Tube experts? I could use some help understanding my Uber Tuber build.

[EBK]

I've built a MBP Uber Tuber:

I've even boxed it up with graphics and everything (an all too rare event for me):
https://www.diystompboxes.com/smfforum/index.php?topic=36392.msg1137865#msg1137865

But, unfortunately, the Thomson 6111 tube I got from Smallbear is producing an output too low for the saturation control to have any effect.

I'm getting a total gain of around 4.8v/v (from input to the top of the saturation diodes, with Drive maxed).
Also, my plate voltages are higher than expected (15.28V and13.07V instead of 11.52V and 11.36V), which suggests (to me, anyway) that my plate-cathode currents are too low (which probably explains the low gain).  I'm just stumbling around with this since I don't really know a lot about tubes....  If anyone wants to explain how this circuit works, I'd eagerly pay attention.

My main question: Is there a way to salvage this build without trying a different tube? 

I basically just want to bump the gain up without messing anything up so that I can justify having that saturation knob on the enclosure.  Should I try reducing the cathode resistors?

[PRR]

The 15V is lower than expected but not alarming.

What is the *heater* voltage?

Sure a 100K is not 10K?

Since you can measure gain, measure each stage separately; also the interstage networks.

Go ahead and *short* the cathode resistors, try that. (No, you can't burn-up with 18V and 100K series.) It may distort at high level. It just-might be better distortion than crystal diodes....

[EBK]

What is the *heater* voltage?

6.34V

Sure a 100K is not 10K?

Pretty sure.  Wouldn't hurt to triple check though, I suppose. 

Since you can measure gain, measure each stage separately; also the interstage networks.

Good idea.  I'll gather some more data and report back.

Go ahead and *short* the cathode resistors, try that. (No, you can't burn-up with 18V and 100K series.) It may distort at high level. It just-might be better distortion than crystal diodes....

Interesting.  I'll give that a shot while I measure the gains.
Thanks!

[vigilante397]

Everything I would have said, Paul already did. Triple check plate resistor values, drop cathode resistor values or short them as necessary.

Now I'm just following the thread out of interest

[EBK]

Triple check plate resistor values

I'm 100% confident on the plate resistors (measured within 1%, not just from correct bin with correct color code). In fact, I've also socketed those so I could try different values. 

[vigilante397]

Triple check plate resistor values

In fact, I've also socketed those so I could try different values.

Have you tried that? Going up in value on the plate resistors may help. Try everything, trust your ears

[EBK]

Yes, I tried upping the plate resistors.
When I increased both to 180k, I got plate voltages of 14.05V and 11.30V.

I took the first plate resistor up to 220k and got a plate voltage of 13.57V.

This did nothing as far as getting that saturation control to work, but it did allow the sound to go from clean to very crunchy with Drive control adjustment (previously, with the stock values, the Drive control only made the sound louder -- still clean).

[thermionix]

I'm getting a total gain of around 4.8v/v (from input to the top of the saturation diodes, with Drive maxed).

Is this calculated with the clipping diodes in circuit?  And you measure the same total gain with the Satch pot min and max?  I reckon you already thought of all that.  Could just be a weak tube.  Low E.  Since you've already double checked values, I don't know any better next step than to try another tube.  Good thing tubes have sockets, right? 

[EBK]

I'm getting a total gain of around 4.8v/v (from input to the top of the saturation diodes, with Drive maxed).

Is this calculated with the clipping diodes in circuit?  And you measure the same total gain with the Satch pot min and max?

Yep and yep.  The waveform didn't change at all over the total travel of the Sat pot.  If I crank the input really high (much higher that my guitar would produce), I can get the Sat pot to work as expected, but I really don't want to have to put an overdrive in front of an overdrive to make it work as expected.

Since you've already double checked values, I don't know any better next step than to try another tube.  Good thing tubes have sockets, right? 

Yeah, about that....

[thermionix]

That's what the winky face was for.  But still no big deal to unsolder.  At least it's not SMT.

[anotherjim]

Rather than increasing plate resistors - because maybe you're only pushing it into less linear behaviour without necessarily increasing voltage gain - look at the cathode bypass and interstage coupling caps.
If you want it for bass, C3, 6 & 7 sizes are not doing the lowest octave any favours.
But C7 in particular seems small considering what happens to the load impedance when the diodes conduct. Isn't 100nF more reasonable?

[thomasha]

Measure cathode voltages, to verify the anode current and bias.

I never had this kind of problem with subminiature tubes, but I also never used plate voltages that low, or cut the terminals that short.

At least ot is working, so there must some small mistake somewhere. If you have an audio probe you can test the signal path. You could remove the tube and test it in a different circuit or protoboard with min. amount of components.

[EBK]

Measure cathode voltages, to verify the anode current and bias.

I've added that to my list of things to measure.  Hopefully, I'll be able to post some useful data tonight.

You could remove the tube and test it in a different circuit or protoboard with min. amount of components.

I'd be willing to do that if nothing jumps out from this next round of measurements.  I could snip the header pins from that daughter board and remove them individually, then solder on a new strip of header pins. Or, I could just remove the individual components from this board that connect to the tube. Much easier than desoldering the tube.  Next time, a socket for sure.

[EBK]

Measure cathode voltages, to verify the anode current and bias.

First cathode voltage is 45.2mVDC.  Second cathode voltage is 60.9mVDC.

Having trouble measuring interstage gain quickly at a few points because my dirt cheap scope is DC-coupled-only, and I can't set the trigger properly.  I'll need to be just slightly more clever with my ground lead placement when I find more time....

[thermionix]

I have no data on the 6111, no experience with subminis or low plate voltages, but shirley those cathode currents are lower than what's expected, right?  And low plate currents would explain your higher-than-expected plate voltages.  I'm still thinking weak tube, the circuit works for others, and I trust your ability to spot build errors.

[EBK]

I have no data on the 6111, no experience with subminis or low plate voltages, but shirley those cathode currents are lower than what's expected, right?  And low plate currents would explain your higher-than-expected plate voltages.  I'm still thinking weak tube, the circuit works for others, and I trust your ability to spot build errors.

Well, there have been a few consistent build reports with high plate voltages, but I'd expect they are all related, and yes, I believe it all has something to do with low current.

Given the low current, it might not be terribly surprising that shorting the cathode resistors didn't do anything noticeable. 

Does this imply that I've basically done as much as I can for gain (with this tube), other than perhaps reducing the plate resistor and simultaneously increasing gain and sacrificing headroom?

[thermionix]

Reducing the plate resistors reduces gain.  IF you have a bum tube, no point trying to adapt to it.  Just get another one.

[PRR]

Re-re-re-check all your connections.

You are 101% sure the cathode resistors are 1.5K, not 15K, not 1.5Meg? (It has happened.)

The tube does light-up, get warm, right?

You never connected it to a very-wrong (high) voltage?

Then, hate to say, contact SmallBear and ask about a swap. He may not have a jig to test these in-house. If he's got a dud batch, he needs customers to tell him.

[EBK]

Re-re-re-check all your connections.

I promise I will, next time I get to work on this.

The tube does light-up, get warm, right?

Yes. 

You never connected it to a very-wrong (high) voltage?

Nope.  Nothing higher than 18VDC.

Then, hate to say, contact SmallBear and ask about a swap. He may not have a jig to test these in-house. If he's got a dud batch, he needs customers to tell him.

Ok.  Once I've re-re-re-checked everything, I'll do that.

[EBK]

If I, say, accidentally ran this thing with R5 as an open circuit for a few minutes, could that fry the first triode?  I just replaced the tube, but this is unfortunately where I am at.  New tube, but nearly no gain.   

The first plate voltage is only a hair below my supply voltage.