2020 Monkeymatic: Bogen DB114

[Phil_S]

So why only drop the voltage to the 6U8? All the other tubes are connected directly to the filament winding as usual.

I confess that I didn't look at the schematic. That seems rather odd. Do you actually think each filament connection went directly back to the PT (hub style) rather than the typical parallel (ladder style) wiring harness we normally use? Would that make any difference? I think, if you put that 2.7R anywhere in the filament circuit so that it is in series with the filament winding, it has the same effect, it loads down the circuit and eats some voltage. All tubes are affected equally. No?

[sluckey]

I confess that I didn't look at the schematic.

Maybe you should.

[xtian]

Progressing nicely.

IMG_4012.jpg

[Phil_S]

I confess that I didn't look at the schematic.

Maybe you should.

I agree, I should have looked before the first post. I wasn't clear. I looked before the 2nd post where I postulated that it doesn't matter, that anywhere in the circuit in series with one leg of the winding affects all tubes. But I don't really know. I'm asking because I don't have a technical background. In any case, it seems a strange place to locate the 2.7R.

[martin manning]

it's an odd thing for sure. I'd guess the 2R7 would drop the filament voltage on that tube by about 1V, lowering the plate current and the gm in both sections. It would also cause a delay in starting up, which might be desirable.

[sluckey]

I agree, I should have looked before the first post. I wasn't clear. I looked before the 2nd post where I postulated that it doesn't matter, that anywhere in the circuit in series with one leg of the winding affects all tubes. But I don't really know. I'm asking because I don't have a technical background. In any case, it seems a strange place to locate the 2.7R.

Take another look. The 2.7Ω is ***NOT*** in series with one leg of the winding. It's only in series with pin 4 of the 6U8. Therefore, only the 6U8 will have a lower filament voltage.

[xtian]

Finished, and working OK, but not perfect. Need to boost the highs and tame the lows a bit. Too much hum. Original design had gNFB--I may implement that. Or maybe a PI filter (CLC).

Attached schematic shows actual component values and measured voltages. I've seen, in other cathode biased amps, that the screen voltage is sometimes higher than the plates. I think I want to lower my screen voltage. Opinions?

Screen Shot 2020-06-28 at 8.35.07 PM.png

[xtian]

Finished up. Made quite a few changes, all shown on attached schematic. CLC filter helped a little with hum, and gNFB solved the last bit (I demand that my amps are near silent at idle for home playing). She's putting out a little north of 13 watts clean. I used the panel switches for Gain bright cap, V1b cathode bypass, and gNFB. Other than the cathodyne PI, it doesn't resemble the Supro much, but I'm very pleased with the sounds.

Screen Shot 2020-06-29 at 6.10.59 PM.png

IMG_4026.jpg

[Colossal]

Cool amp, man! Nice repurposing. Bet it sounds good. You mentioned adding a CLC filter, but your scheme says CRC up front. Did you swap that 470R out for a choke? Did you end up increasing the screen resistors or the 1k in the power supply to lower the screen voltages?

[xtian]

CRC, yes. I'm used to saying CLC, because we so often use a choke and resistor interchangeably.

I increased the common screen resistor from 470R to 1K5, and kept the individual 100R screen resistors. I guess there's so little screen current, the voltage drops very little.

[sluckey]

CRC, yes. I'm used to saying CLC, because we so often use a choke and resistor interchangeably.

Everytime I hear/see reference to CLC I jump back 45 years when I was traveling with a team of depot level radar techs. Our favorite drink was CLC, Canadian Lord Calvert.

[Colossal]

CRC, yes. I'm used to saying CLC, because we so often use a choke and resistor interchangeably.

I increased the common screen resistor from 470R to 1K5, and kept the individual 100R screen resistors. I guess there's so little screen current, the voltage drops very little.

Nice. Thanks for the update!