DIY Audio Projects Forum

So I've been working on a DIY Tube Amplifier on and off for the past year, and have learned a whole lot from scouring the internet and trial-and-error. The amplifier that I've built is based around an Ampeg Reverberocket, and for a while now I've had the entire amplifier in working condition. Amplifies sound beautifully, tone controls and reverb working well. The only part of the circuit that doesn't work properly is the tremolo circuit. Well, it works somewhat, but I'm getting very weird results.

So I'm using a bias shifting tremolo design, and it's fairly standard. But the thing is that the tremolo only works when the Intensity pot is all the way up. The moment I turn the knob even a hair down, the tremolo immediately cuts off. My guess is that when the potentiometer is all the way up, it creates a short between the coupling capacitor from the tremolo circuit to the bias of the power tubes, but as soon as it is turned down, the tremolo circuit is completely bled to ground rather than only some of it going to ground. Another weird thing is that I currently have an alligator clip attached to where the Intensity's middle lug joins the biasing resistors (GLR6 and GLR7 on the schematic below) and despite how tightly crimped and how much solder I've applied to the joint, the tremolo only works when the alligator clip is attached. The amp still works just fine without the alligator clip, just no tremolo.

I've double and triple checked my design, as well as all the components. I've checked continuity between all the components, and even continuity between components that shouldn't be connected. I've replaced the pots, I've replaced tubes. My next guess would be to rip the tremolo circuit out and rebuild a new one. It was done point-to-point, and is pretty modular, so it wouldn't be too much a problem to do that, I just don't have any confidence that doing so will change anything.

Any ideas as to how I might go about diagnosing this? The behavior seems really strange to me and from all that I've been able to read and learn about tremolo circuits online, the circuit should work. I can provide pictures of the circuit/components if necessary. Thanks

Hi, welcome!

I see you're using a triode as your LF oscillator. The better trem circuits use both sections (Fender) or a pentode, like a 6AU6 (most everyone else) for enough gain to stay oscillating across the dial ranges. This is likely your issue.

See the attached Vibrasonic schematic for study.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

Hey Geek, thanks for taking the time to reply.

This was my first amp build, there are plenty of things I'ld have done differently, but hey, you have to make mistakes to learn. I'm too far along to introduce a new triode or pentode into my design.

Looking at the Vibrasonic schematic you sent, the second half of the 12AX7 used in the trem circuit is a cathode-follower, so essentially just buffering the signal and driving a low output impedance. With my circuit, or how it works when I turn it on, is that the Speed dial works fine. The Intensity dial (which connects the tremolo circuit to the bias grids of the power tubes) acts as a switch. All the way up, the tremolo is on and works fine. Once I turn the dial 1/1000th of a degree, the tremolo cuts off. Turn the dial back up, and the tremolo comes back to life.

I initially thought I had a fault potentiometer for the Intensity control, so I replaced it, and had the same problems. My guess is that the design of my circuit is allowing for the tremolo output signal to be shorted to ground, likely through the Intensity potentiometer. Just for reference however, here is the original Ampeg schematic that my design largely draws from:


Again, my guess is that this is a coupling issue. Yes, an extra triode to build a buffer would be great, but even some of the 6V6 Princetons put out by Fender didn't use a buffer stage either. Hmmm.

Hi, a few things to try:

- Another tube; Yours might be weak.
- Another pot.
- Check for miscroscopic solder bridges (35+ years in as a pro and they still get me now and then >_< )

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

Thanks again Greg for providing your assistance.

Not to say it isn't the case, but I've tried several tubes. They all have the same issue.

I have replaced my Intensity pot once, but I've been reviewing some early Fender and Gibson models, and they mostly use a 250K pot, so I might experiment with a lower resistance pot.

Microscopic solder bridges, or cold solder joints for that matter, could very well be the problem. There isn't anything I can see, even under a magnifying glass, but the very last thing I'll try is redoing the entire tremolo module.

After reviewing some early tremolo amp schematics, I noticed a lot of them use a low-pass filter before the coupling cap to the Intensity control. My understanding is that by using a high resistance low pass filter from the anode of the LFO, you could create a high output impedance. (Correct me if I'm wrong, but that's my guess as to why this was done.) Is this something that could make a difference?

Thanks.