no cathode voltage on 2/3 preamp stages

[cooldude666]

Hola again mi amigos,

Worked out my bias supply and non-dropping resistors issues (thank yous very much) and onto the latest, and hopefully (but not likely) last problem of this build. V1a and V2a's cathodes aren't at voltage. Neither of them will rise above ~200mV. What strikes me as odd is that V1b is at a healthy and happy 1.6V. They are all grounded according to my continuity tester. Both of V1's cathodes have 2.2k resistors with 22uF bypass caps, and V2a has a 1.6k with no bypass. Anode voltage of V1b is 260V. I tried cutting the bypass cap out and changing resistors just to rule out faulty components, but sadly no dice. Not one die. I'm starting to lean towards bad socket just because I don't know where else to turn. Can y'all lend a newb yet another hand?

[g1]

Heater voltage is good on both sides of each preamp tube?

[cooldude666]

Yessir, 6.3vac between pins 4 and 5 on each preamp tube. The glow is really, really dim though. They're definitely lit though.

[g1]

Using pins 4 and 5 would be series heater for 12V heater voltage. With 6.3V heaters, pins 4 and 5 of each preamp tube are connected together. One side of the heater winding goes to pin 9, the other side goes to pins 4 & 5.
You should measure 6.3VAC between pins 9 and 4/5.

[cooldude666]

.. Well that explains everything. I'm a doofus. Have I damaged my tubes? Guess that explains why they're so dim eh?

[g1]

No worry, no damage. Shows you how low the heater voltage can get and still some tubes might work .

[Tom Phillips]

...Have I damaged my tubes?...

No. Not by running the heater at half voltage and not by applying plate voltage without proper heater voltage.
Just correct the heater wiring a go.
Cheers,
Tom

[cooldude666]

thank y'all kindly!! after rewiring the heaters (plus i think they're neater than they were before as well) the amp passes signal! now time to make it loud and mean.

[cooldude666]

Actually V2A still has no voltage from cathode to ground. the B side is fine at cool 1.4V. Heaters have been rewired and I read 3.2V between pin 9 and ground, and pins 4/5 and ground. Cathode resistor is an unbypassed 820 ohm 1/2 watt, and my meter shows 820R between cathode and the ground side of the rectifier. What could be going on?

[Tom Phillips]

Have you checked the plate circuit yet? Verify that there is proper voltage on the V2A plate and report. Measure both at the tube pin and at the other side of the plate resistor.

[cooldude666]

What do you mean "proper voltage"? It's very high right now since no current is being drawn.

[Tom Phillips]

What do you mean "proper voltage"? It's very high right now since no current is being drawn.

Sorry for the confusion. My main goal was to find out if the voltage was getting to the plate at all. Since it appears that it is and, by the nature of your response, you seem to understand what's going on I'd say that the plate connection is not the problem.

You have already verified the cathode circuit by directly measuring the resistance to supply ground.

Other things I would try are:
1) Different tube
2) Verify by sight that both heaters are glowing inside the tube
3) With power off & caps drained, make resistance measurements from the top of the tune socket (i.e. with tube removed) of the plate circuit and the cathode circuit.
We are looking for something that has broken the circuit path.

[cooldude666]

I tried a different tube- no change. Here are my actual voltage readings.
384V at supply side
380V after 100k anode resistor.

[cooldude666]

820 ohms from top of socket to ground

[eschertron]

I'd suspect that one of the heaters in that tube still needs some TLC. Either that, or the cathode connection is iffy. Whatever it is, you are drawing effectively no current through that triode.

edit: basically what Tom Phillips said