Alembic F2B build 6N1P

[Spencerleehorton]

Need to wait for a few more bits but pretty much there with one channel

 

[Matador]

I don’t think that’s a led as to drop to 2v @ 10mA it would have to be 470 ohm, I think it’s a 100mA 6v lamp?
So I will go with the 470R, 1n4007 diode then a led?

It's an LED that's ground referenced. With the hum pot in the middle, the 6.3V AC supply swings between +3.15V and -3.15V. Since there's no cap after the rectifier diode, the peak voltage after the rectifier diode is about 2.5V, so with a 2V LED, you have 0.5V left over. 0.5V over 68 ohms is 7mA peak current. The average current will be less. This is probably plenty bright.

 

[Spencerleehorton]

Would leaving out the 68R, diode and led improve any noise ?

I have a nice 220R 2W pot which is wirewound 10 turn pot so should be able to dial out any hum.

 

[Matador]

Would leaving out the 68R, diode and led improve any noise ?

Nope.

If it were mine, I would ditch the pot, and use fixed 100 ohms resistors. Then use the virtual center tap to elevate the heaters to 20V or so: far more effective than hum balance.

 

[Spencerleehorton]

Sorry I don’t understand what your meaning of elevating the heaters to 20v?
Off the 6.3v winding?

Also

So am I right in thinking I can just add the line out and the other parts, resistors, txf caps like the other schematic?
I have a few 12au7
I can also wind a 5:1 transformer or two.

 

[Tubetec]

Sometimes you find a resistor divider network across the HT , the centre tap of the heater is connected to that point , you still have 6.3v AC across the heaters , but now its elevated from ground by a DC voltage in the same proportion as the resistor divider .

As it is you'll only drive hi-z inputs like a DI box or amp input stage , output stage and step down transformer is required to match into line input (25k)

 

[Spencerleehorton]

Little bit of progress!!

 

[Spencerleehorton]

After a tiny bit of confusion between schematics and what pin should go where, it’s alive and sounding quite nice!!
Still got to put in the transformer and xlr out!
And do channel 2.
Layout has changed a bit now to hopefully reduce a bit of noise.
Bright switch doesn’t seem to do a great deal with 100pf so I’ll try something else.
Also I’ve wired the pots backwards!! So I’ll change all that and test again, need to know ideal voltages on plate and kathodes?
At the moment plates are 128v and kathodes are 0.9v?
I thought it should be more like 160v plates and 1.1v kathodes?

 

[Spencerleehorton]

Sounds great, just need to do the line out and finish ch2, bright switch still not doing much so will change 100pf to 120pf of more and see if I like it.

 

[Matador]

At the moment plates are 128v and kathodes are 0.9v?
I thought it should be more like 160v plates and 1.1v kathodes?

0.9V over 1.5K is 0.6mA. 0.6mA across 100K is 60V drop, which implies your B+ is sitting at 188V. Would need to know what your voltage is after the rectifier diodes, and if you used 1.5K series filter resistors to know if it's working as expected.

 

[Spencerleehorton]

After diode bridge it’s 306v, I’ve then got 2 x 10k followed by a 50k trimmer, so should be plenty to play with

 

[Matador]

If you lower the 50K until the B+ reads 250V, then you will be closer to the Fender operating point (with a quiescent point of roughly 0.8mA for each tube). But if you like how it sounds now there's no reason to change it: the electrons won't care.

 

[Spencerleehorton]

That’s perfect thank you, exactly what I wanted

 

[Spencerleehorton]

after 50k im read about 200v now and sounding better, up to 250v yeah?
also ive put in the line level output but its quite distorted and farts on low notes on bass? obviousy need to change a few value to bias it differently.
Also sounds extremely bassy compared with HI-z out?

 

[Spencerleehorton]

my main findings is that the line out and the unbalanced out sound so different, the line out doesnt have much top end and is a bit distorted, using a ECC81 for V2 and 12ax7 for V1.
Using a 4:1 transformer and a 2.2uf cap.
The overall signal is very loud and would help with having a pad on it for bass guitar, or also a gain control and a master.
some good sounds in there just need to know what to tweak now.

here is some voltages:
B+ = 193v (need to alter the values on the PSU so i can get 250v)

V1 = 12AX7

Pin1 = 129v
Pin3 = 0.907v
Pin6 = 136.7v
Pin8 = 0.911v

V2 = 12AT7

Pin1 = 137
Pin3 = 1.27v
Pin6 = 190v

i am going by the F2B style preamp Channel 1 schematic below

 

[Spencerleehorton]

Sorted psu so I now can get 250v B+,
Will test this morning and get some voltages.
Also going to look at input and see if I can pad it, also looking at putting a 500k master volume pot if I can fit it in!!

 

[Spencerleehorton]

Have changed input resistor to 33k and in between 2.2uf cap and txf I’ve put a 22k with 47k to gnd, output seems much more improved less distortion very useable now on active bass.
Will stick with this for a while even though outputs still do sound quite different but a blend of the two does achieve a really nice full bodied valve tone, mix in a bus with speaker emulation and it’s a really nice bass sound.