audio-talk

Just to go a little bit OT, the o/p TX is driven from the cathode of the 12B4.

Wots that all about then?

DTB

Its just a cathode follower driving teh TX through a DC blocking capacitor.

Nick wrote:Its just a cathode follower driving teh TX through a DC blocking capacitor.

Right, is that done because it's driving into a low impedence load? The TX is only 600R on the primary.

Sorry about the hijack Colin, I'm just learning a bit more.

DTB

Yes, the output impedance of the cathode follower is aprox 1/g so for the 12b4a its in the orde of 160R

Hi All
Thanks for your reply Nick
Hijack all you like Dave as I helps with my learning too so good stuff
I will try the resistor on the grid cant recall at the mo if the Alps pot is a sealed unit or not some switch cleaner would be good
On an upgrade path how about swapping out the 100k R3a resistor for a choke would there be any benefit in doing this if so what value choke will work

Yes you could (almost), but you would need to leave the 100k resistor there to drop the voltage, and then decpuple it with a electrolytic, then add the choke between the decoupled point an the anode. Take the signal to the next stage from the anode.

If you are unsure what I mean, I will do a sketch tonight.

The 150H hammond choke would be worth trying I think.

Nick wrote:Yes you could (almost), but you would need to leave the 100k resistor there to drop the voltage, and then decpuple it with a electrolytic, then add the choke between the decoupled point an the anode. Take the signal to the next stage from the anode.

If you are unsure what I mean, I will do a sketch tonight.

The 150H hammond choke would be worth trying I think.

Hi Nick
Thanks yes if you can do the sketch that will be great on the subject of chokes how about using a 30H 40mA 595 ohms on the anode of the 12B4a

Thanks yes if you can do the sketch that will be great on the subject of chokes how about using a 30H 40mA 595 ohms on the anode of the 12B4a

you could, but you would have to alter the dc conditions because the DCR of the choke is so much less than the existing cathode resistor. You could do it, but I think you would have to do to a coupling cap, and then you have the problems of the two inductors in series so th eeffective inductance is lower.

Nick wrote:Yes, the output impedance of the cathode follower is aprox 1/g so for the 12b4a its in the orde of 160R

Hi Nick, sorry to bug you again (its lunchtime and I've got nothing better to do ! ) but I've got the data sheet for a 12B4a up and can you tell me where you get the 'g' figure for the equation 1/g?

And 'cos I'm lovely here's a quick link to said data sheet http://www.nj7p.org/Tube4.php?tube=12B4a

DTB

Nick wrote:

Thanks yes if you can do the sketch that will be great on the subject of chokes how about using a 30H 40mA 595 ohms on the anode of the 12B4a

you could, but you would have to alter the dc conditions because the DCR of the choke is so much less than the existing cathode resistor. You could do it, but I think you would have to do to a coupling cap, and then you have the problems of the two inductors in series so th eeffective inductance is lower.

Ah OK wont bother to do that just trying to upgrade and learn more with this little amp so the 150H Hammond choke is one to try if you could suggest any other way to upgrade this amp pleas do so most helpfully time to rebuild to this higher HT sch but still with batt bios on the drive valve just ignore the green magic eye valve wont be adding them
Edit:
New Schematic Should be able to see this one better

Hi Nick, sorry to bug you again (its lunchtime and I've got nothing better to do ! Smile ) but I've got the data sheet for a 12B4a up and can you tell me where you get the 'g' figure for the equation 1/g?

Woops, my fault, I meant 1/gm where gm is the transconductance in mhos or in earlier units siemens, its the inverse of resistance, so its often expresed in amps per volt (from ohms law), or more often in valve terms milliamp per volt.

In the case of this valve the gm value is 6300 ua/v or 6.3 ma/v (note upside down ohm to indicate its units).

So the inverse of conductance is resistance, so the inverse of gm is measured in ohms, so for this valve 1/gm gives us 147 ohm.

Nick wrote: Woops, my fault, I meant 1/gm where gm is the transconductance in mhos or in earlier units siemens, its the inverse of resistance, so its often expresed in amps per volt (from ohms law), or more often in valve terms milliamp per volt.

In the case of this valve the gm value is 6300 ua/v or 6.3 ma/v (note upside down ohm to indicate its units).

So the inverse of conductance is resistance, so the inverse of gm is measured in ohms, so for this valve 1/gm gives us 147 ohm.

Ah, no wonder I'm confused then! Upside down omega is gm, sorted.

OK, so I've got 1/0.0063A which gives me 158.7 R . Got it.

Ta boss,

DTB

PS. (You can have next week off BTW, I'm on hols you lucky peeps )

Nick wrote:Yes you could (almost), but you would need to leave the 100k resistor there to drop the voltage, and then decpuple it with a electrolytic, then add the choke between the decoupled point an the anode. Take the signal to the next stage from the anode.

If you are unsure what I mean, I will do a sketch tonight.

The 150H hammond choke would be worth trying I think.

Hi Nick
had a go at what i think you ment adding the choke??? see sch

Sorry, I never got chance last night.

Your circuit is almost there, but the grid on the second valve needs to be connected the other side of the choke, on the anode of the first valve.

colin.hepburn wrote:

Nick wrote:Yes you could (almost), but you would need to leave the 100k resistor there to drop the voltage, and then decpuple it with a electrolytic, then add the choke between the decoupled point an the anode. Take the signal to the next stage from the anode.

If you are unsure what I mean, I will do a sketch tonight.

The 150H hammond choke would be worth trying I think.

Hi Nick
had a go at what i think you ment adding the choke??? see sch

Hi Nick
|OK see update Sch not sure of the voltage drop across the choke volts at anode of V1 should be 131v also the value of the cap at 100uf can this be lower as preferred to the use polypro if possibly