First D-style (102) build opinions solicited & build thread:

[martin manning]

-42 seems a little high. Can you reduce the value of the resistor between the PT and the bias rectifier? I think you might need another 10V or so.

The additional 10V would increase the sweep on the -42 end then, correct?

Correct. I'm guessing you'll get about -52 at the low end, and that that might just be low enough. It depends upon where the B+ settles out under load, and the tubes that you have.

[Raoul Duke]

Ok, I think I did all right, but will take any advice/suggestions on offer.

Did the complete power amp test as outlined on page 11.

Slight hum and “break-in smell” but nothing alarming. Power tubes and PI glowed nicely, no arcs, no smoke.

B+1 on the plates was 488, biased at 39.8 (V5) and 39.5 (V4) mV measured at the cathode bias sensing resistor. Bias board has the 3k3 parallel with the 3k9, bias pot after biasing sits at about 1/5th of rotation from max negative voltage. All power supply caps were under 8 mV within 10 seconds of power down.

1. Does this sound like I’m in the ballpark?
2. Bias board resistor final value as close to the 1k8 in there now as possible?

[martin manning]

That all sounds good! The numbers you quote are off the limiter, correct? Filters will drain quickly since the tubes will stay hot and conducting for a little while. You can find a 1k8 resistor (or even a bit less (1k5?) since you have room on your bias pot, and solder it in. Just for interest, what is the bias voltage?

[Raoul Duke]

Correct - off the limiter.

For bias voltage, am I measuring off pin 3 or pin 5?

Also just noticed that when coming out of standby, V5 (furthest to left from back) does a bit of a white flash near the tube base. Is that a bad sign?

[Raoul Duke]

1. Doesn’t apply because I only have 2 power tubes? Correct

2. Pin 8 on each tube - separate measurements, so 2 DMMs make it easier. No, this measurement is from one cathode to the other, pin 8 - pin 8
So one probe from the same DMM on each pin 8?

3. This is the audio adapter/signal generator app part (400 to 1000Hz sine wave). Yes, but injected at the PI input
Power amp input?

4. This is the measurement from step 2 set-up. Yes, monitor the AC voltage across the load as you increase the signal level, looking for ~12V RMS
Increasing the signal at the PI trimmer on the main board?

5. Straightforward - no questions. Remember, DC mV

Last question: do you still tweak by ear after final assembly, or should this be “it” (in your experience)? Personally, I call it good when the measurement indicates balance

All of the above is done after you set the static operating point (bias).

Sorry to keep drilling into this, just wanted to make certain I understand exactly what I’m doing.
Here’s my signal generator “in”:

IMG_0175.jpeg

Thanks!

[GAStan]

Also just noticed that when coming out of standby, V5 (furthest to left from back) does a bit of a white flash near the tube base. Is that a bad sign?

Do you have a speaker or dummy load plugged in? I didn't on my first power up and saw a similar flash.

[martin manning]

...just wanted to make certain I understand exactly what I’m doing.
Here’s my signal generator “in”:

Looks like that last adapter plug is combining the stereo output of the Bluetooth into mono, which should be fine. Did you measure the output voltage from the adapter?

For bias voltage, am I measuring off pin 3 or pin 5?
Also just noticed that when coming out of standby, V5 (furthest to left from back) does a bit of a white flash near the tube base. Is that a bad sign?

Pin 5, or the junction of the 220k's on the preamp board.
That flash you are seeing is inside the tube?

[Raoul Duke]

Yes, inside the tube. It’s not a huge flash. Load resistor (verified at 8.3R) into the main speaker out. I just looked at it again with the lights off and it’s both tubes, but ever so faint. With the lights on - you probably wouldn’t notice it.

What’s the best way to measure the output from the adapter? Clamp test leads to the 1/4” when it’s receiving? Just a guess.

Pin 5 on both power tubes is -52vdc and -53.8 at the 220s.

[GAStan]

On my amp with 6L6's an initial glow that quickly diminishes, kind of a purple color, seems to be normal.

What I had with no load on the OT was a bright white flash. This was on my second amps initial power up, a 183 with EL34's.

[Raoul Duke]

Yes, more of a faint purple glow. Exactly what I’m seeing.

I’m probably just being super paranoid, lol.

Thanks Glenn!

[martin manning]

Purple glow is fine. Any time you power up with tubes in you need to have a load connected.

To measure the Bluetooth output just connect it and run a 400 Hz sine into it from the app. Put the DMM probes on the plug and measure AC voltage.

[Raoul Duke]

Thanks again Martin, I’ll check it out now.

Does the bias voltage look right? Also, did you see my questions in blue regarding the PI process? I don’t mean to make it harder than it is - just want to make sure I understand. Being so close, I don’t want to mess anything up.

[martin manning]

Thanks again Martin, I’ll check it out now.

Does the bias voltage look right? Also, did you see my questions in blue regarding the PI process? I don’t mean to make it harder than it is - just want to make sure I understand. Being so close, I don’t want to mess anything up.

The voltage doesn’t matter, it’s the current (mV on the 1 ohm resistors) that counts. I was just looking for some indication of the tube emission.

On the blue questions, yes, yes, and increasing signal at the PI input to get more output power.

[Raoul Duke]

Understood, thanks

On the receiver at 400hz, I’m getting 1.1vac output clipping DMM test leads to the tip and sleeve.

[martin manning]

It would be nice to have a bit more signal level. I don’t know if it’s the Bluetooth adapter or the signal generator app that is limiting the output. Maybe try a different app and see if that makes any difference? The app I used is called Function Generator Pro.

Also try measuring the output of the Bluetooth adapter without the 1/8" -1/4" stereo-mono adapter plug. If the output is still less than 1.5 VAC you can still use it to get a reading on the PI balance, and later you can use it to feed signal into the preamp input as described in the PI trimmer thread.

Since you have the power amp working and biased, you can plug a speaker in and run signal into it to see what it sounds like.