trouble with Vibro Champ

[sdorer]

Friends,
I'm struggling with the Vibro champ. It sounds greaty but it doesn't vibe. I can't get the tremolo to work. I'm building this one and I'm using the Vivbro champ AA764 schematic. I've checked connections and components still no tremolo. My voltages are close to those specified in the schematic. Any ideas?

[martin manning]

Can you post the voltages on the pins of the trem oscillator tube? Also, you will need a good 12AX7 there because of the high cathode-to-filament voltage on the cathode follower. Don't use a Sovtek or Electro Harmonix, JJ ECC83S is ok with 180V rating where the schematic shows 170V.

[Milkmansound]

A simple thing to double check is the footswitch circuit. Some of the old fender amps did not trem unless the footswitch was on - if you don't have a footswitch, you can use an RCA plug shorted to ground to turn the trem on.

[sdorer]

tremolo tube voltages
1.175
2. .5
3. 2
6. 383
7. 175
8. 184
Thanks for your help thus far.

[martin manning]

tremolo tube voltages
1.175
2. .5
3. 2
6. 383
7. 175
8. 184
Thanks for your help thus far.

Are these voltages measured with the footswitch closed (trem off) or open (trem on)?

The voltage at pin 6 is 43V high... pretty far off, but may be ok. Is the voltage at pin 1 (and 7) steady or fluctuating? How about at pin 2? It should be zero with the trem off, unless the caps in the feedback loop are leaking, or oscillating +/- with it on. With pin 8 10V above pin 7 the cathode follower is not conducting, so even if the oscillator is working the signal won't get out. Are you sure that the tube is good?

[sdorer]

I replaced the tube and here are the voltages.
1. 197
2. .8
3. 1.8
6. 383
7. 197
8. 202
Now I have oscillation but the speed control doesn't have as much effect as I would like. I am not using a reverse audio taper. Is that the deal breaker here? As always thanks for any insight offered. This is a very nice sounding amplifier for anyone considering a fun single ended project.

[martin manning]

The reverse taper shouldn't matter, it's just a variable resistor, so all you need is the 3M range. If the speed is too high, a common mod is to increase the value of one or both of the 0.01 caps to 0.02.