Preamp heaters meter 3V, but no glow and no sound. What's up??

[wcrankshaw]

Any help here would be awesome. I just finished up a 5g9 tremolux, fired it up and got glowing normal power tubes and and the 12AX7 phase inverter. Unfortunately the 12AY7 and 12AX7 in V2 are not glowing or warming up at all. The even stranger thing (to me, at least) is that the pins where the heater wires are hooked up are still reading 3ish volts. Even the pin of the tube shows the same voltage, so it's not a continuity issue in the socket. It is also exclusive to those two sockets, independent of what tube is inserted. Any ideas on what to look for next? Is the filament part of the tube mutually exclusive of everything else in the tube, or could another faulty connection coming into the tubes cause the heater to also not work? Thanks in advance.

[Enzo]

Two things. When you say 3v, I must assume you are talking 3v to ground. But what matters is not voltage to ground, but instead, voltage across the heater. SO in the case of a 12A?7, pins 4,5 are wired together and ther is 6v btween them and pin 9. That is what matters.


And then, both the tube heaters themselves and the heater winding on the transformer are very low resistances. Those tubes that don;t light? Pull them from the sockets, and now measure for 6VAC between pins 4/5 and 9 ON each socket. If the connection from a previous working socket is broken, then the low resistance of the tubes will connect the two sides of the socket together. SO your meter will measure the "good" 3v on the one side, and hte low filament resistance will allow hte meter to measure that same 3v on the other side, and it will look the same as if the misssing side w2ere still there. But only with respect to ground. AAcross teh tube, you will find the 6v is missing.

[wcrankshaw]

Much appreciated. You're suggestions are always helpful...however, this turned out to be a much dumber mistake that just needed a fresh look after several hours away. But, now I have a fresh new exploding capacitor problem as detailed in a new thread...

[J M Fahey]

however, this turned out to be a much dumber mistake that just needed a fresh look after several hours away.

Ok, what was it, after all?

[wcrankshaw]

I had wired the filament from pin 9 on the phase inverter to pin 1 of V2, and then pin 9 of V2 to pin 1 of V1... I guess the 3V was just going on around the wire from pin 4/5 of each preamp tube. Like I said, dumb mistake, but it took not looking at it for a few hours to notice the error.

Now the issue is that the tremolo has no effect. I can't find an error in the wiring, but the pots do nothing, and there is no tremolo. I used linear tapers in the values specified in the schematic. I've noticed a lot of people using RA taper in tremolos. Would that have an effect on whether or not it worked?

[J M Fahey]

Hi.
Post the schematic which you followed, there may be different versions.
Pot taper does not stop its functioning, only adjustment becomes less convenient.

[wcrankshaw]

Hi.
Post the schematic which you followed, there may be different versions.
Pot taper does not stop its functioning, only adjustment becomes less convenient.

I used the original Fender copy. I already had to sort out the discrepancy of the polarity of a selenium rectifier compared to a modern diode... I won't be shocked if there's something else that needs help. The one thing I'm concerned about in particular is the 100k resistor that is on the speed pot. If it has a bad solder to the pot body would that kill the entire trem circuit? Thanks in advance for the help.

[J M Fahey]

If that 100k resistor were poorly soldered you would not be able to adjust speed (it would stay on "slow"), buy it would still oscillate nevertheless.
1) Check you have those 260V (+/- 20%) where stated on the 12AX7 oscillator/buffer.
2) Using a needle multimeter (you should own one, you dabble into vintage equipment) check for oscillation on both points (your needle should wiggle back and forth)
3) There's a 1M resistor feeding the "Depth" pot, you should see oscillation on both its ends, although less on the pot side.
4) Putting Depth on "10" you should have oscillation on its cursor (which now touches the "Hot" lug)
5) If you have a few volts of oscillation there, you should have tremolo.
In a darkened room, any blue glow (however faint) in the 6L6 tubes should pulse up and down following tremolo oscillation.
Good luck.

[wcrankshaw]

thanks for the ideas. I'll check that out later on today and get back with what happened.

[wcrankshaw]

EDIT: I found an error with one of the capacitors that connect to the 1M resistor and the trem jack. It was hooked up backwards from how it should have been...

Ok so now I have trem, but it's oscillating a loud grounding noise instead of the guitar. Is that something as simple as one of the trem pots isn't properly grounded?

[wcrankshaw]

I've narrowed down the trem noise to a confounding issue (to me). There is a wire which runs between the depth knob and the 82K resistor in the bias circuit. When I touch the wire with my hand (on the insulation), or touch the exposed part with a pair of pliers, the noise goes away and I'm left with great sounding tremolo. I tried making the connection with a different wire, two wires, etc. etc. I feel like this is some super obvious issue that I just haven't learned about yet.

In addition to the noise, I'm now seeing really high voltages. The plate on the 6V6 is reading 491V....way too high. Everything else is 30-40V higher than it should be on the schematics. EDIT: Realized the HV tap was about 120V higher than it is marked on the spec sheet...so I switched to the lower voltage tap which was right on the money.

The 82k resistor in the bias circuit is showing the same voltage -69V on both sides. There is no significant voltage drop across the resistor...why??

[wcrankshaw]

It seems that there isn't the appropriate voltage drop happening in the bias circuit before it passes through the depth pot and onto the power tubes. the power tubes are also showing -69V on pin 5 rather than the -28V they should.

[madkatb]

Check the electrolytic cap to ground from the one end of the 82k. Remember it's grounded "backwards" ( positive end to ground) cos the bias circuit is a negative voltage.