Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: coreysan on March 23, 2015, 09:08:25 pm
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Could I ask for some help, if possible?! I've just built a Super Reverb kit (AB763), and I'm checking all the voltage readings
to compare with the schematic.
All voltages match except for one very strange place. It's the 12AX7 that's used for the vibrato stage.
The pin 3 cathode is supposed to have 2.1v, and the pin 1 anode is supposed to measure about 280v.
My amp measures a flat 0.0v at the cathode, and a whopping 434v at the anode.
The other cathode at pin 6 is supposed to be around 17v, and the anode side pin 8 should be 390v.
Mine is measuring a flat 0.0v at the cathode, and a normal 390v at the anode.
So its all flat 0.0 at both cathodes. I replaced the tube, and I get the same thing.
I checked the solder joints to ground, and they are continuous.
Does anyone have any ideas what might be going on? I am so baffled!
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Heater wiring.
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What you describe is/are the telltale signs of a tube that simply not conducting. *IF* it was conducting, it would be drawing current. *IF* it was drawing current, you would see (most typically) a couple of volts across the (usually) small 1.5K cathode resistor...like the ~~2 volts you see in the preamp stages....and, in the same way, you would see a voltage drop across the plate resistor.
The most common size of 12AX7 plate resistor is 100K, usually we think of about 1-2 mils going through a given 12AX7, so if we just guess that 1.5 mils is passing through the tube (either half) then across that 100K plate resistor, we should see E = IR = .0015 * 100,000 = 150 volts.
This is exactly what you see on your preamp stages: 2 volts on the cathode, and two tubes which are supplied from a 410 volts source, their plate resistors are 100K's, lo and behold, they have 270 volts on their plates, looking at the schematic. So, there you go, the plate resistors drop 140 volts each. Close enough.
That's for your generic 12AX7. The vibrato circuit is a little different, but the principles are the same. On the first half, we have that (about 2 volts) on the cathode and 280 on the plate. On the second half, the cathode sits a little higher, closer to 7 volts.
It looks to me like your trem circuit is not oscillating. Obviously that tube (both halves) are getting good B+, but it seems like you have trem osc failure. Recheck wiring & parts there. Alternatively, measure grid voltages on the first half-tube (the one with three caps in series---that's the identifying characteristic of the phase shift oscillator.) That tube gets a negative voltage from the bias circuit. If that grid is driven very low for some reason, that tube will be hard-shut-off.
I think you just have an omitted part or connection.
And finally....is the tube lit up?
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Plug in the footswitch and turn the trem on.
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Plug in the footswitch and turn the trem on.
:l2: Excellent!
If I didn't already have a super reverb this is something that I would miss
Interesting that on this design you only get tremelo WITH a footswitch
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That tube gets a negative voltage from the bias circuit. If that grid is driven very low for some reason, that tube will be hard-shut-off.
If the footswitch is not plugged in and turned on that negative voltage will have the tube cut off. That's just the way the AB763 trem oscillator works. No footswitch = no trem.
If you don't have a footswitch, just put a ground wire on the vibrato footswitch jack to enable the oscillator.
Of course if the filaments are not lit you would have the same voltage readings and no trem. I'm betting on the footswitch.
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From now on, whenever you guys see me (Coreysan) log on to ask a question, think to yourself:
"O look - there's idiot friend!"
I told the kit manufacturer I was testing without a vibrato footswitch, and no one complained. But now
here I am - thinking I've got a problem when it's just the stupid switch!!! Once I switched the vibrato on,
everything fell right into place.
So again, after you all stop laughing at me (as you should) just call me idiot. Pizza's on me for the rest
of your natural days!
boy howdy do I feel stupid.
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The AB763 trem footswitch is just a gotcha waiting for the next victim. A lot of us have fallen. There will be more. Welcome to the club.