Can A Fixed Bias ckt. Diode be too Big? No -DC voltage

[JustMike]

 I don't think so, but I'm grabbing at straws here. I'm getting no DC voltage on the diode's anode. I'm sure I'm just not seeing something simple. Please someone, make me feel foolish.

[dude]

Where's your rectifier..? If there, do you have A/C before and after the bias range resistor? If so, maybe the diode is bad.


Modified: I didn't know that was a "full wave bridge rectifier pre-assembled", didn't even know they sold them. That's one complicated schematic.. 

[sluckey]

I don't think so, but I'm grabbing at straws here. I'm getting no DC voltage on the diode's anode. I'm sure I'm just not seeing something simple. Please someone, make me feel foolish.

That bias circuit will not work with a FWB B+ rectifier. You're missing a capacitor. Study this schematic to see what you must do...

     https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm900_25xx_45xx_50w.pdf

[JustMike]

 Thanks again Sluckey- This is new to me and I didn't get it just by looking at the Marshall schematic. But I found Valve Wizard's article on "Capacitor coupled bias supplies" with an explanation here; http://www.valvewizard.co.uk/bias.html
 He does a pretty good job of explaining it for goobers like me. It was another case of me not knowing what I didn't know!

[sluckey]

Another good explanation with scope pics is in my amp scrapbook. Take a look at page 9. You will notice that the funky AC waveform that comes from the transformer never goes negative with respect to ground. There is no negative voltage applied to the diode, it is always reverse biased, and therefore no negative DC produced. But when you use a cap/resistor to "shift" the baseline of that funky AC waveform, the resulting waveform swings positive and negative. Now there is something for the diode to rectify. The diode will block the positive portion and pass the negative portion to become filtered negative DC.

[JustMike]

Something is still not right. After adding the cap and adjusting the 1st gnd R to 56k per Sluckey's scrapbook diag, I'm getting 0VDC at the junction of C1, R1 & the diode. For testing, the rest of the ckt is disconnected at the 100k dropper, but I should have some DC here, yes?

[sluckey]

That 100K is not a dropper. It simulates the load on the bias supply. It should be connected in parallel to the bias cap. I think the circuit will work when you connect that 100K properly. Once you have proper negative voltage across the bias cap you can remove the 100K and connect the rest of the circuit. Use the attached for a reliable adjustable Marshall style bias circuit...

[tubeswell]

For bias supply, make sure the diode is reverse-biased (opposite way around compared to B+ rectifier diodes), and make sure the filter caps are +ve grounded (if you accidentally hooked them up the wrong way around (with a -ve ground), they would probably short out and be no good after that)

[JustMike]

I'm stumped. I believe I wired it exactly like the Marshall diagram, but my results are still the same. The only difference from the Marshall is the values of the bias pot (mine is 47k) and it's range(?) resistor (mine is 22k). My values total the same, I think it will just give me more adjustable range.

[sluckey]

Looks like you have it wired correctly. It would be easier to see if you didn't mark it up with the blue lines. Are you sure you are using a .047 cap? And it's good? I'd replace it just to be sure.

And pull your output tubes until you have this bias issue resolved. It's a simple, proven circuit. You should be able to fix it. Just keep looking.

[JustMike]

  I figured it out. It has to do with what's NOT shown on the Marshall schematic. My build has a standby switch BEFORE any Brute force filtration, so the first filter cap isn't even in the circuit. I tested this observation by switching the standby switch on and Whoomp, there it is.
 So, if I move the standby switch to after the first cap and bypass it with a 47k resistor, I should be good. I know there are a lot of people on both sides of the great standby switch debate, but I'm one who likes them. Now the question is; where in the power supply is the best location for a standby switch?

[thetragichero]

http://www.valvewizard.co.uk/standby.html

[sluckey]

Ampeg liked to put the switch between the FWB negative terminal and ground. Like this...

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_g15_gemini_ii.pdf

[thetragichero]

^^^ traynor (at least yba-1 and yba-1a that i've been in) did the same

[JustMike]

Update- Got it working. Something I didn't mention was that the Iron is from an Ampeg ReverbRocket (newer one, circa 1990's?) that ran 6L6's with a 8Ω only secondary. Since I'm running 6V6's I have a 16Ω speaker and it sounds OK. The PT gets pretty hot though. In order to get the bias ckt to work, I put a bleeder resistor across the standby switch. I wonder if this has anything to do with the hot PT?

[acheld]

If you put a resistor across the standby switch, it will allow  a small current/voltage in the HT while in standby.  Usually, this means that you can hear if a guitar is strummed loudly, but otherwise the amp is quiet -- of course this depends on the resistor used, and many other variables.

This resistor should have NO effect when the standby switch is closed (eg, "on").   

So, no, it should have no effect PT temperatures.

[JustMike]

Yes Acheld, that's exactly how it operates...

[JustMike]

 I'm gonna call it finished. And here's the synopsis; Donor amp was a Kustom Sienna solid state acoustic amp. It had inputs for a hi Z, lo Z instrument and an XLR mic input. Speaker complement was a Kustom "custom" 12 and a little tweeter (not a Peizo). The Iron was from an old (reissue) Ampeg ReverbRocket that I bought from an old eccentric junker in Key West for $20.00. I go down about twice a year to play with my old band.
 The circuit is a 6v6 plexi and I added Sluckey's tube buffered FX loop. I mounted the return level AKA master volume on the front panel using shielded wire. Works great and overall the amp is pretty darn quiet.
Also, because the OT was made for an 8ohm speaker with 6L6's, I had an old Marshall 16Ω speaker that should match to the 6v6's thru this Iron. The only thing left to do is to take some Armor All to it to bring out the shine.