vibrato in BF Vibro champ weak

[dude]

66 BF Vibro-Champ has weak vibrato during the first minute the amp is turned on, then no vibrato at all. I've checked all the resistors in that circuit, try several known good 12Ax7, replaced the three disc vibrato caps. Still the same. The caps I used were not like the old original ceramic caps but what I had on had, just polyester coupling caps. Do I need a special cap there like ceramic? What about the speed and intensity pots, I didn't check them?

[PRR]

Replace the cathode cap, electrolytic.

[dude]

I replaced the cap, it was bad (was a new Sprague a year ago ?). Still not working but all my voltages are off, bouncing around, not just in the vibrato circuit but the whole amp. I'm checking all voltages from the PT down, something's off. I'll post when I find the problem.


I knew this amp was sounding weak, plate voltages on all plates are bouncing around by 50 to 60 volts or more. Hope it isn't the PT, maybe the 5Y3.  I remember the B+ was 405v, now it's 323v a year ago. If not the 5Y3, maybe the PT is malfunctioning. I had zener's strung off the CT to lower the B+ to spec's around 355v, maybe they damage the PT. I'll post after I take a break first.


al

[dude]

Voltages seems good till I get  to the vibrato tube, the 470k from one plate to the other isn't reducing the voltage from 340v to 170 volts, more like 80v to 130v, bouncing back and forth. Looks like that resistor is faulty, not giving a steady 170v to the other plate on pin 1.


Time for a Dogfish, so tomorrow I take it out and check. Hopefully that's the culprit, it's a film R so looks like I replaced it a year ago, it's across the plates of the vibrato tube so maybe I overheated it...?


Another thing the OT on the 6V6, is dropping about 40V more than the schematic but it's not the original small champ OT but from another SE OT (a little bigger). But that replacement OT is 10K, not the 4k original, wonder if that has anything to do with the lower voltage. Probably need to change  the impedance of the speaker?   


al

[PRR]

> OT on the 6V6, is dropping about 40V more than the schematic ....Probably need to change  the impedance of the speaker?

Audio load has NO effect on DC voltage.

Post a link to the schematic of your specific amplifier (fender made too many).

[mresistor]

http://el34world.com/charts/Schematics/files/fender/Fender_Vibrochamp-AA764-schematic.pdf 


I would think if that 470K is bad it would throw off the whole vibrato circuit. The voltage is not steady probably because the vibrato is on and the the tube is oscillating.
 

[sluckey]

Voltages seems good till I get  to the vibrato tube, the 470k from one plate to the other isn't reducing the voltage from 340v to 170 volts, more like 80v to 130v, bouncing back and forth. Looks like that resistor is faulty, not giving a steady 170v to the other plate on pin 1.

That's the way tremolo works. Pin 1 is the plate of the oscillator. It should be jumping around big time! Pin 6 is the plate of a cathode follower. It is tied directly to a B+ node and should be steady.

If you want to see steady voltages on both tubes you will have to plug in the footswitch and turn the tremolo off.

[mresistor]

Or a shorting plug ..  have you checked or replaced the cathode cap on the second amplification stage? Or replaced that AX7?

[dude]

Voltages seems good till I get  to the vibrato tube, the 470k from one plate to the other isn't reducing the voltage from 340v to 170 volts, more like 80v to 130v, bouncing back and forth. Looks like that resistor is faulty, not giving a steady 170v to the other plate on pin 1.

That's the way tremolo works. Pin 1 is the plate of the oscillator. It should be jumping around big time! Pin 6 is the plate of a cathode follower. It is tied directly to a B+ node and should be steady.

If you want to see steady voltages on both tubes you will have to plug in the footswitch and turn the tremolo off.

No, I want the vibrato to work. OK, if it's jumping all around the place, what's next?
 

[sluckey]

Look at the voltage on pin 8. It should be about the same as on pin 1 and should be jumping around also. Is it?

[dude]

> OT on the 6V6, is dropping about 40V more than the schematic ....Probably need to change  the impedance of the speaker?

Audio load has NO effect on DC voltage.

Post a link to the schematic of your specific amplifier (fender made too many).

http://el34world.com/charts/Schematics/files/fender/Fender_Vibrochamp-AA764-schematic.pdf

[mresistor]

Dude - See reply #5  The link to the schematic has already been posted.

[dude]

Look at the voltage on pin 8. It should be about the same as on pin 1 and should be jumping around also. Is it?

pin 1   182v steady  (vibrato off) on, 70 to 220v jumping all around
Pin 2 -1v
pin3    1 or 2v
pin6    365v
pin7   70 to 220v jumping around
pin8   118-210v jumping around


all voltage with vibrato on except where noted.


I take it all these voltage are normal..? Perhaps the polyester caps for the trem are leaking, I checked them and the capacitance where good.
What next?


mresistor, I see your post thanks, missed it, thanks.

[sluckey]

V2 appears to be doing everything right.

Check voltages on V1 pins 6, 7, and 8. Change that e-cap on V1 pin 8. Check the 1500Ω and the 47Ω on V1 pin 8.

[dude]

V2 appears to be doing everything right.

Check voltages on V1 pins 6, 7, and 8. Change that e-cap on V1 pin 8. Check the 1500Ω and the 47Ω on V1 pin 8.

V1
pin1  114v (too low?)
pin2   0
pin3   4v
pin6   124-140 jumping around, vibrato on, off steady 135v
pin7   0
pin8   6v (?)


V1  cathode e cap was changed to 4.7uf from 25uf but vibrato worked fine a while back
      cath R  1.3K
      47 ohm cap reads 49.6 ohms


  4v on the cathode of pin 3 ?


Further checking V1 for odd voltages, found V1 to be bad, replaced and:


 
100k plate R's on V1 read 100K both but the voltage to pin 6 jumps around 240 to 256, while pin 1 is steady at 256v, you'll see I listed the plate voltages on V1 were 114, I change that 12Ax and got much higher voltages...? Looks like that V1 12AX7 was bad.


And guess what the vibrato works great, a bad V1. Thanks for helping me track down that bad tube, I never would have thought V1 - 12AX7 had anything to do with the vibrato. Live and learn.


One thin, my voltages are 20% higher than schematic, the plates on V1 are 250v, is that kind of too high?  B+ is 372v schematic says 355v.
I had two 10v zeners off the the CT dropping 20v but disconnected them. I'm running a 6L6 and a 470ohm bias R, I didn't do the dissipation as the PT and 6L6 seems not to be running hot (same PT as a Princeton running two 6v6s). Maybe the higher voltages are best for a 6L6, think I'm ok with that 470ohm?


thanks Slucky for your help.


 

[mresistor]

have you checked or replaced the cathode cap on the second amplification stage? Or replaced that AX7?

question was asked and completely ignored

[dude]

have you checked or replaced the cathode cap on the second amplification stage? Or replaced that AX7?

question was asked and completely ignored

Sorry mresistor, you did say try another 12AX, had I, problem fixed. I also found the e cap on the V2 in the vibrato circuit to be bad too, reading .1uf, not 25uf. So two problems. Anyway, I'm glad I went through the troubleshooting process to find the bad tube but you did nail the problem, the e cap on second stage was ok but the vibrato cap wasn't.


I have a lot of USA pulled 12AX7, it was one of the pulls, just probably 100's of hours on it. It worked but the plates in the tube weren't giving me the correct voltages.


What's your take on the 470 bias R with a 6L6? I'll take note to pay more attention to your posts, I guess I got hung-up answering Sluckey, again sorry and thank you.


al 

[mresistor]

No worries Dude-  6L6 in a Champ. If its a BF or SF not a big deal as the PT in those are like you have mentioned used for Princetons. A 6L6 would probably be happier with the higher plate voltage. But watch the voltage rating on your filter caps. 470 ohm would be a good place to start but I'd try to find the value that got plate dissipation in the ballpark for the 6L. There is a bit of mismatch with the stock Ot and speaker isn't there? 


A good reference of SE amps is Merlins  http://www.valvewizard.co.uk/se.html


I use JJ 6V6s in some of my Champs, and that tube in a Champ make the Champ loud. I've read that the JJ 6V has some 6L characteristics. I have popped a 6L in a stock Champ before but I wasn't really that impressed, but I wasn't looking for Clean and Loud.

[mresistor]

For a 6L6 in a Champ I might try a larger cathode bypass cap too,   33-47uf

[dude]

For a 6L6 in a Champ I might try a larger cathode bypass cap too,   33-47uf

I'm using a 4.7uf both bypass caps V1, I like the headroom but I've been thinking of raising the first one a bit for a little more grit, I'll raise it see if I like it, thanks.  For distortion, a modded TS10 to 808 specs. I have 450v power supply e caps and a 33uf on the first node. Hope that isn't too high for a 5Y3?  Never checked the bias cathode cap voltage but I have a 100v 50uf there, overkill, all I had on hand.


I'll have to do the math for dissipation, I did add a screen resistor.


What does a 6L6 want to see as far as OT primary? The OT I have is an old paper SE I got out of an old Conn organ. It's much bigger than the tiny original and sounds pretty good. I did the math and it's around 10K, what ohm speaker should I use with that OT?  I'm running a Weber 4 ohm spk. and usually mic the amp, but sounds better with a 10 or 12 which are 8 ohms.


I'll read Merlins SE link, thanks.


al.

[mresistor]

  Never checked the bias cathode cap voltage but I have a 100v 50uf there, overkill, all I had on hand.
Yes , that was what I was suggesting, to fully bypass the power tube for using a 6L6G or similar.

I'll have to do the math for dissipation, I did add a screen resistor.
I add screen stoppers to my single ended builds. What is your cathode current with the 6L6 in there and a 470 ohm resistor?

What does a 6L6 want to see as far as OT primary?
4k would be ideal with a 4 or 8 ohm speaker. It would be nice to have an OT that has selectable 4 or 8 ohm output impedance taps,  and I use one from Classic tone. Although with a 6L or EL34 something like Allenamps TO-11C would be a good choice.   http://www.allenamps.com/parts.php#transformers       or this one       http://www.classictone.net/40-18031.html


, what ohm speaker should I use with that OT?  I'm running a Weber 4 ohm spk. and usually mic the amp, but sounds better with a 10 or 12 which are 8 ohms.
My take is that with higher OT input impedance then you would want to use a lower impedance speaker - someone correct me if I am wrong. Of course the larger coned speakers are going to sound better. They have more area to reproduce sound, bigger better voice coils, etc.
I play my Champ into an 8 ohm EVM-15L, but it's a 5 watt OT from classic tone with selectable output impedance of 4, 8 ohms. And I'm using a JJ 6V6. It's pretty sweet. The bigger ironed OT would be much better for the 6L6 for sure. I installed an oversize Mercury OT in a VibroChamp I built and it really made it sound a lot better. Had to mount it farther away from the OT and angle it some to get it to clear the 12" Alnico speaker that was in that cabinet.

I'll read Merlins SE link, thanks.


al.

[PRR]

> What does a 6L6 want to see as far as OT primary?

Really depends on the available power voltage and current.

Throwing 6L6 into a 6V6 chassis and PT, you stay with 6V6 loading.

It happens that Rk won't be much different and is not worth changing.

All these old amps now run hi-volt on today's wall outlets. Doesn't seem to be a problem.

> I never would have thought V1 - 12AX7 had anything to do with the vibrato

Follow the wire out of the Intensity pot. It kicks V1b upside its butt (cathode), screwing with its bias and thus gain.

If the gain is supposed to go up-and-down, but that V1b section is weak and gain won't go-up much, that's lame trem.

[dude]

> What does a 6L6 want to see as far as OT primary?

Really depends on the available power voltage and current.

Throwing 6L6 into a 6V6 chassis and PT, you stay with 6V6 loading.

It happens that Rk won't be much different and is not worth changing.

All these old amps now run hi-volt on today's wall outlets. Doesn't seem to be a problem.

> I never would have thought V1 - 12AX7 had anything to do with the vibrato

Follow the wire out of the Intensity pot. It kicks V1b upside its butt (cathode), screwing with its bias and thus gain.

If the gain is supposed to go up-and-down, but that V1b section is weak and gain won't go-up much, that's lame trem.

Yes, I see V1 to the vibrato circuit, thanks for the info
al

[dude]

Mresistor,


The cathode voltage is 27.8v


Dissipation for the 6L6 is 18.84 watts. I didn't substrate the screen so maybe 18watts


seem OK?


al

[mresistor]

Looks good to me. If it sounds great that is what matters. Could go higher with more B+.     I'm assuming you're using a 6L6GC..  I think I'll try an old G model in my Champ head this afternoon..

[dude]

It's as good as it gets, sounds great.


You know all those old pulls, half or more ain't worth sh*t. I'm tuning the champ to weak preamp tubes. I was going to lower the first bypass cap but change the 1st preamp tube. Now the cap is fine and the amp sounds killer. People get hung-up on old pulls, but there old and most likely beat to sh*t. New 12ax7's and she screams, Rubies, whatever as long as there new.


Thanks all,
al