomplish this.

I am probably going to build the mini-TOS (which I am excited about) & have been looking at some options here.

Geezer drew up a "switchable" CF so that the OD tone is unaffected by a CF on the clean channel.

Some thoughts on these two designs?

Think the switchable mosfet design would work?    Any noise issues that you'd caution about?  

With respect, Tubenit

[jojokeo]

tubenit, I like the second one

[Fresh_Start]

Tubenit - I think you need a gate stopper resistor for the MOSFET cathode follower.

Respectfully,
Chip

[Geezer]

Tubenit - I think you need a gate stopper resistor for the MOSFET cathode follower.

Respectfully,
Chip

I agree....what, a 100R? 220R?

I am working on putting this preamp into a PP 6BM8-type amp.....

EDIT: Keen suggestes a 220R here>

http://www.diystompboxes.com/smfforum/index.php?topic=73669.0

[tubenit]

Q1:   What is a gate stopper????   Can you show one in a schematic, please?


Q2:   What wattage of a gate stopper resistor?  Would a 1/2 watt work?


Q3:   Does the mosfet need to be anchored on to the chassis like in a VVR or can it be mounted on a layout board?


Thanks for the help!  With respect, Tubenit

[jojokeo]

Here's some interesting reading of a thread that's used mosfet replacing tube stage(s) to get a feel for what you're working with...hope this helps in your quest guys.

http://music-electronics-forum.com/t3509/

[tubeswell]

Q1:   What is a gate stopper????   Can you show one in a schematic, please?


Q2:   What wattage of a gate stopper resistor?  Would a 1/2 watt work?


Q3:   Does the mosfet need to be anchored on to the chassis like in a VVR or can it be mounted on a layout board?


Thanks for the help!  With respect, Tubenit

Hi tubenit

You need a 100-200R (1W) resistor in series with the gate of the IRF820, and you possibly also should have a ~12V zener between the gate and the source (cathode/banded end pointing at the gate). I've had to do this with a mosfet source follower when I've used them in trem circuits a couple of times now. (Schematic attached - the source follower is in the trem circuit)

I mount my ones on a bit of perf board.

[Tone Junkie]

You guys really think of some cool stuff, made my day, taught something good once again.
Thanks Bill  
                                  

   sorry these things remind me of my kids

[tubenit]

Thanks for the replies. 

As I am understanding it, the mosfet could be mounted on the layout board (since a perf board has worked)?

Thank you for the help!  It is appreciated.

With respect, Tubenit

[tubeswell]

The zener is also a good idea in a source follower application, and I only learnt that about 6 months ago

http://music-electronics-forum.com/t18452/

[tubenit]

I drew up a layout to go with the schematic. They're editable and in the SCH library.

http://www.el34world.com/Forum/index.php?topic=11607.new#new

With respect, Tubenit

[chocopower]

Just for share it.

First i tried a IRF830, but the high capacitance was a problem. Them i use a ZVN0545A. Great and easy tweak!!

[jojokeo]

Just for share it.
First i tried a IRF830, but the high capacitance was a problem. Them i use a ZVN0545A. Great and easy tweak!!

To understand the objective - is this to lessen the load created by the hi output impedance together w/ the signal load of the tone stack thereby preserving the gain from the previous stage(s)?
Also, does it help w/ the frequency response and/or tone too?

[Geezer]

First i tried a IRF830, but the high capacitance was a problem. Them i use a ZVN0545A. Great and easy tweak!!

What were the symtoms of the IRF8__ mosfet high capacitance problem?? Loss of highs?

[chocopower]

Yes.

even with the ZVN i have to put a 4,7uf cap in the 820 rk for a more bright sound. A bypass cap for the volumen pot is a must too.

[chocopower]

Just for share it.
First i tried a IRF830, but the high capacitance was a problem. Them i use a ZVN0545A. Great and easy tweak!!

To understand the objective - is this to lessen the load created by the hi output impedance together w/ the signal load of the tone stack thereby preserving the gain from the previous stage(s)?
Also, does it help w/ the frequency response and/or tone too?

Sorry. Your question is far away up for my technical and english level....

[tubenit]

Thanks Chocopower for the information!

Chocopower was referencing an IRF830.  I plan to use an IRF820.  I don't know if that makes any difference or not.  Data sheet is here:
http://www.mouser.com/ProductDetail/Vishay-Siliconix/IRF820PBF/?qs=sGAEpiMZZMsdSsOhgyFDi%252b36OT01M0HUhI%2fjewLPBDo%3d

With respect, Tubenit

[chocopower]

I suposse the output capacitance is the trick.

120pf for the IRF830
61pf for the IRF820
10pf for the ZVN0545A

[tubenit]

I got a 12v zener  with 21ma  & 1w rating.

Will that work to protect the mosfet?

With respect, Tubenit

[Tone Junkie]

I really enjoy the fact that I can just follow along with what you guys are doing and every day you teach me something new.
Thanks Bill

[Fresh_Start]

Simply parotting what R.G. Keen & Steve Connor have written, you should be fine with a 10-15v zener rated at 1/2 watt:

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

http://www.geofex.com/Article_Folders/sstremolo/sstremolo.htm (schematic showing you've got the cathode pointed in the correct direction)

http://music-electronics-forum.com/t18452/ (discussion with tubeswell on AMPAGE)

All that said, I don't see how the gate could go more than a couple of volts higher than the source with an AC coupled cathode follower MOSFET.  Looking at the second link (tremolo), there are no caps between B+ and the gates of the three MOSFETs.  Therefore, in that case, the gate-to-cathode voltage WILL be greater than 20 volts (max spec) for a moment at turn-on.  That's not the case in tubenit's circuit because there is a cap between B+ and the MOSFET's gate.  Of course, the cap theoretically could leak DC which sends us right back to the zener for protection...

Hope that helps,

Chip

[TIMBO]

Thats some great reading Fresh Start, this got me  of a post Tone Junkie posted http://www.el34world.com/Forum/index.php?topic=11645.0. I liked the Preamp and wanted to mate it with an 18W power amp.I'm am in the planning stage and now this has got me  (waaaaaaaaaaay to much  ) It has a gain stage that can be added (blackmore sw) using 1/2 12ax7,V3b but as always there 1/2 that is not used (don't like wasting TRIODES). I was wondering if this triode could be replaced with the mosfet or does it require an existing triode to be effective or could it be linked to V1b. What would it sound like? Thanks for opening a new can of worms