AB763 Tremolo Working Inconsistently

[Ledzepp007]

Hi all,

I built a Super Reverb using the Hoffman two channel board and layout. Everything works great except the tremolo. It is inconsistent and rather thin. I thought that perhaps I had biased the amp a little cold (35ma with 475.5 VDC on the plates), but a hotter bias (40.5ma) does nothing. Using an RCA plug, I’m only able to get noticeable tremolo when the intensity knob is around 8 or 9 and the Speed is between 4 and 5. Also, the channel volume has to be below 8 or 9. Interestingly, when I unplugged the RCA plug the tremolo still was active.

A quick survey of voltages looked good. The reverb works wonderfully and it is a toneful, very quiet amp.

One thing of interest: I connected the 4 ohm output tap directly to the negative feedback resistor, so I’m not sure if that’s making a difference. No other mods.

Any ideas? I’m doubting it’s a weak tube.

[sluckey]

Interestingly, when I unplugged the RCA plug the tremolo still was active.

Hoffman's trem circuit does not need a footswitch plugged in to operate.

Any ideas? I’m doubting it’s a weak tube.

Why? That's the first thing I'd try. That trem circuit needs a strong 12AX7.

If you replace the resistor/cap on the cathode of the oscillator tube with a red LED you will have a much stronger tremolo signal.

[Ledzepp007]

Interestingly, I replaced the RC from pin 3 of the Trem tube with a 5mm red LED. The LED lit up but the trem effect did absolutely nothing. Ideas?

[Ledzepp007]

Even with the LED I can only get the tube to oscillate if I first mimic tremolo by using my guitar volume knob (rocking it back and forth). Weird. I measured a 1.75v drop across the LED. Any other ideas?

[ginger]

I'm sure you've checked your wiring... did you use push-back solid core cloth wire ? I had a non-working trem , and the skimpy 20 gauge wire had broken in the cloth covering

[sluckey]

Is the LED blinking or on constantly?

[Ledzepp007]

Is the LED blinking or on constantly?

Constant. Then it, when it starts to modulate, it blinks in sync. But, even then, the term is weak and usually will only function with the speed between 4 and 7

[ginger]

delayed reaction
?

[Ledzepp007]

Voltages on that tube are as follows:

P1: 184
P3: 1.7
P6: 464
P7: 180
P8: 184.7

When I was measuring voltages, I put the meter lead on the anode side of the LED and it started oscillating. It stopped as soon as I moved the speed pot past 6. So weird.

[ginger]

Wiring error... or bad cap

[Ledzepp007]

You guys were right! Bad cap! Replaced and now the trem works! Too well! I had replaced a 470k resistor and a .1 cap with lower values and now I’m getting thumping badly. I feel like I’m playing Jumanji when I turn the trem on. I’ll replace those components tomorrow. Any other ways to reduce thumping, though in general? I’d also really like to shift the speed range a little lower.

Thanks for the help!

[sluckey]

Take the LED out and put the R/C back in.

[Ledzepp007]

Made some changes, sounds good. Thanks for the help.

In other news, I tried a weird mod. I took the 2 band brown face tone stack (not the one with the weird tapped pot, the other one) and placed it after a MOSFET cathode follower in the normal channel. It seems to function, but I get a super bassy woman tone-ish sound out of it. The Treble control does essentially nothing. I’ve attached the layout I made.

[Joel]

The only way I found to eliminate thumping/ticking in the Hoffman AB763 bias wiggle tremolo is to reduce the supply voltage to the Trem to 300V or less.  After extensive testing and probing, multiple components tried, 300 to 350V yields a ticking Trem.  Over 350V gives thumping.  Under 300V perfectly quiet.

[terminalgs]

After extensive testing and probing, multiple components tried, 300 to 350V yields a ticking Trem.  Over 350V gives thumping.  Under 300V perfectly quiet.

that's interesting.    years ago I read on various forum threads about the ticking being caused by the proximity of the LDR to the neon and unsuccessfully fooled around with it a good bit.  Then one day with the LDR removed from the circuit altogether I still heard the ticking  and figured out it power supply, not audio signal sensitivity.  Its great to have the benefit of your R&D.


On an AB763, do you increase the 100K plate resistor, or so you create another power supply node with it's own filter cap and a bigish (10K-22K?) dropping resistor off the  power supply screen feed? or ?

[sluckey]

I'll try to remember that tip. I've only built that circuit once and had no problems with thump or tick. But my B+ supply was only 388V and I was modulating a pair of fixed biased 7591s which require much less bias voltage than 6V6s or 6L6s.