2 questions: Is "Bias off" the same as putting a tube amp in Standby...and...

[Ramsin]

For the T/R Switch, is it ok to wind the stacked FT50-61s using the same 22awg wire I used for the filter board? or am I desoldering and rewinding them? Thanks in advance! I can't wait to show off my build. I'm planning on incorporating a Nextion 5 inch display as a "remote head"...I think it'll be epic.

[jcve...@gmail.com]

I can't wait to see your build!

The function of 'Standby' probably varies between manufacturers and I'm not a tube amp expert but I seem to recall that 'Standby' meant that the plate and grid voltages were off but the filament was on.

The tandem match circuit is not at all sensitive to wire type so wind it with anything that will fit. The one issue you might have is that the thicker wire might not fit through the PCB which would be a problem.

73,

Jim WA2EUJ

[Randal Warren]

Are you useing an arduino for your project. Bill has been working on the code and I have been beta testing it, life has just been busy for both of us but we should do a write up and send Jim the new version we have come up with.

My amp is all done except I'm battling noise caused by my meanwell power supply. I'm working on adding a DC line filter right now.

One thing to also note in our discovery while testing this the ardrino it is extremely noisy and when useing a 1 wire temp sensor I can hear a clock ticking on the high bands. Adding toriods to every wire that involved the 1 wire temp sensor seems to have mitigated it but occasionally I can find it ticking away.

A 2 wire resistive tempature sensor probably is the way to go. I just haven't had time to swap mine out yet and redo the tempature code.

Randal VE7NHW

[Ramsin]

Man, that's an awesome build! I've been worried about the noise caused by the power supplies, especially since I'm going the 13.8 --> 50v boost route. Andddd including a 12 --> 5v buck. I already planned on making the display a remote/detachable head...but now I'm wondering if I would benefit from putting all the MCUs in the "head" and leave the Switch, Filter, PA boards and the step up in their own enclosure.

[Ramsin]

Anddd this may be the dumb question, what's the procedure to setting the bias...with only a multimeter?

[jcve...@gmail.com]

1. Set the bias pot fully anti-clockwise
2. Terminate the input and output ports with 50 ohms
3. Turn the power supply off
4. Put the multimeter in series with the positive power line, set the meter to read 100 mA DC current (or higher)
5. Turn the power supply on
6. Note the current draw, is should be a few mA through the bias circuit, if not check the fuse in the meter
7. Slowly rotate the bias pot clockwise until the meter reads 90-100 mA higher than the idle current

Notes:

The inrush current to the board may be enough to blow the current fuse in some multimeters. If this happens short the leads of the meter until the power is turned on and stable then un-short them for the meter to read.

The bias current is not linear, nothing happens for the fist half of the pot's rotation then the current comes up quickly.

[Ramsin]

Thank you for that, Jim...it took me a while because I killed my 101....dumb mistake, but it was a $30 lesson well learned. I just received my new 101....and a spare, just in case.

When setting the bias, I had my BK Precision 393 multimeter series'd inline with the PA...with the "pot" all the way counter-clockwise, it was drawing around 1ma. I adjusted until I saw it come up around 100-ish, but when I let it run for a bit, I noticed it started going up on its own. No matter what I tried, I couldn't get it to stabilize, it was either climbing up, or down on its own depending on the direction I was trying to compensate...is this expected behavior?

Also, I know you mentioned it wasn't linear, but I swear it seems that if you look at potentiometer the wrong way, it changes...also normal?

[jcve...@gmail.com]

Bias current is HIGHLY dependent on temperature so if the bias voltage is stable, when the temperature changes, the bias will change. If you want a stable bias; get a good heatsink. Even if you had a perfect heatsink the thermal resistance of the package would still allow the bias to change. The circuit does have a mechanism that reduces the bis voltage as the temperature increases but that is also tuned to a certain heatsink. 

If you have a less than perfect heatsink, set the bias to 90 mA cold then if it goes to 110 or even 120 mA when it's hot that's OK. If your heatsink isn't big enough to let the bias return to 90 mA when the bias is off; you need a bigger heatsink!

73,

Jim WA2EUJ