Finalized Univox U-1221 Tremolo Circuit Modification

[astronomicum]

A few weeks ago, I had posted in a conversation started by Leevi, a design for a modified tremolo circuit for a Univox U-1221. Since, I have refined and finalized a design that I will be incorporating. I decided to post this in a new thread to share it with folks.

I have settled on a custom made vactrol using a red LED and a GL-5528 LDR in the final design. The GL-5528 gives a wider resistance range than the VTL5C1 which fit better on the U-1221 attenuation curve. FWIW, my testing indicates that the VTL5C1 LDR is very similar in performance to the GL-5506 LDR.

One of the final changes to the circuit was to add a pot to adjust the LED driver bias, as Gibson had incorporated into some of their designs. This circuit works best in the U-1221 when the bias is adjusted so that the LDR reads between 1-1.5 Meg with the tremolo off and with the LDR out of circuit. If I need to change the tube, I would adjust the circuit back this pre-determined bias voltage. If the vactrol is changed, I would determine a new bias voltage.

My testing suggests at maximum intensity better than a 15 db attenuation at 2.7Hz when properly adjusted. The attenuation is smooth, deep, and recovers on the upswing (LED off) to +200K LDR resistance minimizing bass and volume attenuation. Overall volume attenuation increases as the tremolo speed is increased and the recovery time of the LDR becomes more of a factor. A perceptible drop in bass response is also created as the LDR only recovers on the upswing to about 100K resistance at 7.7Hz. An unavoidable drawback of using an LDR.

I had also developed but abandoned a version using a 12AU7 LED driver and a white LED in hopes that I could leave the original 12AU7 in the circuit.  I got the circuit to work well but the quiescent current was nearly 6mA (vs. about 1mA for the 12AX7). This would have put a significant drag on the B4 supply.

With additional tubes in the 12A family, a range of LED colors, and lots of other LDR options in the GL line (which respond differently to different wavelengths of light), there are quite a few more versions possible if you don’t mind getting lost down the rabbit hole 

Hope this gives folks a fix for their noisy U-1221s and some options to explore for other projects.