Unique 26R Vibrato Weak

[plexi50]

The vibrato in this amplifier is weak. I have traced and confirmed all resistor values to be good minus one wrong value on the intensity pots wiper which was 270K but is now 47K. I could see that it had been put in long ago. Every other part of the vibrato circuit running on Channel 1 is good. I have a 0.1 from the plate and running to a single .022 and joining (2) .01 caps which are the rate (speed) caps. I usually change these out for GP. Question:

The varistors in this amp are a little different than the magnatone varistors. They look like black ceramic fuses. How do i test these to confirm weather they are good or bad. I would like to find out if the varistors are good before i start with the usual suspected parts replacement.

The rate is present as is the intensity. The intensity is weaker than the rate. The rate sounds like it is doing it's job. But the intensity is shallow and weak (like low volume)

I say capacitor or varistor

[sluckey]

I'd leave the varistors alone and make sure the oscillator is all it can be first. Here's a better quality schematic. It has a note about the resistor you have changed.

[plexi50]

Much better on the eyes. Yeah i better replace those caps first and go from there. Interesting notation.  270 back in place. Thanks Steve*

[HotBluePlates]

Much smaller bypass cap on the oscillator than typical.

[plexi50]

Much smaller bypass cap on the oscillator than typical.

I changed the two .01 & single .022 cap and it is more present but still no thump or throbing depth. HBP are you talking about the 1k .01 bypass cap? The circuit is oscillating. I need to check out that .047 cap going up to the varistors

[plexi50]

Extra weird time

I have beautiful pitch shifting vibrato / ONLY if i turn the reverb pot up half way. I dissconnected the reverb tank in & out. If i leave the reverb tank connected the mix is nasty. Im looking at the footswitch jack now

EDIT: I resoldered all the tube pins and input jacks. Vibrato now sounds very nice with reverb connected but vibrato drops out if going beyond half way point on reverb pot. At that point the vibrato really starts pumping big time but the guitar signal dies out all together

Im still thinking there is something going on with the intensity pot

[plexi50]

I have the Vibrato working again. I found a 43K CC resistor broke off at the body that biased the 6CG7 cathode pin 3.  I need to increase the intensity a hair. Any ideas? I have lowered the  270K from the intensity wiper to 47K. It sounds a lot cleaner with more treble pitch to it. Im quessing and thinking at this point. The swap back to 47K did not change the pot position at which it starts to intensify. Bad 43K circle

[HotBluePlates]

... HBP are you talking about the 1k .01 bypass cap? ...

Yep. I'm sure they had a reason for doing that, but I don't know what it was.

...  I need to increase the intensity a hair. Any ideas? I have lowered the  270K from the intensity wiper to 47K. It sounds a lot cleaner with more treble pitch to it. ...

Try tacking a 25uF cap across that 0.01uF we were talking about. That oughta boost oscillator output and increase intensity.

[plexi50]

Try tacking a 25uF cap across that 0.01uF we were talking about. That oughta boost oscillator output and increase intensity

Thats weird* I tacked a 25uf and no change. Even clipped the .01 and no change. The intensity pot does not want to activate the vibrato until 1 O'clock. I was thinking of lowering that resistor but cant really go much lower than it already is at 1K. It sounds like it has enough thump now after changing 5 caps. Maybe this is normal because it isnt like the Custom 280 or 380 where you have 8 varistors. Just the same i think it should work properly on the CWR

Maybe lowering that 3.3M resistor might change the grid input signal and make it respond sooner. This is an odd situation.

[sluckey]

If you suspect the LFO output is low, measure it at the top side of the intensity pot. A scope is very nice. Gives a peak to peak reading. My Fluke 87V reads the RMS value very well. Your DMM may read it too. The idea is to have a reference level for the LFO signal voltage you have now so you will know if any of the changes increase the LFO voltage.

Have you changed tube? Have you measured the B+ feeding that node? Have you changed filter caps? Have you forgotten about the recent thread "Is your tremolo weak"?

[plexi50]

I think i have another issue that is effecting the intensity. What is causing the major voltage flux in the PI coupling pins 1&3 ? Plate voltage on the tubes is 387VDC.  I have measured all resistors in this area and some have strayed. Like 47k is 58k. Voltage difference shown on schematic

[Willabe]

What is causing the major voltage flux in the PI coupling pins 1&3 ? Plate voltage on the tubes is 387VDC.  I have measured all resistors in this area and some have strayed. Like 47k is 58k.

Looks like the plate voltage is high and the cathode voltage is low.

If the PI tube was drawing more current than it is now, it's plate voltage would drop and it's cathode voltage would rise.

Put the 2- 47K R's back to spec and see if that helps. Change that tube also, it might be weak.


              Brad    

[plexi50]

Thats what i have been looking at the last hour. Change the 2 47K back to spec and or even try a 12AT7 / 12AU7
It is oscillating and sounds real nice except for the intensity pot having to be 3/4 over it rotation to start working.

Edit: No change in coupling voltages. Changed out (3) 47k and (1) 270k. The 2.7k resistor value is fine.

[Willabe]

I'd check the ground on the 47K tail R, including any wire it may have going from the tail end of the 47K R to it's ground connection.

If it's funky then the tube can't pull the current it want's to through the tube from cathode to plate.



              Brad      

[plexi50]

I'd check the ground on the 47K tail R, including any wire it may have going from the tail end of the 47K R to it's ground connection.

If it's funky then the tube can't pull the current it want's to through the tube from cathode to plate.



              Brad      

The terminal that the tail 47k is on is the terminal chassis mount

[HotBluePlates]

Edit: No change in coupling voltages. Changed out (3) 47k and (1) 270k. The 2.7k resistor value is fine.

Then the tube itself is probably weak and not drawing enough current. Try swapping with another.

Or, don't sweat the difference in voltage.

[plexi50]

The LFO is strong. It sounds very nice. A lot of strayed resistor values and speed caps. Im finished.

[Willabe]

Very nice, good job as always Plexi.

Are you able to see any numbers on the variasistors?


              Brad