bias tremolo cuts out

[alange5]

I recently built a hybrid mix of the brown 6G11A Vibrolux and the 6G5A Pro.  Here's my schematic:







The tremolo sounds great and pulses nicely, but at around 3/4 turn of the depth pot, the tremolo disappears.  The last quarter turn is just clean amp with no trem.  I have the amp biased cool at around 32ma with 440V on the plates.  I hooked up a bias probe and monitored the bias swing with tremolo engaged.  Right before the trem cuts out, the swing is very large, moving between 19ma and 55ma.


What's causing the large swing, and how can I reduce it to make use of the full sweep of the depth pot?


Thanks in advance.

[sluckey]

The tremolo causes the swing in tube cathode current. That's just how it works. Monitor the voltage on the wiper of the intensity pot. What happens when the tremolo fails?

[alange5]

The tremolo causes the swing in tube cathode current. That's just how it works. Monitor the voltage on the wiper of the intensity pot. What happens when the tremolo fails?

With zero intensity, the voltage is a stable -48v. As the pot turns, it begins to swing. Right before the trem cuts out, it's swinging between -44v and -51v. When it cuts out, it jumps back to a stable -48v, accompanied by a faintly audible "pop" as it passes the trem threshold 

[shooter]

it cuts out, it jumps back to a stable

dead spot in the pot 

[alange5]

it cuts out, it jumps back to a stable

dead spot in the pot 

I think you're right. I'm measuring zero ohms on the last quarter turn of the pot.

[sluckey]

I would kinda expect to lose bias voltage too if there was a bad spot on the pot. Try this...

Monitor the voltage on the tremolo side of the intensity pot. What happens when the tremolo fails?

=======================
Edit... Couple more things to try if you don't have a spare pot. Set the INT  pot to Max CW. Trem should have dropped out. Now use a gator clip lead to jumper the pot wiper to the trem side of the pot. Does the trem come back?

Or, you can temporarily replace the pot with a 220K resistor. Tie the wire that goes to the tube grid resistors to the cap from the trem circuit. This simulates the INT pot set to max trem. Any joy?

[shooter]

audible "pop"

lose bias voltage too

could the cap hold Bias stable for a few minutes testing?

[alange5]

On the tremolo side of the intensity pot, voltage is swinging between -30v and -55v. When the trem cuts out, the voltage swing changes to about -41 to -44. That margin slowly decreases throughout the last quarter turn, until it's a stable -44v at max intensity.


Jumpering wiper to trem side of the pot does not make the trem come back.

[sluckey]

Jumpering wiper to trem side of the pot does not make the trem come back.

That kinda eliminates the pot as the problem.

The attached pic shows the 6G16 tremolo circuit. Try these three changes. Any joy?

[alange5]

I made the changes to the 6g16 and the tremolo still cuts out at the same spot, and now I have slightly less overall depth than I did before.


Here are my voltages on the trem tube:
On the feedback side, with the speed set to zero, the plate voltage swings between about 115v and 215v, and the cathode between 1.1v and 1.5v. With speed set to max, the plate is between 165-169v, cathode 1.29v-1.3v.


On the output side, the plate is at 303v. With speed at zero, cathode is 120v-220v. Max speed cathode is 173v-176v.

[sluckey]

I didn't have much confidence that those changes would help.  Let's prove/disprove that the oscillator tube quits when the trem drops out. Monitor the plate voltage of the oscillator. It should be changing. Now turn the INT pot until the trem drops out. Does the plate voltage become steady (indicating that the oscillator has quit)? Try this test with the speed set to minimum, then maximum, then about half way. Same results regardless of speed setting?

[alange5]

At max intensity, minimum speed, oscillator plate is moving between about 115v-215v. Halfway speed yields 150-190v, max speed 166-170v (all readings taken with intensity pot set to max with no audible tremolo)

[sluckey]

When you say "max intensity" does that mean the INT pot is turned all the way up and there is no trem effect, even though the oscillator tube is still oscillating?

Does the voltage on the cathode of the cathode follower still swing strongly with the INT pot set to full clockwise?

[alange5]

When you say "max intensity" does that mean the INT pot is turned all the way up and there is no trem effect, even though the oscillator tube is still oscillating?

Does the voltage on the cathode of the cathode follower still swing strongly with the INT pot set to full clockwise?

Correct - all readings were taken with the intensity pot fully clockwise. The cathode still swings strongly between 120v-220v. The swing is still visible past the 220k (now 470k) on the cathode follower. However, on the other side of the .1 cap (at the tremolo side of the intensity pot), the measured voltage stabilizes as the tremolo cuts out.


So, just to recap (no pun intended), with the intensity pot fully clockwise, I have voltage swing at the cathode follower, but stable voltage at the intensity pot. Should I switch out that .1 cap between the cathode and the pot?

[sluckey]

Should I switch out that .1 cap between the cathode and the pot?

Yes. May as well undo those 6G16 changes too since they result in a weaker trem.

[alange5]

No dice. I'm stumped!

[sluckey]

Pull the 6L6s. Now monitor the voltage at the wiper of the INT pot. Does the voltage continue to wiggle even with the INT pot turned max CW?

[shooter]

No dice

could it be swinging the wrong freq like the SSStrandal 

[alange5]

Pull the 6L6s. Now monitor the voltage at the wiper of the INT pot. Does the voltage continue to wiggle even with the INT pot turned max CW?

6L6's pulled - same behavior. Voltage swings -40v to -60v at 3/4 turn of the pot. Past that, it stabilizes around -53v 

[sluckey]

I need to see some hi rez pics of the trem tube socket, trem circuit components, and bias supply. I'd also like to see the 6L6 grid resistors and tube socket.

[alange5]

I need to see some hi rez pics of the trem tube socket, trem circuit components, and bias supply. I'd also like to see the 6L6 grid resistors and tube socket.

I'll post some tomorrow. Thanks for all the help thus far

[alange5]

Here's an overall shot:





trem tube socket:





trem components:





You can see the bias supply in this next photo.  I have a 4.7K resistor in series with a 50K pot for the bias adjustment.  Note: I have a dual-gang master volume in place of the 6L6 grid resistors, however I removed it and installed 220K resistors during my tests yesterday, just in case it was causing problems.  I have since replaced the master volume.





power tube sockets:

[alange5]

Well.... turns out it was the pot.  Resistance climbs gradually until about 200k, then flatlines at zero. I noticed a slight bulge in the body of the pot. I guess the wiper wasn't making contact at one end, which I assume is why the gator clip test didn't work. New pot is installed and all is well. Here's a pic of the amp all buttoned up:





I built it inside an old Maico audiometer used to diagnose hearing loss. I will now use it to create hearing loss.

[PRR]

> an old Maico audiometer

Jealous. 

[davidwpack]

Awesome!  Fine example of irony!