Fender 5F11 Jewel Pulsing with Trem

[jukelemon]

Hi all!  Good morning.

I have a 1958 5f11 with something I have not seen before and I am thinking likely because of some of the mods of this amp.

Lots have been done to this amp so I will start there and then move to the issues.

1- running a copper cap 5ar4
2- running a pair of RCA 6l6GC at ~50ma a tube on 378v plates
3- running what appears to be some late 60's, massive OT. Think like Showman size.
4- replaced (long ago) the .1 coupling cap for a .25 that is feeding the Depth control
5- PT looks original
6- bias voltage on both 6l6 grids measure - 32.58 +/- .05vdv


Issues


1- around 9-10 on the depth dial I am getting significant peak distortion i.e. crackles/pop on the top of the Trem wobble. The bias is not hot in my opinion given the plate values?
2- around 5 on the depth dial the jewel lamp starts to flash along with the Trem.  There is no volume loss that I can hear i.e. gain/signal seems normal and minus the peak distortion on the Trem, the amp sounds normal/healthy.


#2 is what has me puzzled.  I have never seen a jewel lamp/heater circuit be affected by other failures and especially in sync with the Trem. I am seeing swings of 6.55-6.43vac on the heater circuit in time with the Trem.

Any ideas would be much appreciated!


Schem: vibrolux_5f11_layout.pdf (prowessamplifiers.com)


Thank you.

[Dave]

Have you tried running 6V6's as power tubes? Just speculating here, but the load on the filaments is twice for 6L6's than for 6V6's. Maybe you are operating right on the edge so that when the tremolo peaks, the current demand on the HT secondaries combined with the load on the 6.3 secondaries becomes too much and the whole transformer becomes overtaxed. Then the voltage to the 6.3 that feeds the jewel starts to wobble enough that you can see it?


Dave

[jukelemon]

Have you tried running 6V6's as power tubes? Just speculating here, but the load on the filaments is twice for 6L6's than for 6V6's. Maybe you are operating right on the edge so that when the tremolo peaks, the current demand on the HT secondaries combined with the load on the 6.3 secondaries becomes too much and the whole transformer becomes overtaxed. Then the voltage to the 6.3 that feeds the jewel starts to wobble enough that you can see it?


Dave

Hi Dave.  Appreciate the reply/idea.

Yes, put in a known good set from a DR. No change at all minus I could actually see flashing in the 6v6 tubes along with the jewel light and trem wobble.


The stock PT states a 3a heater current which is ~50% used when 6v6s are used and ~80% used when using 6l6GCs if my calcs are correct.


But good thought for sure!

[Dave]

How about this, when the amp is exhibiting this behavior, turn the volume to zero. Does it still do it or does it stop?


Dave

[jukelemon]

Hey Dave.


No change when Volume is at 0


I sub'd in a 12AT7 in V1.  That cleared up the peak distortion on the wobble but the circuit calls for a 12ax so that is a bandaid at the moment.


Jewel pulse is still there.


I have checked every ground on the amp.  0 issues.


I am wondering if the running of these 6l6's for so long has caused permanent issues in the PT?

Also, the spec on the OT is a Stancor 3311.  10k primary. 25w.  Looked bigger before :)

[Dave]

Can you read the B+ while the amp is exhibiting the behavior? Might be easier with an analog meter, but you should be able to get an idea of how much the voltage fluctuates with a digital one.


Dave

[Dave]

I'm thinking that there might be an ultrasonic oscillation originating in the oscillator that is forcing the power tubes to draw tons of current on the peaks. That's why I asked about the B+.


Dave

[jukelemon]

I'm thinking that there might be an ultrasonic oscillation originating in the oscillator that is forcing the power tubes to draw tons of current on the peaks. That's why I asked about the B+.


Dave

Hi.

The swing on the B+ = 325-389vdc

[Dave]

I would start looking around that oscillator for lead dress issues or a flaky tube. I would also check for leaky caps in the area.
I'm hoping that someone who knows much more than I will chime in with some insight.


Dave

[Dave]

Woops... I guess I wasn't paying attention. I just noticed that it is a bias wiggling trem. So, I think it is natural that there is a major shift in current draw when the trem hits peaks and valleys. That also explains the voltage fluctuation. Still, I can't imagine how it would tax the PT so much and to create a visible brightening/dimming of the pilot lamp.


What I said about the ultrasonic oscillation is still possible, though, I was just seeing it riding on the signal through the preamp. Now, if it is present, is going straight into the bias circuit. At this point, I'm tapped out. Sorry, I missed the point early on.


Dave

[jukelemon]

All good Dave and thanks for attempt/info.

I have played a lot of bias based trems and none have ever done this i.e. loss of heater voltage.


Hopefully others may have some ideas.

[jukelemon]

Addtl info..


Just put in a known good set of 6v6s with a 5y3


Peak distortion is gone and the jewel pulse is almost down to nothing at 11-12 on the Depth.  Still there but far from when the 5ar4 and 6l6's were in.

[Platefire]

If it's not effecting the sound, just enjoy it. Who else has got a blinking trem pilot light?

[sluckey]

If it's not effecting the sound, just enjoy it. Who else has got a blinking trem pilot light?

I do.

[jukelemon]

If it's not effecting the sound, just enjoy it. Who else has got a blinking trem pilot light?

Fair enough :)

I would still like to know why the 6l6s created such a fuss with the peak distortion and significant heater voltage swings.  I would not consider a 6l6GC at 50ma and 380v on the plates to be that hot but maybe for this circuit.


All but 1 coupling cap in the PI and a cathode cap on V1 caps are original. 

[HotBluePlates]

I have a 1958 5f11 ...

...
2- running a pair of RCA 6l6GC at ~50ma a tube on 378v plates
3- running what appears to be some late 60's, massive OT. Think like Showman size.
...
5- PT looks original
...

2- around 5 on the depth dial the jewel lamp starts to flash along with the Trem.  ...

I would still like to know why the 6l6s created such a fuss with the peak distortion and significant heater voltage swings.  I would not consider a 6l6GC at 50ma and 380v on the plates to be that hot but maybe for this circuit.

The original amp sported a pair of 6V6s, and the schematic says there's about 330v on the screens and -31v of bias.  Individual tubes will vary, but a data sheet leads me to estimate a 6V6 should have about 20mA of plate current in that condition.  6V6s also pull 450mA of heater current.

You say the 6L6s pulled 50mA each, and a data sheet shows they pull 900mA of heater current.

If the power transformer really is original (and for powering 6V6s), the amp is basically "browning out" when the trem pushes the output tubes to high-current.  It seems this is confirmed by you note that the symptoms subsided when the original 6V6s were installed again.

[jukelemon]

Thanks HBP.

I mean the 6l6's are idling around 50ma.


It makes sense about the tranny stressing out.

The 6v6s sound just as good and I can hear no real change in power/volume. 

Amp is happy and the tone didn't change much.  So...all is well!