Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on November 09, 2016, 11:51:01 am
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I see this once in a while and have always wondered if there was something i overlooked. But i don't think so.
So i have to ask. Here is the issue:
I have a 1971 Fender deluxe reverb / or could be most any other Fender amp with vibrato using an opto photo lamp.
The intensity works fine but the speed pot does not start functioning the vibrato until it is set to at least 4 on the knob (AND) cut's out all together if set above 7.
The vibrato caps are replaced. The optocoupler is new and oscillating voltage is present. The resistors in the vibrato circuit are in perfect spec value.
The vibrato tube is fine. I have also tried other 12AX7 tubes just for GP. (GENERAL PRINCIPLE)
And last the speed and intensity pots along with the 100k resistor to ground read proper as well.
SO! What is it that makes some Fender amps with vibrato more fully functional in pot adjustment ranges?
This is a million dollar question to me and maybe others. Lead dress wires as well have been tinkered with.
No i don't have a million dollars for the answer! :laugh:
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You need a really strong 12AX7 for the oscillator. Get a bucketful and start rotating. Or you could replace the cathode resistor/cap with a 5mm 20mA red LED.
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You need a really strong 12AX7 for the oscillator. Get a bucketful and start rotating. Or you could replace the cathode resistor/cap with a 5mm 20mA red LED.
Great ideas! My tubes test strong but these are newer production tubes. I don't think much of the newer production tubes for consistency. They don't have the balls or maybe say the magic atoms of NOS tubes. I don't know if that came out right but anyway i will try some other older tubes i have. I hate giving them up/selling them.
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What Sluckey said. This form oscillator just-barely works. Needs a gain >29 in the best case, and more at the extremes of the rate pot. HOT 12AX7, GOOD cathode bypass.
The LED bias trick does avoid some problems with bias. Still need a good HOT 12AX7.
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What Sluckey said. This form oscillator just-barely works. Needs a gain >29 in the best case, and more at the extremes of the rate pot. HOT 12AX7, GOOD cathode bypass.
The LED bias trick does avoid some problems with bias. Still need a good HOT 12AX7.
Well PRR im gona find a red hot 12AX7 with audio flames to boot! I think my million dollar question has been answered well.
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The trem circuit will not be nearly as fussy about strong 12AX7s if you put the LED in. I just tried a 5751, 12AT7, and even a 12AU7 in my revibe. The oscillator worked just fine with the lower gain 5751, and would even oscillate over a very narrow range of speed with the 12AU7! I didn't change anything else for this test and I didn't put a meter on it. The blinky LED was my meter. :icon_biggrin: