Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: astronomicum on March 06, 2023, 03:29:45 pm
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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. Leevi had asked how it compared to the circuit in an AB763. I had indicated at that time I had limited experience with the circuit and could not offer a comparison. Although initially reluctant, curiosity got the better of me and I decided to do some further development before putting my breadboards away.
The Bandmaster this circuit was tested on is a complete rebuild with essentially the case, chassis, chassis parts, and PT as the only original parts. The completed amp voltages are right on the mark. Most importantly to note is the roach is also a replacement. Additionally, I must indicate that this is the only Fender I have ever worked on. I used to own one as a teenager so took on this restoration for personal satisfaction.
The circuit developed uses a copy of the original LFO. All the changes are in the LED driver which is almost identical to my Univox version. The main differences are the use of a GL-5537-1 LDR, and the much higher plate voltage of the driver along with the changes necessary to accommodate it.
This circuit works best in this amp when adjusted so that the LDR reads between 3-4 Meg with the vibrato off and with the LDR out of circuit. This produces a resistance change positioned on an optimal portion of the attenuation curve (see attached). If I would need to change the tube, I would adjust the circuit back the pre-determined bias voltage. If the vactrol is changed (should have a very long life), a new bias voltage would be determined during replacement.
My testing has indicated nearly a 12 db attenuation at 3.7Hz when properly adjusted, a couple db better than this amp’s current circuit performance. The attenuation is smooth, deep, and recovers on the upswing (LED off) to +150K LDR resistance allowing for good signal volume to come through. Overall attenuation drops as the speed is increased. The recovery time of the LDR becomes more of a factor and a perceptible drop in volume is created as the LDR only recovers on the upswing to about 50-75K LDR resistance.
The main advantage to this circuit is moving the intensity control off the signal path. As you can see from the attenuation curve, there is about a 4db attenuation created at 50K. Another advantage is being able to reduce the vibrato channel coupling cap value a bit without losing bass response, allowing for the possibility of a normal channel with vibrato clone design.
For obvious reasons, after incorporating this mod, I am shelving further development of this version. Wanted to share this with folks. Have fun.