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

[Raoul Duke]

So I’m sitting here reading the user manual for an old Behringer UB1002FX mixer I forgot I have trying to figure out if I can run the signal into it - then into the power amp. I think I’ve cracked it?

With the I-pad headphones out sent to the CD/Tape In, through the mixer to the main out, I can get 3.5vac measured at the tip and sleeve of the 1/4” plug I’d send into the power amp in.

I see no reason apparent to my untrained eye that says this is a bad idea, but wanted to throw it out there before I hook it up to my amp.

I’m soooo close…

[martin manning]

I think that will work fine, assuming your app and mixer can produce a clean sine at the power amp in, and that's probably a pretty good bet.

[Raoul Duke]

Thanks Martin

FWIW, I tried a half dozen apps including the recommended one and the most I could get from any of them was 1.2+/- .04 vac - no matter which combination of adapters or apps I used. That spurred me to try and think outside the box a little - which led to the mixer experiment.

Always learning something new every day. Speaking of…

I’m going to follow your notes in Gil’s process, so I should go into the “power amp in” with 2vac to start and increase it until I see 18w from the DMM probes in step 2 using the equation Vrms = (P x Rl)^0.5 or am I misinterpreting this?

[martin manning]

Yes correct. Dial up the signal level until you get ~12VAC on the 8 ohm load resistor and you will have 18W output.

[Raoul Duke]

And once I’m dialed up to 12v, switch over to mV and adjust the trimmer until I reach 0, correct?

[martin manning]

Correct, with 12 VAC on the load, measure mV DC across the power tube cathodes, from pin 8 to pin 8, and adjust the trimmer for 0 mV.

[Raoul Duke]

I must be doing something wrong here. I went back and read through the process and notes, but I’m doing something wrong?
Here’s my set-up:

- Apx 2-5vac going into the “power amp in” jack - 400hz sine wave
- 8R load at the main speaker out, impedance switch set at 8R
- test grabbers from one DMM (set to vac) hooked to pin 8 of each power tube
- Amp volume at half way, all tone controls at “4”
- Mains-on, Standby-operate

I get voltage readings that fluctuate from 0.007 to 0.026vac (not even close to 12)
I can hear the signal emanating from the chassis
The load resistor gets really warm (hot)

[martin manning]

The DMM on the load resistor should be reading 12 VAC, and the resistor will get hot. The reading across the cathodes should be near zero volts DC.

[Raoul Duke]

Thanks as always Martin, you’ve gotten me to think through and figure stuff out (even though I’m sure I’m frustrating to guide, lol).

I hooked a second DMM up to the load resistor and got it to “hover” where 12vac was in the middle of about a 1.2 volt swing in either direction, then with the grabber DMM on pins 8 - adjusted the PI pot down to “0”. The pot didn’t have much swing either; about +4 hard left and -3 hard right. Ended up slightly right of center.

Everything is running smoothly and surprisingly quietly (so far). I guess the power amp to include the PI is good then?

What next? Any particular order to connecting what remains?

Thanks again for your guidance and patience!

[martin manning]

Ok, sounds good. The PI trimmer was able to move the pin 8 to pin 8 voltage to +4 and -3 mV? That's about right. You can plug a speaker in and make sound with it.

From here I'd finish up the preamp board and FET board connections and complete the heater ladder. Then you can check out the preamp and FET voltages with everything operating.

[Raoul Duke]

Sure thing

Thanks again!

[Raoul Duke]

Quick question:

Is it wrong to use pin 5 on pre-amp sockets for the next heater in line if you jumper (vs. twist) it with pin 4, or should the heater wires all go to the same pin?

Yep, new guy question, but I’m almost done with the wiring

[martin manning]

As long as the two lugs are jumped together there's no reason you can't use both of them to connect leads. I've often soldered a staple-like jumper across pins 4 and 5 before installing the heater leads.

[Raoul Duke]

Thanks Martin, I did the staple thing as well. The 18 gauge solid core was giving me a hard time, but I figured it out without too much trouble. There’s plenty of room in the pins on the Belton sockets I have; I was just curious for future reference.

All righty then…
Pretty much there I think. Anyone see anything glaringly dangerous or wrong?

IMG_0350.jpeg

[martin manning]

You could do a last-minute resistance check from the power supply nodes to ground just for some reassurance, but I'd put the tubes in and power it up, with the limiter first, and then check voltages on the preamp nodes and preamp tube plates at full line voltage. Get all the supply node voltages too. The chances of anything going up in smoke are very much reduced now since you have checked out all the high-power, high-current stuff.