Help fine tuning tube preamp build

[sonolink]

Hi guys,

After a very long time (5 years) I finally finished building an AB763 (with a few mods) in a Hammond 1590BB2 enclosure. The initial idea was to use it for bass (like an Alembic F2B) but I tried it with a guitar and it sounds really good when plugged into a power amp. I also tried it as a preamp/overdrive/distortion pedal putting it in front of an amp and it also sounds great BUT a bit too thin, especially when cranking up the gain. All this is WITHOUT engaging the clipping diodes. I'm leaving these out of the equation until sorting the sound out.
I guess this thinness probably happens because the original circuit is meant to be before a power amp, not another preamp.
So my question is: is it possible (and a good idea) to add a switch that would add components in the signal path to fix that, like having a "preamp" position to use it in front of a power amp and a "distortion" position to use it in front of a preamp? If so, how would I achieve that, please?
I hope all this makes some sense

This is the schematic of my build.



Thanks for your time and help
Cheers
Sono

 

[Matador]

The output impedance at mid(ish) gain on the master is pretty high, so the sound may change due to the nature of the type of input this preamp is driving. A typical long tail pair (LTP) power amp stage has a high(er) input impedance so it works out.

It's also interesting that you have a split plate resistor on the second triode, but still have 20% reduction built in between the 220K and the master volume. You could make R9 a bit bigger, and R8 a bit smaller (like 20K/80K), then you get the same effect but don't need the 220K resistor in series. When used as a preamp, possibly driving other unknown "stuff", you might consider adding a solid state follower to reduce the output impedance to a few hundred ohms, which also won't negatively impact driving power amps.

Also, R11 should be after the 1M input resistor, directly in series with the first tube, then you don't need R12.

 

[sonolink]

Thanks for your reply Matador

It's also interesting that you have a split plate resistor on the second triode, but still have 20% reduction built in between the 220K and the master volume. You could make R9 a bit bigger, and R8 a bit smaller (like 20K/80K), then you get the same effect but don't need the 220K resistor in series.

I'll try that to see if the fizzy thing goes away

When used as a preamp, possibly driving other unknown "stuff", you might consider adding a solid state follower to reduce the output impedance to a few hundred ohms, which also won't negatively impact driving power amps.

What do you mean by "solid state follower"? Something like an opamp stage or something like that?

Also, R11 should be after the 1M input resistor, directly in series with the first tube, then you don't need R12.

So I can just delete R13, right?

Thanks a lot for your time and help
Cheers
Sono

 

[Matador]

What do you mean by "solid state follower"? Something like an opamp stage or something like that?

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

 

[CJ]

I had the same problem with thin tone, figured it was the hi z output so I stuck a little Neve cored xfmr and was happy, capacitor input so no dc so the nickel would not freak out, don't need a huge turns ratio, 4 to 1 is fine, levels were fairly low so not a ton if turns needed to reduce Bmax

 

[sonolink]

I had the same problem with thin tone, figured it was the hi z output so I stuck a little Neve cored xfmr and was happy, capacitor input so no dc so the nickel would not freak out, don't need a huge turns ratio, 4 to 1 is fine, levels were fairly low so not a ton if turns needed to reduce Bmax

Thanks for the input CJ,
I tried looking for xfmrs 4:1 and can't seems to find any really small ones online...

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

Thanks for the link, Matador,
Would I implement that idea like so?



If so, what would R1 value be please?

BTW, I've changed the values of the split resistor as you suggested and taken the 220K out and the pedal sounds much better
I'm still curious about that solid state follower you linked

Thanks again for your help
Cheers
Sono

 

[sonolink]

Bumpety bump...

 

[Matador]

If you assume 1mA through the tube, you'll drop 80V across the 80K, leaving 250-80=170V on the gate of the FET. If on, you'll drop 1 threshold voltage (say 5V) leaving ~175V on the source. So for 1mA through the FET, R1 = 175V / 1mA = 175K for R1 as a starting point.

 

[sonolink]

How can I determine how many amps go through the tube?

Cheers
Sono

 

[Matador]

I also

How can I determine how many amps go through the tube?

Easiest way is to measure the cathode voltage of 'B' (where C10 and R15 meet), then divide by the cathode resistance. So 1.5V on the cathode is 1mA through the tube (and a typical value).

Also, I forgot that you probably need a Zener across the gate/source of the MOSFET to protect the gate during power up. It should be rated for a bit lower than the max VGS spec of the MOSFET. For example, an IRF820 specifies a max of 20V, so a 12-15V Zener works.

 

[sonolink]

Also, I forgot that you probably need a Zener across the gate/source of the MOSFET to protect the gate during power up. It should be rated for a bit lower than the max VGS spec of the MOSFET. For example, an IRF820 specifies a max of 20V, so a 12-15V Zener works.

Like so?



Cheers
Sono

 

[Matador]

Flip the Zener around: the + side goes towards the gate (you want the Zener to turn on if the source goes more than the Zener below the gate.

 

[sonolink]

Flip the Zener around: the + side goes towards the gate (you want the Zener to turn on if the source goes more than the Zener below the gate.

Oh I see!! Thanks Matador

Cheers
Sono