Help fine tuning tube preamp build

[rock soderstrom]

Pretty bump....

Hey Sono, I don't have any answers, just questions . Aren't C2 and R4 obsolete?

 

[sonolink]

I was wondering that too. I found that schem here and copied it because it uses irf740 and I have a few of those
This is the original schem I copied the cathode follower from:



Cheers
Sono

 

[Matador]

That stuff looks added to give the ability to tap the output from before the follower. It looks like you might not need R2, R3, R4, C1, or C2 and it would still work.

 

[sonolink]

Thanks a lot Matador!! I'll give it a go and report back

Cheers
Sono

 

[sonolink]

A question out of curiosity: R2 and R3 are a voltage divider right? Why lower by half the voltage to the Mosfet?

 

[rock soderstrom]

A question out of curiosity: R2 and R3 are a voltage divider right? Why lower by half the voltage to the Mosfet?

This DC bias the MosFET to half operating voltage (B+) so that it can modulate equally in both directions.

 

[sonolink]

Another question please

Is there a specific reason why the Gain and Master pots are 1M?
Wouldn't it work with say 100K pots?
What would be the consequences in the circuit of using lower resistance pots?

Thanks a lot
Cheers
Sono

 

[Matador]

This DC bias the MosFET to half operating voltage (B+) so that it can modulate equally in both directions.

Indeed.

It isn't needed in this case because the anode of the previous stage is already swings up and down around 160V. The follower MOSFET is capable of swinging within a threshold voltage of the positive rail, and down close to ground already, so has more (useful) swing than the tube stage preceeding it will ever have. You can DC couple it to the anode and save some components. You see the "half supply" biasing mostly when fed from a stage that swings around ground (e.g. it's already AC coupled).

Is there a specific reason why the Gain and Master pots are 1M?

Because common cathode tube outputs are 'mid-ish' impedance, and you want to follow the rule of voltage coupling which is try to maintain 10:1 input to output impedance. A 1M pot can tolerate a 100K output, however a 100K pot can only tolerate a 10K output. A 12AX7 with a 100K plate and 1K cathode has a roughly 35K output impedance, so you want at least 350K. 1M works fine too.

 

[rock soderstrom]

Because common cathode tube outputs are 'mid-ish' impedance, and you want to follow the rule of voltage coupling which is try to maintain 10:1 input to output impedance. A 1M pot can tolerate a 100K output, however a 100K pot can only tolerate a 10K output. A 12AX7 with a 100K plate and 1K cathode has a roughly 35K output impedance, so you want at least 350K. 1M works fine too.

A question regarding the master pot. Wouldn't it be better to position the potentiometer in front of the souce follower?

 

[sonolink]

A question regarding the master pot. Wouldn't it be better to position the potentiometer in front of the souce follower?

Do you mean like this?

That's a very interesting question......

Cheers
Sono

 

[rock soderstrom]

Do you mean like this?

Something like this, perhaps also as Matador described. I wonder whether this has any significant advantages. The source follower acts as an impedance converter, like a transformer that guarantees constant output impedance. A good thing in itself. Are there any disadvantages?

 

[MidnightArrakis]

Any ideas, comments, suggestions before I melt some solder?

[Any ideas, comments, suggestions before I melt some solder?] -- Go ahead and "melt some solder" and then let us all know what happened, OK??? Meanwhile.....I'll stay put here in my underground bunker, just to be safe, while you do so!!! -- GOOD LUCK!!! -- (as hatch clicks shut).....

/

 

[sonolink]

Ok guys, so I tried the mosfet cathode follower and honestly didn't notice much of a difference.
Since I made changes to this circuit following Matador's advices the circuit sounds WAY better, especially with the split plate resistor (82k/18k). I've also taken out R13 and changed the HIGHS pot to a log pot instead of a linear.

After that I added a parallel output featuring Merlin's cabsim followed by a balanced output.

This thing is now VERY usable and sounds good but it requires fine tuning and getting to know it thoroughly, whereas plugging it into a power amp is automatic great tone.

I have to decide whether I'll leave the Mosfet or not. I had to add it p2p



Thanks again for all your help, ideas and suggestions, especially Matador, to whom I owe quite a few beers by now

 

[rock soderstrom]

Thanks for your reply
The idea is to be able to use it in front of an amp with good sound (before a power amp it already sounds great).

On that same thread I quoted before, there's this comment about the huge output this thing has :

I know you use this preamp before an amp/power amp. I would be very interested to know how your amp behaves with the source follower in a standard 10k line input of an audio interface?
Too much, too little gain and most importantly, how does it sound?

 

[sonolink]

I know you use this preamp before an amp/power amp. I would be very interested to know how your amp behaves with the source follower in a standard 10k line input of an audio interface?
Too much, too little gain and most importantly, how does it sound?

Well the idea is to be able to use it before an amp, before a power amp and directly into a mixer or a souncard. I recorded some short clips for you to have an idea how it sounds.

Clips were recorded using a Strat into the pedal, into a line input, a Hi-Z input and a mic input, using the pedal's line out or balanced out (it's indicated on the clips' name). Three clips were recorded using a Behringer XR18 mixer and the other there using a MOTU Ultralite mk3.
First bit of playing is always bypassed. Settings are the same on all clips except on one HiZ where I had to adjust the input gain.

To me it sounds REALLY good as a basic clean amp to which you can add other pedals and avoid carrying an amp, and still enjoy real tube sound and dynamics. The only thing I notice is a TINY bit of hiss that increases as soon as you go into high gain. I don't know if it would be possible to eliminate that?

MOTUMk3-BalOutIntoMicIn
View attachment MOTUMk3-BalOutIntoMicInNOSF.wav

MOTUMk3-UnBalOutIntoHiZIn
View attachment MOTUMk3-UnBalOutIntoHiZInNOSF.wav

MOTUMk3-UnBalOutIntoLineIn
View attachment MOTUMk3-UnBalOutIntoLineInNOSF.wav

XR18-BalOutIntoMicIn
View attachment XR18-BalOutIntoMicInNOSF.wav

XR18-UnBalOutIntoHiZIn
View attachment XR18-UnBalOutIntoHiZInNOSF.wav

XR18-UnBalOutIntoLineIn
View attachment XR18-UnBalOutIntoLineInNOSF.wav


Cheers
Sono

 

[rock soderstrom]

Well the idea is to be able to use it before an amp, before a power amp and directly into a mixer or a souncard. I recorded some short clips for you to have an idea how it sounds.

thanks a lot Sono!!

To me it sounds REALLY good as a basic clean amp to which you can add other pedals and avoid carrying an amp, and still enjoy real tube sound and dynamics.

I agree with you, the preamp sounds like I expected it to - like a Fender amp. Very good, I am convinced of the solution. All in all, a very flexible and easy to set up instrument preamp. The unit will probably also harmonize very well with a Rhodes, Vox organs and similar keyboard instruments.

The only thing I notice is a TINY bit of hiss that increases as soon as you go into high gain. I don't know if it would be possible to eliminate that?

Maybe you can optimize it a little? Surely you have already tried a different tube?
How do you generate B+? What does your power supply look like? Any "switcher" involved or classic tube transformer (or back to back transformers) plus CRCRC filtering?

Edit: can you please measure the total current is flowing on the B+ rail? Is it still under 15mA?

How do the clipping diodes in this circuit sound? Have you tried them yet?

 

[sonolink]

Maybe you can optimize it a little? Surely you have already tried a different tube?

I have tried 3 different tubes and they all hiss.

How do you generate B+? What does your power supply look like? Any "switcher" involved or classic tube transformer (or back to back transformers) plus CRCRC filtering?

This is the complete schematic of the preamp. Basically, it's a Fender AB763 without the Bright Switch, with added clipping diodes, split plate resistor and 6NxPs adaptor switch.

Edit: can you please measure the total current is flowing on the B+ rail? Is it still under 15mA?

I'll try to post that later today

How do the clipping diodes in this circuit sound? Have you tried them yet?

I have indeed
I've used on one pair two red LEDs and on the other pair two 1n914s. This really is a question of personal taste. To me it still needs some tweaking because the LEDs sound creamy but a bit too dark, and the 1n914s compress the hell out of the signal. Maybe some asymetric combination or give it some mids back...


Cheers
Sono

 

[sonolink]

Ok, so I measured current between F1 and the junction R17-R10 and I get 0.850mA......but I'm not sure that's what you meant by TOTAL current flowing on the B+ rail, right?

Cheers
Sono

 

[rock soderstrom]

Ok, so I measured current between F1 and the junction R17-R10 and I get 0.850mA......but I'm not sure that's what you meant by TOTAL current flowing on the B+ rail, right?

Is that with or without MosFET? A source follower normally needs more current...

 

[rock soderstrom]

This is the complete schematic of the preamp. Basically, it's a Fender AB763 without the Bright Switch, with added clipping diodes, split plate resistor and 6NxPs adaptor switch.

I'll try to post that later today

Thanks!

I have tried 3 different tubes and they all hiss.

Perhaps these are artifacts of the NE555 PSU. I was never satisfied with this type of PSU. Maybe it's just me, but there are better solutions, even for pedals.