valve distortion preamp

[the cracken]

Im trying to make a small distortion pedal out of 2 raytheon jan 6418 and i was getting some oscillation. i changed the value of the grid 2 resistor from 100k which i new was a bit low to 1M and it has made it much better is this because it has reduced the gain???
thanks

 

[Nigel Goodwin]

I suggest you post the circuit - but a 'grid resistor' shouldn't affect gain at all.

 

[the cracken]

heres the circuit!
**broken link removed**

 

[the cracken]

before anyone comments it uses the space charge to negatively charge the grid. and also i was wondering why distortion increases when the anode resistor is made bigger??? i have a 1M variable resistor on there at the moment but ill be putting a 36k ohm resistor and a smaller pot to add distortion!
thanks for the help!

 

[Jaguarjoe]

The grid of the second tetrode is driven from the +18v supply? It ought to come from the plate of the first tetrode.

 

[the cracken]

The grid of the second tetrode is driven from the +18v supply? It ought to come from the plate of the first tetrode.

yer some one said on here that it could be used like that. if i had it on the anode it would lower the gain and it would act like a triode.
if you look on the data sheet it say that it put on ht. but this is a valve with a maximum of 30v ht

 

[Jaguarjoe]

I don't believe it. The 18 volt line is fixed and unvarying. How does that provide a signal to the second tetrode? The first tetrode doesn't do anything. Its output should come from the plate, just like it does on the second tetrode.
Where is the data sheet?

 

[the cracken]

OOPS sorry made a few mistakes i was up late **broken link removed**
heres the data sheet :https://www.electro-tech-online.com/custompdfs/2010/11/6418.pdf

 

[the cracken]

its still oscillating though any ideas on fixing it????

 

[audioguru]

The +18V supply voltage might be jumping up and down with the signal which could cause oscillation. It needs a bypass capacitor. Try 220uf to ground.

Poor wiring results in the input and output wires too close together. They are in-phase so capacitive coupling between the wires could cause oscillation.

 

[the cracken]

i tryed putting in a 47uf but not to much avail do you think i should try something bigger? its on vero board so it could be but i don't think it is capacitive coupling.

 

[Jon Wilder]

Before anything else is done here let's make sure the circuit is correct before proceeding. Going off of the posted schematics it appears that your circuit is missing a couple of things.

Looks like you're also missing the grid leak resistor on stage 2. Usually you would use a 1M gain pot with one side grounded for this between the coupling cap and the grid of the 2nd stage.

The grid leak resistor is part of the cathode bias (or "space charge" bias as you call it). Without it there you have no bias at all. The filament supply references the filament cathodes +1.5V above ground, but without the grid leak resistor the grid has nothing there to pull it to zero and as such is undefined. It has to be pulled to zero via the leak resistor in order to be more negative (less positive) than the cathode filament to develop your space charge.

The other thing it functions as is the load resistor for the driving stage. Without the load resistor there the coupling cap has no way of charging/discharging as plate voltage rises and falls, which is how you develop the output signal in the first place.

Perhaps a better way of doing this -

**broken link removed**

Leave your plate resistors fixed and adjust the drive level via the pot, which also serves as the grid leak for stage 2. The 1st stage coupling cap isolates the pot from the plate voltage.

For higher levels of gain use higher value plate resistors to get a higher swing. If you really wanna go wild with the overdrive add another stage at the front end.

I would also consider dropping the value of your coupling caps. Since these are directly heated cathodes you really have no way to shelve unusable low frequencies via partial bypassing since the cathode filaments are referenced to a fixed voltage by default, and you'll just end up distorting everything...even the bass...and that doesn't sound/feel very good as it gives you the "flubby/farty" lows. As such, this leaves only the coupling caps as your method for shelving bass to keep the lows from getting distorted. For tighter cleaner bass I'd start at 0.0022uF (2n2F) and go down from there.