The Much Anticipated (?) Bluesmaster Build Thread

[Raoul Duke]

“Don't understand this... If pin 2 to pin 7 is 6.2, I would think pins 2 and 7 should both be ~3.1 to ground. Is the only load you have on the heater circuit the panel lamp? What kind of panel lamp is it?”

I thought that was odd…

The panel LED strip is wired per the layout with the 1n4003 and a 511R (vs 330) resistor because the LED was too bright. It’s the blue LED in this table:

https://el34world.com/Forum/index.php?a ... 2876;image

Otherwise there’s nothing else but the ACT wired to those pins; one 150r at V5 pin 7 and one on V4 at pin 2.

[martin manning]

Do you have 150k resistors to ground from pins 2 and 7 instead of 150 ohm?

[Raoul Duke]

Ugh! It appears that I do indeed. I guess that would explain it.

I suppose it wouldn’t be one of my builds without a stupid mistake like this, lol. Hopefully I can limit it to just this one…

I have a Mouser bag sitting here with 2 Koa 150R resistors in it that for the life of me I couldn’t remember why I ordered. Maybe I caught this a month or so ago when I was making a pile of parts and forgot… Or maybe I maybe I’m just a goober…

I’ll get to changing those out now.

[Raoul Duke]

Ok, replaced the heater ACTs with 150R and fired it up. Now I have 6.2vac on pin 2 and 0 on pin 7.

Could the PT wires have been incorrectly marked? The clipped/wrapped heater CT has the white stripe on it and the two I’ve got going to V5 are solid green.

What else could it be?

[sluckey]

Most likely a bad solder joint, broken wire, or wiring error in the filament string. Easy to locate with your meter. DON'T MEASURE FILAMENT VOLTAGE WITH ONE PROBE CONNECTED TO GROUND! Connect one probe to one of the PT green wires and the other probe to the other green wire. Voltage should be as expected. Now follow the filament string to the next tube socket. Measure voltage between pin 2 and 7. Continue to the next tube socket and repeat. Stop when you get to a socket that has a bad voltage reading. The problem will be between the last good reading and the bad reading.

[Raoul Duke]

Understood and thank you for the advice

Currently retracing everything and will report back.

[Raoul Duke]

One thing I noticed was having the diode and resistor for the lamp going to the wrong pins - so I fixed that.

Measuring as sluckey advises gets me 6.4vac and has always been right around that. It’s when I measure each pin to ground that I get the stated discrepancy. If measuring that way isn’t reliable - then I think that might be the answer - but I don’t know enough from a technical perspective to be confident in that conclusion.

Everything has continuity and all the joints are shiny with mechanical connections. I added V4 heater wires to measure at the bare leads and still got 6.4vac, but haven’t soldered them at V4 or added any further down the string.

IMG_0215.jpeg

IMG_0216.jpeg

[sluckey]

Remove the lamp limiter and variac. Plug the amp straight into the wall. Plug in the power tubes. Do the filaments light up? Voltage readings any better?

[sluckey]

Sorry for the filament string wild goose chase. I missed you saying you had correct voltage between the filament pins.

[martin manning]

Ok, replaced the heater ACTs with 150R and fired it up. Now I have 6.2vac on pin 2 and 0 on pin 7.

With or without tubes you should get about 6.3 across the heater string and half of that from each side to ground. Are you sure you don't have any part of that network grounded other than the 2x 150R?

[Raoul Duke]

I’ll go over everything one more time, but I’m fairly certain that I don’t.

Is it safe to measure heater voltage with the leads off the socket and just hanging in the air?

The only other thing I wonder about is the terminal strip where the LED connects. Could the phenolic part have absorbed flux and become conductive? I’ll separate that and measure or just test for ground continuity if that’s as good? Terminal strips seem to be trickier than I thought; the way they wick solder down away from the joint sometimes.

Frustrating.

[martin manning]

I’ll go over everything one more time, but I’m fairly certain that I don’t.

Is it safe to measure heater voltage with the leads off the socket and just hanging in the air?

No problem. Measure across the leads since you have removed the ground reference.

The only other thing I wonder about is the terminal strip where the LED connects. Could the phenolic part have absorbed flux and become conductive? I’ll separate that and measure or just test for ground continuity if that’s as good? Terminal strips seem to be trickier than I thought; the way they wick solder down away from the joint sometimes.

Not likely on the flux absorption. It's not a very good conductor.
The 0V on pin 7 and full heater voltage on pin 2 suggests the pin 7 side is grounded somewhere, but I don't see it in your photos.

[Raoul Duke]

I’ve got the ACT, bias pot resistor, and ground wire from the bias supply board all sharing the same ground lug. Pin 7 has the 150R ACT, the blue wire to the resistor side of the LED terminal strip, and a green heater wire.

If I detach the heater leads and measure across them, aren’t I essentially repeating measuring across the pins?

[martin manning]

Yes. If you disconnect the PT heater leads you should measure ~150R to ground from pins 7 and 2. There are two diodes and a resistor across those pins, but that shouldn't affect the measurement significantly.

P.S. If you measure pin 2 or 7 to ground with the heater leads connected, you should see ~75 ohms.

[Raoul Duke]

Understood.
I get 150 from 7 to ground, but open from 2 to ground. When the heater leads were attached, I was getting 150 on either/both sides. Here’s a close up of the socket:

IMG_0217.jpeg