Double Shot..my new project, tube preamp..layout & pcb

[dschwartz]

yes peter, that´s the traditional way when you have 6.3V AC..

i´m using 12VDC regulated.. so it´s not necessary to twist the wires..

[bancika]

It looks it's not much of an issue if you leave them untwisted even for AC, take SLO amp for instance, they use two parallel buses for heaters.

Best,
Bane

[dschwartz]

yeah.. twisting heater wires always sounded kind of "suspicious" to me...maybe it helps..but must be marginal....
anyway, i twist wires on my builds just because it looks cleaner..

[bancika]

Agree, it does look cleaner. It's important not to have signal wires near AC heater wires, but twisted or not apparently doesn't change much.

[dschwartz]

the new PCB for the main board..

I hope this time someone find any errors! 



note: the relays must be connected in parallel, a single 5 v supply for both and ground to the switch.

Changes:
- Corrected the tubes orientation.. (plz check if it´s right!)
- Corrected heater wiring.
- Added 100kR to bypass caps for popless switching
- relays isolated from circuit.
- Distortion channel when deenergized position
- some parts moved around..
- Bigger space for 400v caps

greetings to all!!!

[stephanovitch]

Try to enlarge copper connection of the heater supply (4mm )

[d95err]

The 6V supply for the relays and LED won't work. It will produce 6V as long as you have no load on it, but look at it this way - you're trying to power relays and LEDs through a 100k current limiting resistor...

I'd avoid the active regulator for the heaters. According to Kevin O'Connor, DC-heaters in general will usually create more problems than it could solve, in particular with active regulation. A simple RC-filter should work well enough. A more efficient way to eliminate heater hum is to reference the heaters to a standoff voltage of e.g. 70-80V. You can derive this voltage from the main preamp voltage using a voltage divider and a filter cap. (I haven't tried DC heaters myself though, but Kevin is usually right about these things...)

[stephanovitch]

DC heater work very well but it's right that regulation is not necessary.

In my tube preamp I have only use single power diode and a RC filter and I have no noise.

[Rmanen]

What software do you use, its beautiful!!

Robert

[dschwartz]

i changed the 6V divider for a 7805.. the voltage with 100k ohm would drop too much.. i am learning soooo much doing this thing!!!!

as i already did the supply board, i think i wont change it if it´s not necesary..if heater hum appears, i´ll take out the regulation for heaters and use an rc filter (although its already filtered by two 2200 uF caps) as you said..

i did the layout with a lot of patience with firewoks...draw each part and made a "library" ..with some testing i got the right sizes...
then followed the schematics done with livewire (I dont trust pcb wizard) and draw the lines and dots of the PCB..

i know is more work than using eagle.. but i just don´t find eagle very comfortable and intuitive..and i like freedom i have for drawing the pcb´s..


FIY fireworks is an image editor from adobe, intended for web designing use..

[dschwartz]

i powered the board last night..it didn´t blow up!!!!
and the heaters worked OK..lighten up right away.
same for relays, they worked fine...
I changed the coupling caps for 200v rating.. since the power supply gives around 260 voltage, i think drain resistors will drop it below 200V.. they fitted much better on the pcb...


i saw a schematic where they put a 22uF cap across de relay coil..parallel to the diode..what is that for? slower action? smother contact??

i just have to do the wiring and test the monster!!!!

(..plz god..make it work!!)

[free electron]

i saw a schematic where they put a 22uF cap across de relay coil..parallel to the diode..what is that for? slower action? smother contact??

There's also a resistor with value equal to the relays coil in series, and the relay is powered with 2 x higher voltage. Yes, this slows down the switching action and reduces the spikes.

I changed the coupling caps for 200v rating.. since the power supply gives around 260 voltage, i think drain resistors will drop it below 200V.. they fitted much better on the pcb...

I wouldn'd do that. Think about what is happening when you turn on the power. Since you are not using a stand-by switch, for some time the caps are overvoltaged. It may not damage the caps at first time but it is not good for them, in time they can deteriorate and even blow out! Besides, the el-caps shouldn't work very close to their maximum ratings.
For more safety add a 470k 1W resistor across C3 in the power supply. It will discharge the caps when you turn the power off.

Very nice looking pcb layout!

[jakenold]

Hey Dan,

This looks very good indeed! I'll check everything when I get home from work, and contact you by messenger!

Keep up the good work!

Kind regards, Jake

[d95err]

I changed the coupling caps for 200v rating.. since the power supply gives around 260 voltage, i think drain resistors will drop it below 200V.. they fitted much better on the pcb...

Just remember that the coupling caps need to handle the anode voltage + the signal voltage (e.g. 200V at the anode + 60V signal peak). To be safe, it's best to make sure they can withstand the full voltage of the preamp.

It will be very interesting to follow this project. Getting the power supply working is a good start.

[12afael]

Daniel, you can add a 1V zener or a diode on the ground pin of the 5V regulator to have a voltage near to 6V for the relays.

[dschwartz]

Hi rafa!!

well the relays are rated for 5 volts so 6 volts are not needed (DS2Y - 5DC)

i´ll be posting the progress as i go.. i´m so close!!!

i´m planning to start it up before friday so stay tuned!!!!!

last night took some pics..enjoy!

[DWBH]

God! Now that looks nasty!

Awsome work dude, nice little hours spent on there.

Gotta give it a try sometime.

[petemoore]

  Frequin' B-Dazz-Mahn !
  That looks like some fine work D !
 
 

[dschwartz]

thanks guys!!!
i´ve put a lot of work on this...mi future wife it´s kind of mad hahaha

I´m building this thing for a customer so it HAS to be great..it´s my first tube preamp, well the first good one , i hope(real mctube didnt float my boat).. and well..i like my pedals to be pretty..

i´ve never design a pcb before, and learned so much these days, heaters, voltage ratings, voltage dropping, how much a PS cap discharge shock hurts ( ), how relays works, and since i´ve not finished yet, i feel that´s going to be ALOT MORE learning when i get this thing going!!!!

so thank you all!!!

[d95err]

I took another look at the schematic (the first one you posted, don't know what you've changed since then). Some more notes:

The first three gain stages share the same power supply node. This could be very bad in a high-gain circuit, and could lead to oscillation and "motorboating". The best solution would be to add another RC node on the power supply for the first (and most sensitive) gain stage. Another solution would be to move the third gain stage to the "B" node. Adding more filter nodes seems like a good idea though, considering that a typical high-gain amp will have about 5 nodes including a choke.

Some of the frequency shaping is odd. For instance, the 220p cap across the 50k Clean volume pot will only affect frequencies above 14kHz, so it will let through noise but do nothing with guitar frequencies. The 1n cap across the 1M Dist gain pot seems very big (470p/1M in a Plexi bright channel is considered extremely bright). The 2.2n cap across the 680k resistor before the Dist pot essentially takes the resistor out of the circuit, since everything above about 100Hz will pass through the cap.

The 3M3/50k voltage divider on the clean channel takes the signal down 36dB even at full clean volume. Are you sure you want to attenuate this much?

EDIT: Another thing - how is the grounding layout? Tube stuff is very sensitive about grounding and it's important to isolate high-current ground returns from sensitive signal grounds. For your preamp, the main retifier ground, heater rectifier ground, heater regulator grounds and the relay ground are high current and should not mix with signal grounds. Try to use some kind of "star" grounding scheme. Getting the grounding right is critical in order to reduce hum in a tube circuit.