miniature tube with LM317

[soapamp]

I build tube booster 6088 miniature tube.  There is a high frequency hiss when switch on after a few second that sound will gone.  It will happen again same when switch on.   The source of my power supply switching is dc 12v 500ma  to tube plate and LM317 to get the voltage supply 1.5v for heater.

Anyone know how to solve this problem. 

[Ripthorn]

Are you saying it hisses when you turn it on and then the sound goes away or you turn it on and hiss comes on later?  Depending on filament draw, you might need a current dropping resistor or zener diode or heatsink for the 317.  Or it could be the switching frequency of your power supply.

[frequencycentral]

I build tube booster with miniature 6088 (similar your 5672tube)  tube with LM317 regulator to supply filament 1.25v and plate 12v.  Source of power supply is switching 12v, 500ma.  The problem is high frequency hiss when switch on after a few second that hiss gone. When I switch off & on again it will be same problem.  Can you suggest or advise how to solve the problem.

It may be that the tube or LM317 does not like the switching supply for the heater. Have you tried a non-switching supply? Maybe you could try powering it from a battery and see it it still has the same issue. You could also try adding a 100uf cap to ground from pins 2 and 3 of the LM317 to keep ripple down. But if the hiss is just there on power-up, maybe you can live with it?

[soapamp]

Thank you for both of your rely.

It will happen every time when switch on.  I will try to supply the heater with normal battery.   I have only 10uf cap at pin 2&3 of LM317 to ground.  I also will try to change it to 100uf.

[head_spaz]

This is the result of voltage appearing on the plate BEFORE the heater is warmed up. This can be hard on the tubes.
Would be better to heat the filaments first, and then apply the plate voltage.

[Ben N]

I vaguely recall someone (trying to) design(ing) a timer to delay voltage to the plates, sort of an auto-standby-switch. Maybe someone knows where this can be found?

[punkin]

How about simply adding a second switch (standby)?

[Ben N]

That would be a solution, but it just seems like a lot of hardware just to plug in a stompbox. Maybe something like a 555 could be pressed into service to control a jfet switch between the LM317 and the plate? Just a thought.

I wonder if a tube rectifier would help--there are submini rectifiers out there, you know.

[petemoore]

  I told Greg to just use the volume control on his Classic 30 amp, it's capable of '30watts worth of current' on cold EL84's.
  I figure it's pretty safe and simple, [standby Sw is ''one more electro-mech thing''] since the volume control on the amplifier is smooth and seems to go to exactly 0.0 input, which equates to almost 0.0 output...
  The plates in a booster, though not as large or robust as 4 x EL84 plates, draw little current, and probably don't even 'notice' they make hiss, and their lives are otherwise ~unaffected by being cold-voltaged.

[maarten]

Hello, I have been using 6088's as well and have not yet encountered this problem. Maybe you could post a schematic so we would have some more clues as to what is happening.

Maarten

[soapamp]

I found out that problem.  It's microphonics from tube.  I install tube in the same board of components. I used the small size of enclosure from Japan. The board sat the wall of enclosure and switch.  The vibration from switch on/off also cause tube microphonics.  I tired to fix the tube flow over the board and put some adhesive all tube lead, also put the soft rubber at switch in order to absorb the vibration.