Supro/Wards Airline GIM-9131A Tremolo

[Rhodesplyr]

I'm overhauling a Wards Airline head for a friend--this is the two 6L6GC version--and the tremolo seems a bit slow to me. This circuit doesn't use an LDR; it modulates the cathode voltage on a 12AX7 gain stage using another 12AX7 section as an LFO.

It has three capacitors in the oscillator circuit, two of which are the Z5U ceramic type, and I know that these can suffer aging effects: drift and/or increased ESR. It isn't hard simply to rebuild the entire circuit with new capacitors and resistors, but I was wondering how often this problem surfaces and whether it's usually the capacitors or the resistors. One nominally 150k resistor has drifted up to 170k.

Of course, the capacitors are marked 20% tolerance. The tremolo may always have been a bit off. The build quality was horrendous: four joints that were never soldered and several other joints I had to resolder to get rid of noise.

David

[Gunny]

My experience has been that the 3 caps in the phase shift circuit are the most common culprit. I've seen 2 cases where the oscillator tube is so old that the stage gain is too low to start and maintain the oscillation. In your case I would change all 3 caps first and try it out. The resistor - a long shot I think.

[Rhodesplyr]

Thanks for the reply. I've eliminated the tube as the culprit both through testing and substitution. I think it has to be the capacitors. Unfortunately, I'm out of one of the values, so it will have to wait for the next parts shipment to come in.

My experience has been that oscillator circuits tend to be hard on components from the constant charging and discharging. They often kill the capacitors in Rhodes Suitcase Piano preamps.

[Enzo]

I would have a different take on that last point. I see plenty of old leaky caps in trem circuits causing them not to work. I also see the rest of the caps in the same amp also leaky. it isn;t charging and discharging, it is just age.

Think about how many times a cap in the signal path of a bass amp has to charge and discharge.

In an old amp, I almost expect to have to replace those caps. That is almost always what is wrong if the oscillator won't osc.

Tolerance is not much of an issue. The speed is continuously variable anyway, so if you want 4Hz trem, does it much matter if it is at 3 or 4 on the control? If I want to shift the sped range, it usually requires a gross change of value. Like doubling the caps. SO a couple .01s become a couple .02s or whatever. A few percent either direction woul dbe hard to notice.

ANd too slow? Wow. I usually have to slow them down. Most Fenders - to my ear - come in with the slowest setting about twice as fast as it should be, and by the time you have it one third of the way up, it is so fast as to be useless - sounds like playing through an electric fan. I like my slow end to be about 2Hz more or less. ANything over about, what, 10-12Hz maybe, I can;t use.

[RWood]

I usually have to slow them down. Most Fenders - to my ear - come in with the slowest setting about twice as fast as it should be

I like a slower trem, too. What gets changed in a Fender circuit? I've slowed down a Marshall that used a tube oscillator but not a Fender with a a bug.

[Enzo]

The bug has nothing to do with the speed.

Tremolos always have two parts to consider. One is the oscillator or LFO, and the other is the interface to the amp.

The oscillators are most all about the same, a simple phase shift circuit with a few caps from the plate back to the grid of the oscillator tube. The value of the caps and the resistors in that feedback loop set the speed. Each pair is an RC time constant. SO basically increasing the cap value slows it down. SO would increasing the resistance - hence the speed control. But I find it a lot more reliable to make a .01 cap into a .02 cap than trying to fudge around with 560k resistors and such.

Once you have an LFO going, then you interface it to the signal path. You might use a bug, or you might use a cap, or maybe something else.