Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: khansen810 on September 18, 2020, 09:18:01 am
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Hi, I added this tremolo circuit to an amplifier that I have built. The tremolo doesn't work. I varied the wiring and replaced the photocoupler but no change. I saw on your schematic that the voltage to the 470K is 434 vdc and varies between 148-345 vdc at pin 1 of the 12ax7. My voltage at the 470K is 250 vdc and varies between 125-150 vdc. Could this be the issue as to why its not working? Should the 470K be lowered to increase the voltage before the resistor? If so what would you suggest? Appreciate your feedback. Thank you.
Ken
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Posted in wrong board............ moving
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My voltage at the 470K is 250 vdc and varies between 125-150 vdc.
That's not much. Maybe too little. Look at the plate with a scope and measure the peak to peak sinewave voltage. Also measure the cathode voltage.
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I think the Treminator is a perfect fit with the Blackface circuit where the 50k intensity pot is a drop in replacement for the original trem attachment. I am happily using Treminator in a plexi/bassman circuit but have substituted a 250k reverse audio pot for intensity and removed the ground connection of the pot. I think that would work for you too. Right now you have permanently wired a 50k resistor (pot) to ground from your 1M volume control. That is an always on load that drastically cuts your signal with or without the trem switch on. I would try removing the ground connection from that pot and see how the trem works. I expect you will get great intensity (probably too much even turned down). Increase the value of the pot until you get a good range of intensity with the sweep of the knob. Since this uses the pot as a variable resistor, the higher ohm value on the pot the less intensity you get at both knob extremes. Hope it works for you!
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I don’t have a scope. I changed the 470k for a 220k. Still not working. The voltage at pin 1 varies from 150-189 Vdc. I removed the ground to the intensity pot. Still not working. Background noise increased when I removed the ground. I replaced the 50k pot with a 220k same thing. Instead of connecting the 220k to C, maybe I should try connecting to A. That will increase the voltage a bit. Any comments much appreciated. Thanks.
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I left the 220k (replacing the 470k) connected to C and removed the 10k to ground. The tremolo circuit started to work for a while and stopped. Turned the amplifier off and back on and the circuit started to work for a while and stopped. Seems to be intermittent for some reason. Comments?
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You need more B+. Move to node B. Any better? If, not, move to node A. Any better? If not, you're out of luck.
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I’ll try that. Should I reduce the 220k also?
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I changed the power transformer to increase the voltage. See attached schematic (new project schematic) showing the voltages at A,B,C,D. I'm still having problems to get the tremolo to work. The voltage at pin 1 is 160Vdc. I also attached a schematic (A0-44 with tremolo) for a project I did last year using an older tremolo circuit that worked fine with 160 Vdc at pin 1.
For this project the voltages at the 12AX7 are as follows with the footswitch open: pin 1 is 160V, pin 2 is -3 to 5 mv, pin 3 is 2.1V.
With the footswitch closed: pin 1 is 160 V, pin 2 is 1.2 mv, pin 3 is 1.55 V.
How does this circuit work? It seems to me that the voltage at pin 3 should oscillate. I believe the Vactrol is good. I don't have another one to try. Appreciate your assistance. Thank you.
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Do this test...
Remove the Vactrol. Connect a red LED to pin 3, anode to pin 3 and cathode to ground. You won't have any tremolo effect but you will have a visual indicator. The LED will flash at the trem speed. Does it flash? If not there is a problem with the circuit that needs to be fixed.
Once the LED is properly flashing you can replace it with the Vactrol, positive to pin 3, negative to ground. Should have tremolo effect.
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I removed the Vectrol and put an led. The led lights up but is steady except there is a slight flicker at times as if it’s on the border of wanting to pulsate. I checked the caps and resistors and swapped the tubes. No change. I removed the 10k to pin 3 and the led is brighter but doesn’t pulsate. I did this because in your previous design you didn’t have this 10k so I thought I would try it without this resistor.
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That's correct. You only need the 10K when using the Vactrol.
But, your oscillator circuit is not working. If it was, the LED would be blinking. Are you sure the LED is connected correctly? LED cathode (flat side) must connect to ground. If you connected it backwards then you will need a new LED.
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Problem found. Cold solder on the tube socket. Thank you for the led idea. Very useful. It works with and without the 10k. Should I re-install the 10k?
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Now that the oscillator circuit is working, reinstall the 10K and the Vactrol. Hopefully you will have tremolo.
BTW, the Vactrol has a LED inside it and it flashes just like the LED, but you can't see it flashing. That's why I suggested using the LED as a troubleshooting aid.