6111 subcaster debug

[HeavyFog]

So iv'e been working on a modified 6111 subcaster and i just can't seem to get things running. It makes a loud humming noise with no signal getting through. Iv'e noticed that the power supply works well when not plugged in to the pedal but when plugged it into the tube the voltage to the heaters drops to around 3.3 volts. After doing a bit of searching on this forum it seems that this might be a known problem, but i'm not sure. I'm running it off a 12vDC adapter that is actually putting out around 18v (500ma). Here's the schematic i based the project off. The power supply is identical to the one in the schematic but the tube stage is modified. Il try and get a schematic of that up as soon as i can.   

[HeavyFog]

Here's the schematic for the tube stage I used

[thermionix]

What is the heater current for a working 6111?  What is the max it draws when the filament is cold (at turn on)?  Are you using a known good tube?  Do you have a way to test it?  Do you have another to sub in?

Can you get 6.3v by adjusting the trimmer with the tube in?

[HeavyFog]

From the datasheet I saw it said 150-300ma for the heater. Definetly within the limits of the wall wart, although considering the one I have Is actually putting out 18v instead of 12 I wouldn't be surprised if it was labeled wrong and I actually got the wrong psu. I got the tube brand new from small bear and it appears to be good. I don't have a sub for it but I could get one if I need to. I'm suspecting the regulator circuit may be to blame. I came across an old post from rick holt (frequency central) where he said he couldn't get a sub caster working until he removed R1 and C1 in the regulator, so I'm definetly suspecting that, but I don't know what his initial issue with it was because I might have a whole different issue.

[HeavyFog]

Also I couldn't get near 6.3v with the tube in, only about 3.3v at max. Without I actually couldn't get it any lower than 6.7v, so it seems obvious that the power supply/regulator circuit Is likely to blame. Perhaps the heater running at only 3.3v is why I'm getting such a strong hum. Also il add that the gain control does nothing at all to the volume of the hum.

[duck_arse]

ohm's law. if the heater pulls 150mA, it has to come thru the 47R, as drawn. that will drop 7V across the resistor, how much of your wall-wart volts is left then?

take the regulator input from the wall-side of the 47R, and you should get another 7V to your plates, and your LM317 may then get enough volts to do some regulating. either way, measure the voltage either side of the 47R, that will show you a few things about your current draw.

[thermionix]

I think the loud hum is a seperate issue.  A tube with too-low heater voltage will make a weak sound or no sound, not a loud hum.  AFAIK.

Since you have no volume control at the output, what does running a resistor across the output jack do?  As in to simulate a full-up volume control.  I don't know if there needs to be a ground reference there, to set output imoedance or whatever.

[HeavyFog]

UPDATE
I tried getting rid of R1 and C2 and now its working! It sounds good and the gain control works, but the hum remains a problem. Its always there but it gets worse as i turn the gain control down. Perhaps this is an issue with the power supply electrolytic. Maybe i should try adding C2 back in?

[thermionix]

it gets worse as i turn the gain control down.

In other words, as the 2nd grid approaches ground.  Maybe a ground loop?  What is the frequency of the hum?

120Hz is very close to the 1st fret of the A string, you have to bend the note a little.  But you're in Canada, are y'all 50Hz or 60Hz up there?  I seem to recall 50Hz.  Either way, hum from full-wave rectified AC will be double the mains frequency.

[HeavyFog]

Il check my grounds. At the moment i have the main board ground going to the 3rd lug of the volume control and from there to the main ground. Perhaps it is a grounding issue. IL redo it anyways

[pinkjimiphoton]

sounds like power supply. check any diodes as well as caps, etc.

if it's not boxed yet, also expect a hell of a racket. it should quiet down when its boxed and shielded.

[HeavyFog]

In other words, as the 2nd grid approaches ground.  Maybe a ground loop?  What is the frequency of the hum?

I think you're right. I'd imagine connecting a tube grid straight to ground would result in noise of some sort. I wonder if this can be fixed without having to move the volume to its usual end of signal placement.

[HeavyFog]

Iv'e noticed that i'm getting a strange overtone with the gain up. Kind of like a very subtle ring mod. This is most noticeable with the gain up. I also noticed that while the hum goes down as you turn the gain up it almost disappears right before max gain, and gets a bit more intense again with the gain maxed out. Looks like it just might be a ground loop caused by the gain control. I did see a few other valve casters online that used the same interstate gain pot but without the same reported issues.

EDIT
Tried adding a master volume at the end (left the gain control in place). Didn't get rid of any hum, but the hum did go down with the output volume when the pot was turned. Also tried wiring it without the interstage gain control and adding the usual  470k resistor to ground and the hum remains, so it looks like that probably isn't the problem. As far as i can tell there isn't anything wrong with the schematic i drew up, but i might be wrong, So perhaps that's not the issue and my ground wiring is bad. Il keep looking

[HeavyFog]

Another update
After measuring the power supply i just noticed that not only is it putting out around out 18.5vDC (something i already knew), its also only putting out around 250ma. The value fluctuated a lot so it might not be entirely right but all thing considered the issue might be the power supply. In a valvecaster, would a really cheap (in my case incorrectly labelled) wall wart really cause a bass heavy hum that is almost as loud as the pedal's output signal?

[thermionix]

Yes, try adding more filtering.  Cheap wall warts skimp there.  Pi filter (cap to ground, resistor in series, another cap to ground) does more than just a bigger cap.

[duck_arse]

put back R1 and C2, they smooth the supply to the plates. MOVE the regulator input to the raw-DC side of R1, that way the heater current won't drag down the plate voltage. AND THEN measure the voltage on both sides of the 47R resistor.

the wall wart will supply what the circuit draws - if the heater only asks 150mA, the 1A (what is the nameplate rating?) wall wart will laugh, and the output voltage may well stay higher than plate rate. at its rated output current, the wall wart output voltage should match the nameplate rate.

[amptramp]

it gets worse as i turn the gain control down.

In other words, as the 2nd grid approaches ground.  Maybe a ground loop?  What is the frequency of the hum?

120Hz is very close to the 1st fret of the A string, you have to bend the note a little.  But you're in Canada, are y'all 50Hz or 60Hz up there?  I seem to recall 50Hz.  Either way, hum from full-wave rectified AC will be double the mains frequency.

We are at 60 Hz here but up until the early 1950's, we had 25 Hz AC.  Someone thought that we should have electric railways running through the Province of Ontario and at the time, this meant series motors.  Series motors run best on DC but can run on low-frequency AC.  Higher frequencies cause inductive reactance that would limit the available torque.  BTW we never did get any electric railways other than streetcars.

[amptramp]

Hum when the volume is turned down is a balancing effect.  With the volume turned up, the hum at the plate of the first tube is divided down by the ratio of Rp / Rp + Rl where Rp is the plate resistance of the tube and Rl is the parallel effect of the plate resistor and the volume control.  Suppose the hum on the plate supply goes positive.  A bit of the plate voltage is fed into the grid of the second tube where the rising grid voltage causes the second tube to draw more plate current and decrease the plate voltage.  At some volume control setting, you may be able to cancel the hum entirely.  But once you turn the volume down, there is no grid voltage to modulate the plate current, so the hum from the final stage goes to the output.

[HeavyFog]

Tried adding back in the 47ohm resistor after the input of the voltage regulator instead of before it. No luck. Didn't make any difference to the hum. The pedal worked as it did without it but still with the hum. I added it after c1 and c2. Would it being after those 2 caps make a difference compared to it being before them?

IL also add that the hum starts the second the power supply is plugged in, before the tube even heats up

[HeavyFog]

Here's the layout I used for the main build. The ground from the tube filaments and the tube cathodes are connected to the board ground strip to save space and reduce messy wiring. Not sure if that has anything to do with the hum