New PR AA1164 build...

[Ed_Chambley]

Hot damn! I was so concerned about my wiring that I forgot to look at my components!
So the tremolo is working now!!!

One thing though, I don't know if that's normal: the speed increases fine but from 8 to 10 it's like the tremolo totally disappears, is it because it is going so fast that the human ear can't discern the oscillation? Or do I have to alter something until I hear the trem at work even on speed 10?

Thanks, I love this board!!!!

Check your pot and see if it changes from position 8-10.  If there is a resistance change, you should be able to hear it.  It is a 3 meg ra pot and I have a couple that do not change much at the end of travel.  The taper just seems to make them get very close to zero very fast as they are increased.

Also, that 1 meg resistor change drives me nuts.  I put in a 220k, then a 270k then a 500k and back to 1 meg.  Right now it is 330k.  I think I am going to put a pot on the thing.  It sounds totally different as I did put a raw mod in thanks to Slucky, but it seems to change.  I do not hear as well as I used to either.  With a mid control and raw there are some really nice tones I am getting, but the overall trem effect seems to change as well.  In reality it doesn't, but a change in tone changes everything.

Your build looks great.  Way better than a RI Bassman I repaired for a friend recently.  Thin wire, cheap pots and broken heater trace on the PCB.  SO yes, your build is better than what is coming from Fender.

Great job.

[Willabe]

I have the same problem with my bias vary term. When I turn up the speed it goes so fast you can't even hear it's on.

The fix is to increase the capacitance in the LFO by changing 1 or more depending, on how slow you need/want it to go, of the .01 disk caps.

The 3-C's/3-R's (+1 pot) that form the FB loop, from plate to grid, around the LFO tube, form a time constant that sets the over all speed. More capacitance (and/or resistance) increases this time constant, because it takes the cap longer to charge up, slowing the LFO down. The speed pot increases/decreases this by adding/subtracting it's resistance from the main TC setting. It acts like a _range_ control for the TC setting.

Edit;

Although I'm sure that changing the cap value will change the speed, I'm not sure I got the TC/cap charging thing right here as the reason? Just re-read about trems/LFO's in a G. Weber and a KOC book.

Weber says it increases/decreases the phase shift of the FB loop, KOC says it changes the frequency of the LFO. It might all be the same thing, just a different way to say it?  I'm thinking the phase shift is set at a frequency or is what sets the frequency of the LFO? And it might be that the TC is what sets the phase shift and/or frequency of the LFO?

Never looked at it this way before, am I close?                     

                           Brad      

  

[tubeswell]

Phase shift = whereabouts in time that the peaks/troughs of the sine/signal wave are.

Frequency = how close-together/far-apart the peaks/troughs are

(Amplitude = how high/low the peaks/troughs are)

And yes - in a LFO circuit you vary the speed/rate/frequency of the trem signal by varying the phase shift of the RC network. The phase inversion of the tube does the first 180 degrees, and the RC network between the plate and the grid does the rest.

[Willabe]

Thanks Pete.

Well alright then, I think I get it.        


                    Brad    

[Ed_Chambley]

I tried many different values out of curiosity after reading lots of info on the trem.  Seems like it only shifted the speed to a different position.  Meaning the tremolo sped did not really change, it was just slower when the speed pot was lower which in turn made it slower at 1 or zero, whichever you prefer.  I did not get a wider range, however I did get a very slow effect which made it slower when full on.

I was attempting to widen the range, but had no success.  I am sure it can be done, but it is beyond my conception.

[SleepLess]

I tried many different values out of curiosity after reading lots of info on the trem.  Seems like it only shifted the speed to a different position.  Meaning the tremolo sped did not really change, it was just slower when the speed pot was lower which in turn made it slower at 1 or zero, whichever you prefer.  I did not get a wider range, however I did get a very slow effect which made it slower when full on.

I was attempting to widen the range, but had no success.  I am sure it can be done, but it is beyond my conception.

All of this is really interesting...

[SleepLess]

Hi there!
I'm in the middle of a Super Reverb AB763 build and I purchased a MM FBFS-P40/100-240 PT. I hadn't noticed it from the website but this particular PT has a bias tap. Do you recommend to use it and if so how and where? What's the use of that tap anyway, does it add or take anything?
Thanks!

[sluckey]

I would use it. Just wire it IAW the schematic.

[SleepLess]

OK. So it goes to the end of the 470ohms/1w resistor on the bias board right?

[phsyconoodler]

No it does not!

[sluckey]

Yes it does.

[SleepLess]

Yes it does... I've just checked it on the schematic + TAD layout...
Sluckey, does this bias tap add a biasing stability or something? I'm interested in knowing its purpose, pros and maybe cons...
Thanks!

[sluckey]

It's just another way to provide input AC voltage to the bias supply. No real pros or cons. Just different (from your PR). The PR was the bottom line Fender reverb amp and chose the cheap way to get bias.

[SleepLess]

Awesome! Thanks Sluckey!
I'll open up a thread once the Super Reverb is finished...

[phsyconoodler]

Sorry! I had the main board in my head when I answered that,not the 'bias' board.Duh!

The trem WILL slow down by upping one cap in the trio.It works every time for me,but you cannot go too large or it won't oscillate.

[PRR]

> changing the cap value will change the speed
> Weber says it increases/decreases the phase shift of the FB loop, KOC says it changes the frequency

An oscillator needs loop (output to input) gain greater than 1.000 and phase-shift of zero (or 360) degrees.

The tube gives gain of 50 and 180 degree phase shift.

We need a trick to add an additional 180 degree phase shift. We like passive tricks. They are lossy, so we must find one with less then 50:1 loss.

One R-C network has phase-shift of zero and loss of zero at infinite frequency, phase shift of 90 degrees at super-low freq where loss approaches infinity, and at some in-between frequency it has phase-shift of 45 degrees and loss about 0.707.

So you would think to use four R-C networks at their 45 degree phase-shift frequency. Loss is about 4:1.

We have gain to spare. Can we be cheaper?

Three R-C networks will give a frequency where phase-shift is 60+60+60 or 180 degrees. At this freq each R-C gives loss near 3:1, total loss near 27:1. We could afford 50:1, so this will work.

For a fixed-frequency oscillator, the number of R-C nets is not too important.

However we want to vary the freq where we get the 180 degree phase-shift and oscillation. Ideally we use a 3-gang pot. This is expensive.

Turns out that you can get a limited range of frequencies by varying just one of the three Rs. The loss increases, which means the oscillator may tend to quit at higher frequency. However with a good 12AX7 and 10:1 range of pot you can get over an octave of useful range, which means you can beat-mesh any tempo.

Replacing one cap is a quickie. It does upset the loss. It may be better to replace all three caps with the next larger value.

> I was attempting to widen the range

To widen the RATE range? Reduce the fixed resistor in series with the Rate pot; however at some point it just quits oscillating. Or becomes too slow to start. Tinkering the gain might find a wee bit more range, but hardly enough to notice.

The direct way would be to switch the caps. The un-clever way would use a 3-pole 1-throw switch to put caps across the existing caps (or not). Using same values (parallel the 0.01 with another 0.01) would cause an octave-down (half-rate) shift.

Another way is to find a 2-gang pot, to vary two of the trhee R-C resistors at once. For the knob to go "right way" this must be a Reverse-Audio taper. These are not common parts. (Straight-audio works if you wire it the other way and remember to turn clockwise for slower.)

[SleepLess]

Hi.
Thanks for such a thorough answer! One question though as it is linked with the Super Reverb optoisolator: One of the two lower legs is soldered with a 10M ohms resistor. can I safely change this for a 8.2M ohms or not? What will happen? Less trem? More? Instability?
Thanks!

[sluckey]

I'm in the middle of a Super Reverb AB763 build

And you need to go ahead and start a new thread. People are responding to your old thread about the PR.