Vibro-Champ trem issue

[angelodp]

Hi, I have a new AA764 Vibro-Champ build. It plays nicely and all my voltages or on par. I do not have Tremolo.
I have been through the amp several times and aside from the the 3M pot being at 2.3M I can not find the issue.
I have the NFB disconnected and I added a RAW pot off of the Tone-stack. I have a bit of DC scratch on the Volume pot as it goes to almost off (1-2). When I shut the amp off it has a quick surge of volume above the level it's at and then it goes quiet. I am hitting voltage within 15% of the AA764 schematic. Any advice on
divide and conquer greatly appreciated.

A

[martin manning]

What voltages do you have on the oscillator tube pins 1, 2, 3, and 6, 7, 8? Are they steady with the trem on or off, meaning the oscillator does not start?

[angelodp]

Hi Martin, will check shortly, thanks for chiming in.

A

[angelodp]

What voltages do you have on the oscillator tube pins 1, 2, 3, and 6, 7, 8? Are they steady with the trem on or off, meaning the oscillator does not start?

Martin, no fluctuation on voltage at any of those nodes with the trem levels up to noon.

[angelodp]

Current status of build. Trem does not Oscillate

[martin manning]

What are the voltages on the trem tube pins?

[Stevem]

Do you have the needed short across the RCA foot switch jack to activate the circuit?

[sluckey]

Do you have the needed short across the RCA foot switch jack to activate the circuit?

The AA764 Vibro-Champ does not require a short to enable the tremolo. Just the opposite. It requires an open circuit to activate.

[Ten Over]

...

[angelodp]

Hi, I notice that the 50kRA pot reduces the volume when engaged....normal yes. But no trem

B+ @ 120VAC 372DC
B+1 365DC
B+2 341DC

Pins
1- 135VDC
2- .01
3- 2.3VDC
4-5 - 3VAC
6 - 364VDC
7- 135VDC
8- 140VDC
9- 3VAC

I have noticed that the Turrets from Antique Electronics are a TAD fussy to solder to.

Given that my Plate voltage is low, I am contemplating lowering the Dropping R from 10k to 5k.
Do I need more gain to get this going?

The amp does play, no hiss and tone knobs work well. I have a NFB switch and a RAW control off of the tone stack.

Thanks A

[angelodp]

Just reduced the dropping R (10k) down to 4.7K (2W), that did not help kick it on.

How does the Red LED swap work? If I want too see if I am getting any oscillation at all, do I replace both the
4.7k R and the 25/25 cap with the LED, LED cathode to ground?

[sluckey]

How does the Red LED swap work? If I want too see if I am getting any oscillation at all, do I replace both the
4.7k R and the 25/25 cap with the LED, LED cathode to ground?

That's correct. Nothing else required.

[angelodp]

Hi Steve, Red LED in and no help, no oscillation? LED does illuminate but very dim.

What steps to take next?

A

[martin manning]

The voltage on the follower's plate (rail voltage) was fine, but the oscillator's plate was low. Got any DC voltage at the top of the speed pot? Might have a leaky cap. Have you tried a different tube?

[angelodp]

Ok so when I swapped in the RED LED I forgot to solder in the 1M, now it's in and the LED blinks and it's all working....I have Trem so thanks Steve and Martin. It seems the 10uf cap was blitzo.

Question, can I just leave that RED LED in place of the RC? Looks cool.

Gotta re-check the bias as I reduced the pre-amp dropping R.