Alembic F2B tube version PCB

[rankot]

I decided to build this preamp using this schematic:



Just added presence control from some other preamp to test if it will work. Also picked transformers that I can buy locally and I hope they are not underpowered.

Resulting PCB can be found here:



I'll be very grateful if anyone of experienced tube builders can comment this PCB. Is it OK or not, and if not, why?

[PRR]

Your Presence pot will not do much to the right and nothing to the left. Where did you get that from?

Your heater supply first cap probably needs to be much more than 470u, to get the 9.3VDC needed to keep the regulator out of buzz.

I absolutely would not plan to put the power parts on the same small PCB as the hi-Z preamp. This invites power buzz to get into everything. A PT right behind the input jacks is daft. Put power on a separate board on the far side of the box.

[rankot]

Thanks a lot, PRR - very helpful as always! I picked that presence control from here: http://hiwatt.org/Schematics/DR103sn727pre.jpg. It looked strange to me, but I decided to try it - if it doesn't work, I'll simply remove that pot.

I will try with 2200u for heater supply cap, or even 4700u.

Regarding transformers - would it help if I put them in back of the case, and then add some kind of grid between them and this PCB? The case I have is pretty big, so it will not be a problem.

[rankot]

What do you think about transformer choice? Would they be sufficient? 

[PRR]

6V 500mA, 3VA, seems much too small for 2 Watts of heater plus rectifier and regulator losses. We have been disappointed by wee-tiny transformers. Without re-designing for you, I would go at least 6V 1A, maybe 2A. (Actually I would go 12V 1A, rectify to ~~16VDC, regulate/resistor down to 12V, and connect the tube for 12V.)

22V 58mA seems over-kill for a 3mA load but smaller won't be cheaper.

> I picked that presence control from here

Mis-copied. The HiWatt plan works. You put it on a stage with NFB, low output impedance. The right-side of Presence pot can't do much. OTOH the left side goes to a bypassed cathode, and another 0.2uFd won't do much with 47uFd already there. Finally your presence pot is 1Meg, not 100K, in a circuit which is under 100K, so it can't do anything much until the very end of rotation.

I suspect the HiWatt wants a Reverse-Audio pot.

https://s9.postimg.org/x607rc573/rankot-1.gif

[Unlikekurt]

I have a few comments and questions to add to the conversation.
1. For the filament supply, your schematic indicates 9V after the bridge rectifier.  This is an overestimate.  In a perfect world without ripple and sag, etc: (6*1.414)-1.4=7.08V.  Your current draw will be around .3A for the filaments and then a couple of mA for the regulator itself.  When you introduce that load and the 470uF cap shown, you are certainly going to see sag well below that 7V; even with a massive 4700uF capacitor.
All of that being said, why bother with the regulator at all?  You've got one tube.  Consider rectifying, filtering and use a resistor on each leg to balance the voltage / supply the desired current under load.
Or, frankly, depending on the secondary voltage from T3, just use that for the heaters along with two 100R resistors to reference ground.  The preamp isn't much of a gain machine and the heaters shouldn't be imparting all that much noise. 

2. Consider putting some kind of buffer before the output.  If you want to stay all tube a cathode follower or, if not, a transistor would be great.  The output off of the plate is fairly high impedance and although the 1M pot will allow you to trim the overall level, the unit will play a lot nicer with the things you plug it into if it had a lower impedance output.

3. What is the jack labeled "+signal-" on the schematic?

4. I agree with PRR that the "presence" control seems rather useless.

[rankot]

+signal- jack is there to provide connection to another different preamp (I have a plan to have two of them, cause case is big enough).

[rankot]

Mis-copied. The HiWatt plan works. You put it on a stage with NFB, low output impedance. The right-side of Presence pot can't do much. OTOH the left side goes to a bypassed cathode, and another 0.2uFd won't do much with 47uFd already there. Finally your presence pot is 1Meg, not 100K, in a circuit which is under 100K, so it can't do anything much until the very end of rotation.

Like this - and I added resistor parallel to Presence pot to mimic antilog behaviour, cause I can't buy ALOG pots here?



I have also changed heaters' supply according to Unlikekurt's comment, if I understood him well?

Regarding output buffer, I am not sure how to do that

[J0K3RX]

This may be of interest to some... It's a pedal but the layout is really nice and all the files needed to build it can be found in the project link.

The link to the project... and have a look at the other projects also, some really cool stuff with great layouts!
http://the7line.clan.su/publ/diystompbox/bass_tube_preamp_alembik_bassman/2-1-0-38

regarding the output buffer, look here..
http://the7line.clan.su/_pu/0/38477320.gif

[rankot]

I think I will go with the supply like this:



I copied filtering section from this post above (green cloud), but I am not sure if it is necessary? I will try without it at first, cause I believe low pass filter created with C4 and R2 is quite sufficient. I would like to go with 6V transformers because I plan to use this same supply for other tubes which can't use 12V for filaments.

[Unlikekurt]

With respect to your first revision schematic:
The Filament supply as drawn is incorrect.
You should have one leg of the transformer secondary going to pins 4&5 of the preamp tube and the other leg of the transformer secondary going to pin 9 of the preamp tube. Additionally, from each leg of the transformer's secondary you should have a 100 ohm resistor.  Not in series as you have drawn, but with the other ends of each resistor connected to each other and ground.  This provides a more balanced ground reference than simply grounding one side of the secondary.
Also each are 3v, not 6.  Together they make 6v.

[rankot]

OK, I hope I got it right now:



Shall this ground point between resistors be connected to chasis, or to DC ground?

[Unlikekurt]

That looks better, but again.  The individual legs are not 6v.  They are 3v.  The resistors are tied to the circuit ground.  This may be directly to the chassis ground point of the proximity to it makes sense, otherwise via a wire, etc.

[rankot]

I've found a very nice article about this here: http://www.valvewizard.co.uk/heater.html They say that heaters can be connected like this:

[Unlikekurt]

That is correct.  So if you have 12v available a series connection is great!

[rankot]

I have connected 6.3V heaters as shown in the middle of this picture above in my last schematic. There is one more thing I am not confident about - when using DC for heaters, 6.3V is voltage with load or without it? Shall I set 6.3V using regulator without the tube plugged in, or I shall plug the tube and then set this voltage with load connected?

Also, is it enough to use dropping resistor to drop heater voltage to 6.3V from 9V DC, or I must use this kind of regulated supply with transistor and zener diode?

[rankot]

When I put this 10 ohm resistor in series with heaters (which are connected parallel for 6.3V DC), I measure 4.2V. Is it OK, or I shall reduce resistor value?

[Unlikekurt]

When you are measuring are you doing so between pins 4/5 & 9 or from 4/5 to ground or 9 to ground?
If the first, then you are too low.
If either of the later, your voltage is too high and you're seeing 8.4 across the heaters.

A suggestion would also be to use two resistors, one from each leg of the DC supply.  This is for the purpose of balancing.  Perhaps not necessary, but ...
Calculate the voltage drop needed.
If actually from 9V it would be 2.7.
Current draw is .3A
So you'd want to drop an equal amount from each side (1.35V)
Use ohms law:
R=V/I
R= 1.35/.3
R= 4.5 Ohm
So a 4.5 ohm resistor on each side
And you'd want to go with 1W resistors.

All of that aside, understand that when under load, your supply voltage is going to change from its unloaded state.
So you will need to do some figuring based upon your actual readings on your actual circuit.
Not a hypothetical 9V.

[rankot]

Thanks for all replies!

Since both pins 4&5 are connected together (heaters are in parallel connection) at the same point, I measure 4.2V between that point and ground. I am talking about DC only circuit, with ground and +9V supply lines (the one I use to experiment and is based on Valvecaster, before I go to higher voltages). So if I put a 10 ohm resistor to lower my voltage from 9 to 6.3V (OK, it should be 9R1, but I don't have anything like that with higher power rating at the moment so I put 10R 5W for testing), I get 4.2V with a tube plugged in. I am not sure if it is correct - it works right now, but I am in doubt if this voltage shall be 6.3 with or without the tube plugged in?



Also, this limiting resistor is radiating almost 1W of heat, so it is unnecessary waste if I want to run this as a part of pedaboard. Shall it be better and with less losses to use transistor/zener circuit shown above?

[Unlikekurt]

Well, first of all why is pin 9 grounded?
Is this actually ground or is this the negative side of your DC supply?
Please show schematic of supply.

That said.  Ohms Law.

With the tube installed, what is the voltage across R9?