My Build - Switch On Looms!

[Shawnobi]

Ok, so.
From my schematic I will refer to the upper power tube as 'B' and the lower tube (with the bypass cap/res) as 'A'.

Voltage Checks
Pins 3 4 5
B 493V 495V 0.02V
A 495V 490V 0.18V

Resistance Checks
B Pin 5 to ground = 240K
A Pin 5 to Ground = OL.

Pin 8 to ground on both pins is 470R

So voltages I believe look ok (a few points positive nothing to worry about on pin 5?)
I've checked the colour code on the 470K resistor and its Yellow/Purple/Black/Orange/Brown so should be a 470K from pin 5 of tube A. So don't understand the low reading.
With regard to the reading of pin 5 on tube B, I wouldn't expect resistance through a cap??

I didn't move onto the tube tests as there is clearly something amiss already!

[Jazz P Bass]

I've checked the colour code on the 470K resistor and its Yellow/Purple/Black/Orange/Brown so should be a 470K from pin 5 of tube A. So don't understand the low reading.
With regard to the reading of pin 5 on tube B, I wouldn't expect resistance through a cap??

!

Pin 5 of tube 'B' has more than one path to circuit ground.
Follow it on the schematic & you will see it.
470K, 7.5K & the treble pot. (depending on where the wiper sits in it's rotation, this will be in parallel with the other two, thus affecting the reading).

As there is not any significant voltages on pin 5 of either socket, it's tube time.
Follow Enzo's troubleshooting path.

[Shawnobi]

Pin 5 of tube 'B' has more than one path to circuit ground.
Follow it on the schematic & you will see it.
470K, 7.5K & the treble pot. (depending on where the wiper sits in it's rotation, this will be in parallel with the other two, thus affecting the reading).

.

But as there are capcitors on both paths I would not expect to see resistance with ground (Unless I'm mistaken).
So I'm still a bit confused by the resistance readings as they don't tie up with what Enzo said to expect. Though I'm off to re-measure with the treble pot rotated.

[g1]

Your 240K reading at one of the pin5 is suspiciously about half of what should be at either tube. And half is what you would get if the two 470K's were in parallel.
Any chance you installed that 470K on the wrong tube?
Some photo's of the power tube sockets and connected parts would be helpful.

[Shawnobi]

Your 240K reading at one of the pin5 is suspiciously about half of what should be at either tube. And half is what you would get if the two 470K's were in parallel.
Any chance you installed that 470K on the wrong tube?
Some photo's of the power tube sockets and connected parts would be helpful.

Thank G-One. I'd mixed up the pin 5 wiring between tubes. Switched that and she works!

Though very quietly, so I need to looked into that. No hum whatsoever. But quite quiet, turning the volume just adds distortion with no rise in volume. But technically something I built from scratch is amplifiying a guitar, so I'm celebrating

[g1]

So are you now measuring approx. 470K from each pin 5 to ground for the power tubes?
If so, time to measure DC for all the tubes on all the pins shown on schematic. Post results.

[Shawnobi]

So are you now measuring approx. 470K from each pin 5 to ground for the power tubes?

Yes indeed.

If so, time to measure DC for all the tubes on all the pins shown on schematic. Post results.

Will do this tomorrow evening now as I have to go out.

[Enzo]

Jazz, look again at the treble pot. The wiper is grounded, so the setting of it will have no effect on the grid return circuit. The 7.5k does not ground through that pot.

[Jazz P Bass]

Yes, you are correct.
The 7.5K in parallel with the 500K pot, even at it's furthest travel, would still equal 7.3K

[Enzo]

No I mean they are not in parallel at all. The only connection they share is both are grounded. The wiper and lower leg of the 500k are wired together and grounded. The upper leg of the 500k has no connection to the 7.5k circuit.

[Shawnobi]

Seems I spoke to soon. The problem hadn't gone away, it was just happening slower. The 250 Ohm grid resistor started to burn up as I was taking readings.
Here they are anyway:

[Enzo]

OK, that is more evidence.

There should be no positive DC on the grids of the power tubes. But you show +1v on one grid, and a little on the other. That is not good.

Now note the two plates on the 12AX7 have substantially different voltages. They should be about the same.

Put those two clues together. There is a cap from pin 6 plate on the 12AX7 to the grid of power tube A with the positive grid voltage. I suspect that cap is leaky, which allows some positive voltage to leak through, and at the same time it allows current to flow through it that drags the plate voltage down.

of course there are other possibilities.

[Shawnobi]

I've also done the tube test now measuring across the resistor. Tube one ramps up to around 25v, then starts to rocket up from there. I shut it down at 50v. Tube 2 gets to about 21V and pretty much sits just creeping slightly. Same result in both sockets.

[Enzo]

If you have one tube that runs away in either socket, and a second tube that works OK in either socket, then that first tube sounds faulty

[Shawnobi]

Is there a fair chance I have damaged it? Reason I ask is bought from a local shop who claims to test his valves before sale. I like to support him so don't want to claim he has sold me something dodgy if there is a reasonable chance I have broken it. Though in fairness I don't think my wiring error could have killed it as the results of the failure have been consistent.

Will check that cap too as soon as I can and around the 12AX7.