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

[dschwartz]

hey that´s great.. but looks asa newer version...fets instead of LDR´s for some switching...
and yes, there´s no cap on the third stage

[stephanovitch]

In my version I have used 4.7nF cap to ground but I have had squealing on this stage.
For canceling it, I have had a 10pF ceramic cap between plate and grid of this stage

[dschwartz]

you could tried that 4.7nF in parallel w/the plate resistor, maybe that would stopped the oscilations..

[stephanovitch]

That will reduce highs and mids on this stage

[dschwartz]

maybe thát´s the point?

[stephanovitch]

Try it and you will see that you will have a sound whith  bass only .
IMO it's awful.

[dschwartz]

THE BOGNER ECSTASY OD channel has a 1nF cap on that point...
i wouldn´t say that´s an ugly sounding amp ...
anyway, if i don´t like it.. i´ll put it out...

[John Lyons]

Putting a .0047 uf cap across a plate resistor will only cut out some very high frequencies.
Mesa, Soldano, bogner etc etc all do this. Mainly this is for citting out the fizzy distortion and possible oscillations.
Maybe a 4uf cap is what you are thinking stephanovitch?

John

[free electron]

Relays are very nice for switching, but often they need special measures to get rid of the spikes and pops they produce. Taking a look at your schem:
I wouldn't leave the cathode caps floating when they're OFF. It will cause a huge POP when you connect the - terminal to gnd, especially at higher gain settings. It's better to put there a 100k to ground and use a switch to short it.
One good rule when using relays in the signal path is not to believe in the caps quality and their leakage current parameter. I'd put the pull down resistors after C5, C7 and before the tone stacks. They will keep the relay terminals at ground.
I'd recommend a use of separate grounds for signal and switching. Looking at your layout: A relative high current spike from 5V relay will travel close to the ground of the first input grid and cathode resistor. Isolate the relay grounds and let them meet the common ground at the power supply.
Another options:
- use a P-FET or N-FET (you'll need a negative voltage to cut it off) or Opto-FET (H11Fx) to switch the cathode caps,
- use LDRs to switch between Gain pots. LDRs do not produce spikes and the closer you are to the input, the more sensitive the circuit is.
- use some tricks to slow down the relay switching voltage: example at Geofex, more examples at Tom's page (the page is in polish, but the schematics are easy to understand)
- use some other tricks like a cutting out the spikes by muting the output for a short period of time, during the switching moment. Take a look at the engl and trace elliot bonneville schematics, these amps are full of relays.
Hope that helps.
There are some nice mods for "rectified" preamp on the Tom's homepage, just take a look at the schematics over there. It's really a mine of ideas
Piotr

[dschwartz]

Hi electron..thanks for your observations!!!
i did noticed the problem with the bypass cap switching, so i already added 100k resistors to avoid popping..i saw that on the original boogie sschems..

i did isolate the relays ground.. cut the traces and wired them to the switch and from the switch to power supply ground..

the ldr´s and fets switching ideas are great (and used on the originals) but if i do that, i have to redo the hole thing..and..lets say i´m not in the mood  ..

i don´s have easy acces to vtl´s optos here in chile, only some cheap ldrs..i´m not shure if they will do the work correctly, so i didnt take that chance and used relays for switching..

i plan to add leak resistors when i test the preamp running..only if the switches pops..just adding some resistors here and there maybe will do the job..

[dschwartz]

no one noticed..that the tube pins are backwards!!!???
damn!!all over again!! damn!!

[dschwartz]

im doing the pcb again with all your suggestions (well the ones i liked)..so..
Any idea how to take the relays out of the board without damaging it? i´ll have to recycle the parts for the new board...

[stephanovitch]

Use a desolder pump.

Which pins of yours tubes have you supply for heater?

For 12V supply, pin 9 of 12AX7 is the mid point of filaments.
You must supply pin 4 and 5 (two filaments in serial)

Use a TO220 package for the 12V regualtor with heatsink.
In 12V supply heater of a 12AX7 use 300mV.
Your regulator will dissipate 2X 0.3A X (17V- 12V)= 3W

[dschwartz]

Use a desolder pump.

Which pins of yours tubes have you supply for heater?

For 12V supply, pin 9 of 12AX7 is the mid point of filaments.
You must supply pin 4 and 5 (two filaments in serial)

Use a TO220 package for the 12V regualtor with heatsink.
In 12V supply heater of a 12AX7 use 300mV.
Your regulator will dissipate 2X 0.3A X (17V- 12V)= 3W

yes i got it all wrong..my first layout was a complete disaster...i put the heaters in parallel with 12 volts.. and pin 9 to ground.. so.. WRONG!!.. i put the pins mirrored on the PCB so .. WRONG!!!..

i shared the ground with  relays..WRONG!!
the deenergized state of the relay was the clean channel... WRONG!!!

at least the PS is working.. i used the TO220 regulators, a 7812 and a 7805..planning to put the heatsink soon..

[free electron]

Don't worry, the next pcb will be better

[dschwartz]

i wired the heaters like this:


12v--->tube 1 pin 4
pin 9 no connection
tube 1 pin 5---> tube 2 pin 4
tube 2 pin 5 ---> ground

is that OK???

[free electron]

12v--->tube 1 pin 4
pin 9 no connection
tube 1 pin 5---> tube 2 pin 4
tube 2 pin 5 ---> ground
is that OK???

No, now you have four heaters in series supplied with 12V.
Put 12V between 4 and 5 pin of each tube, leave all the 9s.

[dschwartz]

that would be:

12V--> tube 1 pin 5 ---> tube 2 pin 5

ground -->tube 1 pin 4 ---> tube 2 pin 4

i can´t believe i´m so confused with something so simple.....

[free electron]

one schem says more than 100 words:
http://s236.photobucket.com/albums/ff176/314_tr/?action=view&current=heat.gif

[petemoore]

  Tap     F------F------F------F
             I        I        I        I
  Tap     L------L------L-------L
  The tube filaments are paralleled between the heater taps voltage, or this is the way I've always seen it done.
  The two wires ~tightly twisted together between all connection points to reduce noise.