Slowing Tremolo Speed in Univox U65RN

[neddyboy]

I'm working on a little Univox for a friend who wants the tremolo speed slowed down. I know it's a question of raising the value of a few caps in a Fender circuit, but what about this one? I'm not in my comfort zone with solid state and I can't find any info on this question online. Currently the tremolo is too fast to be usable even at the slowest setting, and he's looking for maybe two sweeps per second. Possible? Schem attached, and thanks in advance...

[galaxiex]

You could try increasing those three 0.1uf caps to say, 0.22, 0.33, or even 0.47.
If you try, make sure you change all 3 or the trem might not oscillate.

I'm not a tech so I could be out to lunch about this.
Someone with more experience will probably have a better suggestion.

[neddyboy]

Thanks Galaxiex. I did that first off since that was how I'd do it in a Fender. I changed all three to .05mf, but if there was a change to the oscillation speed, it was not enough for me to hear. I suppose I could keep increasing the values, and normally I would, but since it's not my amp I'm trying to be more cautious than usual!

[galaxiex]

Yep, can't hurt anything by increasing those cap values.
You can always put it back to original.

I recently went thru this on an Audition amp.
I used sockets to easily swap parts.
Got the trem working to my liking, removed sockets and soldered in the parts.

Have fun!

[neddyboy]

Cool, thanks man!

[Enzo]

Agree.  what does solid state have to do with it?  Doesn't that one transistor circuit look a LOT like a one triode Fender oscillator?  Same speed strategy applies.

[neddyboy]

I was looking for lamp and photo resistor kind!

[Enzo]

Fair enough, but even the lamp and photoresistor kind has an oscillator behind it.  Look at the typical Fender, it has the triode with the three feedback caps, the the output of that drives a second triode to work the lamp.  The oscillator determines the speed, the lamp is just a way to get that oscillation over to the signal path.  When the trouble is too fast or too slow, it won't be the lamp and photocell's fault, it will be the oscillator.

I'm not picking on you, just pointing out that circuits are circuits, and many of them work about the same regardless of the technology under them.  And troubleshooting is troubleshooting, a systematic approach to solving problems.  if you know anything about fixing tube circuits, you probably know quite a bit about troubleshooting that applies to transistors as well.

[neddyboy]

No offense taken! Any knowledge I have about repairing amps has come from fooling around trying to get my own stuff working. I'm not much on theory, and that's limiting. But I still have a blast!

[Roly]

The oscillator is the phase shift type and if you google image search "phase shift oscillator" you will see dozens of circuits using different gain devices, valve in a Fender, transistor in most guitar amps, perhaps an op-amp, but all sharing the ladder of three CR's between input and output that is the heart of the PSO.  As long as the forward gain element can provide a voltage gain of about x30 it will oscillate.  Paste it in your hat because this is what almost all guitar amps use in some form.



galaxiex is right, doubling the cap values should half the speed, but it will do it to the whole range, so some compromise between slowest and fastest may be needed.  And don't bother trying to increase the range - you can't.  You can only shift it up and down.

Changing the resistor values to ground will also change the speed but it can get fraught because that can change the gain as well so most straightforward to simply pad up those three existing caps.

[Enzo]

I often slow down Fenders.  Too often, the slowest speed is way too fast, and by the time it is half way up, it has turned into a buzz rather than a trem.  I like the slow end to be a slow sexy throb,  Maybe 2Hz,  and no matter how slow I make it, the upper speed end stil winds up way to fast for anything.

[galaxiex]

Roly, it does my heart good to see you posting. 

A question if I may... How to calculate the forward gain of said phase shift oscillator?
Thanks!

[neddyboy]

Wow, great explanation Roly! I've already been playing around with caps in parallel on the back of the board, and I'm up from .01 to .1 but it's still waaay to fast. I don't keep many high value film caps around, so I'll be hitting the electronics store on Friday. I'm thinking 1mf will get the speed where I want it, maybe even 2mf. I assume BP electrolytics will serve, yes? Cheaper anyway.

Enzo, yeah, same with this. With .01mf caps, the slow speed was a good bit faster than the fast speed on a Leslie, and it topped out at buzz.

[Roly]

If it oscillates it's more than x29, if not, it isn't.     {at 180deg phase shift the loss across the PSN is ~x29}

In the example above the gain is nominally set by 1+rf/rb (nominally because a single transistor stage won't have a lot of gain in reserve, perhaps x100 open-loop, while any op-amp may have an open-loop gain of x100,000 in reserve and so will produce a much more accurate result).  A 12AX7 has just enough gain, a 12AU7 not nearly enough.


As far as the values go, the original range suggested, up to 0.47uF (polyester/greencap) should more than do it, no need to go higher, or the complication of using NP (non-polarised electrolytics).

[neddyboy]

One last question for you gentlemen. I've replaced nearly all the electrolytic caps and a few resistors that had drifted, and the noise level has dropped substantially, but it still hisses. As I mentioned before, I'm mostly a tube guy, and this kind of noise is new for me. My battles are usually with hum!

Does hiss just come with the territory in an old, cheap SS amp? It doesn't sound like a fizzing resistor, so I'm guessing it's coming from the all-germanium transistors. Anything I can do to limit the hiss without replacing transistors?