Need help tweeking a schematic for 6k6 amp

[RyanThomas13]

Ok I think I got everything.

Yes I also think its best to have individual bias pots, its not that much more work. I will also add in the extra fuse as suggested.

Just want to clarify on the OT.

I could get the Hammond 1760H (deluxe reverb) which is 20 watts, 6600 ohms with 4-8-16 ohm taps. For flexibility with speaker selection I could use an ohm selector on the output and leave the 16ohm wire unconnected, take the 8 ohm wire connect it to the 16 ohm position, and the 4 ohm wire and connect it to the 8 ohm position. Just double checking this is correct?

[cdemike]

Just want to clarify on the OT.

I could get the Hammond 1760H (deluxe reverb) which is 20 watts, 6600 ohms with 4-8-16 ohm taps. For flexibility with speaker selection I could use an ohm selector on the output and leave the 16ohm wire unconnected, take the 8 ohm wire connect it to the 16 ohm position, and the 4 ohm wire and connect it to the 8 ohm position. Just double checking this is correct?

Yup, that's correct. The idea is that there's no intrinsic resistance in a transformer, but rather ratios of windings in the primary vs secondary. You need 2x higher reflected impedance than the "design ratio," so by doubling the speaker impedance, you double the reflected impedance.

[RyanThomas13]

Ok, so the amp is finally "finished". I've been slowly picking away at it over the last two months. I've gotten as far as putting it in standby mode but haven't gone further because the negative voltage for the bias is only at -0.25, I was expecting something in the -45vdc range. I went back to the schematic I was using for this (which again was a modified version of Mark Huss's Plexi6v6 schematic) and double checked everything. (see the attached power supply schematic)

6K6 power supply.png

So we agreed earlier in this thread that the Hammond 261M6 would be a good PT for this because the voltages where lower, which was necessary to accommodate the lower power handling of the 6K6 vs 6V6. But one thing I noticed is that the 261M6 does not have a heater C.T. or secondary C.T. as is shown in the schematic, I don't know what difference this makes but I thought I mention it.

I pasted in and used a two control bias circuit that I've used on a 50 watt EL34 amp, but I knew that I would likely have to make adjustments once I saw where negative voltage was at. But could the bias circuit values be so far off as to account for almost no negative voltage? Where's the best place to start troubleshooting?

[martin manning]

I pasted in and used a two control bias circuit that I've used on a 50 watt EL34 amp...

The bias supply needs to be capacitively coupled to the HV when using a FWB. See: http://www.valvewizard.co.uk/bias.html, bottom of the page.

[RyanThomas13]

Ok I just read thru it, I think I understand, I will modify my schematic accordingly and re-upload. Thank You!

[RyanThomas13]

Is this correct then?

6K6 power supply.png

[RyanThomas13]

Head scratcher ...I implemented the revised schematic and still only -0.2vdc.

[johnnyreece]

What voltage are you getting right after the diode in the bias supply?

[sluckey]

Prior to installing the Hammond 261M6 did you ever use a different PT with a center tap connected to ground ***AND*** connected to a FWB rectifier as shown in the attached pic? If so, the two diodes circled in blue are now open. That will prevent you getting good bias voltage.

[RyanThomas13]

@sluckey @johnnyreece, I will check on both these tomorrow morning and report back, gotta take my kid to his baseball game.

[LOUDthud]

You can't run a full wave bridge with the minus side grounded and ground the center tap. Basic Power Supplies 101.

[RyanThomas13]

Did some voltage checking, this is what I found.

6K6 power supply.png

[sluckey]

Change the 100nF to 47nF. Change the 33K to 56K. Replace the 1N4007. Your voltages should look like this...

[RyanThomas13]

I changed the cap to 47nf, the resistor to 56k, and I swapped in a new diode. The negative voltage after the diode changed only slightly to -0.177.

With the voltage information that I have obtained so far can we eliminate the PT and the FWB as both working correctly?

Can the bias circuit after the diode account for the low negative voltage?

[sluckey]

Can the bias circuit after the diode account for the low negative voltage?

Sure. But the circuit in your schematic is good. Maybe your actual circuit is not wired the same as the schematic. Maybe the 100µF cap is installed backwards. Maybe the diode is installed backwards. Disconnect the 10K so the only thing connected to the diode's anode is the cap. Got voltage now?

Did you verify the FWB diodes are all good? How much B+ voltage do you have at the choke?